EC3: mRNA splicing, genetic code, and tRNA Flashcards
What are the three post-transcriptional modifications of eukaryotic mRNA?
- Capping
- Tailing
- Splicing
At which end is eukaryotic mRNA capped?
5’ end
In chemical terms, what is the cap on eukaryotic mRNA?
7-methylG
(a cationic base therefore positive charge added)
Why is the cap relevant?
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
At which end of eukaryotic mRNA does tailing occur?
3’ end
What is the tail in chemical terms? How long is it?
poly-A
approx. 200 A residues
What is the relationship between the age of eukaryotic mRNAs and the tail?
As the mRNAs age, the tail gets shorter. The mRNA is inactive when n<10
Why are mature eukaryotic mRNAs typically much shorter than the genes from which they are transcribed, but the primary transcripts are not?
Introns have been spliced out of mature mRNAs but they are still present in the primary transcript
What are introns?
Junk DNA
What are exons?
The fragmented coding sequence
What is an alternative term for the primary transcript?
pre-mRNA
Where does splicing occur?
The nucleus
Define splicing
The removal of introns and the joining together of exons in the same order as in the gene
What are splicosomes?
Congregations of snRNPs (up to 150). They are used up so need to be replaced
What are snRNPs?
Small nuclear RNAs plus proteins
What are RNPs?
Ribonucleoproteins
What diseases can be caused by mistakes in splicing?
- Breast cancer (BRCA1)
- Duchenne muscular dystrophy (DMD)
- Neurofibromatosis (NF-1)
- Thalassaemias
To what does the term ‘genetic code’ refer?
The relationship between the base sequence of a gene and the amino acid sequence of its protein product
What is a codon?
A sequence of three DNA or RNA nucleotides that corresponds with a specific amino acid or stop signal during protein synthesis
What is the coding number (i.e. how big is a codon)?
3
Why is it relevant that 42 = 16 whereas 43 = 64?
A triplet code allows for 64 different possible codons
Is the genetic code degenerate?
Yes
What is a ‘sense triplet’ and how many are there?
A triplet that encodes an amino acid
There are 61 that encode for 20 amino acids
What is the explanation for 61 triplets coding for 20 amino acids?
There are multiple synonym codons, i.e some amino acids are coded for by multiple codons, and some have multiple tRNAs
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
Which three base triplets mean STOP?
UAG, UAA, UGA
Do codons within a given mRNA overlap?
A point mutation within a gene changes a single amino acid in the encoded protien, so no
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.
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
What is the start codon? Which amino acid does it code for?
AUG = Met
What is Shine-Dalgarno recognition?
A sequence typically 4-7 nucelotides upstream of the coding region allowing the ribosome to detect the coding region
What are tRNAs?
Transfer RNAs
They are ‘genetic adapters’ that translate the code
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
What secondary structure do all tRNAs have?
Cloverleaf
What is an anticodon?
The part of a tRNA that recognises the mRNA codon
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)
