DNA Replication
Accurate DNA replication is what passes genetic information down from generation to generation. However, the exact mechanism of DNA replication was not known at the time that Watson and Crick came up with their DNA model. The rough idea was that the two DNA strands separate and serve as template strands for their respective complementary strands. This is known as the semi-conservative model of replication, where each of the two daughter strands include one parent strand and one complementary strand. Nevertheless, other possibilities such as conservative replication (where the daughter strands are broken again so that the original parent strands join back together, leaving the two new strands to pair up) and dispersive replication (where both daughter strands contain a mixture of parental and new DNA). The semi-conservative model is the most appealing because it is the simplest.
In order to find out which model of replication is correct, Matthew Meselson and Franklin Stahl conducted an experiment in which they let bacterial DNA with the nitrogen-15 isotope to replicate in a medium with the lighter nitrogen-14. Then the DNA is extracted from the bacteria and centrifuged so that the heavier the DNA is, the lower it sinks. After the first replication, the DNA is all near the middle of the tube. This suggests that the parent strands with the heavy nitrogen isotope did not "rejoin", thus eliminating the conservative model. After a second replication, half the DNA is near the top of the tube and the other half is still near the middle. This meant that the denser parent strands were not dispersed among the daughter strands, thereby invalidating the dispersive model. That leaves only the semi-conservative model as a valid option.
In order to find out which model of replication is correct, Matthew Meselson and Franklin Stahl conducted an experiment in which they let bacterial DNA with the nitrogen-15 isotope to replicate in a medium with the lighter nitrogen-14. Then the DNA is extracted from the bacteria and centrifuged so that the heavier the DNA is, the lower it sinks. After the first replication, the DNA is all near the middle of the tube. This suggests that the parent strands with the heavy nitrogen isotope did not "rejoin", thus eliminating the conservative model. After a second replication, half the DNA is near the top of the tube and the other half is still near the middle. This meant that the denser parent strands were not dispersed among the daughter strands, thereby invalidating the dispersive model. That leaves only the semi-conservative model as a valid option.
You should already the familiar the process of DNA replication by now. But feel free to refer back to the process through the below link.
However, there are a few points that we did not talk about in that earlier section, namely nucleotide excision repair and telomeres.
In nucleotide excision repair, a damaged portion of one strand of DNA is removed by the nuclease enzyme. Then, DNA polymerase adds the correct complementary bases to the undamaged strand. Finally, DNA ligase attaches the phosphate backbones of the nucleotides together. This process is important for repairing damage caused by radiation and chemicals. For example, when adjacent thymine bases form covalent bonds together (also known as thymine dimers), it can interfere with correct DNA replication. Individuals with the disorder xeroderma pigmentosum are prone to skin cancer because one or more of their nucleotide excision repair enzymes are not working properly. Therefore, they must avoid harmful radiation (such as ultraviolet rays from the sun) and reactive chemicals to avoid the accumulation of DNA damage.
Telomeres are nucleotide sequences at the ends of a DNA polymer. Because DNA polymerase cannot replicate the very last few nucleotides of the 5' end of DNA, every time the DNA replicates, the DNA strand is slightly shortened. This can threaten to harm the DNA sequences which code for vital proteins. To delay this, DNA molecules have telomeres, which serve as buffers that protect critical genes from disappearing. Also, note that the telomeres themselves do not code for any genes. To prevent the shortening of DNA over generations, eukaryotic cells that give rise to gametes (germ cells) have an enzyme called telomerase, which have short RNA molecules that act as templates to elongate the telomeres. This ensures that gametes produced from the germ cells will have long telomeres.
In nucleotide excision repair, a damaged portion of one strand of DNA is removed by the nuclease enzyme. Then, DNA polymerase adds the correct complementary bases to the undamaged strand. Finally, DNA ligase attaches the phosphate backbones of the nucleotides together. This process is important for repairing damage caused by radiation and chemicals. For example, when adjacent thymine bases form covalent bonds together (also known as thymine dimers), it can interfere with correct DNA replication. Individuals with the disorder xeroderma pigmentosum are prone to skin cancer because one or more of their nucleotide excision repair enzymes are not working properly. Therefore, they must avoid harmful radiation (such as ultraviolet rays from the sun) and reactive chemicals to avoid the accumulation of DNA damage.
Telomeres are nucleotide sequences at the ends of a DNA polymer. Because DNA polymerase cannot replicate the very last few nucleotides of the 5' end of DNA, every time the DNA replicates, the DNA strand is slightly shortened. This can threaten to harm the DNA sequences which code for vital proteins. To delay this, DNA molecules have telomeres, which serve as buffers that protect critical genes from disappearing. Also, note that the telomeres themselves do not code for any genes. To prevent the shortening of DNA over generations, eukaryotic cells that give rise to gametes (germ cells) have an enzyme called telomerase, which have short RNA molecules that act as templates to elongate the telomeres. This ensures that gametes produced from the germ cells will have long telomeres.