Question

Distinguish between (a) unidirectional and bidirectional synthesis, and (b) continuous and discontinuous synthesis of DNA.

Short answer

Question: Explain the differences between unidirectional and bidirectional synthesis and continuous and discontinuous synthesis during DNA replication. Answer: The differences between unidirectional and bidirectional synthesis lie in the direction of replication fork movement. Unidirectional synthesis occurs in only one direction starting from a single origin of replication, while bidirectional synthesis proceeds in two directions simultaneously from an origin of replication. Continuous and discontinuous synthesis refer to the mode of new DNA strand synthesis based on the leading or lagging strand. Continuous synthesis occurs on the leading strand, where the new complementary strand is synthesized continuously in the 5' to 3' direction. Discontinuous synthesis occurs on the lagging strand, where the new strand is synthesized in short fragments called Okazaki fragments, which are then joined together to form a complete strand.

Step-by-step solution

1. Introduction to DNA replication

DNA replication is a fundamental cellular process in which a double-stranded DNA molecule is copied into two identical daughter molecules. This process is highly accurate and necessary for the growth, development, and maintenance of genetic information in living organisms.

2. Unidirectional vs. bidirectional synthesis

DNA synthesis can occur in two ways: unidirectional and bidirectional. (a) Unidirectional synthesis: In unidirectional synthesis, the DNA replication proceeds in only one direction, i.e., it starts at a single origin of replication and moves away in a single direction. Prokaryotic DNA replication is an example of unidirectional synthesis, where the replication fork moves around the circular DNA molecule. (b) Bidirectional synthesis: In bidirectional synthesis, DNA replication begins at a certain point (the origin of replication) and proceeds in two directions simultaneously. This process creates two replication forks that move away from each other, eventually meeting at the opposite end of the DNA molecule. Eukaryotic DNA replication is an example of bidirectional synthesis, where multiple origins of replication allow the genome to be replicated more rapidly.

3. Continuous vs. discontinuous synthesis

During DNA synthesis, both strands of the DNA molecule are replicated. However, due to the antiparallel nature of the DNA strands, the synthesis process differs between the two strands. Continuous and discontinuous synthesis refer to the way the new DNA strands are synthesized: (a) Continuous synthesis: On the leading strand, DNA synthesis is continuous, meaning that the new complementary strand is synthesized continuously in the 5' to 3' direction, moving along with the replication fork. The DNA polymerase enzyme adds nucleotides to the 3' end of the primer, following the unwinding of the DNA helix. (b) Discontinuous synthesis: On the lagging strand, DNA synthesis occurs discontinuously. It happens through the formation of a series of short DNA fragments called Okazaki fragments. These fragments are later joined together by DNA ligase to form a complete, new strand of DNA. Each Okazaki fragment requires a separate primer that is removed and replaced by DNA later on, resulting in the discontinuous synthesis of the lagging strand. In conclusion, unidirectional and bidirectional synthesis are different in the direction of replication fork movement, whereas continuous and discontinuous synthesis refer to the mode of new DNA strand synthesis based on the leading or lagging strand.