Module 7 : Concept of GENE, GENOME and GENE EXPRESSION

Translation.

• Translation is the first stage of protein biosynthesis. • During translation, the genetic code in mRNA is read and converted into protein by means of the protein synthesizing machinery comprising of ribosomes, tRNA, amino acids, and a number of enzymes/ proteins. • Translation occurs in the cell cytoplasm, where the large and small subunits of the ribosome bind to the mRNA. • The ribosome facilitates decoding by inducing the binding of tRNAs (anticodon) with complementary sequences of mRNA (codon). • The t RNAs carry specific amino acids that are chained together into a polypeptide as the mRNA is "read" by the ribosome.

Transfer RNA (tRNA) :

• tRNA is a small RNA chain, about 80 nucleotides long ,that transfers a specific amino acid to a growing polypeptide chain at the ribosomal site of protein synthesis during translation. • It has sites for amino acid attachment and an anticodon region for codon recognition. • One end of the molecule attached to a specific amino acid. • The other end has an exposed sequence of 3-bases. These are called the anticodon. • There are different tRNA molecule for each of the possible triplets meaning 64 anticodons. There is at least one t RNA for each aminoacids. • The anticodons of the tRNAs each have a complimentary codon in the mRNA. For example the codon GCC would be the compliment of the anticodon CGG.

Ribosomal RNA(rRNA) :

There are 4 kinds of ribosomal RNAs : In eukaryotes, these are 18S rRNA. One of these molecules, along with different protein molecules, constitute the small subunit of the ribosome. The other three are 28S, 5.8S, and 5S rRNA. One each o f these molecules, along with different proteins, form the large subunit of the ribosome. rRNA (ribosomal RNA) - associates with proteins to form ribosomes.

The Role of Ribosomes :

• Ribosomes are the ‘decoding’ units of the cell. • Each ribosome consists of two subunits, and is an assemblage of risosomal RNA(rRNA) and proteins. Subunits are separate in the cytoplasm, but join during protein synthesis (translation). • Ribosomes have binding sites for both tRNA and mRNA molecules. • The ribosome has two primary functions : -> decode the mRNA. -> form peptide bonds.

Translation, like replication and transcription also occurs in three steps of Initiation Elongation and Termination Initiation :- A ribosome binds to the mRNA and starts to code at the FMet codon (START codon: AUG, GUG or UUG). Elongation :- tRNA brings the corresponding amino acid to each codon as the ribosome moves down the mRNA strand. Termination :- Reading of the final mRNA codon (STOP codon: UAA, UAG, UGA), which ends the synthesis of the peptide chain and releases it.