DNA Structure and Function

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DNA Structure and Function
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Griffith

• Griffith showed some heredity material could move
  into live harmless bacteria and make a lethal
  strain

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Griffiths Experiment
Mice injected with live cells of harmless strain
R.
Mice live. No live R cells in their blood.
Heat killed S strain, but releases the killer
genes that the R strain incorporated.
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Virus

• Basically only two parts
• DNA inside
• Protein Coat outside
• Carries genetic material in which part?

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genetic material
bacterial cell wall
plasma membrane
viral coat
sheath
base plate
tail fiber
cytoplasm
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Hershey-Chase

• Experiment with viruses showed that the genetic
  information was in DNA, not protein.

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virus particle labeled with 35S
virus particle labeled with 32P
bacterial cell
Hershey and Chase showed DNA carries genetic
information
label inside cell
label outside cell
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The Hershey-Chase experiment phages
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Fig. 9.5a
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Fig. 9.5bc
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Fig. 9.6a
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Fig. 9.6b
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Watson and Crick
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Rosalind Franklins X-ray Crystallography
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DNA

• Deoxyribonucleic Acid DNA
• Made up of nucleotides
• Nucleotides have three parts
• Sugar
• Phosphate group
• Nitrogenous base
• Sugar-phosphates make the DNA back bone that is
  covalently bonded

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adenine (A) base with a double-ring structure
guanine (G) base with a double-ring structure
phosphate group
sugar (ribose)
thymine (T) base with a single-ring structure
cytosine (C) base with a single-ring structure
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Nitrogenous bases

• Four different nitrogenous bases
• Have one or two rings
• Form 2 or 3 hydrogen bonds
• Bases can only pair one way
• A-T
• C-G
• The sequence of nitrogenous bases carries the
  genetic information

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or
or
one base pair
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DNA Structure

• Forms a double helix
• Two complementary strands held together by
  hydrogen bonds

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Fig. 9.5a
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Fig. 9.5bc
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Meselson- Stahl

• Heavier isotope falls to bottom of flask
• Timed to capture each new generation of bacteria
• Shows radiation diluted by half each generation,
  didnt stay together.
• Showed semi-conservative replication

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Fig. 9.6a
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Fig. 9.6b
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DNA replication

• Semiconservative one old and one new strand in
  each daughter molecule
• Each original strand acts as a template to form a
  new complementary strand

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DNA Replication
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Three enzymes

• Helicase unwinds DNA
• DNA Polymerase adds new nucleotides off the
  template
• Works in one direction only
• One side makes separate fragments
• Ligase seals up the fragments
• Proofreads DNA, fixes mistakes

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Three Enzymes
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Helicase Unwinds helix
Polymerase adds nucleotides Ligase Seals
fragments
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continuous assembly on one strand
discontinuous assembly on other strand
newly forming DNA strand
one parent DNA strand
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DNA Replication

• Starts in several spots
• Pretty rapid process.
• Very accurate, few errors

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Chromosomes

• DNA Replication forms the sister chromatids just
  before Mitosis or meiosis

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Fig. 9.10
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Mutations

• When cells are dividing, the DNA strands are
  apart.
• A change in the DNA has no complementary strand
  to fix it.
• These changes get incorporated into new strand
• They are passed on in all the new divisions.
• Dividing cells collect mutations, can become
  cancerous
• Skin, lungs, liver

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Transcription
Translation
protein
DNA
RNA
nucleus
cytoplasm

• DNA to RNA
• Copies only select genes, not all at once
• Each gene is on only one strand of DNA, not the
  complimentary strand

• RNA to Protein
• In cytoplasm
• Uses ribosome
• Can make multiple copies
• Relatively short lived

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RNA

• Always a single strand
• Use Ribose as a sugar
• Uses Uracil
• and Adenine, Cytosine, Guanine
• mRNA carries genetic info. From nucleus to
  cytoplasm
• tRNA carries amino acids to ribosome, links the
  genetic code
• rRNA makes up most of ribosome

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URACIL (U) base with a single-ring structure
phosphate group
sugar (ribose)
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DNA ? RNA ? protein
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Chromosome during transcription
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Transcription

• At Initiation RNA polymerase binds start of gene
  and uncoils DNA.
• At Elongation RNA polymerase moves along the gene
  briefly binding nucleotides to DNA (only about 10
  nucleotides at a time), as the RNA nucleotides
  join together in a making a single complimentary
  strand
• At Termination the mRNA moves out of nucleus,
  detaches and DNA recoils

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RNA polymerase
DNA
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transcribed DNA winds up again
DNA to be transcribed unwinds
newly forming RNA transcript
DNA template at the assembly site
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Fig. 9.11
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growing RNA transcript
3
5
3
5
direction of transcription
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3
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m RNA modification

• new pre-mRNA includes extra nucleotides called
  introns must be cut out.
• The exons remain to go on to the cytoplasm
  carrying the information for the protein
  synthesis.

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Fig. 9.17
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Translation

• mRNA code directs sequence of amino acids in
  protein.
• Uses ribosomes to assemble proteins
• At Initiation a tRNA attaches to the mRNA and the
  ribosome subunits combine.
• Start codon is AUG
• At Elongation the ribosome moves down the mRNA
  assembling the amino acids
• Only 6 nucleotides at at time
• Each triplet codes for one amino acid
• At Termination a stop codon causes the protein
  chain and the ribosome and mRNA to separate from
  each other.

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base sequence of gene region
mRNA
amino acids
arginine
glycine
tyrosine
tyrosine
tryptophan
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Genetic Code uses triplets of Nucleotides to
place amino acids in sequence
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Fig. 9.13
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Fig. 9.14
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Fig. 9.15
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Fig. 9.16
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Mutations

• a Point Mutation is a single base pair nucleotide
  substitution
• May cause a single amino acid change, or none
• Insertions and Deletions (adding or removing
  nucleotides) reset the reading frame and change
  subsequent amino acids.
• Missense makes a new amino acid chains
• Nonsense adds stop codons and synthesis cuts off.

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Fig. 9.23
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original base triplet in a DNA strand
a base substitution within the triplet (red)
During replication, proofreading enzymes make a
substitution
possible outcomes
or
original, unmutated sequence
a gene mutation
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Mutations
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mRNA
parental DNA
amino acids
arginine
glycine
tyrosine
tryptophan
asparagine
altered mRNA
DNA with base insertion
altered amino- acid sequence
arginine
glycine
leucine
glutamate
leucine
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Polyribosomes make multiple copies of the
protein at the same time on the same mRNA
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Fig. 9.18
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Transcription
mRNA
rRNA
tRNA
protein subunits
mRNA transcripts
ribosomal subunits
tRNA
Translation
amino acids, tRNAs, ribosomal subunits
Protein
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From DNA to protein