இந்தக் கட்டுரையில், TNPSC குரூப் 1, குரூப் 2, குரூப் 2A, குரூப் 4 மாநிலப் போட்டித் தேர்வுகளான TNUSRB, TRB, TET, TNEB போன்றவற்றுக்கான முறைகள் இலவசக் குறிப்புகளைப் பெறுவீர்கள்.தேர்வுக்கு தயாராவோர் இங்குள்ள பாடக்குறிப்புகளை படித்து பயன்பெற வாழ்த்துகிறோம்.
Chromosome
The human body is made up of million cells.
The nucleus of each cell contains thin thread like structures called
chromosomes.
The term ‘chromosomes’ was first coined by Waldeyer in 1888.
The chromosomes are the carrier of genetic material which contain the
heredity information.
The chromosomes are highly condensed coiled chromatin fibres packed
with the DNA (Deoxyribonucleic acid) that forms the genetic material.
Structure of a Chromosome:
The chromosomes are thin, long and thread like structures consisting of
two identical strands called sister chromatids. They are held together by the
centromere.
Each chromatid is made up of spirally coiled thin structure called
chromonema.
The chromonema has number of bead-like structures along its length which
are called chromomeres.
The chromosomes are made up of DNA, RNA, chromosomal proteins
(histones and non-histones) and certain metallic ions.
A chromosome consists of the following regions
1. Primary constriction:
The two arms of a chromosome meet at a point called primary constriction
or centromere.
The centromere is the region where spindle fibres attach to the chromosomes
during cell division.
2. Secondary constriction:
Some chromosomes possess secondary constriction at any point of the
chromosome.
3. Telomere:
The end of the chromosome is called telomere.
It maintains and provides stability to the chromosomes.
4. Satellite:
Some of the chromosomes have an elongated knob-like appendage at one
end of the chromosome known as satellite.
The chromosomes with satellites are called as the sat-chromosomes.
Types of Chromosomes based on the position of Centromere:
Based on the position of centromere, the chromosomes are classified as
Telocentric, Acrocentric, Submetacentric and
Metacentric:
Telocentric– The centromere is found on the proximal end. They are rod
shaped chromosomes.
Acrocentric – The centromere is found at the one end with a short
arm and a long arm. They are also rod-shaped chromosomes.
Submetacentric – The centromere is found near the centre of the
chromosome. Thus forming two unequal arms. They are J shaped or L
shaped chromosomes.
Metacentric – The centromere occurs in the centre of the chromosome
and form two equal arms. They are V shaped chromosomes.
Types of Chromosomes based on function:
The eukaryotic chromosomes are classified into autosomes and allosomes.
Autosomes:
The chromosomes that determine the somatic (body) characters.
Male and female have equal number of autosomes.
Allosomes:
The chromosomes which are responsible for determining the sex of an
individual.
They are also called as sex chromosomes or hetero – chromosomes.
There are two types of sex chromosomes, X and Y- chromosomes.
Human male have one X chromosome and one Y chromosome and
human female have two X chromosomes.
Karyotype:
Karyotype is the number, size and shape of chromosomes in the cell
nucleus of an organism.
Ideogram is the diagrammatic representation of karyotype of a species.
It consists of all the metaphasic chromosomes arranged in homologous
pairs according to decreasing length, thickness, position of centromere,
shape etc., with the sex chromosomes placed at the end.
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