DNA sequences are transcribed into nRNA sequences, then translated
into protein sequences.
Transfer RNA (tRNA) serves as an adaptor molecule between mRNA codons
and amino acids in a protein sequence.
Translation uses an mRNA message to
help build a protein sequence of amino acids (a polypeptide sequence).
- Coding units in mRNA are called codons.
Each codon contains exactly three bases (a triplet code).
- Each codon of mRNA matches with a particular anticodon on tRNA.
The tRNA thus serves as an "adaptor", controlling the match-up
between codons and amino acids.
The genetic code is a set of rules for translating codons into amino acids.
- The code is commaless, meaning that nothing separates one codon from
the next. Frameshift mutations, in which a base is deleted or an extra
base inserted, make the bulk of the genetic message unreadable, showing
that the code is commaless:
| ORIGINAL MESSAGE:
|
THE CAT ATE THE RAT AND THE HAT |
|
FRAMESHIFT, WITH COMMAS:
|
TH CAT ATE THE RAT AND THE HAT |
|
SAME FRAMESHIFT, COMMALESS:
|
THC ATA TET HER ATA NDT HEH AT |
- The code is degenerate or redundant, meaning
that different codons may code for the same amino acid.
Example: CGA, CGG, CGC, and CGU all code for
arginine.
- There are three chain-terminating or "stop" codons:
UAA, UAG, and UGA.
- The genetic code is universal: all organisms tested use
the same code.
|