lecture 21- translation part 1 Flashcards
genetic code
- in the triplet code, three consecutive bases specify an AA, creating 64 possible codons
- DNA sequence encoding protein read in triplets- codons
- 61 of 64 are sense codons
- 3 stop codons (UGA, UAG, UAA)
- start codon- Met AUG- starts translation
- code is degenerate (redundant)
2 rules of genetic code
1- non-overlapping triplets read from fixed point
2- open reading frame (ORF)- a run of sense codons before a stop codon is encountered
protein synthesis (translation) requires 4 primary components
1- ribosomes- site of translation
2- mRNA- template
3- tRNA’s- adaptors
4- aminoacyl-tRNA synthetases
synthesis of a single polypeptide chain requires…
a coordinated action of at least 100 proteins and RNA’s
describe tRNA as adaptor
adaptor molecules recognize codons in mRNA and carry specific AA’s
- the structure of the tRNA molecule reveals how it is capable of functioning as an adaptor
describe tRNAs 2D structure
- small molecule, single-stranded, 73-93 nt
- each tRNA represents a single AA which it can covalently bind (3’ terminal A-residue)
- cloverleaf: secondary structure held together by H bonds
- all tRNA’s contain some unusual nucleotides:
T loop- ribothymidine and pseudouridine
D loop- dihydrouridine - each tRNA contains anticodon complementary to the codon representing its AA
describe tRNAs 3D structure
3D (twisted) L-shaped structure is formed by 2 extended helices perpendicular to each other
- D and T loops come together
- the shape must be the same for each tRNA so it can “fit” into ribosome
- anticodon and the acceptor arm are at the opposite ends of the L-structure
each tRNA has an ___ sequence that will base pair with a codon in mRNA, they are ___
anticodon
antiparallel
specificity of the three nucleotides in a triplet codon of genetic code
the first two nucleotides of each codon are the primary determinants of specificity
- the difference between the codons usually lies at the third position
describe wobble base pairing
the wobble base allows a single tRNA anticodon to bind to more than one mRNA codon
- the 3rd base codon and the 1st anticodon can form “unusual” base pairing- the rules are more relaxed b/c of the geometry
- a minimum of 32 tRNA’s are required to recognize all the AA codons (31 tRNA’s for AA’s and 1 for initiation)
- (in Watson-Crick base pairing, 61 diff tRNA’s would be required…)
which nucleotide in the anticodon is in the wobble position
the 5’ nucleotide of anticodon
3’ nucleotide of codon
describe the unusual pairing of nucleotides in wobble pairing
in the wobble position, G can pair with U (normal pairing: G-C & U-A)
in the wobble position, I (inosine) can pair with U, C, or A (I-C, I-A, I-U)
describe charging of tRNA’s
aminoacyl-tRNA synthetases charge tRNA’s (attach correct amino acid)
tRNA^(Ala) is an uncharged tRNA specific for alanine
Ala-tRNA^(Ala) is a tRNA specific for alanine that is charged with alanine
last 3 nucleotides of all tRNA’s are ___
CCA-3’
describe the process of aminoacyl-tRNA synthetases activating and linking AA’s to specific tRNA’s
- it is a 2-step process that uses ATP
- it takes place in the cytosol (the enzyme’s active site)
Step 1: Adenylylation step - the AA is linked to adenylate, forming aminoacyl-AMP
Step 2: tRNA charging step - aminoacyl group has high energy ester bond to 3’ end of tRNA (aminoacyl-tRNA) - high energy bond supplies the energetic driving force for translation
