CELL, HEREDITY AND EVOLUTION
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The
article contains · Chromosomes in detail. · Kinds of cell divisions. · Mitosis in detail. · Meiosis in detail. |
Chrosomes:
THE human body is made of millions of cells. They are about
0.005 mm to 0.02 mm in diameter. Nearly all calls have the same basic
structure, no matter what their function is or what organism they are found in.
Most of the cell have nucleus in the centre. The nucleus
contains long thread- like structure called chromosomes. These are only visible
when a cell is about to divide into two.
Chromosomes contain a complex chemical called
decoy-ribonucleic acid or DNA, which control the development of characteristics
that an organism inherits from its parents. That is, it contains the
“instructions” for making the characteristics of an organism, such as skin
colour, hair color and eye colours.
The chromosomes carry bits of information called genes. They
instruct our bodies to make proteins which determine the shape of the body and
how it behaves.
Chromosomes and genes both are made of DNA. The DNA molecule
is like a twisted ladder called a double helix.
When cells divide the DNA first duplicates itself. One copy
is passed from one generation to next. This is the reason why we inherit
characteristics from our parents.
Kind of
cell division
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Mitosis: it is a
kind of cell division in which the number of chromosomes in the newly- formed
cells remains the same as that in the original parent cells. Cells having the
normal set of chromosomes are said to have the diploid number of chromosomes.
All the cells in animals and plants, except the sex cell, are diploid. When a
cell is ready to divide, long thin threads called chromosomes appear in its
nucleus. The chromosomes become shorten and thicker. Each chromosome makes an
exact copy of itself by a process called replication. The chromosomes become
attached to threads called spindle fibres. The spindle fibres contract, pulling
apart the chromosomes, which moves to opposite end of the cell. The cytoplasm
also divide and two new cells are made. Each newly- formed cell has the same
number of chromosomes as the original parent cell have.
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Meiosis: this is
kind of cell division which occurs only within the reproductive organs. Meiosis
is concerned with the production of sex cell or gametes. Four daughter cells,
with half the number of chromosomes as the parent cell, are produced by
meiosis. During meiosis the chromosomes become shorter and thicker. They form
pairs, the members of which are identical in shape and size. They are called
homologues pairs of chromosomes. Each chromosome replicate it’s self to form
two chromatids. Homologous pairs of chromosomes now arrange themselves around
the middle of the cell. Spindle fibres become attached to the homologous pairs
of chromosomes and pull them apart. They move to the opposite ends of the cell.
The cell begins dividing in two. Spindle fibres now pull the chromatids of each
chromosome away from each other. They separate and move in opposite
directions. The cell divides into four parts, each part containing half the
original number of chromosomes, i.e., the haploid number of chromosomes. The
four cells become four haploid gametes. For this reason meiosis is some time
called reduction division.
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