Question

Can you distinguish between (a) unidirectional and bidirectional synthesis, and (b) continuous and discontinuous synthesis of DNA

Answer 1

Answer:

(a) Unidirectional and bidirectional synthesis of DNA: When both the strands of DNA get synthesized independently and separately it is known as unidirectional synthesis. But in bidirectional synthesis, both the strands of DNA double helix are synthesized simultaneously because two replication forks are formed which proceed in opposite directions.

(b) Continuous & discontinuous synthesis of DNA: When DNA replication is bidirectional at that time one strand which is 'leading strand' is synthesized in continuous manner while another strand which is 'lagging strand' is synthesized in discontinuous manner i.e. in small stretches. These small discontinuous stretches are also known as Okazaki fragments.

Explanation:

(a) Unidirectional and bidirectional synthesis of DNA: Unidirectional DNA synthesis mode is considered as unusual type of DNA replication because it occurs only in mammalian mitochondrial DNA. Mitochondrial DNA is circular and double stranded. It has one inner DNA strand known as 'H strand' while the outer strand is known as 'L strand'. The replication of both the strands is unidirectional and not at all dependent on each other. It is only when 2/3rd portion of 'L strand' has been synthesized, the replication of 'H strand' begins.

Bidirectional DNA synthesis on the other hand is considered as usual mode of DNA replication which is the mode of DNA replication in almost all the living organism's nuclear DNA. It proceeds with the formation of two oppositely moving replication forks. During bidirectional synthesis of DNA, the circular DNA of bacteria assumes a 'theta shape' while linear DNA of eukaryotes assumes 'eye shape'.

(b) Continuous & discontinuous synthesis of DNA: As already explained, the replication of DNA is usually bidirectional. During bidirectional DNA synthesis, two replication forks which are moving in opposite directions synthesize both the strands of DNA simultaneously. Because of this half portion of both the DNA strands is synthesized in continuous manner while half portion of these strands is synthesized in discontinuous manner in the form of small stretches which are known as Okazaki fragments. 'Trombone model' best describes how continuous and discontinuous synthesis of double stranded DNA is possible simultaneously.