RNA processing

Question 1

Important components for initiation of eukaryotic transcription:
3 points

Answer 1

1. “TATA box” – DNA sequence within the promoter region of DNA
2. Transcription factors – proteins that bind to DNA and influence transcription
3. RNA polymerase (with sigma subunit)

Question 2

Eukaryotic promoters

Answer 2

1. Eukaryotic promoter includes “TATA”

(non-template nucleotide sequence) box,

~ 25 nucleotides upstream from transcriptional start point

Question 3

Eukaryotic promoters
Several transcription factors (TFs):
3 points

Answer 3

1. one TF recognises
   the TATA box and binds.
2. Additional TFs
   recognise the bound
   TF and bind.
3. RNA polymerase
   recognises TFs - enabling it to bind to DNA in the
   correct position and orientation.

Question 4

Eukaryotic promoters
transcription initiation complex =
2 points

Answer 4

1. Additional TFs bind to DNA with RNA polymerase

2.  RNA polymerase
unwinds the DNA double 
helix, RNA synthesis 
begins at the start codon 
on the template strand.

Question 5

Eukaryotic termination site

Answer 5

1. When RNA polymerase reaches termination site, RNA transcript is released from template

Question 6

Eukaryotic termination site

2 Important components for termination of eukaryotic transcription:

Answer 6

1. “polyadenylation signal sequence” – DNA sequence within the termination site region of DNA, codes for polyadenylation signal in the mRNA
2. An enzyme – recognises polyadenylation signal and releases mRNA from RNA polymerase

Question 7

Eukaryotic termination site

Polyadenylation signal sequence codes for:

Answer 7

1. polyadenylation signal (AAUAAA) in the mRNA

Question 8

Eukaryotic termination site

Polyadenylation signals to

Answer 8

proteins associated with RNA transcript to cut it free from RNA polymerase

Question 9

Differences between prokaryotic and Eukaryotic gene expression

Site of transcription & Translation

Answer 9

Cytoplasm

vs

Transcription – nucleus; Translation - cytoplasm

Question 10

Differences between prokaryotic and Eukaryotic gene expression

Gene structure

Answer 10

Complementary to protein structure

vs

noncoding sequences ‘introns’

Question 11

Differences between prokaryotic and Eukaryotic gene expression

Modification of mRNA after transcription before translation

Answer 11

None

vs

Additions to mRNA ends
Intron removal

Question 12

RNA processing
Additions to mRNA ends - 5’ cap:
4 points

Answer 12

1. added to the 5’ end of the pre-mRNA
2. When is it added: during transcription (while mRNA is still being transcribed)
3. What is it: chemically modified molecule of GTP (guanosine triphosphate)
4. Why is it important:
   facilitates binding of mRNA to an enzyme (ribosome) for translation
   Protects mRNA from being digested by ribonucleases.

Question 13

RNA processing
Additions to mRNA ends - 3’ tail:
3 points

Answer 13

1. When is it added: immediately after mRNA transcript has been released from RNA polymerase
2. What is it: “poly A tail”, 100-300 Adenine nucleotides

3. Why is it important: 
Export mRNA from nucleus to cytoplasm
mRNA stability (degradation)

Question 14

RNA processing 
Intron removal (splicing)
7 points

Answer 14

1. eukaryotic contains non-coding sequences
2. Transcribed into mRNA sequence producing “introns”
3. Small nuclear
   Ribonucleoproteins (snRNPs) – proteins that perform the splicing.
4. Exon – expressing region Intron – intervening region
5. RNA polymerase transcribes both = pre-mRNA
6. pre-mRNA can not exit nucleus for translation with interferring introns
7. Introns removed, mRNA contains only exons that exit nucleus

Question 15

Small nuclear Ribonucleoproteins (snRNPs) –

7 points

Answer 15

1. proteins that perform the splicing.
2. Contain RNA “consensus sequence” – complementary to the pre-mRNA intron enables them to bind by complementary base pairing
3. snRNPs bind to consensus sequences near the 5’ and 3’ splice sites
4. Other snRNPs also attach at additional points along pre-mRNA
5. snRNPs join together to form Spliceosome
6. Intron loops out
7. Spliceosome cuts pre-mRNA at intron/exon boundary

Question 16

RNA splicing

4 steps

Answer 16

1. A cut is made between 5’ exon and intron, - intron forms a closed loop
2. The free 3’ OH group at the end of cut exon reacts with the 5’ phosphate of other exon
3. 3’ exon cleaved and spliced to 5’ exon = “mature mRNA”
4. The excised intron is degraded in the nucleus (recycled)

Question 17

Differential RNA processing

Answer 17

One gene can be responsible for many different proteins or

isoforms of a protein

Question 18

RNA processing summary

3 points

Answer 18

1. In eukaryotes, additional step “RNA processing”
2. Prepares pre-mRNA for exit from nucleus and for translation
3. RNA processing two steps:

Additions to pre-mRNA ends (5’ GTP cap, 3’ poly A tail)

Removal of introns (RNA splicing)

Question 19

Is Transcription the key control point in the expression of genes?
3 points

Answer 19

1. If a gene is never transcribed in a particular cell, it can’t be expressed as a protein – there’s simply no mRNA for the ribosome to read.
2. If a geneistranscribed, it has a chance of being expressed as a protein. In many cases, the amount of transcription (and thus the quantity of mRNA) determines how much protein gets made.
3. A gene may be transcribed at high levels, but the mRNA may be degraded as soon as it leave the nucleus leading to very little protein production