EC3: mRNA splicing, genetic code, and tRNA Flashcards

1
Q

What are the three post-transcriptional modifications of eukaryotic mRNA?

A
  1. Capping
  2. Tailing
  3. Splicing
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2
Q

At which end is eukaryotic mRNA capped?

A

5’ end

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3
Q

In chemical terms, what is the cap on eukaryotic mRNA?

A

7-methylG

(a cationic base therefore positive charge added)

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4
Q

Why is the cap relevant?

A

It might be a protective device against nucleases

The camp is recognised by specific proteins that enable ribosomes to bind to the mRNA and initiate protein synthesis

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5
Q

At which end of eukaryotic mRNA does tailing occur?

A

3’ end

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6
Q

What is the tail in chemical terms? How long is it?

A

poly-A

approx. 200 A residues

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7
Q

What is the relationship between the age of eukaryotic mRNAs and the tail?

A

As the mRNAs age, the tail gets shorter. The mRNA is inactive when n<10

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8
Q

Why are mature eukaryotic mRNAs typically much shorter than the genes from which they are transcribed, but the primary transcripts are not?

A

Introns have been spliced out of mature mRNAs but they are still present in the primary transcript

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9
Q

What are introns?

A

Junk DNA

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10
Q

What are exons?

A

The fragmented coding sequence

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11
Q

What is an alternative term for the primary transcript?

A

pre-mRNA

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12
Q

Where does splicing occur?

A

The nucleus

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13
Q

Define splicing

A

The removal of introns and the joining together of exons in the same order as in the gene

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14
Q

What are splicosomes?

A

Congregations of snRNPs (up to 150). They are used up so need to be replaced

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15
Q

What are snRNPs?

A

Small nuclear RNAs plus proteins

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16
Q

What are RNPs?

A

Ribonucleoproteins

17
Q

What diseases can be caused by mistakes in splicing?

A
  • Breast cancer (BRCA1)
  • Duchenne muscular dystrophy (DMD)
  • Neurofibromatosis (NF-1)
  • Thalassaemias
18
Q

To what does the term ‘genetic code’ refer?

A

The relationship between the base sequence of a gene and the amino acid sequence of its protein product

19
Q

What is a codon?

A

A sequence of three DNA or RNA nucleotides that corresponds with a specific amino acid or stop signal during protein synthesis

20
Q

What is the coding number (i.e. how big is a codon)?

A

3

21
Q

Why is it relevant that 42 = 16 whereas 43 = 64?

A

A triplet code allows for 64 different possible codons

22
Q

Is the genetic code degenerate?

A

Yes

23
Q

What is a ‘sense triplet’ and how many are there?

A

A triplet that encodes an amino acid

There are 61 that encode for 20 amino acids

24
Q

What is the explanation for 61 triplets coding for 20 amino acids?

A

There are multiple synonym codons, i.e some amino acids are coded for by multiple codons, and some have multiple tRNAs

25
Q

What is a ‘non-sense’ codon and how many are there?

A

One that doesn’t encode an amino acid, i.e. stop codons

There are 3

26
Q

Which three base triplets mean STOP?

A

UAG, UAA, UGA

27
Q

Do codons within a given mRNA overlap?

A

A point mutation within a gene changes a single amino acid in the encoded protien, so no

28
Q

How was the code deciphered?

A

In 1961, Nirenberg and Matthaei translatted artificial mRNA in vitro (in cell-free protein synthesising systems derived from bacteria).

They worked out that poly U → poly PHE, poly A → poly LYS, etc.

29
Q

What do XXU and XXC always encode for, and what do XXA and XXG usually code for?

A

XXU/XXC: always pyrimidines

XXA/XXG: almost always purines

30
Q

What is the start codon? Which amino acid does it code for?

A

AUG = Met

31
Q

What is Shine-Dalgarno recognition?

A

A sequence typically 4-7 nucelotides upstream of the coding region allowing the ribosome to detect the coding region

32
Q

What are tRNAs?

A

Transfer RNAs

They are ‘genetic adapters’ that translate the code

33
Q

What is the role of tRNAs?

A

They carry amino acids to ribosomes for protein synthesis

Each tRNA is dedicated to one speicific proteinogenic amino acid

34
Q

What secondary structure do all tRNAs have?

A

Cloverleaf

35
Q

What is an anticodon?

A

The part of a tRNA that recognises the mRNA codon

36
Q

What are the different sections of a tRNA molecule?

A

DHU (dihydro-U) loop

Anticodon

Variable arm, where nucleoide length varies

TΨC loop (UUC)

CCA ending (added post-transcription)