Mechanisms of transcription I

Question 1

Explain the concept of differential gene expression

Answer 1

• Human genome contains about 50,000 genes but in any one cell type only 10,000 genes are expressed
• Between different cell types these 10,000 expressed genes are different and rae expressed at different times

Question 2

What 2 aspects is differential gene expression tightly regulated in?

Answer 2

• Time
  
  • Development (i.e. embryos versus adults)
  • In response to hormones, infection, other signals.
• Space
  
  • Different tissues or cell types express different genes (i.e. brain versus muscle cells).

Question 3

Why is it important to tightly regulate gene expression?

Answer 3

• Because if gene expression isn’t tightly regulated it may lead to a variety of diseases/disorders depending on the type of genes that aren’t tightly regulated

Question 4

Explain the process of differentiation from a single cell

Answer 4

• Totipotent cells can differentiate into any cell type
• Pluripotent cells can differentiate into any cell type except placental cells

Question 5

What is the difference between totipotent and pluripotent cells?

Answer 5

• Totipotent cells can differentiate into any cell type
• Pluripotent cells can differentiaste into any cell type except the cells that make up the placental tissue

Question 6

What is reprogramming?

Answer 6

• The process of reverting mature, specialised cells into induced pluripotent stem cells

Question 7

How does reprogramming work?

Answer 7

• Works by isolating the specialised cells and inserting transcription factors into the specialised cells
• These transcription factors then cause the expression of certain genes causing the cell to de-specialise back into a pluripotent stem cell

Question 8

What are the 2 levels at which gene expression can be regulated?

Answer 8

• Transcriptional level: turning on/off DNA to RNA synthesis.
• Post-transcriptional level: regulation of RNA stability/translation.

Question 9

Why is gene expression mainly regulated at the level of transcription?

Answer 9

• Because it is more energetically favorable to the cell to just transcribe genes that are needed by a cell at a particular time than transcribing all the genome and translating a fraction of these RNAs only

Question 10

What is it called when a gene is expressed in every cell type?

Answer 10

• Housekeeping gene e.g. glycolytic enzymes

Question 11

What are tissue-specific genes?

Answer 11

• These are genes that aren’t expressed in one cell type but are expressed abundantly in another cell type e.g. globin

Question 12

What are inducible genes?

Answer 12

• Genes that are only expressed in response to a specific stimuli e.g. Lac operon genes in response to lactose

Question 13

Eukaryotes have 3 RNA polymerases what are they and ewhat are their functions?

Answer 13

• Polymerase I - transcribes 5, 8, 18, 28S rRNA genes
• Polymerase II - transcribes all protein-encoding genes
• Polymerase III - transcribes tRNA genes, 5S RNA genes, some snRNA genes, etc

Question 14

What are the main features of the RNA polymerase II structure?

Answer 14

• DNA enters into a cleft and ʻjawsʼ grip the DNA. The jaws may open and close.
• When the DNA gets close to the active site it is bound by the bridge helix and ʻswitchʼ regions at the base of the clamp.
• Closure of the ʻclampʼ is induced by binding of the DNARNA hybrid to 3 ʻswitchʼ regions during elongation. This stabilizes the elongation complex.
• Linear DNA exit is blocked by the ʻwallʼ and it exits at a 90^o angle.
• Nucleotides (NTPs) enter via a pore below the active site. Transcription requires recognition sequenc