RNA Synthesis

Question 1

How many types of RNA are found in the cell?

Answer 1

3

Question 2

What are the differences between RNA and DNA?

Answer 2

RNA has ribose not deoxyribose
RNA has uracil instead of thymine
RNA is single stranded
RNA is shorter than DNA

Question 3

Where is mRNA made?

Answer 3

In the nucleus

Question 4

How are amino acids attached to tRNA?

Answer 4

Aminoacyl tRNA synthases

Question 5

How does RNA polymerase know where to start transcribing?

Answer 5

in prokaryotic and eukaryotic DNA there are regions called promoter regions which are specific sequences of nucleotides which bind with RNA polymerase

Question 6

Where is the TATA box found?

Answer 6

25 nucleotides to the start of where the gene is found

Question 7

What are the requirements for transcription?

Answer 7

RNA polymerase II
rNTPs 
Promoter regions
Transcription factors
Enhancers and silencers

Question 8

Describe initiation

Answer 8

→TFIID binds to the promoter region (TBP is a subunit of TFIID tata binding protein)
→TFIIB binds to the TBP and it links RNA polymerase to the TFIID
→WHEN RNA polymerase binds to the TFIIB it is already associated with TFIIF
→TFIIE and TFIIH associate with RNA polymerase
→TFIIH unwinds the DNA around the initiation site (cistron) and phosphorylated the RNA polymerase II
ATP is needed

Question 9

Describe elongation

Answer 9

→RNA Polymerase moving along DNA synthesizing the RNA which is released at the other end.
→Transcription Bubble formed which is 12 - 14 bps.

Question 10

Describe termination

Answer 10

→near the stop codon there is a sequence called AAUAAA which is the polyadenylation signal or the termination signal which is after the stop codon.

→A specific endonuclease recognizes the termination signal. The CSPF is the enzyme that does the cleavage after the termination signal.

→Polyadenylate polymerase adds 250 adenines to the end of the 3’ end of the pre-mRNA and this is called the poly A tail.

Question 11

What is the use of the poly A tail?

Answer 11

IT protects pre-mRNA from degradation and promotes the nuclear transport of pre-mRNA and promotes translation in the ribosome.
The longer the polyA tail the more stable the RNA is.

Question 12

Describe how splicing occurs

Answer 12

→Splicing is the removal of introns and the joining of exons.

→There is cleavage at the 5’ site by a splicosome.

→formation of a lariat-like intermediate (GU joining to A at the branch point, making an AGU)

→Then, ligation joining together of exons occurs.

Question 13

What is alternative splicing?

Answer 13

→The ligation of exons can differ and so the sequence of the mRNA also changes.

→The proteins made may have similar functions as there are common exons, but different function as the sequence of exons differs.

Question 14

what do the 3 RNA polymerases do?

Answer 14

→RNA polymerase I= transcribes rRNA genes
→RNA polymerase II= transcribes mRNA
→RNA polymerase III–transcribes tRNA and other small RNAs

Question 15

How does 5’ capping occur and why is it important?

Answer 15

→After 25 nucleotides are synthesized the 5’ end becomes modified.

→the enzyme 7methyltransferase adds a methyl group at position 7 on the guanine so it becomes a 7 methyl guanine cap.

→The cap protects the 5’ end from the activity of 5’ exonucleases

Question 16

How is RNA exported out of nucleus:

Answer 16

→tRNA and microRNAs bind directly to export receptors.

→Large RNAs such as ribosomal RNAs and mRNA recruit specific adaptor proteins.

Question 17

What proteins bind to mRNA for export from nucelus:

Answer 17

→Poly-A binding protein
→Exon junction complex
→Cap-binding protein

Question 18

Importance of export proteins:

Answer 18

→Helps cell distinguish between rare mature mRNA and RNA processing debris.