Download 6.3 Protein Synthesis Translation

SBI 4UI
Protein Synthesis: Translation

The mRNA enters the cytoplasm and binds with the ribosome. The ribosome consists
of a large subunit and a small subunit. The mRNA is “clamped” in between the 2
subunits at the 5’ cap. The end of the ribosome the mRNA binds with is identified as
the 5’ cap.

There are 20 amino acids found in proteins, but only 4 different ribonucleotides. To
code for all 20 amino acids, a sequence of 3 ribonucleotides must be used for each
amino acid. Each triplet of ribonucleotides on a mRNA is called a codon.

The ribosome moves along the mRNA in the 5’ to 3’ direction, adding a new amino
acid to the growing polypeptide chain each time it reads a codon. While the ribosome
is able to assemble the polypeptide chain, it is the tRNA that must deliver the amino
acid to the ribosomes for assembly.

The tRNA has a 3 base sequence called an anticodon that recognizes the codon on
the mRNA. It recognizes it because they are complementary bases. Every tRNA
carries only one specific amino acid, which means that at least 20 different tRNA’s
are required. If a tRNA is charged, it is carrying it’s corresponding amino acid. The
enzyme responsible for adding the appropriate amino acid to each tRNA is called
aminoacyl-tRNA synthetase. There are at least 20 of these enzymes as well.

The start codon AUG ensures that the correct reading frame is used by the ribosome.
AUG encodes for methionine, therefore every protein initially starts with the
amino acid methionine.

The ribosome has 2 sites for the tRNA’s to “lock” into for translation. They are the A
(acceptor) site and the P (peptide) site. The process begins with the tRNA carrying
methionine locking into the P site. The next tRNA carrying the corresponding amino
acid enters A site and the methonine forms a peptide bond with this amino acid. The
ribosome shifts over one codon. The third amino acid enters A site while the second
amino acid moves to P site. A peptide bond forms between the second and third
amino acids.

The process continues leaving the growing polypeptide to “trail” into the cytoplasm.
When the ribosome reaches a stop codon, there are no corresponding tRNAs and no
amino acid to deliver. The ribosome “stalls”. A protein called the release factor
recognizes that the ribosome has stalled and causes the ribosome subunits to
disassemble, releasing the mRNA and newly formed polypeptide.
Perform Activity 6.2 Transcription in Reverse on page 261 by completing the steps in
the procedure and then answering question #1.