DNA, RNA, and Protein Synthesis by q3f0yOUB

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									DNA, RNA, and
Protein Synthesis
I. Griffith’s 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
Experiment 1
Experiment 2
Experiment 3
Experiment 4
Their conclusion:

  Heat-killed virulent bacterial cells release
   hereditary factor
  Transfers disease-causing ability to live
   harmless cells
  In short, transformation occurred
How does transformation
occur?


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II. Avery’s Experiments

  Oswald Avery (1940s)
    Was protein, RNA, or DNA the transforming
     agent?
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
   DNA destroyed S cells + live R cells
 Injected mice with mixtures
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
III. Hershey-Chase
Experiment
 Martha & Alfred Hershey (1952)
 Do viruses transfer DNA or protein when
  they enter a bacterium?
 Bacteriophage
   Virus that infect bacteria
Conclusion

  All of the viral DNA and little of the
   protein entered E. coli cells
  DNA is the hereditary molecule in viruses
                       Review
1. T or F. Griffith’s experiments showed that harmless
bacteria could turn virulent when mixed with heat-killed
bacteria that cause disease.

2. T or F. Avery’s 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.
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 Franklin’s x-ray diffraction
     photograph to develop the model
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)
Complementary bases

 Erwin Chargaff (1949)
     Base-pairing rule
     Complementary base pairs
     Purine (double-ring) + pyrimidine (single-ring)
     Cytosine – Guanine
     Adenine – Thymine
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
                   Review
1. What are the three parts of a nucleotide?

2. State the base-pairing rules in DNA.
3. Distinguish between purines and pyrimidines.

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

  Process by which DNA is copied in a cell
   before a cell divides by mitosis, meiosis,
   or binary fission
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)
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
 Eukaryotic cells
   Chromosomes are
    long, not circular
   Replication begins at
    many points or origins
    along the DNA
   Two replication forks
    move in opposite
    directions
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
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
RNA Structure and
Function
   Contains ribose sugar
   Contains Uracil (not Thymine)
   Single-stranded
   Shorter than DNA
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
Types of RNA

  Ribosomal (rRNA)
  Part of the structure of the ribosome
Types of RNA

  Transfer (tRNA)
  Transfers the amino
   acids to the ribosome to
   make a protein
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
Steps of transcription

  RNA polymerase binds to the gene’s
   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
Transcription
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
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
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
Translation
 Prokaryotes
   Since they have no nucleus, translation can
    begin even before transcription of the mRNA
    has finished
 Eukaryotes
   Translation occurs only after transcription
The Human Genome

								
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