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Table 7.3 Components of Translation in Prokaryotes

Component

Comments

Anticodon

Sequence of three nucleotides in a tRNA molecule that is complementary to a particular codon in mRNA.The anticodon allows the tRNA to recognize and bind to the appropriate codon.

mRNA

Type of RNA molecule that contains the genetic information deciphered during translation.

Polyribosome (polysome)

Multiple ribosomes attached to a single mRNA molecule.

Reading frame

Grouping of a stretch of nucleotides into sequential triplets; an mRNA molecule has three reading frames, but only one is typically used in translation.

Ribosome

Structure that facilitates the joining of amino acids during the process of translation; composed of protein and ribosomal RNA.The prokaryotic ribosome (70S) consists of a 30S and 50S subunit; it is the target of several groups of antibacterial drugs.

Ribosome-binding site

Sequence of nucleotides in mRNA to which a ribosome binds; the first time the codon for methionine (AUG) appears after that site, translation generally begins.

rRNA

Type of RNA molecule present in ribosomes.

Start codon

Codon at which translation is initiated; it is typically the first AUG after a ribosome-binding site.

Stop codon

Codon that terminates translation, signaling the end of the protein; there are three stop codons.

tRNA

Type of RNA molecule that act as keys that interpret the genetic code; each tRNA molecule carries a specific amino acid.

The Role of Transfer RNA

The tRNAs are segments of RNA able to carry specific amino acids, thus acting as keys that interpret the genetic code. They each recognize and base-pair with a specific codon and in the process deliver the appropriate amino acid to that site. This recognition is made possible because each tRNA has an anti-codon, three nucleotides complementary to a particular codon in the mRNA. The amino acid each tRNA carries is dictated by its anticodon and the genetic code (figure 7.13).

Although there are 64 different codons, there are fewer different tRNA molecules. For example, there are three stop codons, which signal the end of the protein; they have no corresponding tRNA molecules that recognize them. Also, the anti-codon of some tRNA molecules can recognize more than one codon. It appears that a certain amount of "wobbling'' is tolerated in the base-pairing so that recognition of the third nucleotide of the codon is not always precise. Due to the degeneracy of the genetic code, however, the correct amino acid is still incorporated into the polypeptide chain.

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