–      Upstream of the gene, RNA polymerase binds to a genes promoter region and unwinds the DNA double helix


–      RNA polymerase starts building an mRNA, using the template strand of the DNA molecule, in the direction of 5’ to3’

–      the strand that is not copied is called the coding strand

–      the sequence of the mRNA is complementary to the template strand and identical to the coding strand, except it has uracil instead of thymine


–      RNA polymerase continues building the mRNA until it comes across a terminator sequence

–      transcription ceases, the mRNA leaves the template strand and RNA polymerase is free to find another promoter region and transcribe another gene

Posttranscriptional Modification

–      the newly synthesized mRNA molecule, the primary transcript must be modified before leaving the nucleus

–      a 5’ cap is added to the start and a poly- A tail is added to the end of the primary transcript

–      the 5’ cap plays a role in the initiation of translation and protects the mRNA from being damaged in the cytoplasm


–      DNA genes contain coding regions (called exons) and non coding regions (called introns)

–      introns do not code for part of the protein and must be removed from the primary transcript; if these regions are translated the protein will not function properly

–      spliceosomes cut out the introns and join the remaining exons together forming the mRNA transcript

–      the mRNA transcript can now leave the nucleus and be translated into a protein

Protein Synthesis: Translation

– mRNA transcript enters the cytoplasm and binds to a ribosome

– the ribosome recognizes the 5’ cap of the mRNA transcript


–      consist of two subunits, a large subunit of 60S and a small subunit of 40S

–      the subunits clamp the mRNA between them, move along the mRNA in the 5’ to 3’ direction, and add new amino acids to a growing polypeptide chain

–      mRNA is read three nucleotides at a time, and the phase in which this is done is called the reading frame

Transfer RNA (tRNA):

–      tRNA deliver the correct amino-acids to the ribosome

–      tRNA is a small, single-stranded nucleic acid whose structure resembles a cloverleaf

–      one arm of the tRNA contains the anticodon which recognizes the codon of the mRNA

–      the opposite arm to the anticodon carries an amino acid

–      the recognition by tRNA of mRNA is facilitated by complementary base pairing

Initiation of the Polypeptide Chain:

–      the first codon recognized by the ribosome is the start codon AUG

–      AUG sets the correct reading frame for use by the ribosome

–      ribosomes have two sites for tRNA: the A (acceptor site) and the P (peptide) site

–      a) the tRNA the carries methionine enters the P site

Elongation of the Polypeptide Chain:

–      b) the next tRNA enters the A site

–      c) the amino acid on the A site tRNA becomes peptide bonded to the growing polypeptide chain

–      d) the ribosome shifts over one codon causing the A site tRNA to move to the P site, the next tRNA enters the A site and the process continues

Termination of the Polypeptide Chain:

–      translation ends when the ribosome reaches a stop codon (UGA, UAG and UAA)

–      stop codons do not code for an amino acid, there are no corresponding tRNAs

–      release factor releases the polypeptide chain from the ribosome and the two subunits fall off the mRNA

author avatar
William Anderson (Schoolworkhelper Editorial Team)
William completed his Bachelor of Science and Master of Arts in 2013. He current serves as a lecturer, tutor and freelance writer. In his spare time, he enjoys reading, walking his dog and parasailing. Article last reviewed: 2022 | St. Rosemary Institution © 2010-2024 | Creative Commons 4.0

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