Molecular mechanism of DNA replication
- DNA replication is semiconservative. Each strand in the double helix acts as a template for synthesis of a new, complementary strand.
- New DNA is made by enzymes called DNA polymerases, which require a template and a primer (starter) and synthesize DNA in the 5' to 3' direction.
- During DNA replication, one new strand (the leading strand) is made as a continuous piece. The other (the lagging strand) is made in small pieces.
- DNA replication requires other enzymes in addition to DNA polymerase, including DNA primase, DNA helicase, DNA ligase, and topoisomerase.
The basic idea
- DNA double helix.
- Hydrogen bonds break and helix opens.
- Each strand of DNA acts as a template for synthesis of a new, complementary strand.
- Replication produces two identical DNA double helices, each with one new and one old strand.
- They always need a template
- They can only add nucleotides to the 3' end of a DNA strand
- They can't start making a DNA chain from scratch, but require a pre-existing chain or short stretch of nucleotides called a primer
- They proofread, or check their work, removing the vast majority of "wrong" nucleotides that are accidentally added to the chain
Starting DNA replication
Primers and primase
Leading and lagging strands
The maintenance and cleanup crew
Summary of DNA replication in E. coli
- Helicase opens up the DNA at the replication fork.
- Single-strand binding proteins coat the DNA around the replication fork to prevent rewinding of the DNA.
- Topoisomerase works at the region ahead of the replication fork to prevent supercoiling.
- Primase synthesizes RNA primers complementary to the DNA strand.
- DNA polymerase III extends the primers, adding on to the 3' end, to make the bulk of the new DNA.
- RNA primers are removed and replaced with DNA by DNA polymerase I.
- The gaps between DNA fragments are sealed by DNA ligase.
DNA replication in eukaryotes
- Eukaryotes usually have multiple linear chromosomes, each with multiple origins of replication. Humans can have up to origins of replication!
- Most of the E. coli enzymes have counterparts in eukaryotic DNA replication, but a single enzyme in E. coli may be represented by multiple enzymes in eukaryotes. For instance, there are five human DNA polymerases with important roles in replication.
- Most eukaryotic chromosomes are linear. Because of the way the lagging strand is made, some DNA is lost from the ends of linear chromosomes (the telomeres) in each round of replication.