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
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

At which end is eukaryotic mRNA capped?

A

5’ end

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

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

A

7-methylG

(a cationic base therefore positive charge added)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

At which end of eukaryotic mRNA does tailing occur?

A

3’ end

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

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

A

poly-A

approx. 200 A residues

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

What are introns?

A

Junk DNA

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

What are exons?

A

The fragmented coding sequence

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

What is an alternative term for the primary transcript?

A

pre-mRNA

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

Where does splicing occur?

A

The nucleus

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

Define splicing

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

What are splicosomes?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

What are snRNPs?

A

Small nuclear RNAs plus proteins

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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)?

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?

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
What is a 'non-sense' codon and how many are there?
One that doesn't encode an amino acid, i.e. stop codons There are 3
26
Which three base triplets mean STOP?
UAG, UAA, UGA
27
Do codons within a given mRNA overlap?
A point mutation within a gene changes a single amino acid in the encoded protien, so **no**
28
How was the code deciphered?
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
What do XXU and XXC always encode for, and what do XXA and XXG usually code for?
XXU/XXC: **always** pyrimidines XXA/XXG: **almost always** purines
30
What is the start codon? Which amino acid does it code for?
AUG = Met
31
What is Shine-Dalgarno recognition?
A sequence typically 4-7 nucelotides upstream of the coding region allowing the ribosome to detect the coding region
32
What are tRNAs?
Transfer RNAs They are 'genetic adapters' that translate the code
33
What is the role of tRNAs?
They carry amino acids to ribosomes for protein synthesis Each tRNA is dedicated to one speicific proteinogenic amino acid
34
What secondary structure do all tRNAs have?
Cloverleaf
35
What is an anticodon?
The part of a tRNA that recognises the mRNA codon
36
What are the different sections of a tRNA molecule?
DHU (dihydro-U) loop Anticodon Variable arm, where nucleoide length varies TΨC loop (UUC) CCA ending (added post-transcription)