Translation is the process by which mRNA
transcripts are converted into the amino acid sequences of proteins. A cell
uses special organelles called ribosomes to translate RNA messages into
protein. Ribosomes are themselves composed of RNA and protein, and consist of
two components, a large subunit and a small subunit. Together, these form a
complex that moves along the messenger RNA molecule and decodes its sequence.
The translation process begins as the small ribosomal subunit, depicted in
pink, binds to proteins called initiation factors, shown here as green and
yellow spheres. Then, the small subunit binds to an mRNA molecule, shown here
as a blue strand, near its 5’ end. Two important structures called the P site
and the A site are located within the small subunit. As shown, the segment of
the mRNA positioned at the P site binds to a molecule called an initiator
tRNA that is attached to the amino acid methionine.
This is almost always the first amino acid of a protein sequence and is
depicted here as the blue sphere. The initiator tRNA carries three
nucleotides, most often U, A, and C. These are shown here as the small green,
red, and yellow cylinders and are collectively called an anticodon.
The anticodon forms base pairs with a set of three
complementary nucleotides on the mRNA which are referred to as the start
codon and most commonly have the sequence AUG. Next, the large subunit binds
to the complex, and a second tRNA carrying another amino acid enters the A
site of the ribosome. The second tRNA base pairs with the
next set of three mRNA nucleotides, or codon, as before. Note that a
peptide bond forms between the first amino acid and the second amino acid,
and that the first initiator tRNA begins to leave the ribosome as a third
tRNA approaches. The third tRNA adds its amino acid to the growing peptide
chain, depicted here as the linked blue spheres, and
the second tRNA exits the ribosome. This process continues until the ribosome
recognizes a set of three specific nucleotides called a stop codon along the
sequence of the mRNA and releases the completed polypeptide chain. As shown
here, when one ribosome moves away from the initiation site, other ribosomes
are free to bind to the mRNA and begin translation. Many ribosomes will
oftentimes simultaneously bind to and translate the same mRNA molecule in a
configuration called a polyribosome or polysome.
We thank NASA for providing support to the
DasSarma laboratory (grant 80NSSC19K0463).