Question

Outline the current model for DNA synthesis.

Short answer

Answer: The key components and enzymes in the current model for DNA synthesis include DNA helicase, single-strand binding proteins, topoisomerase, DNA primase, DNA polymerase and DNA ligase. DNA helicase unwinds the DNA double helix and separates the two strands, while single-strand binding proteins stabilize the single-stranded DNA. Topoisomerase relieves the stress generated by the unwinding. DNA primase synthesizes an RNA primer, which enables DNA polymerase to add new nucleotides in the 5' to 3' direction, forming a complementary strand. Finally, DNA ligase joins the Okazaki fragments together on the lagging strand. This process leads to semi-conservative replication, producing two identical DNA molecules, each with one original parental strand and one newly synthesized daughter strand.

Step-by-step solution

1. Understanding DNA structure

DNA, or deoxyribonucleic acid, is a double-stranded helical molecule that encodes the genetic information of an organism. It is composed of four nucleotide bases: adenine (A), cytosine (C), guanine (G), and thymine (T). The two strands of DNA are held together by hydrogen bonds between complementary base pairs, with A pairing with T and C pairing with G.

2. Semi-conservative replication

The current model for DNA synthesis is known as semi-conservative replication. In this process, each original strand of DNA acts as a template for the synthesis of a new complementary strand. The result is a replication process where each of the two newly synthesized DNA molecules consists of an original parental strand and a newly synthesized daughter strand.

3. Key enzymes and their functions

Several enzymes and proteins are essential for DNA synthesis: - DNA helicase: unwinds the DNA double helix and separates the two strands. - Single-strand binding proteins: stabilize the single-stranded DNA to prevent them from re-annealing. - Topoisomerase: relieves the stress generated by the unwinding of the helix. - DNA primase: synthesizes an RNA primer, providing a 3' end for DNA polymerase to begin synthesis. - DNA polymerase: adds new nucleotides, always in the 5' to 3' direction, to form a complementary strand. - DNA ligase: joins the DNA fragments (Okazaki fragments) together on the lagging strand.

4. Leading and lagging strand synthesis

DNA synthesis occurs in a continuous fashion on the leading strand, as DNA polymerase can synthesize new DNA in the 5' to 3' direction of the original template strand. On the lagging strand, multiple small fragments called Okazaki fragments are synthesized, which must be later joined together to form a complete strand.

5. Replication fork

DNA replication starts with the formation of a replication fork. This is where the DNA helicase enzyme unwinds the double helix and separates the two strands, creating a Y-shaped structure known as the replication fork. The replication process proceeds in both directions, creating two replication forks that move away from the origin of replication.

6. Process of DNA synthesis

Here's an outline of the DNA synthesis process: 1. DNA helicase unwinds the DNA double helix and separates the two strands. 2. Topoisomerase and single-strand binding proteins stabilize the DNA and prevents supercoiling. 3. DNA primase constructs an RNA primer on both strands. 4. DNA polymerase begins synthesizing the new complementary strand on the leading strand continuously in the 5' to 3' direction and in short fragments called Okazaki fragments on the lagging strand. 5. RNA primers are eventually removed and replaced by DNA nucleotides. 6. Okazaki fragments are joined together by DNA ligase to form a complete lagging strand. Once the replication forks meet, DNA synthesis is complete, resulting in two identical DNA molecules, each consisting of one original parental strand and one newly synthesized daughter strand.