In this 3 types of chromosomes post we have briefly explained about general history of chromosomes, and different types of chromosomes such as viral chromosome, prokaryotic chromosomes and eukaryotic chromosomes.
The word chromosome has been derived from two Greek words “Chroma” meaning colour and “Soma” meaning body. They are the unique cell organelles made up of chromatin material which is the most important and permanent constituent of the cell nucleus. They are capable of self-reproduction. They control cell’s structure and metabolism, and play an important role in the differentiation, heredity, mutation and evolution.
Hofmeister in 1848, discovered nuclear filaments in the nuclei of pollen mother cells of Tradescantia. First accurate count of chromosomes was made by W. Flemming in 1882, in the nucleus of a cell.
In 1884, W. Flemming, Evan Beneden and E. Strasburger demonstrated that the chromosomes double in number by longitudinal division during mitosis. Beneden in 1887 found that the number of chromosomes for each species was constant.
The term “Chromosomes” was coined in 1888 by W. Waldeyer for the nuclear filaments. W.S. Sutton and T. Boveri suggested the role of chromosomes in heredity in 1902, which was confirmed by Morgan in 1933.
All cells have the capability to give rise to new cells and the encoded information in a living cell is passed from one generation to another. The information encoding material is the genetic or hereditary material of the cell.
Different Types of Chromosomes
Different Types of Chromosomes: Viral chromosome
In viruses there is a single chromosome one types in different types of chromosomes bearing a single nucleic acid molecule (DNA or RNA) surrounded by a protein coat called Capsid. It may be linear or circular. The viruses having DNA as genetic material are called DNA viruses and those having RNA as genetic material are known as RNA viruses.
A limited amount of genetic information is present in the viral chromosome which codes for little more than the production of more virus particles of the same kind in the host cell. In RNA viruses, often the RNA directs the synthesis of DNA complementary to itself by reverse transcription in the host. The RNA is then transcribed by the DNA for the formation of new virus particles. Such ribovirus is called retrovirus. The AIDS causing virus is a retrovirus.
Different Types of Chromosomes: Prokaryotic Chromosomes
The prokaryotic (bacterial) genetic material another different types of chromosomes is usually concentrated in a specific clear region of the cytoplasm called nucleoid. The bacterial chromosome is a single, circular, double stranded DNA molecule mostly attached to the plasma membrane at one point.
It does not contain any histone protein. Escherichia coli DNA is circular molecule 4.6 million base pairs in length, containing 4288 annotated protein-coding genes (organized into 2584 operons), seven ribosomal RNA (rRNA) operons, and 86 transfer RNA (tRNA) genes.
Certain bacteria like the Borrelia burgdorferi possess array of linear chromosome like eukaryotes. Besides the chromosomal DNA many bacteria may also carry extra chromosomal genetic elements in the form of small, circular and closed DNA molecules, called plasmids.
They generally remain floated in the cytoplasm and bear different genes based on which they have been studied. Some of the different types of plasmids are F plasmids, R plasmids, virulent plasmids, metabolic plasmids etc.
Different Types of Chromosomes: Eukaryotic Chromosomes
The eukaryotic chromosomes an complex different types of chromosomes in all are present in nucleus and in certain other organelles, like mitochondria and plastids. These chromosomes are called nuclear and extra nuclear chromosomes respectively. Nuclear chromosomes are double stranded long DNA molecules of linear form.
Proteins are associated with them. They are surrounded by nuclear envelope. More DNA is involved in coding far more proteins than the prokaryotic chromosomes. Extra nuclear chromosomes are present in mitochondria and plastids. They are double stranded short DNA molecules of circular form. They lack protein association.
Less genetic information is available for the synthesis of only some particles of proteins for the organelles containing them. Other proteins are received from the cytoplasm where they are synthesized under the direction of nuclear chromosomes.