Biomolecules

14.33

The different types of RNA found in the cell are listed below:-

(i) Messenger RNA (m-RNA)

It carries the genetic message code from the DNA to ribosomes. It is produced by the DNA; m-RNA is also single stranded and constitutes about 15% of total RNA.

(ii) Ribosomal RNA (r-RNA)

It is found in the ribosomes and it is usually associated with protein to form the ribosomes. It is synthesised in the nucleus by DNA. It is single stranded, comprising about 80% of total RNA. It is metabolically stable.

(iii) Transfer RNA (t-RNA)

It is synthesised in nucleus by DNA. It is also called soluble RNA. It is single stranded. There are 20 different kinds of t-RNA and each type has specificity for a particular amino acid. It constitutes about 5% of total RNA. It has very short life.

14.31

In the helical structure of DNA, the two strands are held together by hydrogen bonds between specific pairs of bases.

Cytosine from hydrogen bond with guanine, while adenine forms hydrogen bond with thymine. As a result, the two strands are complementary to each other.

DNA consists of two strands of nucleic acid chains coiled around each other in the form of a double helix. The base of one strand of DNA is paired with bases on other strand by means of hydrogen bonding.

This hydrogen bonding is very specific as the bases can only base pair in a complementary manner.

Adenine pairs with only thymine via 2 hydrogen bonds and guanine pairs with cytosine through 3 hydrogen bonds. 

Thus, the two strands of DNA are complementary to each other in the sense that the sequence of bases in one strand automatically determines that of the other. So the DNA stands cannot be identical, but they are complementary to each other.

14.29

Nucleic acids are Biomolecules which are found in the nuclei of all living cells, inform of nucleoproteins or chromosomes (proteins containing nucleic acids as the prosthetic group). Nucleic acids are of two types: – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Nucleic acids are also known as Polynucleotide as they are long- chain polymers of nucleotides.

The two important functions of nucleic acids are listed below:-

(i) DNA which is responsible for the transference of hereditary effects from one generation to another, which is due to their property of replication during cell division as a result of which two identical DNA strands are transferred to the daughter

(ii) Nucleic acids (both DNA and RNA) are responsible for synthesis of all proteins needed for the growth and maintenance of our body. Actually, the proteins are synthesised by various RNA molecules in the cell but the message for the synthesis of a particular protein is given by DNA molecules