Our cells are constantly changing due to a series of events that take place during the phases of the cell cycle. Among these events, DNA replication stands out, as it is essential for cells to renew themselves.
What does cellular renewal consist of?
Cellular renewal is the ability of cells to reproduce and produce identical cells that replace the originals. This process is responsible for maintaining our cells and is a continuous process.
One of the key steps in this process is DNA replication, during which the cell prepares its cellular machinery to make an exact copy of all its genetic material.
How is DNA replication carried out in human cells?
Human cells belong to the group of eukaryotic cells. These cells contain linear DNA in the form of a double helix. The double helix consists of two complementary DNA strands held together by hydrogen bonds. Both strands run in opposite directions, with one strand oriented 5’ → 3’ (leading strand) and the other oriented 3’ → 5’ (lagging strand).
To create a copy of DNA, the following events must occur:
- The cellular machinery must access the interior of the double helix to read the information we want to copy. To achieve this, the enzyme helicase breaks the hydrogen bonds that hold the two DNA strands together, creating a structure called the replication fork.
- To relieve the tension in the DNA strands caused by the supercoiling of the double-helix structure, topoisomerase cuts and reattaches fragments of the DNA strand being replicated. We can imagine it as a structure responsible for unwinding the double helix.
- SSB proteins bind to the DNA strand to stabilize it and prevent it from rejoining its complementary strand, as this would unable DNA replication.
- The enzyme primase generates an RNA fragment (primer) complementary to the DNA segment that is to be replicated. This fragment is essential for DNA replication to begin. The primer provides a free 3’ OH end, which serves as the binding site for the enzyme responsible for copying the information, DNA polymerase
- DNA polymerases bind to the free 3’ OH end of the primer and add complementary nucleotides to the DNA strand that is being copied.
DNA polymerases only work in the 5’ → 3’ direction. In practical terms, this means that for the leading strand, a single primer is sufficient for the DNA polymerases to perform their function, while the lagging strand will require multiple primers to replicate the entire strand.
This creates replicated DNA fragments on the lagging strand known as Okazaki fragments. In the case of the lagging strand, the enzyme ligase is required to complete replication by "joining" these fragments together.
