Topic 8 From Gene to Protein

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Topic 8 From Gene to Protein
Biology 1001 October 17, 2005
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IIIA. Details of Transcription

• 5-GCCAGGCTAAACTA-3 DNA coding strand
• 3-CGGTCCGATTTGAT-5 DNA template strand
• 5-GCCAGG-3 RNA synthesis
• 3-CGGTCCGATTTGAT-5
• 5-GCCAGGCUAAACUA-3 messenger RNA (mRNA)
• RNA is similar to DNA except that it is always
  single-stranded, the sugar has a 2-OH group, and
  the nitrogenous bases are A, C, G, Uracil
• Messenger RNA is transcribed or copied from a
  DNA template according to the base pairing rules,
  except that U pairs with A
• The mRNA sequence is the same as the DNA coding
  strand sequence, except that there are Us instead
  of As

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Figure 17.7 -Transcription has three
stages initiation, elongation and termination
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IIIB. Translation the Basic Concept

• Translation is the synthesis of a polypeptide
  under the direction of the mRNA
• The nucleotide sequence of the mRNA is converted
  to the amino acid sequence of a polypeptide
• The site at which this occurs is in the cell is
  the ribosome
• Amino acids are carried to the growing
  polypeptide chain by tRNA molecules

Figure 17.13
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Figure 17.14 The structure of transfer RNA
Figure 17.16 The anatomy of a ribosome
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IIIC. Features of Gene Expression in Eukaryotes

• Transcription occurs in the nucleus, translation
  in the cytoplasm
• Transcription initially results in a pre-mRNA or
  primary transcript which undergoes RNA processing
  before being translated
• The ribosomal subunits (rRNA proteins) are
  assembled in the nucleolus

Figure 17.26
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IV. The Genetic Code

• Four nucleotides need to specify 20 amino acids,
  so the genetic code is a triplet code ? 41 4,
  42 16, 43 64
• A codon is a sequence of three nucleotides that
  specifies a particular amino acid 3 of the 64
  possible codons function as stop signals and one
  (AUG) codes for the amino acid methionine
  functions as a start signal
• The code is read from the mRNA which is
  transcribed from the template strand of DNA
• The code is degenerate it is redundant but not
  ambiguous
• The sequence needs to be read in non-overlapping
  tandem groups of three and in the correct reading
  frame
• The code is nearly universal

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IV. The Genetic Code
Figure 17.4 Figure 17.5
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V. The Effect of Mutation on Protein Function

• Mutation a change in the genetic material
• Point mutation a change in just one nucleotide
  (base) pair
• A substitution of one nucleotide pair for another
  produces silent, missense, or nonsense mutations
• An indel (insertion or deletion of a base pair)
  produces a frameshift mutation
• Mutations are either spontaneous errors during
  DNA replication or caused by mutagens physical
  or chemical agents that interact with and damage
  DNA
• Egs. Physical UV, X-rays
• Egs. Chemical base analogs, intercalating agents

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Figure 17.24 Base-pair substitution
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Review Videos of Transcription and Translation
(These animations are intended to aid your
conceptual understanding. I wont test you on the
content unless I mentioned it elsewhere in Topic
8)