eukaryotic gene expression

Question 1

what is the function of centromere

Answer 1

It is a constricted region holding 2 sister chromatids together.
It is a attachment site for sister chromatids and associate with kinetochore microtubules.
Without centromeres there will be improper alignment of chromosomes leading to aneuploidy.

Question 2

What is the end replication problem?

Answer 2

when DNA replication happens in the S phase, DNA polymerase is unable to replicate the sequence completely.They require free 3’OH group to bind nucleotide too. However when the 5’ primer is excised from the newly synthesised strand there is no 3’OH end for a nucleotide to replace the primer. Therefore 5’ end of the newly synthesised strand will be shorter than the parental template strand at the 3’ end

Question 3

Describe the structure of telomeres.

Answer 3

Non coding DNA consisting a series of short tandem repeat sequences.
They can be degraded by exonuclease as they hydrolyse the phosphodiester binds between adjacent nucleotides.

3’ overhang fold back to form a T loop to hide the 3’ overhang.
T loop capping proteins binds to the T loop to maintain stability.

Question 4

What is the function of telomeres.

Answer 4

-Each round of DNA replication shortens chromosomal ends which shortens telomeres.
Genes will not be corroded and there will not be any loss of vital genetic information.
-Telomeres also provide a counting mechanism on how many cell divisions it can undergo (Hayflick limit)
As the cell replicates telomere shortens to critical length. This trigger signal transduction pathways leading to apoptosis. This prevents cancer as the cell will not be able to proliferate uncontrollably.

• telomeres protect and stabilise the terminal ends. The 3’overhang of one chromosome will anneal to a complementary single strand at a terminal end of another chromosome. This is because they resemble broken chromosomes. Telomeres prevent chromosomes from joining to each other
• prevents ends of chrome from being degraded by exonuclease and open ends are detected as DNA damage which trigger signal transduction pathways leading to apoptosis.
• the 3’ overhang resembles DNA damage from double stranded breaks . This triggers apoptosis.

Having a T loop and telomeres prevent chromosomes from fusing with each other and prevent DNA repair machinery from recognising them as DNA breaks