Table of Contents
What Is Chromosome?
Chromosome is the rod-shaped, dark-stained bodies seen during the metaphase stage of mitosis. Term initiated as (Chroma= color and soma= body).
When division starts in the nuclei, very fine thread like structures are produced in it from chromatin material, they are known as chromosomes. They contain small bodies, called genes. The genes contain hereditary characteristics, the chromosomes transfer the hereditary characteristics from one cell to another or from one generation to another.
Plant chromosomes are generally 0.8-7 micrometers in length whereas animal chromosomes are 0.5-4 micrometers in length
Chromosomes were discovered by a German Walther flaming. He placed the larva of a salamander in a chemical Perkins aniline, then he observed that the chromosomes become darker than other organelles of a cell and also show different structures.
The number of chromosomes in the cells of a body is constant and usually, they are diploid that is chromosomes are found in two sets. It occurs both in plants and animals.
Structure Of Chromosome:
Chromosomes are visible during cell division only. A visible and prominent chromosome consists of two thread-like structures, called chromatids. These chromatids are attached by a small body, known as the centromere. Within the centromere, a disc-shaped structure is present, called a kinetochore. The short arm of the chromosome is labeled the “p” arm. The long arm of the chromosome is labeled the “q” arm. It is attached to spindle fibers, during cell division.
The chromatids are composed of two thin, highly coiled filaments, called chromonema(sing-chromonema). Each chromonema contains single double-stranded DNA. Both chromonema are coiled in a spiral manner and they also contain bead-like swelling on their surface, called chromo meres.
The two chromatids of each chromosome are called sister chromatids and the chromatids of two different chromosomes are called non-sister chromatids. The chromosomal region between the secondary constriction and nearest telomeres is a called satellite. A small chromosomal segment separated from the main body of the chromosomes by a secondary constriction is called a satellite.
Types Of Chromosomes
Types of chromosomes according to the position of the centromere are as follows:
1. Telocentric:
These are rod-shaped chromosomes. These have centromere at their proximal end
Acrocentric:
These are also rod-shaped chromosomes, but the centromere is between two chromatids in such a manner that one arm is short and another is long.
2. Sub-metacentric:
These are J or L-shaped chromosomes. In these chromosomes, the centromere is present in the center in such a manner that two unequal arms are formed.
3. Metacentric:
These are V-shaped chromosomes. In these chromosomes, the centromere is present almost in the center and two equal arms are formed.
According to the number of the centromere:
4. Acentric:
Without any centromere.
5. Mono centric:
With one centromere.
6. Dicentric:
With two centromeres.
7. Polycentric:
With more than two centromeres.
Euchromatin And Heterochromatin Region Of Chromosomes
When in the prophase stage, the chromosome is treated with Acetocarmine, two distinct regions of chromosomes are visible according to the number of proteins, DNA and RNA.
Euchromatin region:
These regions have a light color, they form a larger part of chromosomes. These are active regions and form RNA. This region doesn’t show contraction except in cell division.
Heterochromatin Region:
These regions have a dark color. They contain double the amount DNA. These are present between small folds. These are inactive and don’t take part in the formation of RNA. This region show contraction.
Kinds Of Chromosomes
There are different kinds of chromosomes, which are as follows:
- Homologous chromosomes
- Autosomes
- Sex chromosomes
1. Homologous Chromosomes
In higher plants and animals the chromosomes are found in pairs, one set comes from the male and another from the female. These paired chromosomes are identical in all respects and are termed homologous chromosomes. Because two sets of chromosomes are present in a body, it is called diploid (2n). When gamete formation takes place, the number of chromosomes is reduced to half, they are knowns as haploid (1n).
2. Autosomes
These chromosomes contain genes of various characteristics except reproductive organs. They have no concern with the determination of sex organs.
3. Sex chromosomes:
These chromosomes contain the genes of reproductive organs. They determine the sex. These chromosomes are of two types, x chromosomes and y chromosomes. The x chromosome determines female and the y chromosomes determine male.
Chemical Structure Of Chromosomes
Chemically the chromosomes are composed of deoxyribonucleic protein. These nucleoproteins consist of the following parts:
1. DNA:
It is 30 to 40% of the chemical structure of chromosomes.
2. Protein:
It has two types:
A) Histone
B) Non-histone protein
Histone:
These are basic(non-acidic) protein
Non-histone protein:
These are acidic proteins
3. RNA:
It is amount is very less, about 1-10% of the chemical structure.
DNA consists of many thousands of units called a nucleotide. Each nucleotide is made up of three components.
- Deoxyribose sugar(5-carbon sugar that is pentose sugar)
- Phosphoric acid(H3PO4)
- Nitrogen bases
Chromosomes Aberrations
When the changes occur in chromosomes, they are called chromosomal aberration. These are obvious and observable changes. They can be visible in the structure of chromosomes. These changes are as follows:
- Deletion
- Duplication
- Translocation
- Inversion
1. Deletion:
When a small part of the chromosome is missing, it is called a deletion.
Example: A B C D E F is a normal gene A B C D (deletion).
2. Duplication:
When a part of chromosomes is present more than the normal chromosomes, it is called duplication.
Example: A B C D Norma chromosome C D A B C D (duplication).
3. Translocation:
When a part of chromosomes are transferred to a non-homologous chromosomes called translocation.
Example: A B C D E F G H I J K L
Normal chromosome
A B C D G H E F I J K L
After translocation.
4. Inversion:
When there is a change in the sequence of genes within the chromosomes, called inversion
Example: A B C D E F G A B E F G C D
Effects Of Chromosomal Aberrations
In chromosomal aberration amount of genetic material or sequence of genes are changed. This change may cause a great variation in hereditary characters.
The effects of chromosomal aberrations are as follows:
1. Deletion:
If deletion occurs in one chromosomes, it is harmful and it occurs in both the chromosomes, death may occur.
2. Duplication:
It causes abnormality in structure and function of the body.
3. Translocation:
Due to translocation changed may be produced in species.
4. Inversion:
Process of crossing over becomes reduced due to inversion.