Can someone explain why are the primers removed and is that the cause for the ov
ID: 102563 • Letter: C
Question
Can someone explain why are the primers removed and is that the cause for the overhangs or shortening of the necleotide? What exactly is meant by the "The end-replication problem?"
replica-ion Primers 5' 3' ding shrond insshrand Replication forks move outusourd SaN 5' 3' 5' Removal of pnmers overhangs A real eukaryotic chromosome would have multiple origins of replication and multiple replication bubbles, but the end-replication problem would be the same as shown above. Image modified from "Telomere shortening," by Zlir'a, public domain.Explanation / Answer
Answer: -
1.the chromosomes of eukaryotes are linear (rod-shaped), meaning that they have ends.
2.These ends pose a problem for DNA replication. The DNA at the very end of the chromosome cannot be fully copied in each round of replication, resulting in a slow, gradual shortening of the chromosome.
3.When DNA is being copied, one of the two new strands of DNA at a replication fork is made continuously and is called the leading strand.
4.The other strand is produced in many small pieces called Okazaki fragments, each of which begins with its own RNA primer, and is known as the lagging strand.
5.In most cases, the primers of the Okazaki fragments can be easily replaced with DNA and the fragments connected to form an unbroken strand.
6.When the replication fork reaches the end of the chromosome, however, there is (in many species, including humans) a short stretch of DNA that does not get covered by an Okazaki fragment—essentially, there's no way to get the fragment started because the primer would fall beyond the chromosome end.
7.Also, the primer of the last Okazaki fragment that does get made can't be replaced with DNA like other primers.
8.part of the DNA at the end of a eukaryotic chromosome goes uncopied in each round of replication, leaving a single-stranded overhang.
9.Over multiple rounds of cell division, the chromosome will get shorter and shorter as this process repeats.
10.the single-stranded overhangs produced by incomplete end replication in humans are fairly long, and the chromosome shortens significantly with each round of cell division.
11.To prevent the loss of genes as chromosome ends wear down, the tips of eukaryotic chromosomes have specialized DNA “caps” called telomeres.
12.Telomeres consist of hundreds or thousands of repeats of the same short DNA sequence, which varies between organisms but is 5'-TTAGGG-3' in humans and other mammals.
13.Telomeres need to be protected from a cell's DNA repair systems because they have single-stranded overhangs, which "look like" damaged DNA.
14.The overhang at the lagging strand end of the chromosome is due to incomplete end replication .
15.The overhang at the leading strand end of the chromosome is actually generated by enzymes that cut away part of the DNA.
16.In some species (including humans), the single-stranded overhangs bind to complementary repeats in the nearby double-stranded DNA, causing the telomere ends to form protective loops.
17.Proteins associated with the telomere ends also help protect them and prevent them from triggering DNA repair pathways.
18.The repeats that make up a telomere are eaten away slowly over many division cycles, providing a buffer that protects the internal chromosome regions bearing the genes
19.Some cells have the ability to reverse telomere shortening by expressing telomerase, an enzyme that extends the telomeres of chromosomes. Telomerase is an RNA-dependent DNA polymerase.
20.The enzyme binds to a special RNA molecule that contains a sequence complementary to the telomeric repeat. It extends (adds nucleotides to) the overhanging strand of the telomere DNA using this complementary RNA as a template.