NEET Biology Notes Biomolecules and Enzyme Nucleic Acids
Nucleic acids are polymers of nucleotides and also known as polynucleotides. A nucleotide is a compound containing carbon, hydrogen, oxygen, nitrogen and phosphorus. A molecule of a nucleotide is in turn composed of three smaller molecules i.e. phosphate (P), sugar (S) and nitrogen base (N).
The phosphate group is represented by phosphoric acid (H3P04). The sugar molecule in the nucleotide is a 5-carbon pentose sugar. It is represented by either ribose sugar (C5H10O5) or deoxyribose sugar (C5H10O4). Both the sugars have a furanose ring structure.
The nitrogen base is represented by compounds having nitrogen and carbon in the ring structure.
Two types of nitrogen bases are
- Purines, which have a double ring structure.
- Pyrimidines, which have a single ring structure.
Purines are of two types adenine (A) and guanine (G). Pyrimidines are of three types cytosine (C), thymine (T) and uracil (U). The nitrogen base molecule is attached to the sugar molecule by a glycosidic bond. A combination of nitrogen base with sugar is called nucleoside. Nucleosides involving ribose sugars are called ribonucleosides. Similarly, nucleosides involving deoxyribose sugars are called deoxyribonucleosides.
A nucleoside combines with a phosphate group to form a compound called nucleotide. Nucleotides formed by ribonucleosides are called ribonucleotides. They form the monomers of ribose nucleic acid (RNA). Nucleotides formed by deoxyribonucleosides are called deoxyribonucleotides. They form the monomers of deoxyribose nucleic acid (DNA).
There are two types of nucleic acids found in a cell, i.e., Deoxyribose Nucleic Acid (DNA) and Ribose Nucleic Acid (RNA).
DNA is the genetic material in all living organisms except viruses, where genetic material may be either DNA or RNA. A small amount of DNA is found in the cytoplasm (i.e. in cell organelles like mitochondria and plastids, known as extra-nuclear DNA).
DNA is a polynucleotide of adenine, guanine, cytosine and thymine. Uracil nucleotides are absent in DNA. The DNA molecule is composed of two polynucleotide chains.
The structure of the DNA molecule and the arrangement of nitrogenous bases in the two polynucleotide chains are very well explained by the double helix model proposed by Watson and Crick in 1953.
The two polynucleotide chains are coiled around each other like a spiral stair case (double helix). The cross-rungs (steps) are formed by the nitrogen bases, while phosphates and sugars form the uprights. The two-polynucleotide chains run in opposite directions (antiparallel). One chain runs in the 3-5′ direction, while for the other chain 5′-3′ direction is opposite.
The average distance between the two chains (B-DNA) is 20 A. One full turn of the helix, called gyre, measures 34 A in length. The distance between two successive sugar molecules is 3.4 A. Thus, each gyre accommodates 10 nucleotides. .
The nitrogen bases of the two opposite chains exhibit highly specific base pairing. A purine in one chain always pairs with a pyrimidine in the opposite chain.
Among purines adenine (A) pairs only with the pyrimidine thymine (T) and vice-versa. Similarly, guanine (G) pairs only with cytosine (C) and vice-versa. There are two weak hydrogen bonds between A and T or T and A and three weak hydrogen bonds between G and C or C and G. The total amount of purines is equal to the total amount of pyrimidines (A + G) = (C + T). The two polynucleotide chains of DNA molecule are not identical to each other but complementary to each other. The fact that the total amount of purines will be equal to the total amount of pyrimidines, was first enunciated by Chargaff in 1950. Both the strands of DNA do not take part in controlling heredity and metabolism. Only one of them does so.
The DNA strand, which functions as template for RNA synthesis is known as template strand, minus (-) strand or sense strand. Its complementary strand is named nontemplate strand, plus (+) strand,antisense strand. The latter name is given because by convention DNA genetic code is written according to its sequence.
Functions of DNA
- It is the genetic material in all prokaryotes and eukaryotes.
- It is capable of replication by which it can be faithfully passed on to successive generations.
- It is involved in the synthesis of RNA.
- It provides the code for protein biosynthesis.
- It is involved in mutations and genetic recombinations, which bring about variations.
RNA occurs mostly in the cytoplasm in the eukaryotic cells. A small amount occurs in the nucleus of the cell, as a constituent of nucleolus. RNA is a single polynucleotide chain composed of nucleotides of adenine, guanine, cytosine and uracil.
Thymine nucleotides are absent. There are three types of RNA, i.e., ribosomal RNA (rRNA), messenger RNA (mRNA) and transfer RNA (fRNA).