Category: Biology Second Paper

A molecule of phosphate and a molecule of nucleoside combine to form a nucleotide. That is, one molecule of pentose sugar, one molecule of phosphate and one molecule of nitrogenous base are called nucleotides. Nucleotide is the structural unit of nucleic acid. A nitrogenous base is attached to the 1st carbon of a pentose sugar and a phosphate is attached to the 3rd or 5th carbon to form a nucleotide. It is of two types. Purine nucleotides and pyrimidine nucleotides.
(i) Purine Nucleotide: If the nitrogenous base forming the nucleotide is purine, it is called purine nucleotide. It is of two types. Adenosine nucleotide and guanosine nucleotide. Pentose sugars and adenine combine to form adenosine nucleotides and pentose sugars and guanine combine to form guanosine nucleotides.
(ii) Pyrimidine Nucleotide: If the nitrogenous base forming the nucleotide is pyrimidine, it is called pyrimidine nucleotide. It is of three types. Thymidine nucleotide, cytidine nucleotide and uridine nucleotide. Pentose sugars and thymine combine to form thymidine nucleotides, pentose sugars and cytosine combine to form cytidine nucleotides, and pentose sugars and uracil combine to form uridine nucleotides.
Different types of nucleotides are AMP, GMP, CMP, UMP, TMP, dAMP, dGMP, dCMP, dUMP, dTMP etc.

A molecule of phosphate and a molecule of nucleoside combine to form a nucleotide. That is, one molecule of pentose sugar, one molecule of phosphate and one molecule of nitrogenous base are called nucleotides. Nucleotide is the structural unit of nucleic acid. A nitrogenous base is attached to the 1st carbon of a pentose sugar and a phosphate is attached to the 3rd or 5th carbon to form a nucleotide. It is of two types. Purine nucleotides and pyrimidine nucleotides.

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.

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.