translation

 occurs in a 5’ to 3’ direction, start codon is nearer the 5’ end than the stop codon • moves along the mRNA to 3’ end

A site= aminoacyl site P site = peptidyl site E site = exit site

• OH attaches an amino acid • anticodon loops binds to the codon of an mRNA • base sequences of tRNA anticodons are complementary to the sequences of mRNA and relate to the same amino acid

 the small ribosomal subunit binds to the 5’ cap of the mRNA transcript with the help of an initiation factor (enzyme)  the large subunit joins the complex and the initiation factor dissociates  the first amino acid which initiates translation is always the same, an ordinary methionine amino acid  the codon for this initiation amino acid is AUG (start signal), it only causes initiation when it occurs near the beginning of the mRNA  the initiation tRNA (carrying methionine) binds to the P site by complementary pairing its anticodon with the mRNA codon cytoplasm outside nucleus

 a second amino acid on the tRNA is controlled by the second codon on the mRNA strand  binding of amino acids requires an enzyme  methionine disassociates from its tRNA, and then forms a peptide bond with the second amino acid (which is still attached to its tRNA)  the ribosome moves down the mRNA strand (called translocation) one codon length to the right  the tRNA which has lost its amino acid moves into the E site and then leaves the ribosome, the tRNA with two amino acids moves over to the P site and the next tRNA complimenting the new codon moves into the A site  the process continues until the ribosome reaches the last codon in the message

Step 3: Termination  three codons cause the termination of protein synthesis UAG, UAA andUGA  known as the stop signals  the stop signals do not code for an amino acid, also called nonsense codons  every gene has at least one and frequently two of these stop codons at the end after the last codon for an amino acid  termination is triggered by a release factor which causes the ribosomes to fall off the mRNA and release the polypeptide chain from the last tRNA

 cells which must rapidly synthesize many copies of different proteins will stockpile mRNA transcripts in their cytoplasm  a polysome is a cluster of many ribosomes translating the same mRNA transcript at the same time  free-floating ribosomes synthesize proteins for use primarily within the cell  ribosomes associated with ER synthesize proteins primarily for secretion or for lysosomes

nonsense codons are the stop signals do not code for an amino acid during third step of translation

the ribosome moving down the mRNA strand is translocation, one codon length to the right

20

20x3= 60 therefore 60 as there are 20 amino acids in a protein

64

UAG UAA UGA

There are many variations to genetic code and for a specific amino acid to minimize errors which lead to mutation

A change in the nucleotide sequence of a section of DNA coding for a specific trait.

ribosomes associated with ER synthesize proteins primarily for secretion or for lysosomes