The chromatin plexus of the interface nucleus condenses during cell division to form threads or rod-like structures. These structures are called chromosomes. They are called carriers of genetic traits. Strasburger discovered them in 1875 and Waldeyer used the term chromosome (coloured body) for them in 1889.
The number of chromosomes is the same in all the organisms of a species, but their number is different in the organisms of different species. In taxonomy, the number of chromosomes is used to determine the species of organisms and establish relationships among species. The number of chromosomes in some simple organisms is given in the following table: Chromosome number
|1. paramecium aurelia||30-40|
|2. Ascaris lumbriquadis||24|
Shape and Size
In the anaphase stage of cell division, they appear as V, J, L or rods. Although chromosomes are of equal thickness throughout their length, but all chromosomes have a narrowing. This is called primary narrowing.
Each chromosome has two coiled thread-like chromonemata. Each chromonemata represents one chromatid. On the chromonemata there are dark colored chromomeres at short distances. The light colored part between the chromomeres is called the interchromomeres. Chromomeres appear on the chromonemata as beads on a string. In the zygotene stage of the prophase of meiosis, the conjugation of homologous chromosomes occurs between each chromomer.
It was believed that genetic units or genes are located on chromomeres, but according to modern discoveries, chromomeres represent specific regions of chromonemata filaments where the filaments coil to form dense chromatin material.Chromosome structure
Chromosomes are genetic carriers. They are made up of DNA molecules. In which messages are contained for various actions. Enzymes maintain coordination in various activities. They are made up of complex molecules of proteins. The information for the synthesis of protein molecules is contained in the DNA molecules in the form of a defined sequence of nitrogenous bases. These messages are written in the form of a triplet code, that is, a sequence of three nitrogenous bases forms a unit. This is called a codon. Three of the four nitrogenous bases combine to form only 64 codons. The sequence of amino acids in each protein chain is determined by these codons. A codon represents the position of a specific amino acid in a protein chain. Chromosome function
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