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Structure + Function 1 > transcription > Flashcards

transcription Flashcards

1
Q

Compare and contrast transcription in prokaryotes and eukaryotes

A

Prokaryotic

  • Cytoplasm
  • Coupled transcription and translation (simultaneous)
  • No definite phase of occurrence
  • Single RNA polymerase
  • No initiation factors
  • Polycistronic

Eukaryotic

  • Nucleus
  • No coupling
  • Occurs in G1 and G2
  • 3 RNA polymerases
  • Multiple General Transcription Factors (GTFs)
  • Monocistronic - 1 ORF
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2
Q

what parts are included in the structure of a eukaryotic gene

A
  • control elements
  • promoters
    -enhancers
  • introns
  • exons
  • silencers/repressor proteins
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3
Q

define control elements

A

Non-coding DNA segments regulating transcription by binding transcription factor

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4
Q

Define promoters

A

Sequences of DNA bound by first components of the pre-initiation complex

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5
Q

Define enhancers

A

Distal control elements

Short nucleotide sequences influencing rate of transcription

May be bound by silencers or repressor proteins

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6
Q

Define transcription

A

Process by which information in a DNA strand is copied into a new molecule of mRNA

Takes place within nucleus and mature mRNA is exported to cytoplasm for translation

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7
Q

Why is RNA processing essential before exporting mature mRNA from the nucleus?

A

To ensure that mRNA doesn’t degrade in the cytoplasm by enzymes

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8
Q

Which type of DNA in eukaryotes is transcribed like prokaryotic transcription?

A

Non-nuclear or extrachromosomal DNA e.g. mitochondrial DNA

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9
Q

Name 4 RNA polymerases

A

RNA polymerase I

RNA polymerase II

RNA polymerase III

Mitochondrial RNA polymerase

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10
Q

What are the products of each RNA polymerase?

A

RNA polymerase I - pre-rRNA

RNA polymerase II - pre-mRNA

RNA polymerase III - pre-tRNA

Mitochondrial RNA polymerase - all mitochondrial RNA

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11
Q

Name the 4 stages of transcription

A

Initiation

Elongation

Processing

Termination

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12
Q

Name the 4 stages of transcription

A

Initiation

Elongation

Processing

Termination

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13
Q

Explain the events of the initiation stage of transcription

A
  • DNA unwound
  • Chromatin remodelling exposes promoter
  • Pre-initiation complex binds to recognition within promoter
  • Control elements facilitate binding of complex components
  • Transcription occurs within a transcription bubble opened by RNA polymerase
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14
Q

Name 3 additional proteins involved in the initiation stage of transcription

A

Mediator complex

Transcriptional regulatory proteins

Nucleosome modifying enzymes

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15
Q

What is the PIC made up of?

A

General Transcription Factors and RNA Polymerase II

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16
Q

explain how the pre-initiation complex is formed

A
  • » TATA-binding protein (TBP) binds first to the TATAA box (a recognition site within the promoter with the nucleotide sequence TATAA..)
  • » TBP (a subunit of TFIID) recruits General Transcription Factors made up of TAFs (TBP- Associated Factors)
    ○ TFIID recruits TFIIA and TFIIB
    ○ TFIIB binds to its B Recognition Element (BRE)
    ○ RNA Pol II – TFIIF complex recruited
    TFIIE and TFIIH recruited to form the pre- initiation complex
17
Q

Name 3 processes that pre-mRNA can undergo during elongation

A

Capping

Splicing

Poly-adenylation

18
Q

Explain what happens during the elongation stage of transcription

A
  • RNA transcript carries the same information as non-template (coding) DNA strand but contains uracil instead of thymine
  • RNA polymerase opens DNA in transcription bubble from which the template strand is read
  • RNA produced in 5’ to 3’ direction complementary to 3’ to 5’ template strand of DNA (antisense)
  • Non-template (coding) strand not involved but has the same sequence as mRNA except thymine replaced by uracil (sense strand)
  • Only one PIC required, it can keep recruiting a new RNA polymerase II to elongate another mRNA molecule resulting in multiple transcripts from same gene
19
Q

How can pre-mRNA molecules be modified by the addition of chemical groups?

A

Add a 5’ cap or 3’ tail (protects transcript and helps it export from nucleus)

Introns removed and remaining exons spliced together

20
Q

how does capping work

A

As RNA polymerase is elongating the transcript, guanyltransferase attaches a methylated GTP cap to the 5’ end of the mRNA

21
Q

What is the function of a methylated GTP cap on the 5’ end of mRNA?

A

Regulation of nuclear export

Prevention of degradation by exonucleases (not recognised 5’ to 5’)

Promotion of translation

Promotion of 5’ proximal intron excision

22
Q

Define splicing

A

Removing introns from the transcript to produce mature mRNA

23
Q

Define proteome

A

Portion of DNA encoding polypeptides

24
Q

What is the spliceosome?

A

a complex of RNA’s and proteins that remove introns

25
Q

What are the specific sequences found towards the end of a gene (AAUUAA) called?

A

Polyadenylation signals

26
Q

How is a poly-A tail added to RNA?

A
  • polyadenylation signals bind to cleavage factors which manipulate 3’ end of RNA into correct configuration for cleavage
  • 3’ end is cut and cleavage factors dissociate
  • Poly-A polymerase adds 100-250 adenine nucleotides to the cut ends to form a poly-A tail
27
Q

What is the function of a poly-A tail?

A

Stabilises transcript

Aids in transcription termination

Aids in exportation from nucleus

28
Q

Describe the termination mechanism for RNA polymerase I

A

TTF1 (Transcription Termination Factor) causes transcript to bend

PTRF-1 (Polymerase I Transcript Release Factor) and thymine rich sequence causes dissociation

29
Q

Describe the termination mechanism for RNA polymerase II

A

Poly-A tail doubles as TTS (Transcription Termination Signal) and 3’ end modification to stabilise pre-mRNA

30
Q

Describe the termination mechanism for RNA Polymerase III

A

Thymine resides repeat in ‘terminator sequence’

Similar to RNA polymerase I

Structure + Function 1 (18 decks)

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