Meiosis is a phase in sexually reproductive organisms, wherein cell-division takes place. It is of great importance, because it creates genetic diversity in the population.
Meiosis is a process of gamete formation in which diploid germ-line cells, i.e., the cells that are set aside early in animal development for sexual reproduction, yield four genetically different haploid cells. It occurs only in sex cells, which are eggs and sperms.
Phases
Meiosis takes place in two stages – Meiosis I, where DNA replication takes place and crossing-over occurs; and Meiosis II, which lacks DNA replication, but is similar to Mitotic cell division.
Meiosis I
- Prophase 1
- Metaphase 1
- Anaphase 1
- Telophase 1
Meiosis II
- Prophase 2
- Metaphase 2
- Anaphase 2
- Telophase 2
The Process
- In meiosis, during the formation of gametes in animals and spores in plants, the chromosome number is reduced to half. These chromosomes contain the basic DNA chain.
- During the first meiotic reduction division, the chromosomal pairs are divided so that each gamete or spore contains one of each chromosomal pair, it becomes a haploid.
- When haploid gametes unite during fertilization, they form a zygote. Zygotes, having received one chromosome of each pair from each parent become diploid.
- Meiosis involves two successive nuclear divisions, which produce four haploid cells. The meiosis I is the reduction division, meiosis II separates the chromatids, which are the daughter strands of a duplicated chromosome joined together by a centromere.
- In mitotic cell division, new cells genetically identical to the parent cell are produced. Meiosis is responsible for increasing genetic variation in the population.
- Each diploid cell, which undergoes meiosis can produce 2n different chromosomal combinations, where ‘n’ is the haploid number.
- In humans, the number is 223, because there are 23 pairs of chromosomes. This number is greater than eight million different combinations.
- The variation increases, because, during meiosis I, each pair of homologous chromosomes comes together.
- In a process known as synapsis, each pair of homologous chromosomes may exchange parts.
- The relative distance between two genes on a given chromosome can be estimated by calculating the percentage of crossing-over that takes place between them.
Tasks of Meiosis
- Production of haploid gametes to maintain the diploid number of species, generation after generation.
- Crossing-over, which brings together new gene combination of chromosomes.
- A mechanism for comparing the two copies of each chromosome, provided with the purpose of error correction or repairing.
Importance
- In meiosis, variation occurs, because each gamete (either sperm or egg) contains a mixture of genes from two different parent chromosomes in sexual reproduction. In other words, the genetic coupling of non-identical DNA takes place in meiosis.
- It results in an offspring, which has the genetic material of two different individuals.
- These chromosomes contain the basic DNA chain, which determines the physical and genetic characteristics of the child.
- A new combination of genetic information is produced in the gametes. Therefore, in meiosis, the characteristics of parent chromosomes are combined with the characteristics of offspring chromosomes, which ultimately results in a new and unique set of chromosomes.
- It enables individuals to produce physically and genetically unique offspring. Because of this, a high genetic diversity of a population is maintained.
- With mitosis only division would have been possible and there would have been no sharing of genetic information.
- In such a situation, there would have been only clonal populations, which would eventually suffer from diseases or natural disasters.
- What is the explanation for the diversity in populations? How can they survive variations in the environment? The reason is meiosis. Genetic variation plays the role of a raw material for natural selection.
- Some individuals who are favored by natural selection have greater fitness than others because of their alleles (pair of alternative forms of gene).
- In case of animals, males that are unable to compete for mates, for example, succumb to predation or disease or fail to reproduce; small and weak organisms don’t survive for long time. These are the best examples of natural selection.
- You can also take an example of a disease to which some individuals will be at least partially resistant while others are susceptible to it.
- A population can adapt to changes in the environment as a result of the genetic variation resulting from meiosis. However, in clonal asexual populations, organisms are not able to adapt to changes without mutations.
- Organisms which adapt to changes in the environment, survive, while others get eliminated by natural selection. In this way, a population contains fit individuals and the process continues for generations together.
- The diversity afforded by meiosis is beneficial for the population as a whole.
Thus, meiosis helps to create a population that is not only physically and genetically different but also one, which is perfectly fit to survive.