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path 19.01 and 19.07 > 19.01.07 Transcription, mRNA processing (excluding splicing) and translation > Flashcards

19.01.07 Transcription, mRNA processing (excluding splicing) and translation Flashcards

Transcription, mRNA processing (excluding splicing) and translation

1
Q

Central dogma

A

The expression of genetic information in a cell is almost exclusively unidirectional. DNA>RNA>protein

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2
Q

Where does transcription occur

A

Nucleus (limited extent in mitochondrial)

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3
Q

What direction is RNA synthesised?

A

5’ to 3’, complimentary to template (anti sense strand) with same bases except U/T.

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4
Q

how many classes of RNA polymerase molcules

A

3 (I, II, III)

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5
Q

how does transcription start

A

-Transcription factors bind to the promoter region and position RNA polymerase to initiate RNA synthesis.

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6
Q

Are transcription factors trans- acting?

A

Yes, migrate to sites of action following synthesis by remotely located genes

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7
Q

Are promoters trans-acting

A

No, they are cis-acting. Function is limited to the duplex on which they reside.

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8
Q

What other elements are cis-acting

A

Other than promoters, enhancers and silencers are also cis-acting

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9
Q

Elements of promoters

A

TATA box, GC box, CAAT box

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10
Q

TATA box

A

TATAAA sequence. 25-35 bp upstream of transcriptional start site. Able to define direction of transcription, indicates the DNA strand to be read. Mutations can cause transcription to begin at an incorrect location

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11
Q

GC box

A

Variant of GGGCGG. 110bp upstream of transcriptional site. Can function in both directions.

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12
Q

CAAT box

A

GGCCAATCT. 80bps upstream. Strongest determination of promoter efficiency. Functions in both directions

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13
Q

Mutations in promoters

A

1% of single base pair substitutions causing genetic disease occur in promoter regions. Disrupt transcription initiation (alter/abolish cis-activing DNA sequence motids for trans acting TFs), leading to altered amounts of mRNA and thus protein.

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14
Q

enhancers

A

Enhance transcriptional activity. Variable distrance from start site. Function independent of orientation. Cause DNA between promotor and enhancer to loop out.

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15
Q

silencer

A

inhibit transcriptional activity

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16
Q

Main steps of RNA processing

A

5’ capping, 3’ polyadenylation

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17
Q

5’ capping, when does it occur

A

During elongation

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18
Q

functions of 5’ capping

A

protect from 5’ - 3’ exonuclease activity. Facilitate transport from nucleus to cytoplasm, facilitate RNA splicing. Attaches 40s subunity of ribosome to mRNA

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19
Q

What happens during 5’ capping

A

methylated nucleoside, m7g (7-methylguanosine), is linked to the 5’ end of RNA by a phosphodiester bond.

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20
Q

3’ polyadenylation

A

AAUAAA (or AUUAAA) signal sequence is sequentially added by pol(A)polymerase, around 200 added, to form a poly(A) tail. Acts as a signal for 3’ cleavage for RNA pol II transcripts

21
Q

Functions of 3’ polyadenylation

A

Facilitates mRNA transport to cytoplasm, stabilises mRNA molecules, facilitates translation

22
Q

Mutations affecting 3’polyadenylation

A

rare. Causes read0-through and reduced accumulation of alpha-haemoglobin in haemoglobin H (HbH) disease.

23
Q

rRNA processing

A

RNA polymerase I makes a large pre-rRNA molecule. Then cut into 3 pieces: 18S, 5.8S and 28S

24
Q

What makes 5S (smallest eukaryote rRNA

A

RNA polymerase III

25
Q

Translation

A

mRNA transcribed from genes in nuclear DNA, migrates to cytoplasm and engages with ribosomes, tRNA and other components to direct polypeptide synthesis.

26
Q

Functions of untranslated regions (UTRs)

A

Stabilise mRNA on ribosomes for translation of the central segment

27
Q

Mutation in 5’UTR

A

BRCA1 has an alpha and beta promoter which encode transcripts with different 5’UTR lengths. The longer is translated less efficiently. Cancerous breast tissue has the longer 5’UTR transcript so BRCA1 protein expression is inhibited in cancer tissue.

28
Q

Ribosomes

A

RNA-protein complexes

29
Q

Structure of a ribosome

A

2 subunits: 60S and 40S subunits.

30
Q

Ribosome 60s composed of

A

28S, 5.8S and 5S rRNAs, with various ribosomal proteins (50)

31
Q

Ribosome 40s composed of

A

18S rRNA and >30 ribosomal proteins

32
Q

How many ribosome biding sites

A
  1. A site (aminoacyl-tRNA), P site (peptidyl- tRNA), E site (for tRNA released following peptidyl transferase reaction)
33
Q

What is tRNA

A

75-95 nt ribonucleotides in length

34
Q

What do tRNAs do

A

Mediate decoding of mRNA sequence. Have a specific amino acid covalently bound to acceptor arm by amino acyl tRNA synthetases

35
Q

How many types of tRNA synthetases

A

20 (1 specific to each amino acid).

36
Q

What does the tRNA acceptor arm contain

A

Sequence to allow discrimitation of tRNAs by tRNA synthetases.

37
Q

tRNA anti codon sequence

A

This recognises the complimentary mRNA codon.

38
Q

Steps of translation

A

Initiation, elongation, termination

39
Q

What initiates mRNA translation

A

Binding of tRNAfmet to P (peptidyl) site.

40
Q

What is then delivered to A (aminoacyl site)

A

tRNA in complex with elongation factor= EF-Tu-GTP

41
Q

What happens when codon-anticodon pairing occurs

A

Activates GTPase centre of ribosome, causing hydrolysis of GTP and the release of aminoacyl end of tRNA from EF-TU.

42
Q

What also happens when tRNA binds

A

COnformation changes in rRNA, optimally orientates the peptidyl-tRNA and aminoacyl-tRNA for the peptidyl-transferase reaction to occur.

43
Q

What happens during the peptidyl-transferase reaction

A

transfer of the peptide chain onto the A-site tRNA

44
Q

How does the ribosome shift in the 3’ mRNA direction

A

GTPase EF-G casuses the deacylated tRNA at the P site to move to the E (exit) site and the peptidyl-tRNA at the A site to move to the P site upon GTP hydrolysis.

45
Q

When is the deacylated tRNA in the E site released

A

Upon binding of the next aminoacyl-tRNA to the A site.

46
Q

When does elongation end

A

when stop codon is reached. No tRNAs complimentary to the stop codon. Leads to hydrolysis of the bond between tRNA and polypeptide at the P site. They are then released and ribosomal subunits and template dissociate

47
Q

How many combinations of codons for amino acids

A

64 possible codons (4 bases at 3 positions, 4^3) with 20 amino acids

48
Q

Wobble hypothesis

A

allows interpretation of all 64 codons. 5’ end of anticodon can bind any of the several bases at 3’ end of codon.

49
Q

When does nuclear-encoded mRNA translation stop

A

When a termination codon is encountered (UAA, UAG, UGA)

path 19.01 and 19.07 (45 decks)

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