Key Points for DNA Replication

What is DNA Replication?

• Process of copying DNA to produce two identical DNA molecules.

Why DNA Replication is Essential?

• Ensures genetic information is passed to daughter cells during cell division.
• Allows for DNA repair, growth, and development.
• Introduces occasional mutations for genetic diversity and evolution.

Models of Replication

1. Conservative Model: Original DNA remains intact, and a new DNA molecule is synthesized alongside.
2. Semiconservative Model: Each new DNA molecule has one parental strand and one newly synthesized strand.
3. Dispersive Model: Original DNA segments are dispersed and mixed with newly synthesized segments.

Experiments related to DNA

• Meselson and Stahl Experiment- Supported the semiconservative model of DNA replication using isotopes of nitrogen.
• Hershey and Chase experiment- Radioactive phosphorus (32P) and and radioactive sulphur (32S) was used to culture bacteriophages. These bacteriophages were then made to infect bacterial cells. It was seen that the bacterial cells contained radioactive viral DNA, but not radioactive viral protein because DNA contains phosphorous, but protein does not. Hence this experiment proved that the DNA, and not protein, is the genetic material passed from the virus to bacteria.

Requirements of Replication

• Template DNA strands, DNA polymerases, primers, nucleotides, enzymes, and proteins.

Enzymes Involved in DNA Replication

• DNA polymerases, helicases, primases, ligases, and topoisomerases.

Direction of Replication

• Bidirectional with two replication forks moving from the origin.

The Mechanism of Replication

• Involves unwinding DNA, synthesizing complementary strands, and error-proofing.

Bacterial DNA Replication

• Starts at the origin, bidirectional, forms a replication bubble.

Initiation of Replication

• Initiated by recognition of the origin, binding of helicases and primases.

Elongation of Nucleotide Chain

• DNA polymerase adds nucleotides in a 5’ to 3’ direction.

DNA Polymerase of Prokaryotes

• DNA polymerase III synthesizes the new strand.
• DNA polymerase I removes RNA primers and proofreads.

Termination of DNA Replication

• Involves the termination of replication forks and joining of DNA fragments.

Eukaryotic DNA Replication

• Similar to prokaryotic replication but with multiple origins.

Origin Sites in Eukaryotes

• Multiple origin sites on chromosomes.

DNA Polymerase of Eukaryotes

• Various DNA polymerases, including α, δ, and ε.

DNA Synthesis at Chromosome Ends

• Telomerase maintains telomere length.

RNA

• RNA generally mutates at a faster rate compared to DNA due to the inherent properties of RNA polymerases and the lack of proofreading mechanisms in some RNA viruses. Viruses with RNA genomes tend to have shorter life spans and higher mutation rates, which can lead to faster evolution. This is because the replication machinery of RNA viruses often lacks the ability to correct errors that occur during replication, contributing to their higher mutation rates and ability to evolve rapidly.

• Coding strand have same sequence as of mRNA but at the place of U, T is found in coding strand