Translation Flashcards
What is translation?
The synthesis of a protein from a transcribed sequence into mRNA
Where does translation occur?
cytoplasm
In prokaryotes (70S), translation is __-_________
co-transcriptional
In eukaryotes (80S), translation occurs after _______
transcription
How big is a ribosome?
10nm
The different units of ribosome are characterised based on the rate at which they sediment (measured in S) in a _______
centrifuge
What does the small ribosomal subunit contain?
Decoding centre needed for reading mRNA
What does the large ribosomal subunit contain?
Catalytic RNA-peptidyl transferase centre
What is a polyribosome?
Multiple ribosomes on an mRNA strand
tRNA carries amino acids that occupy A,P and E sites that occur mostly in the large/small subunit, but are completed in the large/small subunit
large
small
What is the A site?
acceptor site where amino-acyl tRNA lands
What is the P site?
Peptidyl tRNA site occupied by the last amino acid
What is the E site?
exit site where tRNA leaves
What is tRNA?
Adaptor molecules that deliver amino acids to the ribosome
What is the secondary structure of tRNA?
- clover leaf shape
- contains intra-tRNA H-bonding sites
The specific sequence of the anticodon is ________ to a sequence on mRNA
complementary
What are codons?
3 nucleotide triplets
Describe the method of the Nirenberg experiment
- They synthesised mRNA with repeating nucleotides, and they added these to a test tube containing E.coli lysate.
- The proteins were isolated to see which amino acids were incorporated
What were the results of the Nirenberg Experiment
UUU is an mRNA codon for phenylalanine
AAA is an mRNA codon for lysine
CCC is an mRNA codon for protine
A triplet codon based on 3 base codons determines amino acids
List the 3 stages of translation in prokaryotes
- Initiation
- Elongation
- Termination
Define translocation
Movement of a ribosome along mRNA
What does the initiation complex of translation include?
- ribosome
- mRNA
- initiator tRNA
Initiator tRNA recognises the ____ codon and enters the __- site, and the following tRNA’s only enter the __-site
AUG
P
A
What else is required for initiation?
- 3 initiation factors
- GTP
What 3 steps are involved in elongation?
- Peptide bond formation
- Translocation
- Aminoacyl-tRNA delivery
After initiation, the __-site is occupied whilst the __-site is empty
P
A
3 elongation factors are recruited to the initiation complex and they can all bind to ___ or ___
GTP or GDP
What do elongation factors do?
- delivers aminoacyl-tRNAs
- generates release of EF-Tu-GDP complex
- Translocation uses energy from GTP hydrolysis to eject tRNA from P-site, and move peptidyl-tRNA into the P-site from the A-site
How is frameshifting prevented?
The ribosome complex maintains a 6bp contact with the mRNA
When does elongation stop?
When a termination codon appears at the A-site
What is the process called in which the ribosome dissociates from the mRNA?
Termination
Protein release factors (RF) interact with these termination codons to release the polypeptide chain.
Describe what each RF does
- RF 1 - recognises UAA + UAG
- RF 2 - recognises UAA + UGA
- RF 3 - facilitates RF 1 + RF 2
RF causes ____________ to transfer the polypeptide to water so the protein is released
peptidyltransferase
RF dissociates the ribosome complex from the _____ and removal of tRNA from __-site
mRNA
P
What is co-translation?
When transcription and translation occur simultaneously.
What is an example of a co-translational control?
Tryptophan operon (trp operon)
What is the trp operon?
Group of genes that encode enzymes for Tryptophan
When is the trp operon expressed?
When tryptophan levels are low
When is the trp operon repressed?
When tryptophan levels are high
What regulates the trp operon?
trp repressor
How does the trp repressor work?
When bound to tryptophan the trp repressor blocks expression from the operon
What is attenuation used as?
A control mechanism when tryptophan levels are high and stops ribosome progressing along the mRNA
The attenuator polypeptide contains 2 tryptophan residues. What happens if there is NO tryptophan?
The ribosome stalls and an anti-terminator hairpin follows allowing transcription and translation to proceed
What happens if there is a lot of tryptophan?
The mRNA stalls and stops due to the terminator hairpin
What are the sequences needed to generate anti-terminator and terminator?
anti-terminator - 2 +3
terminator - 3 + 4
List the properties of proteins
- contain C,H,O,N
- can absorb light in the UV range
- contain charged/uncharged/hydrophilic/hydrophobic amino acids
