The hard outer shell of arthropods and insects like beetles is primarily made up of chitin, a naturally occurring biopolymer. The following BiologyWise article elaborates more on the structure, function, and uses of chitin.
Did You Know?
Behind cellulose, chitin is the second-most abundant natural biopolymer in the world.
If one is to observe a lobster closely, he cannot fail to notice its tough outer covering. This protective outer shell, referred to as the exoskeleton is a distinguishing feature of arthropods that include crustaceans (crabs, lobsters, shrimp), arachnids (ticks, mites, scorpions, and spiders), and even insects (beetles, grasshoppers, butterflies). Chitin, a naturally occurring biopolymer is an important component of this exoskeleton. The internal shells of cephalopods and radulae of mollusks are also primarily composed of chitin.
Chitin is essentially a linear homopolysaccharide (long chain polymer) consisting of repeated units of N-acetyl-glucosamine, which is a monosaccharide derivative of glucose. These units form covalent β-1,4 linkages. Chitin with the chemical formula (C8H13O5N)n is considered as a complex carbohydrate, whose structure resembles that of cellulose, with one hydroxyl group on each monomer replaced with an acetyl amine group.
- This skeleton on the outside of the body appears hard and rigid due to the presence of chitin that is known for its tough elastic properties. Although chitin is the dominant constituent, other compounds such as proteins and calcium carbonate also play a crucial role in the formation of exoskeleton. The main function of this chitin-containing exoskeleton is to keep the inner soft tissue safe from any sort of injury.
- Most importantly, it prevents these delicate tissues from becoming dry. In short, it acts as a watertight barrier against dehydration, which is crucial for their survival.
- The hard chitin-containing exoskeleton of arthropods also acts as a defense mechanism against predation. This outer covering can tolerate strong compressive stresses, which can provide protection from predation because predators exert a compressive force on the exoskeleton to injure their victim.
- The fungal cell wall that protects the micro-organism from the outside environment is also made up of chitin.
Chitin is released from the animal’s outer skin (epidermis) to form the protective covering. After the exoskeleton fully develops, the growth of epidermis stops. Moreover, the exoskeleton is found to be relatively rigid, since it does limit growth with the increase in the size of the animal. So when there is a mismatch between the anatomy of the arthropod and the size of the exoskeleton, it can cause suffocation. To avoid this, the animal gets rid of the exoskeleton and begins to form a new one. This process of shedding the current skeleton is done periodically, which is necessary for their proper growth.
As a Fertilizer
One of the most important benefits of chitin is its use in making fertilizers. Chitin-containing fertilizers are organic, non-toxic, and have shown to increase crop productivity. Chitin in fertilizers helps in increasing soil organisms and enzyme activities, which positively affects soil health. This in turn increases crop yield.
As a Food Additive
Chitin has a long history of use as a food additive. It is commonly obtained from crabs, and shellfish that include shrimp. Sometimes cell walls of eumycetes (a type of fungi) are used as a source for extracting chitin. Microcrystalline chitin (MCC) as a food additive can be helpful to enhance taste and flavor.
As an Emulsifying Agent
Use of food chitin can also help in creating stable food emulsions. It essentially acts as an excellent emulsifying agent, which helps to prevent the breaking of emulsion when exposed to other fluids. For instance, whipped dessert toppings often contain chitin that provides uniformity and stability to the product.
This naturally-occurring fiber-forming polymer can also help to lower cholesterol levels as found out through animal studies. Chitin molecules tend to mop cholesterol and fat in the digestive system. So chitin in the diet may help to reduce cholesterol absorption efficiency.
As a Surgical Thread
Chitin is also used for manufacturing strong and flexible surgical threads. Quite a few dissolvable stitches used to close wounds are made from chitin. These stitches start decomposing during the wound healing process. Reports also suggest that stitches composed of chitin may help to facilitate the healing of wounds.
Chitin in its supplemental form may help to reduce cholesterol. Moreover, chitin is said to have antioxidant, anti-diabetic, anti-inflammatory, antimicrobial, anticoagulant, antihypertensive, and anticancer properties. So taking it in the supplemental form may be beneficial for overall health.
Receptor-mediated endocytosis is a process in which receptors are used for importing material from extra-cellular matrix into the cells. This BiologyWise post elaborates more on this cellular process critical for the growth and development of cells.
Did You Know?
If endocytosis, an important cellular process is not taking place properly, it may cause chronic ailments like leukemia and Alzheimer’s disease.
Endocytosis is an important process used by body cells for their survival. It is essentially a cellular process in which cells ingest nutrients in the molecular form. This is how cells eat for their survival. It is a mechanism of nutrient absorption, which is critical for cellular growth.
In most cases, the material that needs to be absorbed by cells is available in the form of large molecules that are simply unable to penetrate the cell membrane. This is where endocytosis comes into picture, wherein the cell simply swallows the substance (molecule) and enters the cytoplasm. Thus, it is through endocytosis that the substances can move inside the cell.
Depending upon how endocytosis is carried out, it is classified into 3 main types, viz. phagocytosis, pinocytosis, and receptor-mediated. Here we will be discussing receptor-mediated endocytosis.
Receptor-mediated Endocytosis (RME)
This type of endocytosis makes use of receptors (attached to the cell membrane) to engulf molecules. It is also referred to as clathrin-dependent endocytosis, since clathrin (a type of protein) is crucial for the proper execution of this cellular process. In this technique, specific molecules that get bound to the receptors can only be engulfed by the cell.
- Whenever a receptor (molecule) that is embedded within the plasma membrane, detects a molecule (that it can bind to) outside the cell, it immediately hooks onto it. The molecule (ligand) attached to the receptor then travels all the way to the clathrin-coated pit.
- A clathrin-coated pit is a special area located on the plasma membrane that initiates the uptake of molecules present in the extracellular region. The pit shows a distinct polygonal lattice of clathrin chains on its inner surface. When the receptor-molecule reaches the clathrin-coated pit, it is observed that the pit folds inwards and then that part of the membrane detaches itself to form a closed coated vesicle.
- These vesicles act as tools to move molecules inside the cells. Simply put, they transport molecules within the cells. If the receptor-bound molecule is a pathogen, opsonization mechanism is activated, meaning the molecule is tagged as a pathogen for subsequent destruction. After opsonization, the protein coat of clathrin is shaken off to allow the vesicle to merge with an early endosome.
- An early endosome is an organelle, a sorting compartment that helps separate the molecule from its receptor. Once the vesicle fuses with the endosome, multiple compartments are formed within the endosome and at the same time the molecule detaches itself from the receptor.
- Chemical changes occur within the endosome to form a late endosome. The late endosome splits into two, in which one endosome contains the molecule, while the other contains the receptor.
- The molecule-containing endosome then combines with a lysosome, which is essentially a membrane-bound cell organelle that stores digestive enzymes. The main job of the lysosome (also referred as the stomach of the cell) is to promote breakdown of the molecule, which can either be a protein or fat. The lysosome can also ingest pathogens including bacteria and viruses, in turn helping to clear cellular debris.
- The lysosome is essentially involved in digesting the material present in the endosome. The receptor in the other endosome is then recycled and sent back to the cell surface.
Body cells use the mechanism of receptor-mediated endocytosis to absorb cholesterol, growth factor EGF, and the iron transport protein transferrin from the bloodstream.
Any change in the normal sequence of DNA is called a mutation. Most mutations in organisms are deleterious by nature. This post explains this type of mutation in a comprehensive manner.
Deleterious mutations do not persist in haploid populations, since they only possess one copy of genes, which if mutated, proves to be fatal to the organism. Thus, the mutated gene is not passed onto further generations.
All living beings possess genetic material that is composed of a sequence of nucleotides. Errors in this sequence are known as mutations, and they exist in the genome of all living beings. It is impossible for the genome of a biological entity to be devoid of any mutations. With each new generation, almost 100 – 200 new errors are incorporated into the genome. Considering the vast timeline of the existence of life on Earth, the amount of accumulated mutations is staggering. To put this in perspective, if we consider humans, each generation includes 100 new errors into the human genome, and at the current population growth rate, each generation of humans on the entire planet has a cumulative 100 billion mutations. Over the years, this vast aggregation of mutations has provided the raw material for the development of various genetic alleles, which increase the genetic variation and diversity, thereby providing the groundwork for the process of evolution and natural selection.
Despite the usefulness of mutations with respect to genetic variability, not all of them are desirable with regards to the overall fitness of the organism. Hence, they are segregated into three types: neutral, beneficial, and deleterious. Neutral mutations have no observable effect on the organism. They merely increase the genetic variation. Beneficial mutations provide the organism with a vital advantage for its survival and proliferation. Finally, deleterious mutations, as the name suggests, pose a threat to the fitness of the organism, as they have harmful effects of the general health of the organism. In general, when considering population genetics, mutations are usually deleterious in nature; very few are beneficial or neutral.
As mentioned earlier, these mutations reduce the fitness of an individual. But what is meant by fitness? It refers to the capacity and ability of an individual to carry out normal life activities, to live a relatively disease-free existence, and to pass on their genetic material to the next generation by reproducing. This is applicable to all living beings, from plants to animals, from microorganisms to birds, and from insects to humans. The fitness of the individual enables it to reach maturity, where its healthy constitution is conducive to reproduction, via which its genetic legacy is passed on to the next generation.
However, in case of deleterious mutation, any one or all of these stages may be compromised, causing the individual to be unfit. The mutation may cause health problems, reproductive failure, and even premature death, depending on the gene in which it occurs. With respect to humans, these three effects can be explained by the following examples.
A mutation in the LMNA gene that produces laminin, a protein that provides support to the cellular nucleus, gives rise to a condition called ‘progeria’. It is a disease that causes accelerated aging, and is characterized by sclerotic skin, baldness, bone abnormalities, growth impairment, etc. Almost all genetic disorders are caused due to the presence of deleterious mutations.
A mutation in the AZF (azoospermia factor) or the SRY (sex-determining region Y) gene gives rise to a non-functional gene-product, resulting in infertility in case of men, thereby inhibiting that particular male to be able to pass his genes to the next generation.
In case of animals like peacocks, who attract mates by the display of their bright tail feathers, any mutation that adversely affects this phenotype would result in the peacock’s failure to attract a mate (peahen) and reproduce.
Mutations that give rise to conditions like spina bifida, metabolic genetic disorders, and Marfan syndrome greatly reduce the quality and length of the affected individual’s lifespan, causing that person to die a premature death. Also, some mutations that occur in embryo development genes result in the premature death of the embryo, leading to miscarriages or still births.
Persistence of Deleterious Mutations
Despite their harmful nature, why do deleterious genes persist in the genome of organisms? This could be due to a number of reasons, such as the rate of elimination of these mutations may be low compared to the rate at which they appear. Other probable reasons are as follows.
This is the condition where the possession of two different copies of a gene (wild-type and mutant) is beneficial to the organism, rather than detrimental. An example of this is the mutation that occurs in the hemoglobin gene, resulting in the condition called sickle cell anemia (SCA). In this case, the homozygote for the mutant allele will show a deleterious effect, i.e, the individual will suffer from SCA (all RBCs will be sickle-shaped). However, if the individual is a heterozygote, the recessive nature of the condition will render him a carrier (Partial sickling of RBCs) of the condition. This is beneficial, since the malarial parasite P. falciparum that infects red blood cells and deprives them of oxygen will be unable to infect the sickled cells and lead to a malarial infection. In other words, the partial sickling of RBCs of a carrier render that person immune to malaria. On the other hand, a wild-type homozygote individual would be susceptible to the malarial infection.
No Effect on Reproductive Fitness
In some cases, the deleterious effect of the mutation is exhibited at a later stage in life, by when the reproductive stage of the organism has already elapsed. Hence, the mutations are passed on despite their harmful nature, as the effect does not interfere or exhibit itself during the reproductive stage. An example of this is the trinucleotide repeat mutations seen in the HD gene that causes Huntington’s disease. In this case, the effects of the disease are seen after the age of 40, and till then, the individual has already reproduced and passed this deleterious mutation onto the offspring. Despite this affecting the fitness of the individual, it persists, since it does not affect the reproductive fitness of the individual, but merely shortens the lifespan.
Maintained by Mutations
Some mutations may keep arising in certain genes despite the elimination efforts taken by the organisms genome. This may be due to the hyper-mutable nature of the gene, and also because the gene maybe too vital to tamper with (to prevent the induction of other accidental errors). An example of this is the NF gene, which when mutated, gives rise to a condition called neurofibromatosis, that causes tumors of the nervous system. Here, it may be difficult to remove the mutation, since any unwanted disruption in the gene sequence will only cause further damage. Also, even in case this mutation is eliminated, the gene does have a high tendency to mutate; almost 1 in every 4,000 gametes possesses new mutations of this gene.
Maintained by Gene Flow
This refers to the prevalence of a mutated gene copy in a population to its introduction by another population that has migrated to the same location. As mentioned above, the SCA mutation is beneficial to areas with rampant malaria, as is the case with the regions of the African continent. However, when the carriers residing in this area migrated to other countries with a low incidence of malaria, the SCA mutation was introduced into the populations of those countries. Therefore, human migration brought about the flow of genetic material from Africa to other countries, where this mutation, in the absence of malarial incidence, was purely detrimental.
Polyploidy of Genome
Deleterious mutations are usually recessive in nature. If a haploid organism possesses a deleterious mutation, the effect can be readily observed, crippling the organisms fitness, and resulting in its demise. However, in case the organism is a diploid or polyploid with multiple alleles of a gene, the detrimental effect can be silenced or overridden by the presence of a fully functional wild-type allele. While this prevents the expression of the mutated allele, it does not eliminate it, causing it to persist in the population, till two individuals with the same allele reproduce and give rise to an offspring that will suffer the deleterious effects of the mutation.
Although the cellular repair machinery, along with the proofreading mechanisms, try to eliminate the mutations, certain mutations are not rectified or are actively conserved (as explained above). The accumulation of mutations, by this way, over the course of several generations, leads to an effect called Muller’s ratchet, which may lead to the extinction of the species of that organism. This effect is a principle studied in reference to the extinction of species, and the effort to conserve those on the brink of extinction.
Genetic diversity is considered to be the baseline of biodiversity. It is the cumulative sum of all the genetic traits or characters present in the genome of any given population or species.
Africa exhibits higher genetic diversity levels than most other areas of the world.
The term ‘biodiversity’ refers to the amount of variation in all biological entities within ecosystems. It is often defined as being the cumulative sum of all genes, species, and ecosystems of a particular region. Hence, it can be studied on three interrelated levels: genetic, species, and ecosystem. However, the genetic level forms the baseline. Genetic diversity in organisms forms the root of any genetic adaptation or variability. It is the basis for evolution and natural selection. The genetic diversity of a species paves the way for the species to be able to adapt to its changing environment. It refers to the possession of various genetic traits in a species, that may or may not be expressed depending on the prevalent environmental conditions. However, this term should not be confused with that of genetic variation.
Genetic variation refers to the prevalence of two or more allelic forms of a particular gene in a given population, whereas, genetic diversity refers to the prevalence of a diverse range of genes in the gene pool of organisms. Hence, variation deals with individuals, whereas, diversity deals with entire populations or species. Genetic diversity can be considered to be a combination of variation as well as variability (tendency for variation to occur in a species). This combination helps biological species to be adaptable to changes in its environment. These changes could be a result of change in population sizes, weather conditions, natural disasters, arrival or departure of competing species, etc.
Sources of Genetic Diversity
In germ-line mutations, the alteration in the DNA sequence is passed onto the organism’s offspring. In time, these mutations may accumulate and cause defects in the genome, which could ultimately prove fatal, causing the line to die out. On the other hand, some mutations may be conserved by the genome, and ultimately, over time, lead to the development of a new trait, i.e., cause the evolution of a gene. This would increase the genetic diversity of that organism. In some cases, the evolution of new genes may have a drastic result, causing the formation of an entirely new species. However, this process is incredibly slow, taking place over a long period of time, although, it is the way true diversity manifests in nature and induces evolution.
It refers to the creation of a new species. The differentiating factor between the old and new species is that, they are unable to breed with each other. Speciation takes place via multiple processes, which include geographical isolation, competition, and polyploidy.
◆ Geographical isolation occurs when natural phenomenon cause a habitat to be split in such a way that a part of the population is separated from the main population, with no means of rejoining it. Eventually, the separated individuals adapt to their new surroundings, and evolve at a different rate and in a different direction than the parent population, causing them to diverge to an extent where they are no longer sexually compatible with the main population. In essence, they have evolved to form a new species. This same effect is observed when a species migrates from one habitat to a new one (immigration).
◆ Speciation by competition occurs when a particular food source becomes limited, or by the arrival of a different species in the habitat that survives on the same food source. In such a scenario, certain parts of the population will undergo changes and adapt themselves to become better at acquiring the food, and protecting it from being taken away. Hence, in terms of species propagation, the better adapted individuals have a better chance at reproducing, due to their enhanced ability to secure food in comparison to the other individuals who have diverted to other food sources. Therefore, the selective breeding of the enhanced part of the species would eventually lead to them becoming a separate species altogether
◆ Polyploidy is observed more in case of plants than in animal species. This is so, because, animal reproductive processes are much more sensitive to gross chromosomal changes as compared to plants. In plants, however, polyploidy is quite a common phenomenon. Polyploidy refers to the presence of more than two copies of a chromosome in the genetic material of the organism. In plants, instances of self-fertilization are quite common, and there are also cases where similar species have been successfully crossed with each other. Due to this, the polyploid effect can be observed instantaneously in the subsequent offspring. The most common example of this is that of common wheat (Triticum aestivum), which contains six sets of chromosomes. It is the result of the crossing between a diploid Triticum urartu and a diploid Aegilops speltoides, which resulted in Triticum turgidum, that possess 4 sets of chromosomes. Finally, Triticum aestivum was obtained by the crossing of Triticum turgidum with a diploid Aegilops tauschii.
Errors in Meiosis
During prophase of meiosis, in plants and animals, crossing over of chromatids occurs. This causes an exchange of genetic material between the two homologous sister chromatids. The intermixing of genes from the parental chromatids causes an increase in the genetic diversity of the resulting offspring. In case the crossed over chromatids do not separate and remain joined during chromosomal segregation, one daughter cell gains an additional chromosome, while the other is devoid of one. This increases the diversity in one cell while, at the same time decreases it in the other.
During meiotic divisions, translocations and chromosomal structural changes may lead to the formation of dissimilar sister chromatids. In such an event, the load of genes being distributed to each daughter cell will vary, causing the induction of diversity in the genetic material of each daughter cell. However, in some cases, these errors might also prove fatal to the daughter cell/offspring.
Factors Affecting Genetic Diversity
Any event or gradual process that culminates in the extinction of a species causes the loss of all the genetic diversity represented by that species of plant or animal. Entire species may be wiped out due to mass extinction events like the Ice Age or the meteor and volcanic eruptions that wiped out the dinosaurs. Species may also go extinct individually due to their excessive hunting as a source of food or valuable commodity. For example, the dodo birds were hunted as a convenient food source since they were unable to engage in evasive tactics. Also, the rhino has been driven to the very brink of extinction by hunting it for its horns. Alternatively, species may experience extinction due to alterations to their ecosystem. An example would be deforestation in order to utilize land to set up cities or factories. This would lead to the destruction of that ecosystem and all the indigenous specious therein. In some cases, certain genes may also go extinct in an organism due to its inability to be transferred to the next generation.
As mentioned previously, competition for food resources may cause the adaptation and evolution of a species. However, this is the positive effect. If the negative effect is considered, there is chance that the newly arrived competing species is stronger and better adapted than the indigenous species. This would lead the new species to out-compete the old species, and alienate them from the food source. The unavailability of a food source would cause the old species to die out, and hence, become extinct. Although, this outcome can be averted by the migration of the old species to a new environment which lacks a competing species.
These events refer to situations where a considerable amount of the population die due to natural phenomena or human activities. The surviving population just represents a fraction of the original genetic diversity of the species, and hence, if the population of the species rebuilds, the overall genetic diversity will also be low. The lost diversity will require a considerable amount of time to be recovered. This is coupled with the side effects of inbreeding and genetic homogeneity, leading to an effect called the bottleneck effect.
Significance of Genetic Diversity
A high diversity is vital, since genetic diversity is directly related to biodiversity. Reduced diversity would eventually reduce the overall biodiversity of an ecosystem, whereas, greater diversity would lead to increased biodiversity due to the adapting and speciation of the species when faced with an adverse environment. If we consider an organism with higher diversity of genes, it implies that it has a higher chance of utilizing its diverse gene pool and adapting itself to change along with its changing environment. This allows it to evolve to be better suited to survive. One can equate this to possessing a fully-kitted-out tool box while carrying out repair work. In case some repair fails, one has the necessary tools to deal with it and overcome the setback. However, if the tool box contains a minimal amount of tools, the repairs becomes difficult to manage, and may even lead to further complications. Similarly, if an organism is not genetically diverse, but is homogeneous and uniform, then in the event of a change in circumstances, it will be ill-equipped to adapt to and overcome the adversity, causing it to die out. Even if such an event does not occur, the uniform gene pool will lead to the side effects of inbreeding, such as accumulation of genetic defects, loss of genetic health, and eventual death of species.
◾ The domestication of wolves into various breeds of dogs via selective breeding has greatly increased the genetic diversity of the canine genome as compared to its lupine origins.
◾ In general, woody plants like trees have a higher genetic diversity than vascular plants like grasses. This is due to the size of their geographic range, and also due to the ability to scatter their seeds over a wide area by the use of various seed dispersal methods.
◾ The cheetah population in the world has a very low genetic diversity, as a consequence of a bottleneck event that occurred 10,000 years ago. This low diversity has led to an increase in lethal genetic disorders and low reproductive success.
◾ In potato cultivation, new plants are formed as a result of asexual reproduction, and hence, are clones of the parent plant, implying that there is very low genetic diversity. This proved to be disadvantageous for the potato plant during the potato famine in Ireland in the 1840s, when potato fields were ravaged by a rot-causing oomycete called Phytophthora infests.
◾ In oceanic plankton, ocean viruses help maintain good levels of genetic diversity by a process called genetic shifting. The viruses carry genes of other organisms in addition to their own genome, and when this viral particle infects a cell, these genes are transferred into the infected cell as well, where the host genome changes and incorporates these genes, thereby gaining genes and diversity.
◾ Numerous varieties of corn have emerged as a result of hybridization between multiple related genera, and also due to the presence of mobile genetic elements called transposes. These factors have resulted in a marked increase in its genetic diversity, which can be observed in the form of the diverse colors exhibited in its kernels.
A high level of genetic diversity of a species is essential for its prolonged survival, despite the changing environment. Hence, in order to preserve and maintain the current biodiversity of the various ecosystems on the planet, appropriate conservation measures must be undertaken. Additionally, research could be carried out regarding ways to increase the genetic diversity of the species that lack it.
Reproductive isolation refers to a set of mechanisms that prevent animals of the same group from breeding. These are reproductive barriers that do not allow the species to mate and produce offspring.
Did You Know?
Reproductive isolation that occurs before fertilization is known as prezygotic isolation. However, when it occurs after fertilization and prevents the fertilized egg from turning into a fertile offspring, it is known as post-zygotic isolation.
Reproductive isolation refers to a set of conditions, that can be psychological, ecological, genetic or behavioral, which do not allow animals of closely-related species to unite and mate. In reproductive isolation, there are strong reproductive barriers that keep the related species separate. Two species belonging to the same family are unable to breed and produce offspring successfully due to various differences, which form an important aspect of reproductive isolation.
Types of Reproductive Isolation with Examples
- Here, the breeding or mating calls act as powerful reproductive barriers. If the closely-related species are unaware of their mating rituals (which are often bizarre), inbreeding does not occur. In behavioral isolation, despite being close to each other there is no sexual attraction between the male and female species. This happens because either of the species is unaware of special signals or the ritual that has to be performed before mating.
- Male hooded seals inflate their nasal cavities like a bubble gum to attract their female counterparts. The female seal will mate the one with the most visually appealing nasal balloon. So unless the male blows up his nasal, the female won’t come near its prospective mate.
- Similarly, light production in male fireflies is necessary to draw female attention. Depending upon the flash pattern produced by the male, the female will then respond and light up to signal the mate.
In mechanical isolation, the reproductive barrier arises due to incompatibility between the sex organs of male and female species. The organs do not fit correctly (probably due to variation in shape or size) or cannot be aligned due to their unique body structure, thereby preventing inbreeding. Mechanical isolation is commonly noticed among snails with their shell coiling in the opposite direction.
- Sexual receptivity in male and female species is possible only if their breeding season is the same. In temporal isolation, mating seasons of the closely-related species do not match.
- In some animal species breeding seasons differ and arrive at different times of the year. Also, some species are sexually active in the morning while their counterparts prefer nighttime for mating. These different times of the day for remaining sexually active are also one of the isolating mechanisms of temporal isolation.
- Some species of fruit flies such as Drosophila persimilis are active in the morning, whereas its close relative Drosophila pseudoobscura prefer mating in the afternoon.
- In ecological isolation, the male and female species prefer a different habitat for mating. The difference in mating sites forms the reproductive barrier in ecological isolation. As their places of breeding do not coincide, the species prefer not to mate.
- Rana aurora, one of the species of frogs found in northern California prefer rapidly moving water streams for inbreeding. Whereas, the Rana catesbeiana species prefer the still, slow-moving waters of permanent ponds.
The reproductive barrier occurs due to incompatibility between the sperm and the egg. As we all know, the interaction between a female egg and a male sperm that produces a zygote (fertilized egg) is necessary to produce offspring. However, the sperm is not drawn towards the egg due to genetic or chemical incompatibility. Gametic isolation helps prevent explosion of population.
Many species of female fish simply drop their eggs into the water. Also, the sperms of male fish of similar species are deposited into the water. The mechanism of isolation prevents formation of hybrid species and ensures that the eggs and the sperms of only the same species combine. Gametic isolation is essentially a hybridization barrier to control the population of similar species.
In this type of reproductive isolation, the closely related species mate and even the female eggs get fertilized. However, the zygote formed is immature, hence suffers an early death. So even if the inbreeding of the species takes place, no offspring is produced due to the immature zygotes.
- An offspring that is produced from two different species is essentially called a hybrid. The term ‘inviable’ means something that is incapable of surviving. In hybrid inviability, the zygote formed from combining of the sperm and egg of two different species is incapable of sustaining its own life.
- The fertilized egg formed of different species does not mature beyond its early stages of embryonic development. The mating of a female cat with a male dog or rabbit may produce a fertilized egg, but it will die shortly due to genetic differences.
In hybrid sterility, different species breed and produce an offspring, which does survive to become an adult. However, the adult suffers from very low fertility, thereby proving to be incapable of giving birth to an offspring. For instance, mating between a horse and a donkey gives rise to a species, commonly referred to as mule, which is found to be sterile.
Due to reproductive barriers, closely related species may remain separated for centuries and evolve to become completely different species. This is a perfect example of speciation wherein the two species cannot mate because they are now distinct species with absolutely no similarity.
The Cori cycle is an important metabolic process that helps our bodies produce the additional amount of energy required by the muscles to perform grueling activity. This BiologyWise post provides a brief explanation about the Cori cycle.
Did You Know?
The Cori cycle is named after physicians Carl and Gerty Cori (married couple), who first mapped it in 1929.
To understand how the human body functions, it is essential to analyze the numerous smaller processes that take place within it. These dependent and independent processes work together in tandem, allowing us to live and perform all our daily activities.
The Cori cycle is one such important process that helps the human body produce the energy required by our muscles when performing a strenuous activity. The following is a description of the working and significance of the Cori cycle, starting with a discussion on how the energy required by our muscles is produced.
Energy Production for Muscle Activity
The muscles in our body enable us to perform all our daily activities, including walking, standing, running, lifting weights, etc. They produce an amount of force which is directly proportional to the intensity of the activity that is being performed. For the generation of this force, energy is needed.
For muscular activity, ATP (adenosine triphosphate) is required, which is produced in a process known as glycogenolysis. Glycogenolysis breaks down glycogen, which is stored in the skeletal muscles releasing glucose.
For most of our daily activities, our muscles combine glucose and oxygen aerobically, in a process known as glycolysis, which results in the production of two units of ATP and two units of pyruvate. ATP is directly used for energy generation, and when enough oxygen is available, pyruvate too is further broken down aerobically for generating more energy. Thus, these two metabolic compounds provide energy at the cellular level to the muscles, allowing them to function.
However, when we perform a highly strenuous muscular activity, the amount of oxygen intake becomes disproportional (much less) to the energy requirement of the muscles. In such a scenario, since oxygen is insufficient, glucose is broken down through an anaerobic metabolism process known as fermentation, wherein, the pyruvate is converted to lactate – a soluble milk acid, and then secreted into the bloodstream.
This allows the chemical process responsible for energy generation to continue without the use of oxygen. In this manner, the muscle cells can produce energy anaerobically in this at very high rates, but only for about one to three minutes, after which lactate accumulation in the bloodstream becomes excessive, which leads to fatigue.
What is the Cori Cycle?
If a strenuous activity continues, the body adopts an alternate metabolic route to get rid of the lactate, and keep producing energy anaerobically. This process of energy production is known as the Cori cycle.
In the Cori cycle, lactate accumulated in the muscle cells is taken up by the liver. The liver performs a chemical process known as gluconeogenesis, to convert lactate back to glucose.
Essentially, gluconeogenesis reverses both the processes of glycolysis and fermentation that the body had performed to produce lactate. This first converts lactate to pyruvate, and then finally into glucose.
This glucose is then introduced into the bloodstream, which carries it to the working muscles, where it is used to feed the additional energy demands of the muscles. The subsequent lactate production by the muscles is again taken up by the liver, and thus the Cori cycle resumes.
In case the muscular activity ceases, the glucose generated in the Cori cycle undergoes glycogenesis to replenish the glycogen stored in the muscles.
Limitations of the Cori Cycle
Using the Cori cycle, the human body is able to convert metabolic by products into a source of energy for the muscles. However, it cannot continue to do so infinitely.
Similar to many other natural cycles, the Cori cycle isn’t a completely closed loop. In the muscles, glycolysis results in the production of two units of ATP. However, the liver uses up six units of ATP to carry out the process of gluconeogenesis. The Cori cycle also requires the initial introduction of oxygen, without which it cannot begin. As such, eventually, the muscles are bound to require a new supply of glucose as well as oxygen.
If a physical activity is too strenuous, the energy requirements of the muscles will exceed the capacity of the Cori cycle to regenerate glucose from lactate. This will result in a condition known as lactic acidosis, which is an accumulation of excess lactic acid in the system. Lactic acidosis brings down the pH level of the blood, which can lead to tissue damage. It also induces symptoms associated with panic, such as hyperventilation, abdominal cramps, vomiting, etc., all of which are the body’s natural defense mechanisms designed to slow down the rigorous activity, and prevent permanent damage from occurring.
The process of spermatogenesis, i.e., the formation of sperms, is an essential part of reproduction in humans and all kinds of animals. In this article, we will learn about where and when spermatogenesis occurs, and what are the stages that the cells need to go through to complete the process.
Did You Know?
In humans, spermatogenic cells need to be maintained at around 2°C below the body temperature to function. This is done by the regulation of blood flow and the positioning of the muscles, which keeps the scrotum away from the heat of the body.
Spermatogenesis can be defined as ‘the process occurring in the male gonad of sexually reproducing organisms, wherein the undifferentiated male germ cells develop into spermatocytes, which then transform into spermatozoa’. Spermatozoa are mature male gametes that are present in organisms which are sexually reproductive, and it is similar to oogenesis in females. Spermatogenesis usually occurs in the seminiferous tubules of the testes in a series of stages, followed by maturation in the epididymis, where they become ready to be passed out as semen along with other glandular secretions. This process begins at puberty due to the actions of the hypothalamus, pituitary gland, and Leydig cells, and the process only ends at death. However, the quantity of sperms reduces gradually with age, eventually leading to infertility.
Purpose of Spermatogenesis
The purpose of spermatogenesis is to create mature male gametes, which can effectively fertilize female gametes in order to form a single-celled organism called a zygote, which eventually leads to cell division and multiplication to form a fetus. Also, to have a healthy offspring, the number of chromosomes must be maintained at a fixed number across the body, for which, failure can lead to abnormalities such as Klinefelter’s syndrome, Down’s syndrome, or abortion of the fetus. Spermatogenesis works to avoid this.
Process of Spermatogenesis
The process of spermatogenesis is very similar in animals and humans. Let us look at each stage of the spermatogenesis process in some detail.
- Stage 1: The original diploid spermatogonium located in the seminiferous tubules has twice the number of chromosomes, which replicate mitotically in interphase before meiosis 1 to form 46 pairs of sister chromatids.
- Stage 2: The chromatids exchange genetic information by the process of synapsis, before dividing through meiosis into haploid spermatocytes.
- Stage 3: In the second meiosis division, the two new daughter cells further divide themselves into four spermatids, which have unique chromosomes that are half in number to the original spermatogonium.
- Stage 4: These cells now move through the lumen of the testes to the epididymis, where they mature into four sperm cells by growing microtubules on the centrioles, forming an axoneme, i.e., a basal body, and some of the centrioles elongate to form the tail of the sperm, facilitated by testosterone.
It is important to note that each division in the process is incomplete, and that the cells are always attached to each other by cytoplasm to allow them to mature at the same time. Also, some spermatogonia replicate themselves, rather than change into spermatids, which ensures that the supply of sperms does not run out. Throughout the entire process, spermatogenic cells interact with sertoli cells, which provide nutrition and structural support to them.
Factors Affecting Spermatogenesis
- The process of spermatogenesis is very sensitive, and can be affected by the slightest change in the levels of hormones such as testosterone produced by the hypothalamus, pituitary gland, and Leydig cells.
- The process is also very sensitive to changes in temperature.
- Deficiencies in diet, exposure to strong drugs, alcoholism, and presence of diseases can adversely affect the rate of sperm formation.
- Stress of oxidation can cause DNA damage to the sperms, leading to problems in fertilization and pregnancy.
The process of spermatogenesis in humans transpire over a time period that is more than two months long. During this time, over 300 million spermatozoa are produced on a daily basis. However, by the end of the process, only around 100 million become mature sperms. It can take another month to transport the new sperms on the ductal system.
Calvin Bridges and Thomas Hunt Morgan discovered the process of nondisjunction in dividing cells in the year 1910. This is one of the most common forms of chromosomal aberration that occurs in humans. This BiologyWise post explains what is nondisjunction, how does it occur, and some of the causes and effects of this condition.
Did You Know
Most human aneuploidy syndromes seem to be of maternal origin. Female meiosis is more prone to error because these cells are arrested in their diplotene phase and have relatively fewer crossovers as compared to that of the male gametes.
Almost all cells in the bodies of higher eukaryotic animals contain two sets of chromosomes―one that is inherited by the mother, and the other is of paternal origin. Such cells are termed as diploid cells (denoted as 2n).
The sex cells, or gametes, are usually haploid in nature. They arise when the diploid cells of the germinal epithelium undergo reduction division, i.e., meiosis. The male haploid gamete fertilizes the female haploid gamete to give rise to a diploid zygote. This zygote may undergo several rounds of mitotic divisions to give rise to a new individual.
Cells usually double their number of chromosomes in the S phase―before undergoing a round of cell division (either meiosis or mitosis). During cell division, there is either separation of the homologous chromosomes (pair of chromosomes derived from each parent) or the sister chromatids (identical copy of the chromosomes generated in the S phase) into newly formed daughter cells and is known as disjunction of chromosomes.
Nondisjunction can be defined as a state in which the chromosomes fail to separate from each other during cell division. This gives rise to cells with abnormal number of chromosomes, and this condition of the cells is known as aneuploidy.
Types of Nondisjunction
Depending on the stage in which nondisjunction has occurred, it can be classified into three types.
■ Nondisjunction in meiosis I
■ Nondisjunction in meiosis II
■ Nondisjunction in mitosis
In Meiosis I
In this process, the separation of homologous chromosomes in anaphase I of meiosis doesn’t take place. This results in two daughter cells carrying an extra chromosome (n + 1), and two daughter cells with one less chromosome (n – 1). The effects of nondisjunction in meiosis I are more far-reaching than that in meiosis II. This is because all four gametes that arise have altered number of chromosomes.
In Meiosis II
In this process, separation of sister chromatids in anaphase II fails, resulting in an uneven distribution of the chromatids into the newly formed daughter cells. If this type of nondisjunction takes place, two cells have normal number of chromosomes (n), whereas in two cells, the number of chromosome has increased by one (n + 1). There is a good chance that this aneuploidy might go unnoticed in females as only one of the newly formed daughter cells develops into an ovum.
In mitosis, there is a separation of sister chromatids into newly formed daughter cells. When nondisjunction occurs, the sister chromatids fail to separate from each other in the anaphase and results in aneuploidy of cells―(2n + 1) in some and (2n – 1) in others. This is also known as the chromatin or the anaphase bridge. This may lead to mosaicism (a condition some cells are normal while other show aneuploidy) of somatic cells in an individual.
Effects of Nondisjunction
Nondisjunction can lead to the loss of a chromosome and give rise to a condition known as monosomy, denoted as (n – 1) or (2n – 1). It can also lead to the addition of a chromosome and is known as trisomy, denoted as (n + 1) or (2n + 1). These abnormalities can give rise to a number of conditions. Here are a few of them.
Turner Syndrome: In this condition, there is monosomy of the sex linked chromosome, the resulting zygote has only one X chromosome (X + 0). As only X chromosome is present the resulting zygote, it develops into a female. These females are usually sterile with underdeveloped sexual characters. Their stature is usually short, have a webbed neck and low-set ears. They have been observed to suffer from heart defects, diabetes, and hypothyroidism. Intelligence is normal in these females.
Down Syndrome: This results from the trisomy of the autosomal chromosome 21. The frequency of this condition is one in every thousand births. Nondisjunction is mostly of maternal origin. Individuals with this syndrome usually have a lower intelligence and poor immunity. These individuals usually have slanting eyes and experience a stunted growth. The mouth is usually small, and the tongue may be protruding. These individuals usually suffer from heart defects and thyroid abnormalities.
Klinefelter Syndrome: It occurs due to trisomy of sex-linked chromosomes, due to the nondisjunction of paternal sex chromosomes in meiosis I. The individuals suffering from this syndrome exhibit the development of breasts as well as underdeveloped male sex characteristics. These males are more susceptible to autoimmune disorders, breast cancer, and osteoporosis―conditions that usually affect females.
Retinoblastoma: Nondisjunction in mitosis can lead to abnormalities like cancer. Retinoblastoma protein is a tumor suppressor protein located on chromosome 13. Mutations in the gene encoding for this protein, RB1 gene on one chromosome could cause a loss of the wild type gene from the other chromosome in subsequent rounds of replication. This loss of the functional suppressor prompts cells to divide unchecked. Retinoblastoma is a rare cancer that develops in immature cells of the retina. It is usually found to affect young children and results in the loss of vision in the affected eye.
Nondisjunction is one of the most common causes of aneuploidies, it amounts to about 25 percent of the aneuploidies that may occur in human oocytes.
Tight junctions are a type of cell junctions that play a role in cell adhesion and permeability of paracellular barrier. This BiologyWise post elaborates on where these junctions are found, their structure as well as their function.
Pathogens Target Tight Junction Proteins
Proteolytic enzymes from pollen, many viruses, dust mites, and enterotoxins from bacteria, like ,.Clostridium perfringens, interact with these junctions to bring about the loss of the epithelial barrier function.
A cell junction is a multiprotein complex that links two neighboring cells or a cell to the extra cellular matrix. These complexes form the barriers guarding the inter cellular spaces, and thus, control the para cellular transport. They help in establishing communication between neighboring cells.
There are three basic types of cell junctions: anchoring junction, communicating or GAP junctions, and tight junctions. Anchoring junctions are protein complexes that are used to anchor the cells of a tissue either to each other or to the extra cellular matrix. Communicating junctions bring about direct chemical communication between adjacent cells. Tight junctions act as barriers that regulate the movement of ions, water, and other molecule via the para cellular space in the epithelial cells. We will now elaborate on the tight junctions in this article.
What are Tight Junctions?
These are also known as occluding junctions or zonulae occludentes. These junctions form the closest contacts as compared to the other cell junctions and can therefore form a barrier that is virtually impermeable to fluids. These are the most apical structures of the apical complex, and they form the demarcation between the apical and the basolateral membranes of the domains.
Where are Tight Junctions Found in the Body?
Tight junctions are required for cell adhesion in various tissues of the body. These structures are seen to be present on the epithelium cells that form the internal lining of the body. These are usually of one or two layers of cells. Recent studies have also highlighted their role in barrier function in the skin as well.
Numerous and highly complex tight junctions are usually found in the epithelial lining of the distal convoluted tubules, the collecting duct of nephrons, the blood brain barrier, and the part of the bile duct that transverses the liver. These linings are thus given the name “tight epithelia”.
Relatively fewer number and less complex tight junctions are present on the epithelial lining of the proximal tubules of the kidney. These linings are called “leaky epithelia”.
What is the Structure of Tight Junctions?
Tight junctions are usually made of trans-membrane proteins that are linked to a cytoplasmic plaque. Trans-membrane proteins are usually of two types: tetra-span and single-span trans-membrane proteins. Tetraspan proteins contain four membrane spanning domains, these include proteins like occludins, claudins, and tricellulins.
Occludins regulate the diffusion of hydrophilic molecules; they are usually associated with the intramembrane strand of the actin filament. Claudins determine the ion selectivity of tight junctions and are required for junction assembly. Tricellulins are found in junctions with three cells and are required to bring about cell-cell adhesion.
Single-span trans-membrane proteins include Junctional Adhesion Molecules (JAMs). JAM protein is required for adhesion between the endothelial cells and leukocytes as well as for maintaining cell polarization.
The cytoplasmic plaque is formed by a network of scaffolding and adaptor proteins, that are bound to cell signaling components as well as to the components of the cytoskeleton such as actin filaments. This complex acts as an interface between junctional membrane proteins and the cytoskeletal protein. ZO-1 is a scaffolding protein that interacts with membrane proteins like claudins and cell signaling protein. ZO-2 and ZO-3 are adapter proteins that bind to membrane proteins like occludin.
Tight junctions occur in a belt completely encircling the cells, for a solute, ion, or molecule to pass through the layer of cells, it has to be first taken inside the cell from one end and given out from the other side. The junction membrane proteins are arranged like beads on a thread of the cytoskeletal filaments and are cross-linked to each other.
What are the Functions of Tight Junctions?
They two main functions of Tight Junctions include para-cellular permeability and regulation of cell proliferation and polarization. As these multi-protein complexes are negatively charged, they selectively allow positively charged ions to pass through. These junctions are also known to be size selective―molecules with radii greater than 4.5 °A are usually excluded. These junctions may also determine the permeability of certain hydrophilic molecules via the para-cellular space. Physiological pH also seems to determine the permeability of these molecules.
Cell proliferation and regulation seems to play a major role in the development of differentiated tissues. Occludins present in tight junctions are required for suppression of cell proliferation, and the absence of these proteins may lead to uncontrolled cancerous growth of cells. Certain biochemical studies indicate that tight junctions are required for the maintenance of apico-basal polarity. Proteins that are required for cell polarization usually form the complexes at tight junctions.
Occludins are also seen to regulate migration of neutrophils across the epithelial cell layer. Claudins also function to regulate cell migration.