7.3 Transcription and 7.4 Translation

Direction of transcription
Transcription is carried out in a 5’ to 3’ direction.

Distinguish between the sense and antisense strands for DNA

Anti-sense strand serves as a template for mRNA; partner strand is the sense strand.

Process of transcription in prokaryotes

RNA polymerase binds to promoter region which initiates transcription. DNA polymerase splits DNA into two strands, but rewinds it after it passes and transcription is completed for that segment. Nucleoside triphosphates bind to complementary bases on antisense strand. RNA polymerase synthesizes in 5’ to 3’ direction until it reaches the terminator sequence. Then polymerase and mRNA detaches.

Removal of introns in eukaryotic RNA
Eukaryotic RNA needs removal of introns to form mature RNA.

tRNA molecule recognized by a tRNA-activating enzyme

Each different tRNA molecules has a unique shape / chemical composition that is recognized by a specific tRNA-activating enzyme. Enzyme (aminoacyl-tRNA synthetase) binds amino acid to molecule of ATP (to form amino-acid-AMP complex linked by high energy bond). Amino acid transferred to 3 and of appropriate tRNA, attaching terminal CCA sequence to acceptor stem and releasing AMP molecule.  tRNA molecule with amino acid is charged and can go translate. Energy in bond-linking the tRNA molecule to amino acid will be used in translation to form a peptide bond between adjacent amino acids.

Structure of ribosomes

Ribosomes made of protein (stability) and rRNA.

There are 2 subunits which consists of a small subunit contains mRNA binding site and a large subunit contains 3 tRNA binding sites (Aminacyl (A), peptidyl (P), exit (E) sites).

Translation

Translation contains initiation, elongation, translocation, termination.

Translation direction

Translation occurs in 5’ to 3’ direction.

Structure of peptide bond

Peptide bond between two amino acids: find the O=C-N-H bond.

(chemicalconnection.org.uk)

Process of translation

Initiation: Small ribosomal subunit binds to 5’ end of mRNA and moves until it reaches AUG start codon; large ribosomal subunit then binds to small one. Initiator tRNA in P site, next codon signals another tRNA to bind, it occupies the A site; peptide bond formed between A and P sites.

Elongation: Ribosomes translocates 3 bases along mRNA, moving tRNA in P site to E site, freeing and allowing tRNA with appropriate anticodon to bind to next codon and occupy vacant A site.

Translation: Process continues until stop codon is reached when free polypeptide is released.

Free ribosomes and bound ribosomes

Free ribosomes synthesize proteins for use primarily within the cell and that bound ribosomes synthesize proteins primarily for secretion or for lysosomes.