Nucleic Acids

NUCLEIC ACIDS

First discovered by Meischer.

Nucleic acids are giant molecules having a variety of functions.

There are two major types of nucleic acids Deoxyribonucleic Acid or DNA and Ribonucleic Acid or RNA.

DNA is found mainly in the nucleus but also occurs in chloroplasts and mitochondria.

It is the genetic material and contains all information needed for the development and existence of an organism.

RNA occurs as genetic material in some viruses.

Nucleic acids are linear polymers of purine and pyrimidine nucleotides.

The nucleotides are linked serially by phosphate groups, each linking the C₅(5 – C) and the C'₃(3' – C) of the pentoses of the successive nucleotides.

The DNA molecule consists of a double chain of nucleotides, whereas RNA consists of a single chain.

The nucleotides of DNA contain the bases adenine (A), thymine (T), guanine (G) and cytosine (C), while RNA contains A, G, C and uracil (U) instead of T.

The backbone of the nucleic acid is uniformly made up of alternating pentose and phosphate groups.

The pentose in DNA is deoxyribose (C_sH₁₀O₄) and that in RNA is ribose (C₅H₁₀O₅).

In the double stranded DNA, the bases of the opposite strands pair in a specific relationship by means of hydrogen bonds.

'A' always pairs with 'T' and 'G' always pairs with 'C'.

This complementarity is known as the Base-Pairing Rule.

Nucleic acids exhibit a wide variety of secondary structures. For example, one of the secondary structures exhibited by DNA is the famous Watson-Crick model.

This model says that DNA exists as a double helix.

The two strands of polynucleotides are antiparallel i.e. run in the opposite direction.

The backbone is formed by the sugar-phosphatesugar chain.

The nitrogen bases are projected more or less perpendicular to this backbone but face inside.

A and G of one strand compulsorily base pairs with T and C, respectively, on the other strand.

There are two hydrogen bonds between A and T.

There are three hydrogen bonds between G and C.

Each strand appears like a helical staircase.

Each step of ascent is represented by a pair of bases.

At each step of ascent, the strand turns 36°.

One full turn of the helical strand would involve ten steps or ten base pairs.

Attempt drawing a line diagram.

The pitch would be 34 Å.

The rise per base pair would be 3.4 Å.

This form of DNA with the above mentioned salient features is called B-DNA.

In higher classes, you will be told that there are more than a dozen forms of DNA named after English alphabets with unique structural features.

In 1950, Erwin Chargaff found that in any DNA molecule:

(i) The amounts of purines and pyrimidines are equal i.e., A + G = T + C.

(ii) The amount of adenine is always equal to that of thymine; and the amount of guanine is always equal to that of cytosine (i.e., A =T and G =C).

(iii) The base ratio (A + T)/(G + C) may vary from one species to another, but is constant for a given species.

(iv) The deoxyribose sugar and phosphate components occur in equal proportions.

RNA is usually single-stranded, but sometimes (as in Reovirus and Rice dwarf virus), it is double-stranded.

RNA does not follow Chargaff's rules i.e., 1 : 1 ratio does not exist between purines and pyrimidines bases due to single-stranded nature and lack of complementarity.

There are three types of non-genetic RNA.

(i) Messenger RNA (m-RNA) : It is produced in the nucleus and carries the information for the synthesis of proteins; it was discovered by Jacob and Monod (1961).

(ii) Ribosomal RNA (r-RNA) : It is the largest RNA and constitutes about 80% of total cellular RNA. Found in the ribosomes where protein synthesis takes place.

 

(iii) Transfer RNA or Soluble RNA or adaptive RNA (s-RNA, t-RNA) : It is the smallest type of RNA and constitutes about 10-15% of total cellular RNA. These are found in the cytoplasm and are different types (as many types as the types of amino acids -usually 20). Their function is to collect amino acids from the cytoplasm for protein synthesis.

t-RNA molecule is folded to form a clover leaf -like structure. This structure was given by Holley.

Points to Remember

1. Study of X-ray diffraction patterns of DNAs isolated from various organisms by Wilkins, Franklin and Astbury revealed that DNA has a right handed helical structure.

2. Using all the available chemical and physical information, James Watson and F.C. Crick of Cambridge gave the double helix model of DNA for which they were awarded the Nobel Prize in 1962.

3. The width between the two backbones is constant and equal to the width of a base pair (i.e., width of a purine + a pyrimidine).

4. Along the axis of the molecule, the base pairs are 'spaced at intervals of 0.34 nm. Therefore, one complete turn of the double helix comprises 3.4 nm (10 base pairs).

5. There is no restriction on the sequence of bases in one chain. However, due to rule of base pairing, the sequence of one chain determines the sequence in the other. The two chains are thus said to be complementary. As a result, the (purine) adenine in either chain is associated with (pyrimidine) thymine in the other. Similarly, the (purine) guanine in either chain is associated with the (pyrimidine) cytosine in the other.

6. The two chains are held together by hydrogen bonding between the bases (joined together in pairs)-a single base from one chain being hydrogen-bonded to the complementary base from the complementary chain.

7. Adenine-thymine pair has two hydrogen bonds and the guanine–cytosine pair has three hydrogen bonds.

8. The double helix has a diameter of 20Å i.e., the distance between two strands is 19.8Å (or 20Å).

9. DNA with higher percentage of G  C have more density than those with higher percentage of A = T.

10. Upon heating at temperatures above 80-90º, the two strands of DNA uncoil and separate (denaturation). On cooling, the strands come closer and are rejoined together (renaturation/ annealing). Low melting area of DNA is A = T base pairs.

11. 1 µm of DNA contains about 3000 base pairs.

12. The DNA is mostly right handed. This type of DNA exists in 4 forms -

(a) B form - the usual DNA, having 10 base pairs per turn.

(b) A form - having 11 base pairs (instead of 10 base pairs per turn), the base pairs are not perpendicular to the axis, but are tilted.

(c) C form - like B form, but having 9 base pairs per turn.

(d) D form - like B form, but have 8 base pairs per turn.

13. DNA with left handed coiling is called Z-DNA. In it, repeating unit is dinucleotide.

14. In some cases, as in × 174 and S-13 viruses, the DNA is single stranded.

Palindromic and repetitive DNA: DNA duplex possessing areas of same sequences of nucleotides is called palindromic DNA. Repetitive DNA has sequence of nitrogen bases repeated several times in tandem.