Transcription and Translation

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Transcription and Translation
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• Transcription
• the process by which chemical information encoded
  in DNA is copied into mRNA (messenger RNA)
• transcription is classically described in three
  distinct steps initiation, elongation and
  termination.

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• Initiation
• an enzyme binds to specific sequence of bases on
  a DNA strand called the promoter region and
  begins to break H of the complementary bases,
  unwinding the DNA.
• a segment of DNA of around 17 base pairs stays
  unwound throughout the transcription process.

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• Elongation
• an enzyme helps free floating ribonucleotides add
  to the unwound strand

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• each ribonucleotide is inserted into the growing
  RNA strand following the rules of base pairing
• for each C encountered on the DNA strand, a G is
  inserted in the RNA
• for each G, a C
• for each T, an A
• however, each A on the DNA guides the insertion
  of the pyrimidine uracil, U instead of A
• Termination
• when the enzyme reaches a specific sequence of
  nucleotides, it and its newly transcripted RNA
  are released from the DNA.

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• finally, two major processes are done to the new
  mRNA strand before it's ready to be used to make
  protein
• introns are regions of mRNA that do not code for
  proteins and must be removed by spliceosomes
  leaving only exons or coding regions that code
  for proteins
• this is now considered mature mRNA
• a cap and tail are added to the ends for
  stability/protection

Splicing Out Introns Movie
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Translation

• process of converting the information in mRNA
  into a protein.
• 3 main parts- Initiation, Elongation, Termination

University of Colorado Translation Movie
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1. Initiation

• an initiation complex is formed when
• small subunit of the ribosome binds to a site on
  the mRNA
• it proceeds downstream until it encounters the
  start codon AUG.
• here it is joined by the large ribosomal subunit
  and a special initiator tRNA.
• in eukaryotes (cells with a nucleus), the
  initiator tRNA with the anticodon UAC carries the
  amino acid methionine (Met) that binds to the
  first site on the large ribosomal unit

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2. Elongation

• the process of adding more a.a. to the protein
• another tRNA covalently bonded with its amino
  acid base pairs with the next codon on the mRNA
  at the next site on the ribosome
• the preceding amino acid methionine (Met) is then
  bonded to the incoming amino acid
• the first tRNA is released and the ribosome moves
  one codon downstream

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3. Termination

• the point where the translating ribosome reaches
  a stop codon
• stop codons are UAA, UAG, and UGA because none of
  the tRNAs have anticodons that are complimentary
  to these three codons
• instead, a protein binds to the mRNA and releases
  the polypeptide from the ribosome which splits
  back into its two subunits

McGraw Hill Movies.
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