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Genetic Recombination
Polymerases use nucleoside triphosphates to synthesize DNA or RNA from scratch using templates. They add a nucleotide to the 3’ hydroxyl aspect of a polynucleotide chain so that they add to the length of the chain. They always work from the 5’ end to the 3’ end and have active sites that both recognize the DNA template and have a nucleotide to add to the growing chain. They are classified according to the type of template they use.
In DNA replication, it is the DNA polymerase enzyme that does all the work in making new copies of the DNA chain. It needs to be very precise in order to make sure there are no mistakes in replication. Many DNA polymerases have a proofreading aspect that will recognize the occasional mistakes that can happen during replication and will repair any aspect of the DNA that has been incorrectly replicated.
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If an accidental replication has occurred, a 3’ to 5’ exonuclease is activated and the incorrect nucleotide base is removed before the correct one will be added to the growing polymer. This often happens as part of a huge replisome complex that has many subunits for the correct placing of a nucleotide on a growing DNA polymer.
There is a special type of DNA polymerase that will copy a sequence on an RNA molecule and will make a strand of DNA out of it. These include reverse transcriptase, which comes from viruses that infect cells by making DNA out of their RNA. Telomerase is unique in that it is a special polymerase that is necessary for the replication of the telomeres at the ends of each chromosome. Telomeres prevent chromosomes from fusing together during the replication process by capping the end of the chromosome.
Transcription happens because of the action of RNA polymerase that simply makes an RNA strand from a DNA template strand. This usually involves messenger RNA that takes the genetic message by first attaching to a promotor region, transcribes a section of the DNA segment, and then breaks off to go to the ribosome in order to translate the message into the correct protein. They often act within a large complex that can do all the steps necessary to turn a DNA message into a protein.
In most cases, segments of DNA do not interact with other and are actually kept in separate sections of the nucleus, known as “chromosome territories”. This allows the DNA to function without risk of DNA accidentally combining with another chromosome. One of the few times where crossover is allowed is during sexual reproduction, when genetic recombination is an evolutionary advantage. Two DNA helices break apart, switch sections and then join back together again.
