DNA Replication



DNA replication is the first step in both mitosis and meiosis. This process requires that a cell unwind the length of an entire chromosome in segments to be copied. These copies must be exact; without any errors. The process will end with an exact copy of one chromosome's DNA.

Several enzymes are necessary for DNA replication to occur:

DNA Polymerase
Primase
DNA Helicase
DNA Gyrase

The process begins when an enzyme known as helicase, unwinds the DNA. Helicase accomplishes this by breaking hydrogen bonds between each nucleotide. In human DNA, it would be impossible for the cell to handle the unwinding of the entire DNA molecule at one time. There simply would not be enough room in the nucleus of the cell. As a result, helicase will only unwind as much DNA as is necessary. As soon as it is replicated, the DNA molecule can wind back up into a more compact structure.

The replication of DNA also uses origins of replication to start the process. Organisms with very large genomes would take days to replicate due to their size. As a result, multiple origins of replication usually form to replicate simultaneously and speed the process along. This means that the replication of DNA will proceed in two directions until the entire molecule is copied.

The easiest way to understand DNA replication is through the use of a visual known as a replication fork. This shows the 5' (pronounced five prime) end of the DNA and the 3' end. The 5' end always ends with a phosphate and the 3' with a hydroxide. This also makes it easier to understand why DNA is considered semi-conservative. This means that when DNA is replicated the parental strand is split. Although there are 2 new DNA molecules made, both of the molecules only contain one half parent and the other half newly made DNA.

The next step in DNA replication is performed with an enzyme known as DNA polymerase. DNA polymerase is responsible for adding new nucleotides to the new strands according to the rules of base pairing. That is, A binding with T and G binding with C. DNA polymerase can add nucleotides only in a 5' (prime) to 3' (prime) direction. This means that on the leading strand it can add the nucleotides in a continuous fashion. On the lagging strand the DNA is going in the opposite direction. Therefore, it must add the newly synthesized DNA in small fragments. These fragments are known as Okazaki fragments. They will eventually be glued together to make a seamless strand of new DNA. It should also be noted that DNA polymerase does have some proofreading ability. It will check to make sure it has added the correct nucleotides during the replication process.

The last step in DNA replication is the putting together the Okazaki fragments so they are one seamless piece. This involves using an enzyme called DNA ligase. DNA ligase glues together the pieces of DNA to make one long molecule. The DNA can now be recoiled so that it will fit inside the cell. The recoiling process occurs when DNA winds around a histone protein. It is then packaged into chromatin. It is in this manner that DNA is able to fit inside the tiny nucleus of a cell.

There are now two complete strands of DNA. This means the cell must now commit to either mitosis or meiosis. This is because the cell has two complete stands of DNA (double the genetic material). It is imperative that the cell divide in order to have the correct amount of DNA in the cells. This entire replication process occurs during the S phase (synthesis) of the cell cycle. If the cell is destined for mitosis, it will only divide one time. This means the amount of genetic material will be the same as the parent cell. If the cell is meant to be a gamete, the cell will undergo meiosis (two divisions). This will result in a cell that is haploid (half the genetic material of the cell).