DNA, RNA, and Protein Synthesis

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DNA, RNA, and Protein Synthesis
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I. Griffiths Experiments

• Frederick Griffith (1928)
• British medical officer
• Streptococcus pneumoniae (bacteria causing
  pneumonia in mammals)
• Develop a vaccine against the virulent strain (S
  strain) of bacterium
• R strain is not pathogenic

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Experiment 1
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Experiment 2
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Experiment 3
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Experiment 4
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Their conclusion

• Heat-killed virulent bacterial cells release
  hereditary factor
• Transfers disease-causing ability to live
  harmless cells
• In short, transformation occurred

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II. Averys Experiments

• Oswald Avery (1940s)
• Was protein, RNA, or DNA the transforming agent?

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Steps

• Destroyed agents using enzymes
• Protease proteins
• RNase RNA
• DNase DNA
• Separately mixed experimental batches of
  heat-killed S cell with live R cells
• Protein destroyed S cells live R cells
• RNA destroyed S cells live R cells
• Protein destroyed S cells live R cells
• Injected mice with mixtures

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Conclusion

• Cells missing RNA and protein were able to
  transform R cells
• Cells missing DNA were not able to transform R
  cells
• In short, DNA is responsible for transformation
  in bacteria

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III. Hershey-Chase Experiment

• Martha Alfred Hershey (1952)
• Do viruses transfer DNA or protein when they
  enter a bacterium?
• Bacteriophage
• Virus that infect bacteria

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Conclusion

• All of the viral DNA and little of the protein
  entered E. coli cells
• DNA is the hereditary molecule in viruses

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Review
1. T or F. Griffiths experiments showed that
harmless bacteria could turn virulent when mixed
with heat-killed bacteria that cause disease.
2. T or F. Averys experiments clearly
demonstrated that the genetic material is
composed of DNA.
3. T or F. The experiments of Hershey and Chase
cast doubt on whether DNA was the hereditary
material.
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DNA Double Helix

• James Watson and Francis Crick (1950s)
• DNA is made up of two chains wrapped around each
  other in the shape of a double helix
• Used Rosalind Franklins x-ray diffraction
  photograph to develop the model

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DNA Nucleotides

• Repeating subunits making up the two long chains
  (strands) of DNA
• Three parts
• Five-carbon sugar (deoxyribose)
• Phosphate group (P atoms bonded to 4 O atoms)
• Nitrogenous base (N and C atoms accepts hydrogen
  ions)

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Complementary bases

• Erwin Chargaff (1949)
• Base-pairing rule
• Complementary base pairs
• Purine (double-ring) pyrimidine (single-ring)
• Cytosine Guanine
• Adenine Thymine

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Why base pairing is important in DNA structure

• Hydrogen bonds between base pairs help hold the
  two strands.
• Complementary nature of a DNA molecule replicates
  before a cell divides
• One strand serves as template for the other

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Review
1. What are the three parts of a nucleotide?
2. State the base-pairing rules in DNA.
3. Distinguish between purines and pyrimidines.

• Use the base-pairing rules to determine the base
  sequence that is complementary to the sequence
• C-G-A-T-T-G.

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DNA Replication

• Process by which DNA is copied in a cell before a
  cell divides by mitosis, meiosis, or binary
  fission

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How does replication occur?

• Helicases separate DNA strands by breaking
  hydrogen bonds along the DNA molecule producing a
  replication fork
• DNA polymerases add complementary nucleotides
  floating inside the nucleus.
• DNA polymerases finish replication and fall off
• Results in two separate and identical DNA
  molecules (original strand new strand
  semi-conservative replication)

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Prokaryotic vs Eukaryotic Replication

• Prokaryotic cells
• Circular chromosomes
• Replication begins at one place
• The two resulting replication forks are copied in
  opposite directions
• Replication continues until they meet

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• Eukaryotic cells
• Chromosomes are long, not circular
• Replication begins at many points or origins
  along the DNA
• Two replication forks move in opposite directions

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Errors in Replication

• One error in every billion paired nucleotides
  occur
• DNA polymerases proofread the DNA
• Mutation
• A change in the nucleotide sequence of a DNA
  molecule
• Can have serious effects on the function of genes
  or disrupt cell function
• Some can lead to cancer (tumors)
• Some allow individuals to survive and reproduce
  better

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Protein Synthesis

• Forming proteins based on information in DNA and
  carried out by RNA
• Flow of genetic information
• DNA ? RNA ? Protein
• Transcription DNA is the template for the
  synthesis of RNA
• Translation RNA directs the assembly of
  proteins

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RNA Structure and Function

• Contains ribose sugar
• Contains Uracil (not Thymine)
• Single-stranded
• Shorter than DNA

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Types of RNA

• Messenger RNA (mRNA)
• Single-stranded, carries the instructions from a
  gene to make protein
• In eukaryotes, it carries the genetic message
  from DNA in the nucleus

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Types of RNA

• Ribosomal (rRNA)
• Part of the structure of the ribosome

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Types of RNA

• Transfer (tRNA)
• Transfers the amino acids to the ribosome to make
  a protein

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Transcription

• The process by which the genetic instructions in
  a specific gene are transcribed or rewritten
  into an RNA molecule
• Eukaryotes takes place in nucleus
• Prokaryotes takes place in the DNA containing
  region in the cytoplasm

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Steps of transcription

• RNA polymerase binds to the genes promoter and
  the two DNA strands unwind and separate
• Complementary nucleotides are added and joined
• RNA polymerase reaches a termination signal in
  the DNA, the DNA and new RNA are relesed

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Transcription
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The Genetic Code

• The rules that relate how a sequence of bases in
  nucleotides codes for a particular amino acid
• Codon a three-nucleotide sequence in mRNA that
  encodes an amino acid, start, or stop signal
• 64 mRNa codons
• AUG start codon
• UAA, UAG, UGA stop codons

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Steps in translation

• Initiation
• The ribosomal subunits, the mRNA, and the tRNA
  carrying methionine bind together
• Elongation
• tRNA carrying the amino acid specified by the
  next codon binds to the codon
• Peptide bond forms between amino acids
• Ribosome moves the tRNA and mRNA

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Steps in translation

• Elongation, cont.
• First tRNA detaches and leaves its amino acid
  behind
• Elongation continues and chain grows
• Termination
• The process stops when a stop codon is reached
• Disassembly
• Ribosome complex falls apart and polypeptide is
  released

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Translation
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• Prokaryotes
• Since they have no nucleus, translation can begin
  even before transcription of the mRNA has
  finished
• Eukaryotes
• Translation occurs only after transcription

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The Human Genome