Initiation:

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

Elongation:

–      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

Termination:

–      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

transcription Protein Synthesis: Transcription

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

Splicing

–      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

processing Protein Synthesis: Transcription

Protein Synthesis: Translation

– mRNA transcript enters the cytoplasm and binds to a ribosome

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

Ribosomes:

–      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

c7.17.16ribosome Protein Synthesis: Transcription

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

trna diagram Protein Synthesis: Transcription

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

protein synthesis Protein Synthesis: Transcription



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