- Chromosomes.

 Chromosomes

Each cell contains a nucleus within which there are several thread-like structures known as chromosomes, so called because of their affinity for certain stains. (Chroma = color   soma = body (

Chromosome: is DNA thread-like double-helix tightly coiled around histones.

Human somatic cells have 23 pairs of chromosomes (diploid number), with one copy of each chromosome inherited from each parent:

ü  22 pairs of chromosomes are autosomes;

ü  The remaining pair is called the sex chromosomes (XX in female or XY in male).

Human genetic material contains 3.1 billion bases.

Each germ cell contains 23 single chromosomes (haploid number):

ü  22 autosomes +

ü  1 sex chromosome ( x in ovum - Y or X in sperm)

 The non-dividing chromosome appears as thread-like structure which has:

·       Short arm (p) (for ‘petit’ or small)

·       constriction (centromere): localize genes and categorize chromosomes.

·       Long arm (q) (as next letter in the alphabet!)   .

In the duplicated state (in metaphase, prior to cell division) the chromosome appears as X shaped structure with two chromatids connected together at one point by centromere.

Each chromosome arm is divided into regions or bands which can be seen microscopically with special stains. The bands are labelled counting out from the centromere, with p1 and q1 being closest to the centromere. Within the bands, there are sub-bands (only seen at very high resolutions), which are also numbered outwards from the centromere.

The end of each chromosome arm is termed telomere. This is a specialized structure thought to play an important role in maintaining the integrity of chromosomes. The chromosomal regions adjacent to telomeres , the sub telomeric regions are particularly gene rich.  

Classification of chromosomes

According to size of chromosomes and the position of the centromere.

ü  Acrocentric: centromere at one end

ü  Metacentric: centromere in the middle

ü  Submetacentric: intermediate position.

 

ü  Group A        1-3               large               metacentric

ü  Group B        4,5               large              submetacentric

ü  Group C        6-12,X          medium            submetacentric

ü  Group D        13-15            medium            acrocentric

ü  Group E        16-18            short               submetacentric

ü  Group F        19,20            short              metacentric

ü  Group G       21,22, Y      very short          acrocentric