What is Chromosome?
The term chromosome (Chrome = Color; Soma = body) was first coined by W. Waldeyar (1888). Meaning of chromosome is nothing but the darkly stained body.
Chromosome Types : Based on the function of chromosome
- Sex chromosomes
Autosomes control the determination of the somatic characters of an organism.
The number of autosomes is always more in the nucleus.
E.g. in a human being out of a total of 46 chromosomes, 44 chromosomes are autosomes and 2 are the sex chromosomes.
Sex chromosomes are responsible for the determination of the sex of an individual.
These are of two types i.e. X and Y chromosomes.
The X chromosome is longer than the Y chromosome. It is straight, rod-like, and sub-metacentric.
It contains a large amount of euchromatin with DNA and a small amount of heterochromatin therefore X chromosomes are genetically active.
The Y chromosome is short slightly curved and acrocentric.
It contains less DNA and a large amount of heterochromatin; therefore Y chromosome is genetically inactive.
Chromosome Types : Based on the position of the centromere
The shape of the chromosome changes from phase to phase of cell division. In the resting phase or interphase stage, the chromosomes occur thin, coiled, elastic, threadlike structures called chromatin threads. In metaphase and anaphase, they become thick and filamentous.
Depending on the position of the centromere on the chromosome, it has different shapes as follows,
- Metacentric – The centromere is present nearly in the middle of the chromosome. The two arms of a chromosome are approximately equal. They appear ‘V’ shaped during anaphase movements.
- Submetacentric – Centromere is some distance away from the middle. One arm of the chromosome is shorter than the other. Such chromosomes are called submetacentric. They appear ‘L’ shaped during anaphase movements.
- Acrocentric – The centromere is a little away from the end of a chromosome. One arm of the chromosome is very short and the other arm is very long. These chromosomes are called acrocentric. They appear ‘J’ shaped during anaphase movements.
- Telocentric – When the centromere is located at the tip of the chromosome then it is called telocentric. There is no division of chromosomes into arms. These chromosomes appear ‘i’ shaped during anaphase movements. Telocentric chromosomes are very rare.
Chromosome Types : Based on number of centromeres present
- Monocentric – They having only one centromere present.
- Holocentric – They having diffused centromere and microtubules are attached along the length of a chromosome.
- Acentric – These chromosome may break and fuse together to form a chromosome without a centromere.
- Dicentric – These chromosomal aberration where chromosomes break and fuse together with two centromeres.
Chromosome number of some species
- Man (Homo sapiens) = 46
- Rhesus monkey (Mucaca mulatt) = 42
- Horse (Equus calibus) = 64
- Rat (Rattus norvegicus) = 42
- Fruit fly (Drosophila melanogaster) = 08
- Housefly (Musca domestica) = 12
Chromosome Types : Special Types Of Chromosomes
Normally chromosomes are not visible during interphase but polytene chromosomes are the exception.
They are visible during the interphase.
They were first observed by Balbiani (1881) in salivary glands of Chironomus tetanus; hence the name salivary gland chromosome.
These chromosomes are also found in some other organs like the foregut, midgut, trachea, Malpighian tubules, and fat body cells of dipteran insects.
These chromosomes have a large number of chromonemata hence they are called polytene chromosomes.
In Drosophila larvae, the polytene chromosome is about 100 times longer and 300 times big than the normal somatic chromosome.
This enlargement of the chromosome is due to endomitosis or polyteny. Endomitosis is a process in which chromosomal constituents undergo several extra duplications without cell division.
As a result, chromosome gets enlarged very much in length and width.
Due to extra duplications number of chromatids increases in the polytene chromosome between the range of 500 to 1000.
After staining, polytene chromosomes show alternate dark and light bands (interband). The number and arrangement of the bands are species-specific.
The bands are possibly composed of the chromomeres of the chromonemata. According to Bridges (1935), the linear order of bands corresponds with the linear arrangement of genes on the chromosome. However, each band does not represent a gene.
The interband are also found to be genetically active.
During larval development of some dipterans, some of the bands become expanded, called ‘puff’. When a puff becomes very much enlarged, it is called a ‘Balbiani ring’.
The pattern of puffing is a characteristic of the phenotype of a cell. Pelling (1964) showed that the puffs are sites of RNA synthesis. Puffing activities increase during larval development due to the release of the hormone ecdysone (molting hormone).
The formation of puff and Balbiani rings are reversible.
The lampbrush chromosomes were first observed in Salamander (amphibian) oocytes in 1882.
These are also found in immature eggs or oocytes of amphibians, birds, mammals, and spermatocytes of certain insects, birds, and other vertebrates.
These chromosomes in shark oocytes were first described by Ruckert (1892). These are named lampbrush chromosomes due to their appearance like a brush used for cleaning the chimneys of kerosene lamps.
The size of the lampbrush chromosome may be more than 1000 microns in length. However, it contracts greatly and reduces in size.
These chromosomes are very elastic and can be stretched up to about two times before they break.
During the early prophase, lampbrush chromosomes are in the form of a pair of homologous chromosomes with few points of contact between them.
Each chromosome consists of two chromatids or axial filaments.
Along the axial filaments, there is a row of dense granules or chromomeres. These are tightly coiled points on the axial filament. From each chromomere arises a pair of lateral loops.
The axial filaments and the chromomeres consist of DNA.
The loops represent a lateral extension of the axial filaments. Loops are the active sites for RNA synthesis.
It has been suggested that each loop corresponds to a single functional unit (gene) that codes for a single primary polypeptide product.
So, a number of loops may decide the number of functional units (genes).
The function of the lampbrush chromosome is a synthesis of RNA and proteins by the loops. They are also concerned with the production of yolk in the egg.