Introduction to the control of gene expression

Question 1

What do all human cells contain?

Answer 1

They all have the same genetic blueprint which is known as the genotype.

Question 2

How many genes does the human genome contain and how many of these are actually expressed?

Answer 2

The human genome contains ~50,000 genes and only 10,000 genes are expressed.

Question 3

What is differential gene expression?

Answer 3

It is the biochemical processes that determine which genes are actively transcribed and translated into mRNA and proteins in a cell and under what conditions.

Question 4

Explain how differential gene expression is tightly regulated with respect to time and space.

Answer 4

1. TIME:-

• Development (i.e. embryos versus adults is different) in response to hormones, infection, other signals.

2. SPACE:-

• Different tissues or cell types express different genes (i.e. brain versus muscle cells).

Question 5

Why is it important for gene expression to be regulated?

Answer 5

Failure to regulate gene expression tightly may lead to different diseases and disorders. Examples include:

• metabolic diseases (if gene expression involved in metabolism is compromised)
• metastasis (if gene expression involved in cell shape/motility is compromised)
• congenital disorders (if gene expression involved in cell differentiation is compromised)
• cancer (if gene expression involved in cell proliferation is compromised)

Metastasis meaning - the development of secondary malignant growths at a distance from a primary site of cancer.

Question 6

Give two examples of drosophila homeotic mutations.

Answer 6

1. Bithorax - When the fly has 2 thoraxes.
2. Antennapedia - When the fly has legs in place of its antenna.

Question 7

Where are totipotent stem cells found? And what can they do?

Answer 7

• Totipotent stem cells are found in spores and zygotes.
• They can differentiate into any type of cell (this includes placenta stem cells unlike pluripotent stem cells).
• It is possible for a fully differentiated cell to return to a state of totipotency.

Question 8

What can pluripotent stem cells do?

Answer 8

• They can differentiate into any tissue in the body (like blood stem cells) apart from placenta cells.

Question 9

What is beta-thalassaemia?

Answer 9

• It’s a blood disorder that reduces the production of haemoglobin.
• In most types of β-thalassaemia, the β-globin protein is structurally normal (unlike sickle cell disease).
• It is caused by the insufficient expression of beta-globin.

Question 10

What sites are included in the beta-globin gene mutations in beta thalassaemia?

Answer 10

• Base changes to the TATA box which results in the prevention of the binding of RNA polymerase.
• Alteration of bases at intron - exon boundary prevents spliceosome from recognising the boundary.

Question 11

What is the translational control in early embryogenesis?

Answer 11

• There are no genes expressed in the first 4-8 cell divisions.
• Once the blastocyst is formed the first genes to be expressed are due to up-regulation of translation from maternally-deprived pre-formed mRNAs.

Question 12

What other factor contributes to translational control?

Answer 12

Environmental stress.

Question 13

What is the function of the 5’ UTR?

Answer 13

• The 5’ UTR determines the rate of protein synthesis (translation).

Question 14

What is the function of the 3’ UTR?

Answer 14

The 3’ UTR causes proteins to bind which protects the RNA from degradation.

Question 15

Describe how the intracellular levels of iron are translationally controlled.

Answer 15

Ferritin binds to iron and retains it in the cytoplasm as a store for excess. We only need ferritin in times of iron excess.

1. When there are low levels of iron, an inhibitor blocks the ribosomal subunit from interacting with the ferritin mRNA. Thus, no translation of the ferritin mRNA takes place.
2. When there is an excess of iron, the iron binds to the inhibitor, taking it away from the ferritin mRNA. This allows the ribosomal subunit access to the mRNA, so translation occurs.

Question 16

What are miRNAs?

Answer 16

• A microRNA (miRNA) is a small non-coding RNA molecule that is transcribed by RNA polymerase II.
• These miRNAs act to control the silencing and post transcriptional regulation of as many as one-third of all human genes.

Question 17

How are miRNAs synthesised?

Answer 17

1. MicroRNAs are transcribed by RNA polymerase II as large RNA precursors called pri-miRNAs and comprise of a 5’ cap and poly-A tail. The pri-miRA are processed in the nucleus by the microprocessor complex.
2. The resulting pre-miRNAs are aproximately 70-nucleotides in length and are folded into imperfect stem-loop structures.
3. The pre-miRNAs are then exported into the cytoplasm.
4. Once in the cytoplasm, the pre-miRNAs undergo an additional processing step by the RNAse III enzyme generating the miRNA, a double-stranded RNA aproximately 22 nucleotides in length.
5. The RNAase III enzyme also initiates the formation of the RNA-induced silencing complex (RISC).
6. RISC is responsible for the gene silencing observed due to miRNA expression and RNA interference.

Question 18

What is the mechanism of action of miRNA?

Answer 18

1. The miRNA assembles with a set of proteins into a complex called RISC, which then searches for mRNAs that have a nucleotide sequence complementary to its bound miRNA.
2. Depending on how extensive the region of complementarity is, the target mRNA is either rapidly degraded by a nuclease within the RISC (this happens when there is an extensive match) or is transferred to an area of the cytoplasm where other cellular nucleases destroy it (this happens when there is a less extensive match).