Transcription: Initiation, Elongation, Termination

Transcription Overview

• Transcription converts DNA into mRNA (messenger RNA).
• It occurs in three phases: initiation, elongation, and termination.

Initiation

• RNA polymerase binds to the promoter region of DNA.
• The promoter region has a TATA box sequence (TATA double A) in eukaryotic DNA.
• The TATA box is 25 nucleotides upstream from the transcription start site.
• RNA polymerase separates the two DNA strands.

Elongation

• RNA polymerase adds nucleotides to the growing mRNA strand.
• mRNA is synthesized from the 5' end to the 3' end.
• RNA polymerase reads the DNA strand in the 3' to 5' direction.
• The template strand (antisense strand) is used to synthesize mRNA.
• The non-template strand (sense strand or coding strand) matches the mRNA sequence, except uracil replaces thymine.

Termination

• RNA polymerase and the mRNA strand separate from the DNA template.
• Poly A polymerase adds a poly-A tail to the 3' end of the mRNA.
• The 5' end is capped to protect the mRNA from degradation.

RNA Splicing

• Pre-mRNA contains introns (non-coding sequences) and exons (coding sequences).
• Introns are removed during RNA splicing, and exons remain.
• A completed mRNA strand is produced.

DNA to mRNA Practice

• Remember that in RNA, uracil (U) replaces thymine (T).
• G corresponds to C, and C corresponds to G.
• A in DNA corresponds to U in mRNA.
• T in DNA corresponds to A in mRNA.

Translation Overview

• Translation uses mRNA to construct proteins.
• mRNA leaves the nucleus and enters the cytosol.
• mRNA interacts with free ribosomes or ribosomes attached to the rough ER.
• Ribosomes interact with tRNA (transfer RNA).
• Sets of three nucleotides on mRNA form codons that match anticodons on tRNA.
• Each codon corresponds to a specific amino acid.

Translation Steps

• Translation also occurs in three steps: initiation, elongation, and termination.
• The ribosome is composed of small and large subunits and has E, P, and A sites.

Initiation

• The start codon AUG binds to the anticodon UAC.
• The tRNA molecule carries methionine (Met) and enters the ribosome at the P site (peptidyl site).

Elongation

• Another tRNA molecule enters the A site (aminoacyl site).
• A covalent bond forms between the two amino acids.
• The tRNA in the P site moves to the E site (exit site) and loses its amino acid.
• The tRNA molecules enter the A site and exit from the E site.
• A growing polypeptide chain of amino acids is formed.

Termination

• Termination begins when a stop codon is read which are: UAA, UAG, and UGA.
• Stop codons (nonsense codons) do not code for a specific tRNA molecule.
• A release factor enters the A site, causing the ribosomal subunits to disassemble.
• The protein leaves the ribosome and goes to the Golgi body for processing and folding, to form a specific shape for a specific job.

Transcription is the process of converting DNA into mRNA. It involves initiation where RNA polymerase binds to the promoter, elongation where nucleotides are added to the mRNA strand, and termination where the mRNA strand separates from the DNA template.