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BMS242 Cell and Molecular > L15 - Post Transcriptional Control > Flashcards

L15 - Post Transcriptional Control Flashcards

1
Q

At what levels can regulation of gene expression occur at

A

Almost every level: Transcription/translation etc.

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2
Q

% of genes in Drosophila and humans that have alternative splicing

A

40% of Drosophila

75% of Human genes

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3
Q

Describe alternative splicing: Optional exon

A

Middle exon is spliced out of the transcript

Acceptor site used of the first intron and the donor site of the last intron (so intron-exon-intron) spliced out

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4
Q

Describe alternative splicing: Optional intron

A

Where an intron is able to behave as an exon so is not spliced out

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5
Q

Describe alternative splicing: Mutually exclusive exons

A

Where exons are skipped but the two exons are mutually exclusive of each other (i.e. so if exon 1 is in one transcript an exon 2 is not) They are mutually exclusive of each other

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6
Q

Describe alternative splicing: Internal splice sites

A

Where there is a donor site within the intron so only a part of it is spliced out

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7
Q

Why do splice variants occur so frequently

A

Because donor and acceptor sites are only two bases hence they appear very frequently

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8
Q

What is the most spliced gene known

How many isoforms are there

A

Dscam - 38’000

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9
Q

What is an isoform

A

Functionally similar proteins but they do not have the same amino acid sequence - this could be as a result of alternative splicing

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10
Q

Describe the splicing of Dscam gene

A

38000 permutations of the gene

4 sets of mutually exclusive exons

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11
Q

What is special regarding the chromosomes in male and female drosophila

A

Male X
Female XX
There is no Y chromosome in drosophila

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12
Q

Sxl

A

Sex lethal

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13
Q

Tra

A

Transformer

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14
Q

Dsx

A

Double sex

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15
Q

What occurs to Sxl in males

A

Normal splicing occurs - non functional protein is produced

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16
Q

What occurs to Tra in males

A

Normal splicing occurs - non functional Tra protein is produced

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17
Q

What occurs to Dsx in males

A

Middle exon is spliced out

Forms a protein with 400AA at the N’ then a further 150 AA at the C’ this form of the protein is male specific

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18
Q

What occurs to Sxl in females, WHY?

A

Due to the DOUBLE X chr, some active Sxl is made

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19
Q

What is the function of active Sxl made in females (2)

A

Firstly feedbacks on splicing of Sxl (positive feedback) by causing the splicing out of a middle intron ==> leads to more of the active Sxl being made

Then binds to the intron;exon boundary of Tra to alter the splicing

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20
Q

What occurs to Tra in females

A

Different splicing due to the binding of sxl leads to production of functional Tra protein

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21
Q

What binds to the mRNA of Dsx in females to alter splicing

A

Functional Tra and Tra2

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22
Q

What occurs to DSx in females

A

Binding of Tra and Sxl to the mRNA of Dsx - changes the pattern of splicing and a protein with a 400AA at the N’ and then 30AA at the C’ is formed

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23
Q

Is N - 400 - 150 - C Dsx male or female specific

A

Male

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24
Q

Is N - 400 - 30 - C Dsx male of female specific

A

Female

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25
Q

Active sxl represses splicing by blocking …

A

U2AF

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26
Q

What type of feedback does Sxl use

A

Positive feedback - feedbacks on its own regualtion

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27
Q

What is special about an antibody gene

A

It has two possible positions for cleavage and poly adenylation

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28
Q

Describe what occurs when the cell produces the long transcript of an antibody gene

A

First stop codon is spliced out

Leads to the translation of a transmembrane domain

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29
Q

Describe what happens when the cell produces the short transcript of an antibody gene

A

Splice acceptor site is lost and so the first stop codon IS NOT spliced out
Translation terminated at the first stop codon
TM domain is not translated

30
Q

What type of antibody is produced from the long transcript

A

Membrane bound (TM domain translated)

31
Q

What type of antibody is produced when the cell produces the short transcript

A

Secreted

32
Q

What does the optimal Kozk sequence describe

A

The sequence containing AUG which the ribosome is most likely to begin translation at

33
Q

What is the Kozak sequence

A

ACCAUGG

34
Q

What occurs if the sequence around the start AUG is less than perfect

What is this known as

A

Ribosome can scan past the first AUG until it reaches the next or the next

Leaky scanning

35
Q

What cellular conditions would favour the ribosome to begin translation at the first AUG

A

High levels of eIF-4F inside of the cell

36
Q

Describe the genome of HIV - What happens to it

A

Small genome which is integrated into the host genome

37
Q

Describe the transcription of HIV

Why does this present a problem

A

Transcribed in one piece - alternative splicing allows for many protein products to be made
HOWEVER the full length RNA is required to make the new virions but it is unable to leave the nucelus as it is unspliced

38
Q

How does HIV overcome the problem of not being able to get out of the nucelus without being spliced

A

By using Rev protein

39
Q

What is the function of Rev protein

A

Binds to HIV introns and interacts with the nuclear pore to allow the exit of unspliced mRNA

40
Q

What can levels of REV be used for

A

By clinicians to distinguish between the two phases of infection

41
Q

Signals in the UTR of mRNA are able to …

A

Direct it to a part of the cell

42
Q

What can form in the 3’ region of the UTR

What is the singificance of this

A

Intermolecular forces allow the formation of stem-loop structures - these can be recongised by cellular proteins and then subsequently targetted to regions of the cell

43
Q

What is the functional of ferritin

A

Stores Fe - leading to a decrease of Fe levels within the cell

44
Q

What is the function of transferrin

A

Imports Fe - leading to an increase of Fe levels within the cell

45
Q

What needs to happen (in terms on imported and stored) when Fe is lo

A

Fe needs to be imported and not stored

46
Q

What needs to happen (in terms of imported and stored) when Fe is hi

A

Fe needs to be stored and not imported

47
Q

Describe what happens to Ferritin mRNA when Fe is lo

A

Aconitase binds to the 5’ UTR region - this physically blocks the ribosome and no ferrtin protein is made

48
Q

Describe what happens to Transferrin receptor mRNA when Fe is lo

A

Aconitase binds to the 3’ UTR region - this stabilises the mRNA and translation is able to proceed

49
Q

Describe what happens to aconitase when Fe is hi

A

Fe binds to aconitase - this induces a conformational change so that aconistase is no longer able to bind to the UTRs of the two mRNAs

50
Q

Describe what happens to ferritin mRNA when Fe is hi

A

Aconitase is unable to bind due to Fe binding to it.

Thus there is no block to translation and it is able to proceed

51
Q

Describe what happens to transferrin receptor mRNA when Fe is hi

A

Aconistase is unable to bind due to Fe binding to it. Thus the mRNA is more unstable so it is degraded

52
Q

Where does aconitase bind to transferrin R mRNA

A

3’ UTR

53
Q

Where does aconitase bind to ferritin mRNA

A

5’ UTR

54
Q

What initation factor must bind to the small ribosomal subunit in order for scannning to begin

A

EIF-2 with GTP bound

55
Q

EIF-2B is a

A

GEF (guanine nucleotide exchange factor)

56
Q

What is the effect of eIF-2B on eIF-2

A

GDP –> GTP

Activates

57
Q

What is the effect of phosphorylation of eIF-2

A

Causes eIF-2B to bind very tightly - so eIF-2 remains associated with GDP and so is inactive

58
Q

When does phosphorylation of eIF-2 occur
(3 situations)
What is the effect of this

A

Cells entering resting G0 (quiescent), cells that ae infected by a virus or cells which are lacking nutrition

Turns down global translation

59
Q

What does IRES stand for

A

Internal ribosome rentry sites

60
Q

eIF-4g binds to

A

Tail

61
Q

eIF-4e binds to

A

Cap

62
Q

What are IRES, what are they able to do

A

Stem-loop strucutures which can initiate formation of the ribosome independent of the cap/polyA initiation complex being present

63
Q

What is required for IRES bases initiation

A

eIF-4G

64
Q

Where is IRES often found

A

In viral transcripts

65
Q

What is the benifit of IRES being present in viral transcripts

When else is this seen

A

It favours translation of their transcripts bu cleaving eIF-4G into a form which cant bind to eIF-4e but can still bind to IRES

Also occurs during apoptosis - where certain genes which are involved in the process utilise IRES until they can be translated

66
Q

What is the half life of mRNA

A

This is highly variable - from several minutes to hours

67
Q

What length to poly A tails start at …
What can cleave them …
How short can they be cleaved …
What is the effect of cleavage this short …

A

300
Exonucleases (cut the terminal A off)
30
Causes them to be degraded

68
Q

What occurs in the cytoplasm to extend the life of an mRNA

A

Can be readenylated

69
Q

What can bind to the mRNA to extend their half life

A

DAN able to compete with eIF-4E - if DAN binds translation is promoted and degradation of the mRNA is blocked

70
Q

What should be wirtten in italics

A

Gene and species names

BMS242 Cell and Molecular (26 decks)

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