Chromosome

What is Chromosome?

Chromosomes are thread like structures which are located inside nucleus of all plant and animal cells. Each chromosome comprises of protein and a single molecule of deoxyribonucleic acid (DNA). DNA are passed from parents to offspring. DNA contains those specific instructions that build each type of living creatures unique among themselves.

Chromosomes are a key part of the important process that ensures if DNA is accurately copied and distributed in vast majority of cell divisions. Changes in structure or number of chromosomes in new cells may lead to several serious problems. For instance, in human beings, one type of leukemia and some of other cancers are caused by the defective chromosomes which are made up of joined pieces of broken chromosomes.

Chromosomes are made of chromatin fibers. Chromatin fibers are nothing but DNA wounded to the circular protein molecule called Histone.

What Chromosomes do?

The chromosomes have unique structures which helps keep DNA wrapped tightly around the spool-like proteins known as histones. Without such tight packaging, it will not be possible for DNA molecules to fit inside cells as their length is too long for this. For an interesting example, if all the DNA molecules in a single human’s cell are unwounded from their histones and then placed end-to-end, they would stretch approx 6 feet long.

Why the DNA molecule is wound around the histone?

Because the length of the DNA is usually about hundred thousand times of cell length and placing this much length DNA into the cell’s nucleus is only achieved through wounding of DNA to this histone protein and in turn condensed to form chromosomes.

 

How the DNA is wounded around  histone protein?

The answer is Histone protein consists of positively charged amino acids it interacts with the negatively charged phosphate group of DNA.

 

Then, what is nucleosome?

Possible segment of DNA wound around eight histone to form Nucleosome and they combine and condensed to form chromosomes which are visible under microscope during metaphase of mitotic division of cell.

    • Number of chromosomes in each species remains constant due to meiotic division of cell.
    • A complete set of chromosome responsible for making of organism called Genome present in reproductive cells such as ovum and sperm comprise haploid(n).While somatic(body) cells consist of haploid set of chromosomes formed during sexual reproduction(male(n) + female(n) = 2n).

 

   Chromosome consist of primary constriction called centromere or kinetocore, it divides the chromosome into two arms(telomere). Based on the position of centromere, chromosomes are classified into
chromosome
 Chromosome by KES47 (CC BY-SA)
    • Metacentric : centromere present at the center form two equal arms.
    • Sub-metacentric : centromere located near one end of the chromosome forms one short and one long arm.
    • Acrocentric : Here centromere located very close to one end forming one very short arm and a long arm.
    • Telocentric: Has centromere at terminal forming single arm.

 

   Sometimes chromosomes may contains secondary constriction results in formation of small  fragment called “satellite”.
Chromosome in human:
      Human chromosomes are divided into two types
    • Autosomes : Chromosomes other than sex       chromosomes(allosomes)
    • Allosomes : Sex chromosomes(X and Y chromosome in human)

 

      Human cells consists of 23 pairs of chromosomes of which 22 pairs are fall into autosome category while 1 pair of chromosome(x and y falls into sex chromosome).Thus human cell comprise total of 46 chromosomes.

Chromosomes and sex determination:

      We know that human cells consist of two sex chromosomes namely X and Y chromosome.These chromosomes decide the fate of zygote’s sex(male or female)
       During meiotic cell division two types of sperm cells fromed(one containing x and
the other containing y).However only one type of ovum cells(either x or y chromosome) are produced during meiosis.
        Now comes the interesting part, if the sperm cell containing x chromosome fertilise the ovum bearing x chromosome results in female(XX) zygote.If sperm cell containing y chromosome fertilise the ovum with x chromosome results in male(XY).
        Thus chromosomes play major role in maintaining hereditary traits of an organism.

By | 2015-07-05T17:37:23+00:00 June 7th, 2013|Genetic material|0 Comments

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