There are different types of chromosomes present in a cell during a particular stage of cell division. We shall learn about these chromosome types in the article discussed below.
The nucleus of plant and animal cells contains thread-like structures called the chromosomes. These chromosomes are made up of a single molecule of deoxyribonucleic acid (DNA) and a protein. These chromosomes are passed from the parents to their progeny and contain all the genetic material related to the particular organism. The word chromosome comes from Greek words. Chroma in Greek means ‘color’ and soma means ‘body’. As these structures tend to stain very strongly with color dyes used in labs, these structures were named as chromosomes. These chromosomes have a very important function without which an individual or organism cannot exist.
What are the Functions of Chromosomes?
Chromosomes have a unique structure that keeps the DNA in a tightly wrapped position. The DNA strands are placed around a spool-like structure of protein called the histones. If the DNA structure was not tightly wound, it would have been very difficult to accommodate the strands in the cell nucleus. The strands of DNA molecules in a single cell of humans are almost 6 feet long, if unwound. Thus, the strands being wrapped tightly help them to be placed in the tiny, microscopic cell nucleus. Functions of chromosome also includes ensuring that during cell division, the DNA is copied exactly as it is and distributed evenly after cell division. If there are any changes in the structure or number of chromosomes, it could lead to serious birth defects in the progeny. Thus, it is absolutely essential for every reproductive cell, that is the eggs and sperms, to contain the right number and correct structure of chromosomes. Failing this, it will lead to serious complications in the offspring produced.
What is a Centromere and Telomere in Chromosome?
When we speak about chromosome types, you will find that the words ‘centromere’ and ‘telomere’ will be mentioned a couple of times. It is important to understand the structure of a chromosome before we have a look at their types. If you have a look at the structure of the chromosome, you will find a constricted region in the linear chromosome. This constricted area is called the centromere and it is not necessary that it will be located exactly in the center of the chromosomes. It may be placed right at one end of the chromosome. The structures on either side of the chromosome are called the chromosome arms. These centromeres are very important during cell division. They help in aligning of the chromosomes properly and serve as an attachment site for the two halves of the replicated chromosomes. These replicated chromosomes are called sister chromatids that are copied for making a new cell.
The repetitive stretches of DNA on the ends of the linear chromosomes are called telomeres. They help in preventing the DNA from unraveling from their tightly wound state. In some cells, the telomeres tend to lose their DNA bit by bit after each cellular division. This continues to happen, till it loses all the DNA and the cell dies. In some cells, there are special enzymes that prevent this loss of DNA during division. Thus, these cells are able to live a long life as compared to other cells. Apart from the centromere and telomere, other parts of the chromosome include the primary and secondary constriction regions with their satellite, pellicle, matrix, chromonemata and kinetochore.
Chromosomes Found in Humans
Chromosomes are highly dispersed and difficult to locate before cell division. When the cell begins to divide, the chromosomes are seen clearly and can be photographed as well as their morphology, size and shape can be studied. The chromosome types are divided into two categories, autosomes and sex chromosome. These types are explained below:
Autosomes are structures that contain the hereditary information. They do not contain information related to reproduction and sex determination. They are identical in both sexes, i.e., male and female species of humans. There are 46 (2n) chromosomes in humans. Of these 46 chromosomes, there are 44 pairs of autosomes and contain information related to the phenotypic characters.
♦ Allosomes/ Heterosomes
The allosomes are sex chromosomes that are different from autosomes in form, behavior and size. There are a pair of allosomes in humans. The X chromosomes are present in the ovum and either the X or Y chromosome can be present in the sperm. These chromosomes help in determination of sex of the progeny. If the offspring receives X chromosome from the mother as well as father, it results in a female child (XX). If the offspring receives one X and one Y chromosome from the parents, it results in a male child (XY). In simple words, it is the donation of X or Y chromosome by the father that helps in determination of the sex of the child.
Apart from these two categories, chromosomes can further be divided according to the location of the centromere and number of centromeres.
♦ Chromosome Types: Based on Centromere Position
Chromosomes are divided into four types based on the centromere position. These four types are as follows:
⟿ Metacentric Chromosome
The metacentric chromosome has its centromere centrally located between the two arms. This gives the chromosome a typical ‘V’ shape that is seen during the anaphase. The arms of this chromosome are roughly equal in length. In certain cells, fusion of two acrocentric chromosomes leads to formation of metacentric chromosome.
⟿ Submetacentric Chromosome
The arms of the submetacentric chromosome are said to be unequal in length. This is because the kinetochore is present in the sub median position. This gives rise to the ‘L’ shape of the submetacentric chromosome.
⟿ Telocentric Chromosome
Also known as the monarchial type of chromosomes, they have a centromere that is located towards the end of the chromosome. Thus, telocentric chromosomes have a ‘rod’ shaped appearance. In some cases, the telomeres extend from both the chromosome ends. The telocentric chromosome is not present in humans.
⟿ Subtelocentric Chromosome
Chromosomes that have a centromere that is located closer to the end than the center, are called subtelocentric chromosomes.
⟿ Acrocentric Chromosome
The location of the centromere in the acrocentric chromosome is subterminal. This causes the short arm of the chromosome to become really short making it very difficult to observe.
⟿ Holocentric Chromosome
In holocentric chromosomes, the centromere runs through the entire length of the chromosome. These chromosomes are very common in cells belonging to organisms in the animal and plant kingdom.
♦ Chromosome Types: Based on Centromere Number
The number of centromeres present on the chromosome help in determining the type of chromosome. These different chromosome types based on the number of centromeres are as follows:
⟿ Acentric Chromosome
Acentric chromosomes are those that lack centromeres, i.e., the centromere is totally absent on the chromosome. These chromosomes are observed due to effects of chromosome-breaking process like irradiation.
⟿ Monocentric Chromosome
Monocentric chromosomes are those that contain a single centromere. This type of chromosome is present in most of the organisms. The monocentric chromosomes can be called acrocentric, if the centromere is located at the end of the chromosome. If the centromere is located towards the center it is called metacentric chromosome. And, telocentric chromosomes are those in which the centromere is present at the chromosome’s end.
⟿ Dicentric Chromosome
Dicentric chromosomes are those that have two centromeres that are present on its arms. These chromosomes are formed after two chromosomal segments with a centromere each, are fused end to end. This causes them to lose their acentric fragments, leading to formation of dicentric chromosome.
⟿ Polycentric Chromosome
Polycentric chromosomes are those that contain more than two centromeres. These chromosomes are very common in plants, for example, the Adder’s-tongue fern has 1262 chromosomes.
The types of chromosomes in humans are divided into two categories as seen above. They include the autosomes and sex chromosomes. Thus, in all, humans contain 22 pairs of autosomes and one pair of sex chromosome. This leads to presence of 46 chromosomes present per cell. Chromosomes are very important for each and every organism as the future of their offspring depends on the normal division and separation of chromosomes.