Molecular Genetics: DNA Structure and the Genetic Code

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Molecular Genetics DNA Structure and the Genetic
Code
Textbook Readings Ch. 16-background only Ch.
17-most important Ch. 19 Sec. 19.1 also
important Ch. 18 and 20-not yet Ch. 15 (reminder)
Secs. 15.3-15.5
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Molecular Genetics DNA

• Four roles for DNA
• Information storage
• Replication (reproduction)
• Day to day instructions to the cell
• Mutation

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Molecular Genetics DNA

• Consists of two strands of nucleotides.
• Nucleotides bonded together in each chain with
  phosphodiester bonds.
• Hydrogen bonds hold the two chains to one another
• Adenine hydrogen bonds with Thymine.
• Cytosine hydrogen bonds with Guanine.
• Antiparallel
• Structure is compatible with four roles of
  DNA-but there is one major problem-length of a
  DNA molecule

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DNA from a bacterial nucleoid illustrates the
major problem with DNA
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Eukaryotes have a more difficult job to do
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Molecular Genetics Folded fiber model of
chromatin structure
Eukaryotic DNA must be highly folded to fit in
the nucleus.
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Molecular Genetics-information storage role for
DNA

• The sequence of bases carries a coded message
• Each base is like a letter in a book
• The letters are organized into words called
  codons-3 bases long
• The codons are arranged in sentences or messages
  called genes
• Each gene carries the information for a final
  product that is either a protein or an RNA
  molecule

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Molecular Genetics-information storage role for
DNA

• DNA does not carry out its own instructions.
• It relies on RNA or protein to accomplish tasks
  for the cell.
• DNA makes RNA (transcription)
• RNA makes protein (translation)

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Molecular Genetics overview of transcription and
translation
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Molecular Genetics-summary of transcription

• Catalyzed by enzymes (not shown)
• Produces different types of RNA (rRNA, mRNA, tRNA
  are the 3 main types)
• Some of the RNAs go on to produce protein
• Others have different jobs

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Molecular Genetics-codons

• mRNA contains a chain of successive codons
• It has start and stop signals in the chain
• Also has extra nucleotides at the end help to
  regulate its activity
• Single- not double-stranded

A cartoon of an mRNA showing codons but not
regulatory nucleotides
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Molecular Genetics-codon dictionary
Each codon corresponds to an amino acid But
there are more codons (64) than amino acids
(20)
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Molecular Genetics-codon dictionary properties

• Degenerate (more than 1 codon per AA)
• Unambiguous (each codon always corresponds to the
  same amino acids)
• Has start (MET) and stop (nonsense) codons
• Universal-almost
• The reading frame determines the sense of the
  message
• Most important

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Molecular Genetics-codon properties

• Because each codon has 3 bases
• There are 3 ways to read each sequence
• 3 reading frames
• Different messages

One sequence-3 reading frames with different
messages, 3 with premature UAG stop
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Molecular Genetics-codon definitions

• Open Reading Frame (ORF) start codon to stop
  codon
• Initiator codon AUG (methionine, MET)
• Termination codon UAA, UGA, UAG

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Molecular Genetics-information storage role for
DNA-summary

• The sequence of bases carries a coded message
• But the molecule that carries the messages has to
  be very long-much longer than the cell.
  Solution???
• The bases are organized into words called
  codons-3 bases long and the messages contain
  start and stop codons.
• This leads to additional problems.