If the nitrogenous base forming the nucleoside is purine, it is called purine nucleoside. Carbon 1 of pentose sugar is linked to nitrogen 9 of base by glycosidic bond to form purine nucleoside. It is of two types. Adenosine nucleoside and guanosine nucleoside. Pentose sugars combine with adenine to form adenosine nucleosides and pentose sugars combine with guanine to form guanosine nucleosides.

A molecule of pentose sugar and a molecule of nitrogenous base is called a nucleoside. It is of two types. Purine nucleosides and pyrimidine nucleosides.
(i) Purine Nucleosides: If the nitrogenous base forming the nucleoside is purine, it is called purine nucleoside. Carbon 1 of pentose sugar is linked to nitrogen 9 of base by glycosidic bond to form purine nucleoside. It is of two types. Adenosine nucleoside and guanosine nucleoside. Pentose sugars combine with adenine to form adenosine nucleosides and pentose sugars combine with guanine to form guanosine nucleosides.
(ii) Pyrimidine Nucleosides: When the nitrogenous base forming the nucleoside is pyrimidine, it is called a pyrimidine nucleoside. The 1st carbon of the pentose sugar is joined to the 1st nitrogen of the base by a glycosidic bond to form a pyrimidine nucleoside. It is of three types. Thymidine nucleoside, cytidine nucleoside and uridine nucleoside. Pentose sugars and thymine combine to form thymidine nucleosides, pentose sugars and cytosine combine to form cytidine nucleosides and pentose sugars and uracil combine to form uridine nucleosides.

A molecule of pentose sugar and a molecule of nitrogenous base is called a nucleoside. It is of two types. Purine nucleosides and pyrimidine nucleosides.

Chromosomes that contain satellites are called sat chromosomes. The satellite is separated from the parent chromosome by a thin chromatin. Sat chromosome plays a major role in the formation of cell nucleolus. In 1930, scientist Heitz first used the term SAT.

One or more extra chromosomes other than the normal chromosomes in the nucleus are called B chromosomes or supplementary chromosomes. B chromosomes are small, non-vital and have heterochromatin. This is sefish genetic material and can be easily distinguished. It contains few genes and does not follow Mendelian pattern. In 1905, Wilson first discovered the B chromosome from the beetle Metapodius. B chromosome is found in 475 species of plants belonging to 163 genera in 42 families. B chromosome is found in organisms such as Ulat Chandal, Bhuta, Brachycome iberidifolia, Lilium callosum, Secale cereale etc.

Chromosomes that are large, multi-armed and consist of thousands of DNA are called polytene chromosomes. It is also called salivary gland chromosome. Its length is 2000 micrometers. It increases the nucleus and cell shape. Their multicopy genes result in high levels of gene expression in the organism.
Polytene chromosomes have 5 long and 1 short arm. Each arm has a black and white band. Black arms are euchromatin regions. Some parts of it swell and form large formations. It is called chromosomal puff or Balbiani ring. Chromosomal puff or Balbiani ring synthesizes mRNA. This chromosome is found in the salivary glands of insects such as Drosophila, Chironomus, Rhynchosciara etc. In 1881, French entomologist E.G. Balbiani discovered polytene chromosomes.

Chromosomes that look like chimney brushes or lamp brushes are called lamp brush chromosomes. These chromosomes have several pairs of loops centered on their axis and look like chimney brushes or lamp brushes. The axis of the chromosome is made up of chromomeres and inter-chromomeres and loops of transcribing DNA. Its length is 1500-2000 micrometers. It is required for the formation of mRNA and egg yolk. In 1882, the embryologist Flemming first discovered this chromosome from the egg of the amphibian salamander (Amblystoma maxicanum). It is found in oocytes or immature eggs of fish, amphibians, reptiles, birds and insects. However, it is not found in the ovum of mammals.

A chromosome that has its centromere at the extreme end is called a telocentric chromosome. During the anaphase phase of cell division, the telocentric chromosomes look like English letters. Telocentric chromosomes are rare in organisms. Humans do not have telocentric chromosomes. Telocentric chromosomes are found in gymnosperms (Welwitschia) and onion (Allium).

A chromosome that is close to the end of the centromere is called an acrocentric chromosome. During anaphase of cell division, acrocentric chromosomes look like the English letter J. All acrocentric chromosomes are sat-chromosomes. Human chromosomes 13, 14, 15, 21, 22 and Y are acrocentric types.

A chromosome that is slightly away from the center of the centromere is called a sub-metacentric chromosome. During the anaphase phase of cell division, the sub-metacentric chromosomes look like the English letter L. Human chromosomes 2, 4, 5, 9, 10, 12, 17 and 18 are of sub metacentric type.