Biology 12
4.3 Gene Expression Day 2: Translation
Translation
Translation is the second process in gene expression. It
requires several enzymes and different types of RNA
molecules that include mRNA, rRNA, and tRNA.
Translation relies on the genetic code to convert mRNA
sequence to the amino acid sequence of a protein. The
genetic code
• consists of triplets of RNA nucleotides called codons
• has codons that code for amino acids
• is degenerate and almost universal (suggesting it
dates back to first organisms on Earth and a common
evolutionary history)
Transfer RNA
The transfer RNAs (tRNAs) bring amino acids for protein
synthesis to the ribosomes.
• Each tRNA is single-stranded and folds back on itself
to form a boot-like shape
• One end has an amino acid
• The other end has an anticodon that is complementary to an mRNA codon
Biology 12
Transfer RNA (cont.)
Although there are 64 different codons, there are only 40 different tRNA molecules. This is
because the third nucleotide in some mRNA codons can vary, which is called the wobble effect.
For translation, the anticodons of tRNA-amino acid complexes pair with mRNA codons.
If the mRNA codon is CGG, what is the tRNA anticodon and amino acid attached to it?
Overview of Gene Expression
During transcription, the base sequence in DNA is copied
into a base sequence in mRNA.
During translation, tRNAs bring amino acids to ribosomes in
an order determined by the sequence of codons in the
mRNA.
Ribosomes and Ribosomal RNA
Ribosomes are composed of many proteins and ribosomal
RNA (rRNA) and consist of large and small subunits.
• They have binding sites for mRNA and three tRNAs,
and help pair tRNA anticodons with mRNA codons
Biology 12
Ribosomes and Ribosomal RNA (cont.)
As ribosomes move along the mRNA, new tRNAs enter and the amino acids bond together,
forming a protein.
• Several ribosomes translate an mRNA at one time, forming a polyribosome
Translation Requires Three Steps
During translation, codons on the mRNA base pair with anticodons on the tRNAs that are
carrying specific amino acids. The order of the mRNA codons determines the sequence of
amino acids in the protein being synthesized.
Translation is an orderly process that must produce a protein of a particular sequence. It
involves three main steps
• initiation (requires energy)
• elongation (requires energy)
• termination (does not require energy)
Initiation
Initiation involves
• assembly of components (initiation factors,
small and large ribosomes, and mRNA)
• a start codon, AUG, on the mRNA
• tRNA with the first amino acid binding in the
P site of the ribosome
Elongation
Elongation is the protein
synthesis step, when a
polypeptide increases in
length one amino acid at a
time. Four steps are
repeated until a codon
that signals to stop is
reached.
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Termination
Termination is the final step in protein synthesis.
• A stop codon on the mRNA indicates where
termination occurs
• A release factor cleaves the new protein
from the tRNA
• Components for translation disassemble
• The protein begins to fold into its proper
shape
Review of Gene Expression
A gene is expressed when a protein it codes for is
made. It involves
A. Transcription
• DNA serves as a template for mRNA
synthesis. Bases in mRNA are
complementary to the DNA. Each codon in
mRNA is a triplet.
B. In eukaryotes, the mRNA is processed and transported to the cytoplasm
C. Translation
• tRNAs with amino acids pair with mRNA codons until the protein is synthesized
Figure 4.15 Review of gene expression. Messenger RNA is produced and
processed in the nucleus during transcription, and protein synthesis occurs at the
ribosomes (in cytoplasm and rough ER) during translation.