Regulation Of Transcription And Translation

Question 1

Why are cells specialised or differentiated?

Answer 1

• during development, totipotent cells translate only part of their DNA resulting in cell specialisation
• for a cell to become specialised certain genes must be ‘switched on’
• achieved through transcription

Question 2

basics of gene expression= transcription has to occur, pre-mRNA has to be spliced, translation has to occur- but what decides when this happens and at which section of DNA?

Answer 2

• genes don’t start to transcribe themselves spontaneously (if that was the case, cells in your pancreas would produce adrenaline, and cells in testicles would begin to release oestrogen!)
• our bodies contain regulatory proteins called transcription factors

Question 3

What are transcription factors?

Answer 3

Protein that switches on the gene to begin translation- moves from cytoplasm into nucleus and binds to a promoter region

Question 4

How do transcription factors work?

Answer 4

1- for transcription to begin, the gene is ‘switched on’ by a protein that moves from the cytoplasm into the nucleus and binds to a promoter (transcription factor)
2- each transcription factor has a DNA binding site that binds to a specific base sequence of DNA in the nucleus
3- when it binds, it causes this region of DNA to begin the process of transcription
4- mRNA is produced and the information it carries is then translated into a polypeptide
5- when a gene is not being expressed (I.e. ‘switched off’) the site of the transcriptional that binds to the DNA is not active
6- as the site on transcriptional factor binding to DNA is inactive it cannot cause transcription and polypeptide synthesis

Question 5

What about when the gene doesn’t need to be expressed? How could you stop transcription factors from stimulating DNA?

Answer 5

inhibitor molecule = When a gene is not being expressed, the DNA binding site on its complimentary transcription factor is blocked and this inhibitor stops the transcription factor from binding to DNA, thus blocking transcription from occurring

Question 6

Transcription and translation though, are regulated via the other hormone mechanism, which involves lipid-soluble hormones such as

Answer 6

oestrogen

Question 7

Explain how oestrogen can switch on a gene and thus start transcription?

Answer 7

• oestrogen is a lipid-soluble molecule and therefore diffuses easily through the phospholipid bilayer of cell-surface membrane
• once inside the cytoplasm of a cell, oestrogen binds with a site on a receptor molecule of the transcriptional factor- the shape of this site and the shape of the oestrogen molecule complement each other
• by binding with the site, the oestrogen changes the shape of the DNA binding site on the transcriptional factor, which can now bind to DNA (it is activated)
• transcriptional factor can now enter the nucleus through a nuclear pore and bind to specific base sequences on DNA
• the combination of the transcriptional factor with DNA stimulates transcription of the gene that makes up the portion of DNA

Question 8

in eukaryotes and some prokaryotes the translation of mRNA produced by a gene can be inhibited by breaking mRNA down before its coded information can be translated into a polypeptide- one type of small RNA molecule that may be involved is small interfering RNA (siRNA) and the mechanism involving small double-stranded sections of siRNA operates as follows:

Answer 8

• an enzyme cuts large double-stranded molecules of RNA into smaller sections called small interfering RNA (siRNA)
• 2- one of the two siRNA strands combines with an enzyme
• 3-The siRNA molecule guides the enzyme to an mRNA molecule by pairing up its bases with the complementary ones on a section of mRNA molecule
• 4- once in position, the enzyme cuts the mRNA into smaller sections
• 5- mRNA no longer capable of being translated into polypeptide
• 6- this means gene has not been expressed- has been ‘blocked’

Question 9

One of the two strands of siRNA combines with an enzyme and guides it to an mRNA molecule which it then cuts. Explain why the mRNA is unlikely to be cut if the other siRNA strand combines with the enzyme

Answer 9

• the other strand would have complementary bases I.e. GCUA instead of CGAU respectively
• it is unlikely that these opposite base pairings would complement a sequence on the mRNA
• the siRNA with enzyme attached would therefore not bind to the mRNA and so would be unaffected

Question 10

Suggest how siMRNA could be used to:
A- identify roles of genes in a biological pathway
B- to prevent certain diseases

Answer 10

A= some siMRNA that blocks a particular gene could be added to cells; observing the effects (or lack of them) we could determine what the role of the blocked gene is

B= siMRNA could be used to prevent the disease by blocking the gene that caused it