BOTANY Higher Secondary Second Year - Textbooks Online

3. CELL BIOLOGY AND GENETICS
In the previous unit, you have studied several types of cells and their
organization to form tissue and tissue systems. Now, we shall study how
characters and traits are inherited from one generation to another. Sexual
reproduction, besides producing individuals, introduces variability in the
offspring by combining traits of parents. How are these traits inherited?
Now, we know that the units of heredity are genes that are transmitted from
one generation to another. The genes are arranged in a linear manner at
specific positions on specific chromosomes. Differences in gene expression
are the basis for differentiation of the organisms. This unit will acquaint
you with various aspects of genetics.
3.1. Chromosomes
Chromosomes are the physical carriers of genes, which are made up
of DNA and associated proteins. The term chromosome was introduced
by Waldeyer in 1888. Chromosomes occur in all the living organisms.
The bacterial chromosomes are circular. It has closed circular DNA. Linear
chromosomes are found in eukaryotes. Bridges in 1916 was the first to
prove that the genes are carried on the chromosome.
Structure of chromosome
Each chromosome consists of similar structures called
chromatids. They are identical and are called sister chromatids. A
Kinetochore
Telomere
Secondary
constriction
Satellite
Secondary
constriction
Primary
constriction
Fig. 3.1 Structure of chromosome
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typical chromosome has narrow
zones called constrictions. There
are two types of constrictions
namely primary constriction and
secondary constriction.
The primary constriction is
made up of centromere and
kinetochore. Both the chromatids
are joined at centromere, which is
essential for the movement of
chromosomes at anaphase. If the centromere of the chromosomes is
damaged, such chromosome fails to move at anaphase. The number of
centromeres varies from chromosome to chromosome. The monocentric