Gene Expression

Question 1

How many genes in the human genome?

Answer 1

50,000

Question 2

In any given cell, how many genes are expressed?

Answer 2

10000

Question 3

How is gene expression regulated in time and space?

Answer 3

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 4

Give an example of animal that has abnormalities because of wrong gene expression

Answer 4

Antennapedia- extra pair of legs where antaenna is supossed to be

Question 5

What is the goal of gene expression?(2)

Answer 5

• The ability to isolate adult stem cells from all developmental compartments
• The ability to drive these cells to develop either in vivo or in vitro in a completely controlled manner so that we can regenerate tissues

Question 6

How does change in promotor region B thalassaemia arise?(2)

Answer 6

1. Change in the promotor region means RNA polymerase II cannot bind and unwind the DNA
2. This disrupts the TATA box and there is no expression of the gene

Question 7

How do changes in the intron and exon junctions give arise to B thalassaemia?

Answer 7

Nucleotides AG that promote splicing.

This change means that the intron is not spliceable so the intron remains in the mature mRNA.

It will not form the lariat structure.

Question 8

How do changes in the 5’ prime UTR give rise to B- thalassaemia?

Answer 8

Mutations in the 5’ prime UTR so RNA is highly unstable and they are degraded quickly

Question 9

At what level is regulation of gene expression?

Answer 9

At the post transcriptional level

Question 10

Give examples of environmental influences on gene expression(3)

Answer 10

• Early embryogenesis – during first 4 – 8 cell divisions there is virtually no gene expression.At the end of blastocyst formation first genes to be expressed are due to up-regulation of translation from maternally derived pre-formed mRNAs. Highly abundant amount of mRNAs
• Environmental stress – exposure to heat shock or pathogens can cause global changes in translation.
• When cells are infected with viruses cells can respond to the viral infection by transcribing genes that code for antiviral proteins.

Question 11

Where does translation occur on the mRNA?(2)

Answer 11

1. The first AUG sequence after the Kozak sequence. There are many AUG sequences
2. EIF3 and EIF4 recognize the cap and start moving along the RNA, these two proteins regulate whether you have binding to the RNA.

Question 12

How do viruses survive in cells?

Answer 12

Viruses inactivate EIF3 and EIF4 because these two recognise the cap and start moving along the mRNA so they inactivate so translation cannot start

Question 13

What is ferritin and how are they stored?(2)

Answer 13

1. An iron storage container which is found in the bone marrow, iron atoms floating in the blood are collected by transferrin and are stored as ferritin
2. They undergo a modification in structure that prevents its escape

Question 14

What happens in the absence of low iron levels?(2)

Answer 14

1. The UTR loops because of complementarity of the nucleotides in the region in 5’ untranslated region.2. This looping is recognised by Fe Starvation inhibitor.
2. Ribosomal subunit cannot bind and move along because of the inhibitor.

Question 15

What happens in the presence of excess of iron in the blood?(3)

Answer 15

1. Iron bind to the inhibitor and changes the conformation of the protein and so cannot bind to the ferritin.
2. The ribosome can bind to the cap site and move along the mRNA molecule.
3. Ferritin attaches to the excess iron.This is an example of cells working quicker because the cell generates the protein from already existing mRNA

Question 16

How is plasma ferritin used?(2)

Answer 16

1. An indirect marker of the total amount of iron stored in the body- diagnostic tool for anaemia
2. Serum ferritin is secreted from damaged cells as a leakage product

Question 17

What is the role of the 5’ and 3’UTRs?

Answer 17

• The 5’ UTR is not the bit that determines whether a ribosome binds, but it DOES play a major role in determining how efficiently the ribosome initiates translation eg. globin

The 3 UTR sequences cause proteins to bind which protect the RNAs from degradation.There are 4 initiation factors associated with the UTR

Question 18

How is globin translated and why do they have 3’UTR stability?

Answer 18

Globin (very efficiently translated) RBCs do not have DNA so the only way the RBC can express beta globin is by translating pre mRNAs already in the RBC.Globin 3’UTRs confer stability because RBCs do not have DNA so to produce globin from pre existing RNA is to ensure they are highly stable.

Question 19

Why do immune stress hormones have unstable UTRs?

Answer 19

They are degraded as soon as they are translated because you don’t want to be stressed all the time. Less stable means less hormones produced

Question 20

What is miRNA?(4)

Answer 20

• The human genome encodes over 500 small non-coding RNAs that are transcribed by RNA polymerase II- miRNA
2.MicroRNAs are specialized RNA molecules encoded as parts of larger RNA precursors.

3. miRNA are really short- about 21 nucleotides long and can be synthesised.
4. They form hairpin structures

Question 21

What is the role of miRNA?(2)

Answer 21

Control the post-transcriptional regulation of as many as one-third of all human genes

• Any given miRNA can regulated several target genes miRNA have large regions of complementarity

Question 22

Describe the synthesis of miRNA(5)

Answer 22

1.Each precursor miRNA transcript is processed to form a double-stranded intermediate
→ 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 transferred to an area of the cytoplasm where other cellular nucleases destroy it (this happens when there is a less extensive match)

Question 23

What is RISC protein and how are they involved in degrading RNA?(6)

Answer 23

1. The RNA-induced silencing complex
2. Cleaves mRNA
3. RISC proteins recognise and bind to double stranded RNA
4. They unwind the double stranded RNA
5. The single stranded RNA now looks for a complementary target strand
6. The RNA is then degraded by RISC if it is an extensive match

Question 24

How do miRNA degrade mRNA molecules?

Answer 24

Cleavage to mRNA means ribosome cannot move along mRNA and eventually degraded.

Question 25

How does miRNA regulate genes?

Answer 25

1) cleavage of the mRNA strand into 2 pieces 2) destabilization by shortening the poly A tail 3) less efficient translation in the ribosomes.

Question 26

Why does the beta gene have one of the most stable 3’UTR?

Answer 26

because red blood cells do not have a nucleus and cannot produce their own RNA.

Question 27

What can proteins be used for

Answer 27

Metabolism
cell shape/ motility
cell differentiation
proliferation