Translation (College Board AP® Biology): Study Guide
Translation
Translation involves converting the genetic code in the mRNA into a polypeptide
Ribosomes are the site of translation; these are present:
in the cytoplasm of both prokaryotic and eukaryotic cells
on the rough endoplasmic reticulum of eukaryotic cells
Translation requires energy, and involves the following steps:
Initiation
Translation is initiated when the start codon of an mRNA molecule interacts with the rRNA of a ribosome
Elongation
tRNA molecules with complementary anticodons bind to codons on the mRNA
Each tRNA anticodon corresponds to a particular amino acid, which is attached to the amino acid binding site of the tRNA molecule
Two tRNA molecules fit onto the ribosome at any one time, bringing the amino acids that they are carrying side by side
A peptide bond forms between the two amino acids
This process repeats many times to form a chain of amino acids
Termination
The stop codon acts as a signal for translation to stop and at this point the amino acid chain coded for by the mRNA molecule is complete
The polypeptide is released from the ribosome
Reading the genetic code
Each mRNA codon corresponds to a specific amino acid
A genetic code chart can be used to determine which amino acid is encoded by a particular triplet
Many amino acids are encoded by more than one codon, e.g., GAU and GAC both code for the amino acid aspartic acid
Nearly all living organisms use the same genetic code; this is considered to be evidence of the common ancestry of all living organisms
Worked Example
Use the rules of base pairing and the genetic code chart above to deduce the amino acid sequence coded for by the following DNA sense strand sequence:
TTC GAG CAT TAC GCC
Answer:
Step 1: convert the sense strand into the template strand
We first need to convert the DNA sense strand into the DNA antisense strand, which is the same as the template strand
This is done using base pairing rules: A-T and C-G
The sense strand = TTC GAG CAT TAC GCC
The antisense, or template, strand = AAG CTC GTA ATG CGG
Step 2: convert the DNA template strand into mRNA codons
Base pairing can be used again to work out the mRNA sequence that will form during transcription
Remember that mRNA has U instead of T
DNA template strand = AAG CTC GTA ATG CGG
mRNA strand = UUC GAG CAU UAC GCC
Step 3: use the genetic code chart for the first amino acid
First base in codon = U, second base = U, third base = C
So we're looking in the top-left box of the table; this amino acid is Phe
Step 4: repeat for the remaining 4 codons
GAG = Glu
CAU = His
UAC = Tyr
GCC = Ala
So the sequence = Phe-Glu-His-Tyr-Ala
Translation in prokaryotes
In prokaryotes transcription and translation can occur simultaneously, i.e., one end of the mRNA might still be being synthesized while the other end is attached to a ribosome
This is because the processes are not separated by the nuclear membrane, which is not present in prokaryotes
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