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  Cell Biology and Genetics

Introduction to Cell Biology and Genetics

In the preceding topics, you have studied various types of cells and the organization of them to create tissue and tissue systems. Here, we shall study how characters and traits are inherited from one generation to other.  Sexual reproduction, above and beyond producing individuals, introduces variability in the offspring through combining traits of parents.   How are these traits inherited? Here, we identify that the units of heredity are genes which are transmitted from one generation to other. The genes are arranged in a linear way at particular positions on particular chromosomes. Variations in gene expression are the basis for separation of the organisms.

Chromosomes

The physical carriers of genes are known as Chromosomes that are built up of DNA and related proteins. The word chromosome was come into existence by Waldeyer in the year 1888. Chromosomes take place in all the living organisms. The bacterial chromosomes are circular. It comprises closed circular DNA.  Linear chromosomes are created in eukaryotes.  Bridges in the year 1916 was the first to show that the genes are taken on the chromosome.

Structure of chromosome

Every chromosome contains identical structures called chromatids.  They are similar and are termed sister chromatids.  A usual chromosome has narrow zones are termed as constrictions.  There are 2 kinds of constrictions that are primary constriction and secondary constriction.

The primary constriction is built up of kinetochore and centromere. Both of the chromatids are connected at centromere that is necessary for the movement of chromosomes at anaphase. If the centromere of the chromosomes is damaged, such type of chromosome not able to move at anaphase. The no. of centromeres changes from chromosome to chromosome. The monocentric chromosome comprises one centromere and the polycentric chromosome has several centromeres.

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Diagram: Structure of chromosome

The centromere consists of a complex system of fibres termed as kinetochore. Every centromere has 2 kinetochores lined along with chromosomal arms. The kinetochore is built up of protein fibres and microtubules that help in the creation of spindles throughout mitosis and meiosis.

Other than primary all constrictions are termed as secondary constrictions. In a specific set of chromosomes just one or two chromosomes comprise secondary constrictions. The nucleoli build up from secondary constrictions and such type of secondary constrictions is known as nucleolar organisers.

A satellite is a short chromosomal segment that separated from the main chromosome through a comparatively elongated secondary constriction. A chromosome along with a satellite is known as SAT-chromosome.

Chromatin is a viscous gelatinous substance which consists of DNA, RNA, histone and non-histone proteins. The five types of histones found in the chromatin are:

1. H1,

2. H2A,

3. H2B,

4. H3 and

5. H4

The chromatin is created through a series of repeated units known as nucleosomes. Every nucleosome comprises a core of 8 histone sub units.

The terminal part of chromosome is known as Telomere. It provides stability to the chromosome. The telomere's DNA has particular sequence of nucleotides. Eukaryotic chromosome comprises DNA, RNA, histones, non- histone proteins and metallic ions such as Ca+2, Mg+2, etc.

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