DNA Replication

RNA generally does not participate in being an aspect of the genome. It is only a genome when virus particles are concerned. Viruses have a wide variety of genomes that can include double stranded RNA or even single-stranded RNA.

DNA REPLICATION

There is a completely separate process that goes on with regard to the replication of DNA. Replication happens when the cell needs to divide itself. The enzyme that does most of the replication process is called DNA helicase. Figure 39 shows what DNA replication looks like:

Figure 39.

There are other enzymes involved in the replication process, including topoisomerase and DNA primase. DNA helicase disrupts the hydrogen bonding between the two DNA strands, creating a replication fork. Remember that the DNA strands run in opposite directions from one another. The five-prime end have a phosphate group attached,

while the three-prime end has a hydroxyl group attached. One strand is called the leading strand, while the other strand is called the lagging strand.

The leading strand is easier to replicate than the lagging strand. There is a primer piece of RNA on the three-prime strand that gets made by DNA primase. DNA polymerases then engage in the process of elongation of the new strand. There are DNA polymerases that do most of the actual replication and DNA polymerases that participate in error checking and in DNA repair during the replication process. Replication proceeds in eukaryotes in the five-prime to three-prime direction.

The lagging strand is harder to replicate. There are multiple primers that participate in this process. Each primer is just a few bases long. There are pieces of DNA that get added between the primer strands called Okazaki fragments. These fragments then get joined together.

After the strands are completely made in the replication process, there is an enzyme called an exonuclease that gets rid of the RNA primers. These get replaced with the proper DNA fragments. There is another exonuclease that properly proofreads the DNA in order to replace errors. DNA ligase is important in joining the Okazaki fragments so there is a unified strand.

Telomeres are the protective end of the DNA strands. These telomere sections prevent DNA molecules from attaching to one another. After the telomeres are added by an enzyme called telomerase, the DNA forms its typical double-helical shape.

Topoisomerase is also referred to as DNA gyrase. This is the molecule that unwinds and rewinds the DNA strands in the process of DNA replication. The purpose of this enzyme is to keep the DNA from becoming supercoiled or tangled in the replication process.