Features of the genetic code:

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• Features of the genetic code
• Triplet codons (total 64 codons)
• Nonoverlapping
• Three stop or nonsense codons UAA (ocher), UAG
  (amber) and UGA (opal)
• Degenerate
• Open reading frame starting at the initiation
  codon (AUG)
• Each codon has 5 base and a 3 base e.g.
  5CGU3
• Mutations that modify the genetic code are of 3
  types frameshift (include deletions and
  insertions), missense (lead to an amino acid
  replacement) and nonsense (mutation that
  generates any of the three stop codons leading a
  a premature truncation of the polypeptide.

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• TRANSCRIPTION
• General features
• RNA polymerase (one kind in prokaryote
  containing five subunits and several kinds in
  eukaryotes where each transcribes a different
  type of genes)
• Promoter sequences immediately upstream of the
  gene and contain sequences that have high
  affinity to bind RNA polymerase
• Synthesis of RNA is in the 5 to 3 direction.
  At the transcription bubble, RNA forms a duplex
  with the template DNA strand where A is
  complementary to U and C is complementary to G.
• Transcription is terminated at the the 3 end of
  the gene at sequences know as terminators which
  direct RNA polymerase to stop synthesis.

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• Processing of RNA transcript in eukaryotes
• The immediate RNA transcript is known as primary
  transcript and is subject to three modifications
  prior to moving to the cytoplasm
• A capping enzyme adds a G to the first
  nucleotide in the transcript in the unusual 5-5
  direction (phosphate to phosphate bond). Then a
  methyl thransferase adds methyl groups (-CH3) to
  the G and one or more of the first few bases of
  the RNA transcript. Capping and methylation is
  believed to be critical for efficient
  translation.
• Addition of a poly A tail (100-200 As) at the 3
  end of the primary transcript by a
  poly-A-polymerase. Tailing is believed to
  stabilize mRNA so it remains for longer time to
  be translated to many polypeptide molecules
  before it is degraded and increase the efficiency
  of the initial steps of translation.
• RNA splicing and removal of introns from the
  primary transcript followed by a very precise
  joining of the exons.

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• Mechanism of splicing
• Within each intron three sequences are present
  splice donors (at the 5 end of the intron),
  splice acceptors (at the 3 end of the intron)
  and the branch sites (sandwiched between the
  splice donors and acceptors).
• Splicesome is needed to identify and catalyze
  the sequence of events leading to removal of the
  intron and rejoining of the two successive exons.
  The splicesome consists of snRNP (snRNA 100-300
  nucleotides long proteins). Each splicesome is
  composed of four snRNPs together and each snRNP
  is five snRNA plus about 50 proteins. Some
  snRNAs base pair with the splice donor and
  acceptor in the primary transcript and this way
  can bring them together.
• Alternative splicing (as in the gene encoding
  the antibody heavy chain) and trans-splicing (as
  in C. elegans) are means of controlling gene
  expression in eukaryotes.

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