Control of Gene Expression

Question 1

Out of the human genome,how many genes are actually expressed ?

Answer 1

In any given cell, only 10,000 genes are expressed

Question 2

Where are totipotent cells found?

Answer 2

In zygotes

Question 3

What is β thalassaemia?

Answer 3

Group of genetic blood diseases caused by insufficient expression of β-globin reducing haemoglobin production

Question 4

What is the genotype?

Answer 4

The genetic makeup of cells and organisms determining characteristics

Question 5

What are the different translational control mechanisms in the body?

Answer 5

• Early embryogenesis
• Environmental stress
• 5’ UTR
• Fe starvation
• 3’ UTR
• Small non coding RNAs

Question 6

What is differential gene expression?

Answer 6

The expression of different genes in different cells at different times giving the cell its functions

Question 7

What is the role of miRNAs?

Answer 7

Control part of the post-transcriptional regulations of 1/3 of all human genes by regulating several target genes

Question 8

How does β thalasaemia differ from sickle cell disease?

Answer 8

In β thalasaemia, the β globin protein is structurally normal

Question 9

How are intracellular iron levels controlled?

Answer 9

Via translation of Ferritin

Question 10

How does the genome differ from cell to cell?

Answer 10

It doesn’t, the genome is identical in every cell

Question 11

How is the miRNA able to target genes?

Answer 11

Single stranded miRNA searches for complementary mRNA on target proteins

• If an extensive match is found: mRNA is rapidly
  degraded and RISC released
• If a less extensive match is found: mRNA translation is
  reduced; mRNA sequestered and eventually degraded

Question 12

How is gene expression controlled in early embryogenesis?

Answer 12

During the first 4-8 cell divisions no genes are expressed
After blastocyst formation the 1st genes are expressed
- due to up regulation of maternally derived preformed mRNA translation

Question 13

What is a pluripotent cell?

Answer 13

A cell that can give rise to all cell types except placental cells

Question 14

What role does the 5’ UTR play in translation control?

Answer 14

Determines how efficiently the ribosome initiates translation e.g.

• Globin: very efficient
• Ferritin: very inefficient

Question 15

How many small non coding RNAs are present in the human genome?

Answer 15

Human genome encodes 500+ small non coding RNAs transcribed by RNA Pol II

Question 16

How is differential expression regulated in time?

Answer 16

• During development (i.e in an embryo vs. adult)

- In response to hormones, infections and other signals

Question 17

How many forms of β thalasaemia are there?

Answer 17

There are multiple independently arising forms of the disease

Question 18

How is differential expression regulated tightly in space?

Answer 18

Different tissues and cell types express different genes (i.e. brain vs. muscle cells)

Question 19

How does environmental stress control gene expression in humans?

Answer 19

Exposure to heat shock or pathogens can cause global translation changes

Question 20

How does Ferritin help maintain the bodies cellular iron levels?

Answer 20

Ferritin binds iron and retains it in the cytoplasm as an excess store
Ferritin is only required when iron levels are in excess

Question 21

What is the result of β thalassaemia caused by the single base change at the intro-exon junction?

Answer 21

CACAGCTCCT -> CACGGCTCCT
Deletion causes intron to not be spliced
Gene not exported to cytoplasm

Question 22

What is the translational control of gene expression?

Answer 22

The regulatory mechanisms of the rate at which proteins are formed

Question 23

What are small non coding RNAs?

Answer 23

RNA molecules that aren’t translated into proteins e.g. miRNA

Question 24

What is meant by a totipotent cell?

Answer 24

A cell that can differentiate into every cell type

Question 25

What are the 2 major types of β thalasaemia?

Answer 25

1. single base change

2. single base change at intron-exon junction

Question 26

Why is differential gene expression regulation so important?

Answer 26

Dependent on the protein encoded for, the cell function and outcome differs 
If not regulated properly can cause:
- Metabolic disease
- Metastasis 
- Congenital disorders
- Cancer

Question 27

What is the role of the 3’ UTR?

Answer 27

Determines the stability of mRNA e.g.

• Globin confers stability
• Immune stress hormones are very unstable

Question 28

How does an excess of Iron result in the translation of Ferritin?

Answer 28

1. Inhibitor bound to Ferritin mRNA loop
2. Iron binds to inhibitor
3. Inhibitor no longer able to bind to Ferritin mRNA loop
4. Ribosome can now move along mRNA and translate
   Ferritin protein
5. Ferritin goes on to bind to excess iron molecules

Question 29

What is the consequence of the single base change β thalasaemia?

Answer 29

ATAAA -> ACAAA
Mutation occurs in TATA box so RNA Pol II can’t bind
Results in the gene not being transcribed

Question 30

What is the effect of the Drosophila Homeotic mutants?

Answer 30

Bithorax Gene Mutation
- Fruit-fly has an extra pair of wings

Antennapedia Gene mutation
- Flies express an extra pair of legs as gene over
expressed

Question 31

Where are miRNAs derived from?

Answer 31

Obtained by processing larger precursors

Question 32

How many genes does the human genome contain?

Answer 32

Approximately 50,000 genes present in human genome

Question 33

Outline how miRNA is formed

Answer 33

1. Precursor miRNA molecule with 5’ cap and
   polyAdenylated tail exported into cytoplasm
2. Enzymes cleave off the loop, the single stranded RNA
   on 5’ and 3’ ends
   => producing a short double stranded RNA molecule
   (30 bp long)
3. RISC proteins bind to the double stranded RNA
4. Proteins unwind double strands forming single
   stranded miRNA

Question 34

How are stem cells formed?

Answer 34

We can reprogramme cells using TFs