Chromosomes were first observed by Wilhelm Hofmeister, in 1848. Later on in the year, 1883 Wilhem roux speculated chromosomes as a carrier of genetic information.
How DNA is arranged on the chromosome is a complex process. Let’s start with the DNA itself. DNA is a double-stranded molecule and helical in shape. Imagine a rope and arrangement of threads in a rope. This spiral arrangement creates tension on the remaining strand of DNA. Now the DNA wraps on each other and creates a supercoiled structure.
It binds with Histone protein molecules (H1,H2A, H2B, H3 and H4) and forms a nucleosome. It looks like a solenoid which has the beads-on-string structure and, later turned into a long chromatin. Finally, the chromatid attached with the centromere and creates the chromosome.
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It is a complex arrangement of protein and DNA. Different chromosomes differ in their structure based on the location of the centromere. metacentric, submetacentric, acrocentric are different forms of the chromosome.
DNA is arranged on the chromosome based on it’s coding and noncoding properties. The coding region generally become loosely packed and called as euchromatin region. Non-coding regions are tightly packed are called as heterochromatin region. For more detail on euchromatin and heterochromatin read the article: what is genome
In humans, 22 pairs of autosomal chromosomes and one pair of sex chromosome are present. X and Y are sex chromosomes, XX in female and XY in the male. Total 46 numbers of chromosome carry all the information of our life. Genes are located on chromosomes, actually DNA.
Two copies of the gene for one single phenotype are located on two different chromosomes. For example, the MTHFR gene is located on chromosome number 1 which means that one copy of the MTHFR gene is located on chromosome number 1 and another one copy is located on another chromosome number 1. Here one gene is dominant over the other.
Attend class: Mutation
Alternative forms of the gene are called as an allele. Let’s understand the terminology by taking one example, there are many shades of human eye colour in the world. Brown eye, light blue eye, black eye, slightly whitish- black eye etc, the gene for eye colour is expressed and which colour it will produce, it depends on the type of eye colour allele.
Remember allele is actually a DNA sequence, for a gene one allele is expressed (dominant) over other alleles. We will discuss about the structure of a chromosome in another article.
Article covered and review by Tushar Chauhan