Chromosome Structure Functions and Types

Chromosome Structure Functions and Types

Total Chromosomes

Hello biology lovers. Total chromosomes! I mean “Do you know how much total chromosomes are present in our body?” In this blog, you will get the answer.

The Chromosomes are physical basis of inheritance. They are carries of inheritance and the term chromatin was coined by Flemming. The Chromosome word made of two Greek words Chroma means colored and Soma means body. They were discovered by Strasberger in the plant cell. Waldeyer coined the term chromosomes.

 

Sutton and Boveri proposed the chromosomal theory of inheritance which states that number chromosomes in a species are constant and genes lie on them in the linear fashion.

Number of Chromosomes: –

Somatic cell and germ cell of animals and higher plants are diploid (2n). Gametes are haploid (n) and the endosperms cells of angiosperms are triploid. The haploid number of total chromosomes is called as genome and a diploid number of total chromosomes is called genotype. total chromosomes in the human being are 46. I think you get the answer here.

Do you want to know more about chromosomes?  read the following text.

Following table showing total chromosomes of different organisms.

[table id=3 /]

 

Chromosome structure:-

a) Primary Construction/Centromere:-

It is the highly stained part without pellicle. Chromatids are separated by the centromere. Centromere contains a disc-like protein called kinetochore. The kinetochore is attachment site to spindle fibers, during chromosomal division, the centromere is required for their migration towards opposite poles during anaphase.

b) Secondary construction:-

The sites of chronemata showing less coiling are called as secondary constrictions. It is associated with nucleolar organizer region (NOR). NOR forms the nucleolus. Secondary construction is found in the form of heterochromatin. Secondary construction is found at 13, 14, 15, 21, 22 and Y chromosomes of the human genome.

c) Satellite:-

In the nucleus, the end part of the chromosome that is separated from the main body of the chromosome by a secondary constriction is called as a satellite. It is the part of chromosome beyond nucleolar organizer region is very short appears like a sphere. The chromosomes having a satellite are called marker chromosomes. Because they are always constant in their positions.

d) SAT-chromosomes: –

The chromosome that bearing satellite is called SAT-chromosome. SAT stands for sine acid thymonucleinico (without thymonucliec acid). The DNA present here on this chromosome is called r-DNA or repetitive DNA. Because they contain variable repetitive sequences.

e) Telomere: –

This part found on both ends of the chromosome, which protect the chromosome and prevent binding of both ends of chromosomes with each other.

The replication is facilitated by an unusual DNA polymerase called telomerase(Telomere terminal transferase). Telomere has TG rich repeat. The sequence of nucleotides in telomere is TTAGGG and complementary DNA strain have a sequence AATTCCC.

Telomere was reported by Richard Cawthon. Each time a cell divides 25-200 base pair of the telomere is lost due to this the process of aging in human begins.

Chemical composition of chromosome:-

DNA = 40%

Histone protein = 50%. It is a basic protein.

RNA = 1.5%

Non- histone protein = 8.5%. It is an acidic protein.

Minerals (Ca, Mg, Fe, Na), lipids and salts = traces

Types of chromosome: –

1. On the basis of position of centromere

Telocentric – Telocentric chrosome  is I shaped chromosome in which centromere is present at the terminal end.

Acrocentric Acrocentric chromosome is  J shaped chromosome. Centromere is present at sub-terminal in the position that makes one arm smaller than other.

Sub-metacentric –  L shaped chromosome that have two arms with dissimilar length out of which one is longer than other.

Metacentric – V-shaped chromosome with both arms having equal length centromere is in the middle.

2. On the basis of number of centromeres

Acentric – centromere is absent in chromosome.

Monocentric – chromosome having only one centromere.

Dicentric – chromosome having two centromeres.

Polycentric – chromosome having more than two centromeres.

3. On the basis of function :

Autosomes – They have genes that take part in all general functions except sex determination. Montgomery gave this word.

Allosome or Sex chromosomes – They have the genes which determine the sex of the individual. Willson gave them word “sex chromosome.

Special Chromosome: –

It is also called as the giant chromosome. They are two types –

  1. Polytene chromosomes:

Poly means many and tene means strands. It was discovered by E.G. balbiani in 1881 in the salivary gland of the larva of dipteran insect such as Drosophila, Mosquitoes. It is a type of giant chromosome and it is formed as result of endomitosis. Polytene chromosomes name was given by Koller. The cell bearing such chromosomes never undergo cell division.

Structure of Polytene:- it has many chromonemata so it is called as polytene chromosome which remains connected at chromocenter. Each chromonema has puffed region & unpuffed regions.  Puffed region bear special of Balbiani rings which are made up of DNA & form RNA & proteins. unbuffed regions are without rings but they are made up of alternate bands and interbands. The bands are the more coiled regions. It has many chromatids which remain attached to a common part called chromocenter.

The cell containing polytene chromosome has a metabolic advantage as multiple copies of gene permits the high level of gene expression.

  1. Lampbrush chromosomes:

Walther Flemming discovered theses chromosomes. It appears like a lampbrush so it is called as lampbrush chromosome. It is found in diplotene stage the primary oocytes of sharks, amphibians, reptiles, and birds. They play important role in the synthesis of Yolk.

 

Other Types of Chromosomes: –

  1. Supernumerary or accessory or B-chromosomes:- They were discovered by Wilson. These are formed by breaking the chromosomes. B-chromosomes are unnecessary, chromosomes which simply increase the chromosome number in a genome.

B-chromosomes are smaller than normal chromosomes and found commonly in plants and metapodium insects. They do not form pair during meiosis so they do not take part in cell division.

 

  1. Limited chromosomes or L chromosomes:- These chromosomes are limited to germline cells only. They are reported in the family Sciaridae of order dipteral of insects

 


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