Translation Flashcards
translation
- first step in protein synthesis
- the code contained in a mRNA is translated into a particular series of amino acids to form a polypeptide
codon
-a nucleotide triplet of mRNA
how many possible combinations of codons are there?
4 different nucleotides
3 nucleotides in each codon
64 total possibilities
what direction are the nucleotides read and the proteins synthesized
- nucleotides are read from 5’ to 3’
- proteins are synthesized from N-terminus to C-terminus
Start codon
- start codon is always AUG
- AUG encodes for methionine
- ALL START CODONS ARE AUG BUT NOT ALL AUG ARE START CODONS
methionine
- AUG is the only codon that encodes for methionine
- other amino acids have multiple codons
Stop codons
- DO NOT CODE FOR AN AMINO ACID
- UGA, UAA, UAG
- the first stop codon is THE stop codon
What are the characteristics of the genetic code?
- universal
- specific
- redundant
- nonoverlapping
- commaless
Universal
-genetic code defines the same amino acids in almost all organisms
specific
each codon is specific to a certain amino acid
redundant
- each amino acid may have more than one codon
- each amino acid has between 1 and 6 codons
nonoverlapping and commaless
-a continuous series of 3 bases, no overlap and no bases are skipped o repeated
single nucleotide changes
- point mutations
- silent mutation
- missense mutation
- nonsense mutation
silent mutation
- if the DNA sequence is mutated, so that the codon is changed, but still encodes for the same amino acid
- EX UCA and UCU both encode for serine
missense mutation
- if the point mutation results in a codon that defines a different amino acid
- UCA to UCU goes serine to proline
conservative mutation
a missense mutation that results in an amino acid with similar properties
-non-polar to non-polar amino acid
nonsense mutation
- change results in a change from an amino acid to a STOP codon
- the earlier in the sequence this occurs, the worse
Frameshift
- if the insertion/deletion is NOT a multiple of 3.
- usually results in a premature STOP codon and truncated protein
- generally a pretty severe mutation
What if the insertion/deletion is in a multiple of 3?
-results in the insertion/deletion of an amino acid or amino acids
splice site mutation
-changes in nucleotides involved in splicing RNA
possible results of splice site mutations
- deletion of nucleotides from an exon
- leaving nucleotides from an intron in the final mRNA
- complete deletion of an exon from the final mRNA
- can be single nucleotide mutations
trinucleotide repeat expansion
- regions in a gene where a sequence of bases is repeated many times
- the repeat is amplified significantly
where can trinucleotide repeat expansion happen?
- in the coding region
- in the 5’ or 3’ UTR
result of trinucleotide repeat expansion in the coding region
- a faulty protein
- protein aggregation
result of trinucleotide repeat expansion in the 5’ or 3’ UTR
- decreased production of the protein due to the effect of the UTRs on translation
- disrupts the protein expression
components required for translation
- amino acids
- tRNA
- mRNA
- ribosomes
what if the amino acid encoded on the mRNA is not present?
-translation will stop at the codon specifying that amino acid
CCA-3’ terminus
- amino acid attachment site on tRNA
- if an amino acid is attached, the tRNA is charged
Anticodon
- specific 3 nucleotide sequence that base pairs with the mRNA
- pair the codon, then flip it
aminoacyl-tRNA synthetases
- enzymes that attach amino acids to the corresponding tRNA
- each of these enzymes recognizes the amino acids and ALL of the tRNAs that correspond to that amino acid
How many different aminoacyl-tRNA synthetases are in humans?
20!
tRNA charging
- two step reaction
1. amino acid is covalently linked to the hydroxyl group on the 3’ terminus of the tRNA via its carboxyl group
2. pyrophosphate generated is then cleaved to give energy
is there energy involved in attaching the amino acid?
-yea buddy
subunits of ribosomes
-exist separately until protein synthesis is about to begin
A site
-site on ribosome that binds incoming aminoacyl-tRNA
P site
-site on ribosome that binds peptidyl-tRNA, which carries a chain of amino acids that have already been synthesized
E site
- exit site on ribosome
- contains empty tRNA as it is about to exit the ribosome
- ONLY FOR PROKARYOTIC RIBOSOMES
Do eukaryotic rbosomes have an E site?
-nope
Where are eukaryotic ribosomes located?
-free in the cytosol
OR
-bound to the ER
rough ER
-ER bound with ribosomes
secretory proteins
-proteins translated on the ER are destined for post-translational modifications and or subcellular compartmentalization
Where are prokaryotic ribosomes located?
-only free in the cytosol
Energy requirements for translation
- Charging the tRNA: 2 ATP
- Binding the aminoacyl-tRNA to the A site: one GTP
- Translocation step: one GTP
total energy needed for translation
-4 high energy bonds are required for each amino acid that is added
Wobble hypothesis
- tRNAs can recognize more than one codon for a specific amino acid
- 3rd nucleotide of a codon and the 1st nuceotide of the anticodon can SOMETIMES pair non-specifically
polycistronic
- multiple coding regionson the same genes
- multiple genes on one mRNA
polyribosomal
-multiple ribosomes can bind and work on a single mRNA molecule
what kind of organisms are polycistronic?
prokaryotes only
what kind of organisms can be polyribosomal?
-prokaryotes and eukaryotes can be this
initiation of translation in prokaryotes
- presence of a shine-dalgarno sequence
- 16S rRNA contains a complimentary sequence to the shine-dalgarno
- this allows for correct alignment of the small ribosomal subunit with the AUG start codon
initiation of translation in eukaryotes
-16S rRNA recognizes the 5’ cap structure(scanning event)
prokaryotic initiator tRNA
-bound to a formylated methionine
eukaryotic initiator tRNA
-bound to a methionine
elongation
- the polypeptide chain is elongated by the addition of amino acids to the carboxyl end of the growing chain
- forms peptide bonds
translation outline
- aminoacyl-tRNA to the A site
- move uncharged tRNA into the E site
- move peptidyl-tRNA into the P site
- opens A site for the next aminoacyl-tRNA
termination
- stop codon enters the A site
- no tRNA can come in, so it terminates
- releases polypeptide from the E site into the cytosol
default pathway for proteins
-outside of the cell
secretory pathway
-go into an organelle or another part of the cell
