Lesson 3 — Protein Synthesis


Translation

Read Pages 636 - 641


Translation is the process of turning the mRNA sequence into an amino acid chain (protein). The mRNA codons determine which amino acid is added to the chain. A codon is a set of three bases that codes for an amino acid. For example, the mRNA codon ACU codes for the amino acid threonine.

Table 18.3 on page 637 of your textbook is a table of mRNA codons and the corresponding amino acid for each. Read pages 636 and 637, "The Genetic Code", in your textbook to understand the three important characteristics of the genetic code and to learn how you use this table.  
NOTE:  This table is also found in the Biology 30 data pages so for examinations, including the Diploma Examination, you always will be provided with this table.



Translation. NHGRI. Public Domain.


Overview of Translation
  1. Transcription ends and mRNA strand leaves the nucleus and enters the cytoplasm.

  2. mRNA binds to a ribosome in the cell cytoplasm.

  3. mRNA codon is exposed and read. (Three mRNA nucleotides or one mRNA codon are exposed at a time.)

  4. A transfer RNA (tRNA) with the matching anticodon docks onto the ribosome and brings the appropriate amino acid.

  5. The amino acid starts to form bonds with other amino acids, building a chain.

  6. The ribosome moves in 5' to 3' direction along the mRNA strand and exposes the next codon for tRNA docking.

  7. The amino acid chain continues to grow and a protein molecule is created.


Transfer RNA (tRNA)

Translation of the messenger RNA occurs in the cytoplasm of the cell with the help of transfer RNA (tRNA). tRNA is a single stranded RNA molecule that is folded so that it has a 3 nucleotide anticodon on one end and a binding site for an amino acid on the other end.  Anticodons bind to the complementary codons on the mRNA. The other end of the tRNA has the amino acid that corresponds to the codon on the mRNA.

For example, the tryptophan amino acid is attached to the tRNA with anticodon ACC. The tRNA with anticodon ACC binds to UGG codon of mRNA and links the tryptophan to the amino acid chain. About 45 types of tRNA occur for the 64 codons - some tRNAs can recognize more than one codon. Figure 18.14 on page 639 shows a tRNA molecule that carries the amino acid arginine. 

Ribosomes, which contain ribosomal RNA (rRNA), help by bringing together the tRNA strand, the mRNA strand, and other enzymes needed to build the protein. The initiator or start codon on the mRNA is AUG, and it signals the start of translation. The stop codons are UAA, UAG, and UGA, and no amino acids correspond to these codons. They mark the end of translation. Read Figure 18.15 to see the steps that translation follows.


Example

DNA Sense Strand
 ATG CTG AAT
This is not the template. The sense strand is also called the coding strand because it is the same sequence as the mRNA.
NOTE:  On page 638 of the Inquiry into Biology textbook, the sense and anti-sense strands are incorrectly identified.  
DNA Antisense Strand
 TAC GAC TTA
This is the template strand, complementary to the mRNA strand.
mRNA codons
 AUG CUG AAU
Find these codons in the mRNA table to determine the corresponding amino acids.
tRNA anticodons
 UAC GAC UUA
This sequence is complementary to the mRNA codon.
Amino Acids
 Met - Leu - Asp
You can use the mRNA codon table to determine this.