DNA replication is the process by which a cell copies its DNA prior to cell division. This ensures that each daughter cell receives a complete and identical copy of the genetic information. DNA replication occurs in three stages: initiation, elongation, and termination.
Initiation:
- The double-stranded DNA is unwound and separated into two strands by an enzyme called helicase.
- The separated strands are stabilized by single-strand binding proteins.
- An enzyme called primase adds a short RNA primer to the DNA template strand.
Elongation:
- DNA polymerase adds nucleotides to the 3′ end of the RNA primer to synthesize a new DNA strand.
- DNA polymerase moves along the DNA template strand in the 3′ to 5′ direction, synthesizing a complementary strand in the 5′ to 3′ direction.
- The leading strand is synthesized continuously in the 5′ to 3′ direction, while the lagging strand is synthesized in short segments called Okazaki fragments.
Termination:
- When DNA polymerase reaches the end of the template strand or encounters another replication fork, it falls off the DNA strand.
- The RNA primers are removed by an enzyme called RNase H and replaced with DNA by an enzyme called DNA polymerase I.
- The gaps between the Okazaki fragments are sealed by an enzyme called DNA ligase.
DNA repair is the process by which cells correct mistakes and damage in their DNA to maintain the integrity of the genetic information. There are several types of DNA damage that can occur, including:
- Point mutations, which involve a change in a single nucleotide.
- Chemical modifications, such as methylation or deamination, which alter the chemical structure of a nucleotide.
- DNA strand breaks, which can be caused by exposure to radiation or chemicals.
Cells have several mechanisms to repair DNA damage, including:
- Base excision repair, which corrects chemical modifications to individual nucleotides.
- Nucleotide excision repair, which removes and replaces segments of damaged DNA.
- Mismatch repair, which corrects mistakes made during DNA replication.
- Double-strand break repair, which repairs breaks in both strands of the DNA helix.
Overall, DNA replication and repair are critical processes that ensure the accurate transmission and maintenance of genetic information.