9.3 Dna replication in prokaryotes  (Page 3/16)

Once the chromosome has been completely replicated, the two DNA copies move into two different cells during cell division. The process of DNA replication can be summarized as follows:

1. DNA unwinds at the origin of replication.
2. Helicase opens up the DNA-forming replication forks; these are extended bidirectionally.
3. Single-strand binding proteins coat the DNA around the replication fork to prevent rewinding of the DNA.
4. Topoisomerase binds at the region ahead of the replication fork to prevent supercoiling.
5. Primase synthesizes RNA primers complementary to the DNA strand.
6. DNA polymerase starts adding nucleotides to the 3'-OH end of the primer.
7. Elongation of both the lagging and the leading strand continues.
8. RNA primers are removed by exonuclease activity.
9. Gaps are filled by DNA pol by adding dNTPs.
10. The gap between the two DNA fragments is sealed by DNA ligase, which helps in the formation of phosphodiester bonds.

[link] summarizes the enzymes involved in prokaryotic DNA replication and the functions of each.

The previous section has demonstrated how DNA replicates by breaking existing double-stranded pieces apart, then adding a new second strand to each existing one. This may seem very logical, but scientists weren't always so certain how the process worked. All they knew was somehow a cell started with one piece of double-stranded DNA and then later it ended up with two pieces of double-stranded DNA. Scientists hypothesized three possibilities, one of which was the model described above, called semi-conservative replication. The other models (conservative replication and discontinuous replication) were rejected based on multiple experiments.

DNA replication is not a perfect process. The DNA polymerase sometimes makes mistakes. Cells have multiple ways to fix mistakes: DNA polymerase has a proof-reading ability to check its own work and different types of repair processes exist to fix different kinds of mutations. However, not every mistake is fixed. If a mistake is not fixed, we call it a mutation. Mutations are passed on because a cell can only fix a mistake as soon as it happens, so if it's not fixed right away, it stays in the cell's DNA permanently.

Section summary

DNA replication is required for cells to pass on their DNA when they divide into new cells. Multiple proteins are involved in replicating DNA, and it happens in a semi-conservative manner. The two strands are split apart, and a new, complementary copy of each strand is built. DNA polymerase can only build in the 5' to 3' direction, which means that each strand is replicated in different directions. Unfixed errors during DNA replication are mutations, and can be passed on when that cell divides.