Transcription in Eukaryotes part 3 Flashcards

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1
Q

When does the addition of the 5’ cap happen?

A

As soon as the first 20-30 nt have exited the RNA pol.

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2
Q

What is the role of guanyl transferase

A

It converts and GTP into GMP, and then adds it to the rNDP at the 5’ end to make an unusual 5’-5’ linkage

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3
Q

What enzyme adds a methyl group to position seven of the final guanine on a strand at the 5’ end?

A

ribonucleotide methyl transferase

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4
Q

What are three functions of the 5’ cap?

A
  1. distinguish transcripts made by RNA pol. II
  2. protect mRNA from degradation by exoribonucleases
  3. facilitate interaction with the Cap Binding complex
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5
Q

Define the first transesterfication reaction in the context of splicing

A

A special “A” in a sequence motif in an intron attacks the phospodiester bond present between G/G sequence present at the beginning of the intron sequence, cutting it.

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6
Q

Define the second transesterfication reaction in the context of splicing

A

The free -OH group on the first exon attacks the phosphodiester bond present between the G/G at the end of the intron sequence, fully separating the intron from the second exon.

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7
Q

In the context of the spliceosome, what is responsible for cleaving introns?

A

small nuclear RNA (snRNA) with catalytic activity, linked to a complex of proteins called small nuclear ribonucleoprotein (snRNP). Often have names such as U2, U4, U5, etc.

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8
Q

What two proteins attract U2 snRNP to the 3’ splice site?

A

BBP and U2AF

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9
Q

What three snRNPs form the “triple snRNP?

A

U4, U5, U6

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10
Q

What proteins put the two exons together at the end of splicing?

A

the exon junction complex

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11
Q

What is the role of RNA helicase in splicing

A

It modifies the RNA-RNA hybrids in such a way that the catalytic centers are optimally oriented

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12
Q

What snRNP replaces for U1?

A

U6

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13
Q

What snRNP helps facilitate joining of the exons together?

A

U5

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14
Q

What is the exon definition hypothesis?

A

predicts that the relative constancy of the exon size in eukaryotes (100-200nt) is used to facilitate formation of splice-site machinery

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15
Q

Define hnRNPs

A
  1. heterogenous nuclear ribonucleoprotein
  2. preferentially bind to cryptic bind sites in introns to mask them
  3. condense large introns to help facilitate their excision
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16
Q

Define SR proteins

A

Proteins high in serine and arginine that bind to exon splice sites to help facilitate attraction of snRNPs.

17
Q

Define minor form of spliceosome

A

just recognizes different consensus sequecne

18
Q

Define trans splicing

A

a single 5’ exon is spliced onto the 5’ end of many different mRNAs

19
Q

Phosphorylation of the CTD allows the recruitment of what two factors?

A

CPSF (cleavage and polyadenylation specificity factor) andCstF (cleavage stimulation factor F)

20
Q

What is PAP and what does it do?

A

Poly A polymerase. It adds close to 200 A’s to the 3’ end of the mRNA.

21
Q

What is the role of the poly-A binding protein?

A

Bind to the poly A region at the end of the mRNA. They are responsible for preventing degradation from the 3’ direction

22
Q

What six factors must be present in order for mRNA to be “acceptable” to leave the nucleus?

A
  1. the cap binding complex
  2. SR proteins
  3. PABP
  4. hnRNPs
  5. Export factors
  6. Exon Junction complex
23
Q

The CTD is made of a heptamter that is repeated 52 times in humans. What is the sequence of the heptameter?

A

Tyr-Ser-Pro-Thr-Ser-Pro-Ser

24
Q

What occurs in the individual heptameters of the CTD during the transition from initiation to elongation?

A

Serine residues 2 and 5 are phosphorylated