Translation and the role of tRNA Flashcards
Translation
conversion of information in mRNA to protein
genetic code
rules by which nt sequence of gene (via mRNA intermediate) is translated into an amino acid sequence of a protein
only 4 nucleotides
specify 20 amino acids
Triplet code
mRNA sequence decoded in sets of 3 nucleotides
Transcription RNA
cell cannot directly translate genes into amino acids intermediate
The genetic instructions for a polypeptide chain are written in
the DNA as series of 3 nucleotide words
Codon
each group of 3 consecutive nucleotides (triplet) in RNA
Sequence of nts in mRNA is read
consecutively codon by codon
RNA is a linear polymer
made up of 4 different nts, there are therefore 4 x 4 x 4= 64 possible combinations of 3 nts (i.e. 64 codons)
Only 20 different AAs
therefore code is redundant and some AAs are specified by more than one codon
Codon specifies either
one amino acid or a stop to the translation process (stop codon)
Codons are
adjacent (not overlapping) and not separate by punctuation (comma-less)
In principle, an RNA sequence can be read
in any one of the 3 different non-overlapping reading-frames
Only one of the 3 possible reading frames
encodes the required protein
A punctuation signal at beginning of each RNA message
sets correct reading frame
Start codons
indicated by AUG sequence and codes for Methionine amino acid
Stop codons
indicated by UAA, UAG, UGA sequence and do not code for an amino acid
The RNA codon sequences in the ‘non overlapping reading frames’
to be translated into a protein is known as an ‘open reading frame’ (ORF) and does not contain a stop codon
the protein that codes RNA
mRNA
Other sub-types of RNA are classified as
non-coding RNA (ncRNA)
These subtypes of RNA are also
transcribed from genes on DNA (just not translated into a protein) and they function as RNA molecules
There is a gene for every
sub-type of RNA and these genes are transcribed (not translated) and are functional as a nucleic acid
3 different types of RNA molecules involved in process of translation
- mRNA: messenger RNA carries to message from DNA to code for a protein = coding RNA
- tRNA: Transfer RNA: interprets the message on mRNA and brings the correct amino acid in translation
- rRNA: ribosomal RNA constitutes part of the ribosomal structure which facilitates the process
snRNA
constituent of the SNURPS in the spliceosome (U1, U2, U4, U5, U6) in the process of RNA splicing (RNA processing)
mRNA, tRNA and ribosomes
are critical for translation
Translation
cell interprets genetic message (RNA) and builds a polypeptide accordingly with amino acids (protein)
transfer RNA (tRNA)
The genetic message is a series of codons along an mRNA molecule (the messenger), and the interpreter
tRNA transfers amino acids
from cytoplasmic pool of amino acids to a ribosome
Ribosome adds each amino acid
brought to it by tRNA to the growing end of a polypeptide chain
Therefore the tRNA and Ribosome are
the most crucial molecules to carry out the process of translation on mRNA
Translation of mRNA molecules into proteins depends on
tRNAs that can recognise and bind to both codon and AA
tRNA molecule consists of a single RNA strand
that is only about 80 nucleotides long
Atypical nucleotides on tRNA
D loop = D = dihydrouridine
T loop = The TiC-loop = pseudouracil (i)
4 short segments of folded tRNA base-pair with each other forming
double helical regions, producing molecule that looks like a cloverleaf (2D)
cloverleaf undergoes further folding to form
a compact L-shaped structure (3D)
2 key regions of unpaired nts situated
at either end of L-shaped molecule are crucial to function of tRNA in protein synthesis
Anticodon
set of 3 consecutive nts that pairs with complementary codon in an mRNA molecule
Amino acid binding site
short single-stranded region at 3’ end of tRNA: site where AA that matches the codon is attached to tRNA
Recognition and attachment of correct AA to tRNA depends on enzymes
aminoacyl-tRNA synthetases that covalently couple each AA to its appropriate set of tRNA molecules
20 different aminoacyl-tRNA synthetases
one for each amino acid (x20)
All active tRNA must contain CCA sequence on
3’ end of tRNA as the recognition site for amino acid treatment
ATP is used to
link the amino acid to the correct tRNA producing a high energy bond therefore tRNAs become ‘charged’ after linkage
Example 1
Tryptophan AA is coded for by UGG codon on mRNA. Therefore the tRNA with corresponding ACC anticodon is selected by tryptophanyl tRNA synthase and matches tryptophan AA to 3’ end of tRNA
Example 2
Tyrosine is coded by UAU codon on mRNA. Therefore tRNA with AUA anticodon selected by Tyrosyl tRNA synthase and matches Tyrosine AA to 3’end of tRNA
steps
1: The tRNA and its cognate amino acid enter the active site of the specific synthetase
2: Using ATP, the synthetase catalyses the covalent bonding between the amino acid and the tRNA
3: The tRNA charged with the amino acid is released by the synthetase
Accurate translation go genetic message requires. recognition steps in key sites of tRNA
Step 1: Must be a correct match between a tRNA and an amino acid
-A tRNA that binds to an mRNA codon specifying a particular amino acid must carry only that amino acid
-Each amino acid is joined to the correct tRNA by a specific aminoacyl-tRNA synthetases
Step 2: Must be a correct match between the tRNA anticodon and an mRNA codon
-Ribosomes facilitate the specific coupling of tRNA anticodons with mRNA codons during protein synthesis
