EC3: mRNA splicing, genetic code, and tRNA

Question 1

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

Answer 1

1. Capping
2. Tailing
3. Splicing

Question 2

At which end is eukaryotic mRNA capped?

Answer 2

5’ end

Question 3

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

Answer 3

7-methylG

(a cationic base therefore positive charge added)

Question 4

Why is the cap relevant?

Answer 4

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

Question 5

At which end of eukaryotic mRNA does tailing occur?

Answer 5

3’ end

Question 6

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

Answer 6

poly-A

approx. 200 A residues

Question 7

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

Answer 7

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

Question 8

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

Answer 8

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

Question 9

What are introns?

Answer 9

Junk DNA

Question 10

What are exons?

Answer 10

The fragmented coding sequence

Question 11

What is an alternative term for the primary transcript?

Answer 11

pre-mRNA

Question 12

Where does splicing occur?

Answer 12

The nucleus

Question 13

Define splicing

Answer 13

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

Question 14

What are splicosomes?

Answer 14

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

Question 15

What are snRNPs?

Answer 15

Small nuclear RNAs plus proteins

Question 16

What are RNPs?

Answer 16

Ribonucleoproteins

Question 17

What diseases can be caused by mistakes in splicing?

Answer 17

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

Question 18

To what does the term ‘genetic code’ refer?

Answer 18

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

Question 19

What is a codon?

Answer 19

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

Question 20

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

Answer 20

3

Question 21

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

Answer 21

A triplet code allows for 64 different possible codons

Question 22

Is the genetic code degenerate?

Answer 22

Yes

Question 23

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

Answer 23

A triplet that encodes an amino acid

There are 61 that encode for 20 amino acids

Question 24

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

Answer 24

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

Question 25

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

Answer 25

One that doesn't encode an amino acid, i.e. stop codons There are 3

Question 26

Which three base triplets mean STOP?

Answer 26

UAG, UAA, UGA

Question 27

Do codons within a given mRNA overlap?

Answer 27

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

Question 28

How was the code deciphered?

Answer 28

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.

Question 29

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

Answer 29

XXU/XXC: **always** pyrimidines XXA/XXG: **almost always** purines

Question 30

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

Answer 30

AUG = Met

Question 31

What is Shine-Dalgarno recognition?

Answer 31

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

Question 32

What are tRNAs?

Answer 32

Transfer RNAs They are 'genetic adapters' that translate the code

Question 33

What is the role of tRNAs?

Answer 33

They carry amino acids to ribosomes for protein synthesis Each tRNA is dedicated to one speicific proteinogenic amino acid

Question 34

What secondary structure do all tRNAs have?

Answer 34

Cloverleaf

Question 35

What is an anticodon?

Answer 35

The part of a tRNA that recognises the mRNA codon

Question 36

What are the different sections of a tRNA molecule?

Answer 36

DHU (dihydro-U) loop Anticodon Variable arm, where nucleoide length varies TΨC loop (UUC) CCA ending (added post-transcription)