This is Btec Applied Science Unit 11 Assignment B (Cell division (Mitosis and Meiosis)) which was awarded a distinction. This is an example of a Distinction level assignment, and you may use it as a guide to help you achieve a distinction and finish this assignment.
STRUCTURE OF CHROMOSOMES:
Chromosome is a tiny thread like structure which is made of tightly
coiled DNA (DEOXYRIBONUCLEIC ACID) in the nucleus of every cell.
They are supported by proteins known as histones which help to
maintain its structure. Chromosomes usually appears as tiny dark rod
like body during metaphase and can be seen under light microscope.
Eukaryotic cells contain multiple linear and big chromosomes in the
nucleus. Based on structure there are three parts of chromosomes
which are pellicle, matrix and chromonematic. The outer covering of
the chromosome is called pellicle which is very thin and made of
achromatic materials. The grounded substance of chromosome
which consist of chromonematic is known as matrix and it is made up
of non-genic substances. The chromonematic are two identical
threads which are coiled spirally and embedded in matrix of
chromosome. Each chromonematic has thickness of 800A and
contains eight microfibrils made of double helix DNA. In metaphase
stage of mitosis, different structural features of chromosomes can be
seen under microscope. The tiny structure in the chromonema
known as centromere, which is marked by constriction, divides the
chromosome into two arms. The long arm is called ‘q arm’ whereas
the short arm is called ‘p arm.’ Besides having centromere, the
chromosomes have secondary constriction which identify specific
chromosome in a set. The section in-between this constriction and
nearby telomere is called satellite, so the chromosome possessing
secondary constriction are known as satellite chromosomes or
nucleolus organiser. The end of each chromosome is known as
telomeres which provides polarity and physiological differentiation.
This helps to prevents chromosomes to get fused with each other.
, TYPES OF CHROMOSOMES:
- Autosomes and sex chromosomes: Human chromosomes are of
two kinds of sex and autosomes chromosomes. The genetic
information which is related to the gender of individual are
possessed by sex chromosomes whereas remaining genetic
information is in autosomes. Humans have total 23 pairs of
chromosomes out of which 1 pair is of sex chromosome whereas
remaining pairs are of autosomes, which makes total 46
chromosomes in every cell.
- Based on number of centromeres: The chromosome with only one
centromere is called monocentric. The chromosomes having 2
centromeres are called dicentric. If chromosome have more than 2
centromeres, they are called polycentric. The chromosomes which
do not have any centromere are called acentric. These chromosomes
do not survive for a long period as they are freshly broken sections of
chromosome.The chromosomes having indistinct centromere which
is diffused all over the chromosomal length are called non located or
diffused chromosomes.
- Based on the position of centromere:
Telocentric: In telocentric chromosomes, the centromere is
positioned at the proximal end of chromosomes, so they are long,
and rod shaped in structure. They usually seen as ‘I’ shape during
metaphase of mitosis. Telocentric chromosomes have only one arm
and they are quite rare and present only in few organisms.
Acrocentric: In acrocentric chromosomes, the centromere is present
at one end by creating one very long and one very short arm. This
type of chromosome can be seen as ‘J’ shape during metaphase of
mitosis. All acrocentric chromosomes are satellite chromosomes and
are usually present in grasshoppers. In humans, the chromosomes of
13, 15, 21, 22 number are satellite acrocentric chromosomes.
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