drkemcolian26

· 14 Dec 2021

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What is a chromosome? :

In the nucleus of every cell of all the living organisms, the DNA molecule is condensed into thread-like structures known as chromosomes. Each chromosome is composed of DNA which is tightly coiled over many times around proteins which are called histones that support its structure.

Chromosomes can not be detected in the cell’s nucleus—not even under a microscope—when the cell is in a non-dividing phase. However, the DNA that makes up chromosomes becomes more densely and tightly packed during cell division and is then can be seen under a microscope. Most of the information that researchers know about chromosomes was learned by them by observing the chromosomes during cell division.
Each chromosome has at its center a constriction point called the centromere, which divides the chromosome into two sections, or “arms.” The small arm of the chromosome is tagged the “p arm.” The extended arm of the chromosome is labelled the “q arm.” The locale of the centromere on each chromosome gives the chromosome its distinct shape and can be used to help interpret the location of specific genes.

Chromosomes are generally noticeable under a light microscope just during the metaphase of cell division (in this phase all chromosomes are aligned in the center of the cell in their condensed form forming a metaphase plate). Before all this happens, every chromosome is duplicated (S phase), and both replicas are wedded by a centromere, ensuing in either an X-shaped structure, if the centromere is placed equatorially, or a two-arm configuration, in the case when the centromere is found distally. The inseparable copies are now termed sister chromatids. During metaphase of the cell division, the X-shaped configuration is called a metaphase chromosome, which is highly dense and thus easiest to differentiate and study. In animal cells, chromosomes attain their highest condensing level in anaphase during chromosome segregation.

Chromosomal recombination which occurs during meiosis and subsequent sexual reproduction plays an important role in genetic diversity and variations. If any of these structures are manipulated incorrectly, through processes known as chromosomal instability and translocation, the cell might undergo mitotic catastrophe. Typically, such conditions lead to making the cell initiate apoptosis leading to cell death, but in some cases mutations in the cell hamper this process and thus lead to the progression of cancer.

Sometimes the term chromosome is used in a broader sense, maybe to refer to the individualized portions of chromatin in some cells, either the chromosome may be visible or may not be under light microscopy. Some other times this term may be used in a relatively narrower sense may be to refer to the individualized portions of chromatin during cell division, this refers to the chromosomal portion which is visible under light microscopy due to high condensation.

The number of chromosomes in the cells of an organism belonging to eukaryotes is highly variable. Chromosomes can fuse or break up and can thus evolve into novel karyotypes. Chromosomes may also be made to fuse artificially. For instance, the 16 chromosomes of an organism called yeast have been fused into one giant chromosome and the cells are still viable and we see only somewhat reduced growth rates, in humans, there is a total of 46 chromosomes in somatic cells(arranged in 23 pairs) while only 23 chromosomes (reduced to half) in the sex cells or gonads (egg in female and sperm in the male).


Read more:
https://www.genome.gov/about-genomics/fact-sheets/Chromosomes-Fact-Sheet
#chromosome #metaphase #telophase #centromere #humans #pairs #constriction