what is meant by chromosomes explain its types
Answers
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The nucleus of plant and animal cells contains thread-like structures called the chromosomes. These chromosomes are made up of a single molecule of deoxyribonucleic acid (DNA) and a protein. These chromosomes are passed from the parents to their progeny and contain all the genetic material related to the particular organism.
The chromosome types are divided into two categories, autosomes and sex chromosome. These types are explained below:
♦ Autosomes
Autosomes are structures that contain the hereditary information. They do not contain information related to reproduction and sex determination. They are identical in both sexes, i.e., male and female species of humans. There are 46 (2n) chromosomes in humans. Of these 46 chromosomes, there are 44 pairs of autosomes and contain information related to the phenotypic characters.
♦ Allosomes/ Heterosomes
The allosomes are sex chromosomes that are different from autosomes in form, behavior and size. There are a pair of allosomes in humans. The X chromosomes are present in the ovum and either the X or Y chromosome can be present in the sperm. These chromosomes help in determination of sex of the progeny.
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A chromosome is a string of DNA wrapped around associated proteins that give the connected nucleic acid bases a structure. During interphase of the cell cycle, the chromosome exists in a loose structure, so proteins can be translated from the DNA and the DNA can be replicated. During mitosis and meiosis, the chromosome becomes condensed, to be organized and separated.
Types of Chromosomes
Chromosomes are divided into two parts (p and q arms) with a constriction point called a centromere in the middle.
The centromere can be located in different positions and this forms the basis for the four different classes of chromosome:
1. Metacentric – centromere is in middle, meaning p and q arms are of comparable length (e.g. chromosomes 1, 3, 16, 19, 20)
2. Submetacentric – centromere off-centre, leading to shorter p arm relative to q arm (e.g. chromosomes 2, 4 – 12, 17, 18, X)
3. Acrocentric – centromere severely off-set from centre, leading to much shorter p arm (e.g. chromosomes 13 – 15, 21, 22, Y)
4. Telocentric – centromere found at end of chromosome, meaning no p arm exists (chromosome not found in humans)