Page 17 - CH6 (1)

their corresponding tRNA.
● Ribosomes are the workbenches for translation. Ribosomes have 2

subunits: a large subunit and a small subunit.
● Smaller subunit comes in contact with mRNA to initiate the process of

translation.
● Translational unit in an mRNA is the region flanked by start codon and

stop codon.
● Untranslated regions (UTR) are the regions on mRNA that are not
themselves translated, but are required for efficient translation

process. They may be present before start codon (5′ UTR) or after
stop codon (3′ UTR).

● Initiator tRNA recognises the start codon. (Initiation)
● Then t­RNA­amino acid complexes bind to their corresponding codon

on the mRNA and base pairing occurs between codon on mRNA and
tRNA anticodon.

● tRNA moves from codon to codon on the mRNA and amino acids are
added one by one. (Elongation)
● Release factor binds to stop codon to terminate the translation.

(Termination)

Regulation of Gene Expression

● Regulation of gene expression could be exerted at following levels.

○ Transcriptional level (following of primary transcripts)
○ Processing level (splicing)
○ Transport of mRNA from nucleus to cytoplasm

○ Translational level
● In addition, metabolic, physiological, or environmental conditions

regulate the expression of genes.
● Expression of genes coding for enzymes is required only when
substrate for that enzyme is available.

For example:

Lactose Glucose + Galactose

s

E.coli​ynthesises beta­galactosidase, only when lactose is available.