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Translation translates final mRNA to polypeptides
(proteins) through a process that has three steps:
- Initiation
- Elongation
- Termination
Translation involes multiple molecules including mRNA, tRNA, the two ribosomal units and various enzymes and proteins.
Initiation
During initiation, everything is set up for translation. The various components
comes together. mRNA attaches to the small ribosomal unit, with the initiator
codon, which is always AUG (methionine), being on the unit and the charged tRNA that has the mRNA and binds to the ribosomal unit with three proteins. Once this is done, the complex attaches to the large ribosomal unit with the mRNA-tRNA starting point in the P site of the large unit.
Elongation
During elongation, a tRNA that matches the codon in the A site comes in. With the help of the enzyme peptidyl transferase, an amino bond is formed between the two amino acids, the enrgy coming from the bond between the first amino acid and its tRNA. Once the bond is made, the now uncharged tRNA drifts away, and the whole complex moves over three bases (to the right). Each of the three base mRNA sequences, codons, instruct tRNA to bring a specific amino acid to the ribosome. This step repeats until a specific codon called the stop codon is reached. Termination now occurs.
A table of mRNA codons and what function they serve follows.
Messenger RNA Codons and Their Corresponding
Amino Acids
| First Base |
Second Base |
Third Base |
|
U |
C |
A |
G |
|
| U |
UUU phenylalanine
UUC phenylalanine
UUA leucine
UUG leucine |
UCU serine
UCC serine
UCA serine
UCG serine |
UAU tyrosine
UAC tyrosine
UAA stop **
UAG stop ** |
UGU cysteine
UGC cysteine
UGA stop **
UGG tryptophan |
U
C
A
G |
| C |
CUU leucine
CUC leucine
CUA leucine
CUG leucine |
CCU proline
CCC proline
CCA proline
CCG proline |
CAU histidine
CAC histidine
CAA glutamine
CAG glutamine |
CGU arginine
CGC arginine
CGA arginine
CGG arginine |
U
C
A
G |
| A |
AUU isoleucine
AUC isoleucine
AUA isoleucine
AUG methionine * |
ACU threonine
ACC threonine
ACA threonine
ACG threonine |
AAU asparagine
AAC asparagine
AAA lysine
AAG lysine |
AGU serine
AGC serine
AGA arginine
AGG arginine |
U
C
A
G |
| G |
GUU valine
GUC valine
GUA valine
GUG valine |
GCU alanine
GCC alanine
GCA alanine
GCG alanine |
GAU aspartate
GAC aspartate
GAA glutamate
GAG glutamate |
GGU glycine
GGC glycine
GGA glycine
GGG glycine |
U
C
A
G |
Termination
Since there are no tRNAs that bind with the stop codons, useless proteins plug up the A site. The bond between the last tRNA and the last amino acid on the now long poly peptide is broken and the whole complex breaks up.
It should be noted that mRNA can have many ribosome complexes translating it at the same time - polysomes. The ribosomes are spaced at regular intervals and translate the mRNA, allowing for fast production of many proteins.
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