#15. what is the end-replication problem? why, in the absence of telomerase, do the ends of the linear chromosomes get progressi
vely shorter each time dna is replicated?
1 answer:
The DNA replication by the action of DNA polymerase takes place in the 3' to 5' direction on the leading strand. The lagging strand which has the opposite orientation or polarity as that of the leading strand requires a more time to get synthesised. The DNA replication of the lagging strand happens in short segments where a RNA primer forms a compliment with a part of the DNA segment on its 3' end. This RNA primer helps initiate the replication of the Okazaki fragments. When the replication on the lagging strand reaches its end, the RNA primer forms a compliment with the last bit of the strand. This small segment gets missed in the end as no more DNA is left to form a RNA primer-DNA compliment. Such shortening of the lagging strand in the replication process is the end-replication problem.
Telomeres are protective ends of the DNA strands. These ends contain a poly-A tail. When the lagging strand replication reaches its end, the RNA primer forms a compliment with the telomere and initiates the replication. This leads to the shortening of the telomere and not the coding segments on the lagging strand of DNA. The telomerase repairs the shortened telomere by re-synthesising it.
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Answer:
Option c () is the correct alternative.
Explanation:
Given:
Common ancestor,
= 50 million years ago
Base differences,
= 92
Now,
The predicted mutation rate will be:
=
=
=
or,
=
Thus the above solution is the right one.
I searched for the image and it points out to the liver. According to the choices, the answer is "converting toxins to less toxic forms" as the liver functions to detoxify substances through its cells (particularly in the smooth endoplasmic reticulum) via the cytochrome p450 system. Detoxification involves 2 phases, first is Phase I wherein the p450 system makes the toxic substance more hydrophilic (it means it is easily soluble to water) and can be readily excreted by the kidneys. If Phase I is insufficient, then the toxin enters Phase II which is conjugation with other molecules to make the toxin even more hydrophilic. The liver does not produce urine and excreting salt and urea as this is the function of the kidneys. The liver's main function is not digesting foods and liquids as that is the main function of the gastrointestinal tract, but the liver aids in the absorption of fats by the secretion of bile, thereby emulsifying the fats rendering it ready for absorption.
