Even before the structure of DNA was elucidated, experimental evidence indicated

Question

Even before the structure of DNA was elucidated, experimental evidence indicated that the hereditary material had to have three key properties. Name and explain these properties. Explain how the DNA structure supports its role as genetic materials. (consider all three properties required for genetic materials)

A replication fork moves in one direction, however, both daughter strands are synthesized at the same time. Describe how the two daughter strands are synthesized in eukaryotes.

What is end-replication problem? How do telomeres and telomerase regulate the replication of the ends of chromosomes in eukaryotes?

Explanation / Answer

Telomeres shorten in part because of the end replication problem that is exhibited during DNA replication in eukaryotes only. Because DNA replication does not begin at either end of the DNA strand, but starts in the center, and considering that all DNA polymerases that have been discovered move in the 5' to 3' direction, one finds a leading and a lagging strand on the DNA molecule being replicated. On the leading strand, DNA polymerase can make a complementary DNA strand without any difficulty because it goes from 5' to 3'. However, there is a problem going in the other direction on the lagging strand. To counter this, short sequences of RNA acting as primers attach to the lagging strand a short distance ahead of where the initiation site was. The DNA polymerase can start replication at that point and go to the end of the initiation site. This causes the formation of Okazaki fragments. More RNA primers attach further on the DNA strand and DNA polymerase comes along and continues to make a new DNA strand. Eventually, the last RNA primer attaches, and DNA polymerase, RNA nuclease, and DNA ligase come along to convert the RNA (of the primers) to DNA and to seal the gaps in between the Okazaki fragments. But, in order to change RNA to DNA, there must be another DNA strand in front of the RNA primer. This happens at all the sites of the lagging strand, but it does not happen at the end where the last RNA primer is attached. Ultimately, that RNA is destroyed by enzymes that degrade any RNA left on the DNA. Thus, a section of the telomere is lost during each cycle of replication at the 5' end of the lagging strand. Telomerase is an enzyme that adds DNA sequence repeats ("TTAGGG" in all vertebrates) to the 3' end of DNA strands in the telomere regions, which are found at the ends of eukaryotic chromosomes. This region of repeated nucleotide repeats called telomeres contain condensed DNA material and prevents constant loss of important DNA from chromosome ends. As a result, every time the chromosome is copied only a couple telomeres are lost, which causes no damage to the organism. Telomerase is a reverse transcriptase that carries its own RNA molecule, which is used as a template when it elongates telomeres, which are shortened after each replication cycle. The enzyme telomerase allows for replacement of short bits of DNA known as telomeres, which are otherwise shortened when a cell divides via mitosis.

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