CHROMOSOME

CHROMOSOME

In the nucleus of each cell, the DNA molecule is packaged into thread-like structures called chromosomes. Each chromosome is made up of DNA tightly coiled many times around proteins called histones that support its structure.

Chromosomes are not visible in the cell’s nucleus—not even under a microscope—when the cell is not dividing. However, the DNA that makes up chromosomes becomes more tightly packed during cell division and is then visible under a microscope. Most of what researchers know about chromosomes was learned by observing chromosomes during cell division.

Each chromosome has a constriction point called the centromere, which divides the chromosome into two sections, or “arms.” The short arm of the chromosome is labeled the “p arm.” The long arm of the chromosome is labeled the “q arm.” The location of the centromere on each chromosome gives the chromosome its characteristic shape, and can be used to help describe the location of specific genes.

 TWO CATEGORIES OF CHROMOSOMES

In human beings, there are two types of chromosomes: AUTOSOMES and ALLOSOMES (also known as sex chromosomes). There are 22 pairs of autosomes and 1 pair of allosomes in our genome (and thus in our cells), resulting in a total of 46 chromosomes. The autosome pairs are labeled 1-22 depending on their sizes in base pairs, while the allosomes are labeled by letters X and Y.

A female contains two X chromosomes  in the allosome pair, while a male  human being has X and Y chromosomes in the allosome pair. The X chromosome is present in ovum, while either X or Y chromosomes can be present in the sperm. The female allosome is represented as XX, while the male allosome is XY.

Male and female bodies contain the same 22 pairs of autosomes. 

THE FOUR MAIN TYPE OF CHROMOSOMES

1. Metacentric Chromosomes

Metacentric chromosomes have the centromere in the center, such that both sections are of equal length. Human chromosome 1 and 3 are metacentric.

2. Submetacentric Chromosomes

Submetacentric chromosomes have the centromere slightly offset from the center leading to a slight asymmetry in the length of the two sections. Human chromosomes 4 through 12 are submetacentric.

3. Acrocentric Chromosomes

Acrocentric chromosomes have a centromere which is severely offset from the center leading to one very long and one very short section. Human chromosomes 13,15, 21, and 22 are acrocentric.

4. Telocentric Chromosomes

Telocentric chromosomes have the centromere at the very end of the chromosome. Humans do not possess telocentric chromosomes but they are found in other species such as mice.

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.