Intro to Control of Gene Expression

Question 1

Differential gene expression is tightly regulated in space (eg. different cells express different genes) and time (eg. different genes expressed during development of embryo to adult).
Why is it important for gene expression to be regulated?

Answer 1

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)

Question 2

Give two examples of drosophila homeotic mutations.

Answer 2

• Bithorax (when the fly have two thoraxes)

- Antennapedia (when the fly has legs in the place of its antenna)

Question 3

What is β-thalassaemia?

Answer 3

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).
Mutations that cause this disease are usually single base changes.

Question 4

Give examples of translational control.

Answer 4

• EARLY EMBRYOGENESIS: During the first 4-8 weeks of cell division, there is virtually no gene expression. At the end of blastocyst formation, genes are expressed due to the up-regulation of translation from maternally-derived preformed mRNAs.
• ENVIRONMENTAL STRESS: exposure to heat shock or pathogens can cause global changes in translation

There are many specific examples, such as ferritin.

Question 5

What is the function of the 5’ UTR?

Answer 5

The 5’ UTR isn’t the bit that determines whether a ribosome binds, but it does play a major role in determining how efficiently the ribosome initiates translation.

Question 6

What is the function of the 3’ UTR?

Answer 6

3’ UTRs confer very different stabilities on mRNAs.

Question 7

Describe how the intracellular levels of iron are translationally controlled.

Answer 7

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

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.
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 8

What are miRNAs, and what is their function?

Answer 8

The human genome encodes over 500 small, noncoding RNAs that are transcribed by RNA Polymerase II. These RNAs are referred to collectively as microRNAs or miRNAs.
These miRNAs act to control the post-transcriptional regulation of as many as one-third of all human genes.

Question 9

Describe the synthesis of miRNAs and the mechanism of their action.

Answer 9

Each precursor miRNA transcript is processed to form a double-stranded intermediate, which is further processed to form a mature, single-stranded miRNA. This 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. 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).