Basic Concepts of Protein Biosynthesis
1. Protein biosynthesis is also called translation in which mRNA, which is transcribed from DNA, is translated into protein.
2. The genetic information in the mRNA molecule is read one codon (three nucleotides) at a time, always beginning with AUG, which specifies methionine, and ending with a stop codon, which specifies the end of translation.
3. Three major components in protein synthesis are mRNA, ribosomes and tRNA.
4. The mRNA code is read in the 5' to 3' direction, and the protein is synthesized from the amino or N terminus to the carboxyl or C terminus.
5. An eukaryotic mRNA molecule generally contains coding sequence (codes for the protein) and additional sequences that are not translated into proteins (See figure below).
The mRNA sequence upstream of the start codon (AUG) is termed the 5'-untranslated region (UTR), and the mRNA sequence downstream of the stop codon (UAG, UAA, or UGA) is termed the 3'-UTR. The eukaryotic mRNAs usually contain a CAP structure attached to the first nucleotide of the mRNA, and generally a poly A tail is attached at the end of the 3'-UTR.
mRNA (5') cap - ~ ~ ~ ~ AUG ~ ~ ~ ~ UGA ~ poly (A)n
Protein N ---------------- C
6. The mRNA coding sequence beginning with a start codon and ending with a stop codon is also called an open reading frame, or ORF. Mammalian mRNAs generally contain only one ORF. Post-translational processing can produce multiple proteins from a single polyprotein precursor.
7. An ORF contains codons. A codon consists of three nucleotide residues. There are a total of 64 codons, including 61 amino acid-specifying codons and three stop codons.
Each ORF is specific. Despite degeneracy, a given codon specifies only one amino acid. While there may be more than one tRNA per amino acid, each amino acid is specified by a separate tRNA. Different amino acides ot not share the same tRNA molecule.
ORFs are almost universal in all organisms.
Each nucleotide is part of one codon and an ORF is read continuously from teh beginning to the end without any punctuations.
8. Another basic component of translation is the ribosome. Mammalian ribosomesare 80S (containing two subunits 60S and 40S). The 60S subunit is composed of three RNA species (28S, 5.8S and 5S) plus numerous proteins and the 40S is composed of one RNA (5S) plus numerous proteins. (Note: S = Svedberg unit which is related to the molecular weight and shape of the molecule.)
