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Chapter 12 Molecular Genetics

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Chapter 12 Molecular Genetics 12.1 DNA: The Genetic Material Discovery of the Genetic Material Chromosomes are about 50% nucleic acid and 50% protein, which is the ... – PowerPoint PPT presentation

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Title: Chapter 12 Molecular Genetics


1
Chapter 12 Molecular Genetics
  • 12.1 DNA The Genetic Material

2
Discovery of the Genetic Material
  • Chromosomes are about 50 nucleic acid and 50
    protein, which is the genetic material?
  • Most scientists thought that protein was the
    genetic material because protein is more complex
  • Griffith performed the first major experiment
    that led to the discovery of DNA as the genetic
    material

3
Discovery of the Genetic Material
4
Discovery of the Genetic Material
  • Significance of Griffiths work (1928)
  • One strain of bacteria transformed into another
    strain
  • Did not identify what the transforming substance
    was

5
Discovery of the Genetic Material
  • 1944 Oswald Avery identified that DNA was the
    transforming substance in Griffiths experiments
  • Most leading scientists did not believe him

6
Discovery of the Genetic Material
  • 1952 Hershey and Chase used radioactively labeled
    DNA and radioactively labeled protein and proved
    that DNA is the genetic material

7
DNA Structure
  • DNA is made of subunits called nucleotides
  • Three parts to a DNA nucleotide
  • Sugar
  • Phosphate
  • Nitrogen Base

NUCLEOTIDE
8
DNA Structure
  • Four Different Nitrogen Bases
  • Purine (two rings)
  • Adenine
  • Guanine
  • Pyrimidine (one ring)
  • Cytosine
  • Thymine
  • Uracil (not found in DNA)

9
DNA Structure Chargaffs Rule
  • Chargaff determined in 1950 that the amount of
    adenine equals the amount of thymine and the
    amount of guanine equals the amount of cytosine
  • Chargaffs Rule AT and CG

10
DNA Structure X Ray Diffraction
  • Rosalind Franklins (1951) famous photo of X ray
    diffraction of DNA

11
DNA Structure Double Helix
  • IN 1953 Watson and Crick astounded the scientific
    community with their announcement of DNAs
    structure

12
DNA Structure Double Helix
  • Watson and Crick, using Franklins photo,
    determined that DNA is a double helix with
  • Outside strands of alternating sugar and
    phosphate
  • C bonds with G with three hydrogen bonds
  • A bonds with T with two hydrogen bonds

13
DNA Structure Double Helix
  • DNA called a twisted ladder
  • Sugar is deoxyribose in the upright rails of the
    ladder alternating with phosphate (spacers)
  • Rungs of the ladder have a purine base H-bonded
    to a pyrimidine base

14
DNA Structure Double Helix
  • DNA strands are antiparellel (one strand right
    side up and other stand upside down)
  • Stands named by their Carbon orientation, C-5
    (5) or C-3 (3)

15
Chromosome Structure
  • An average sized chromosome would be 5 cm long if
    the DNA were stretched out
  • DNA is packaged to be condensed in the cells
    nuclues

16
Chapter 12 Molecular Genetics
  • 12.2 Replication of DNA

17
Semiconservative Replication
  • DNA original strand untwists
  • New base pairs bond to open existing strands
    following base paring rules (AT, CG)
  • New strands twist each new helix is half new
    half original

18
Enzymes Control DNA Replication
  • Untwisting by DNA helicase
  • Strands kept apart by single-stranded binding
    proteins
  • Add starter RNA segment by RNA primase
  • Add new nucleotides by DNA polymerase
  • This is only the highlights there are many other
    enzymes involved

19
DNA Replication
  • Because DNA is antiparallel and new nucleotides
    can only be added to the 3 end, each strand
    replicates slightly differently

20
DNA Replication
  • Leading strand replicates by continuous addition
    of nucleotides to the 3 end
  • Lagging strand replicates by producing short DNA
    sections called Okazaki fragments
  • Enzyme ligase glues the fragments together

21
Comparing DNA Replication in Eukaryotes and
Prokaryotes
  • Eukaryotes have multiple areas of DNA replication
    along one chromosome
  • Prokaryotes have one circular chromosome and have
    only one origin of replication

22
Chapter 12 Molecular Genetics
  • 12.3 DNA, RNA, and Protein

23
Central Dogma
  • How does the information in DNA, located in the
    nucleus, allow for the production proteins in the
    cytoplasm?
  • RNA is another form of nucleic acid that relays
    the information.

24
RNA versus DNA
  • RNA
  • Single helix
  • Ribose sugar
  • Bases adenine, guanine, cytosine, and uracil
  • Several types of RNA
  • DNA
  • Double helix
  • Deoxyribose sugar
  • Bases adenine, guanine, cytosine, and thymine
  • One type of DNA

25
RNA versus DNA
26
Types of RNA
  • Messenger RNA (mRNA) long strands (hundreds of
    nucleotides) that are formed complementary to
    DNA leave the nucleus to carry information to
    the cytoplasm
  • Transfer RNA (tRNA) short (80-100 nucleotides)
    T-shaped RNA that transport amino acids
  • Ribosomal RNA (rRNA) along with protein make up
    the ribosomes

27
Types of RNA
28
DNA to RNA to Protein
  • Two step process transcription and translation
  • Transcription (rewrite) RNA is made from DNA
    occurs in the nucleus
  • Translation (change language) protein is made
    from RNA code occurs in the cytoplasm at the
    ribosome

29
Transcription
  • A section DNA (ave. size 8000 nucleotides) in the
    nucleus untwists and unzips.
  • RNA nucleotides, following base pairing rules,
    bond on the leading strand of DNA
  • Like DNA replication controlled by many enzymes

Occurs in the nucleus
30
RNA Processing
  • RNA when it is transcribed must be processed
  • GTP cap is added to 5 end to protect and give
    attach signal to ribosome
  • Introns (intervening sequences) are cut out
  • Exons (expressed sequences) are put together
  • Poly-A tail (30-200 A nucleotides) added to 3
    end to protect and get out of nucleus signal

31
Translation Making Protein
  • Starts when mRNA, tRNA carrying amino acids, and
    small and large ribosomal subunits come together
  • Concludes when a polypeptide chain in produced

32
The Code
  • There are 20 amino acids, each is coded for by a
    sequence of 3 nucleotides called a codon.
  • Discovered during the 1960s.

33
The Code
mRNA Genetic Code
  • mRNA has the codon
  • tRNA has the anticodon (complementary to the
    codon)
  • Example mRNA codon AUG would code for the amino
    acid methionine which is also the start codon
  • Redundancy exists more that one codon per amino
    acid (UAU and UAC codes for tyrosine)
  • Ambiguity does not exist UAU only codes for
    tyrosine not any other amino acid.

34
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35
Translation
  1. All the players come together
  2. First tRNA with anticodon UAC carrying methionine
    bonds with mRNA codon AUG at the P-site of the
    ribosome
  3. Second tRNA with anticodon carrying another amino
    acid bonds with complementary mRNA codon at
    A-site of ribosome
  4. Polypeptide bond forms between two amino acids
  5. Ribosome moves down the mRNA so that the first
    tRNA is now in E-site of ribosome (and is
    released)
  6. A-site is now empty to attach the third tRNA
    carrying the third amino acid
  7. Steps 4-7 repeated until mRNA codon for stop is
    signaled, then polypeptide chain released

36
One Gene-One Enzyme
  • The Beadle and Tatum experiment showed that one
    gene codes for one enzyme. We now know that one
    gene codes for one polypeptide.

37
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38
Chapter 12 Molecular Genetics
  • 12.4 Gene Regulation and Mutation

39
Prokaryote Gene Regulation
  • Ability of an organism to control which genes are
    transcribed in response to the environment
  • An operon is a section of DNA that contains the
    genes for the proteins needed for a specific
    metabolic pathway.

40
Lac Operon
41
Try Operon
What would an off Try operon look like?
42
Eukaryote Gene Regulation
  • Controlling transcription transcription factors
    ensure that a gene is used at the right time and
    that proteins are made in the right amounts
  • Promoters stabilize binding of RNA polymerase
  • Regulatory proteins control rate of
    transcription
  • The complex structure of eukaryotic DNA also
    regulates transcription.

43
Eukaryote Gene Regulation
  • Hox genes are responsible for the general body
    pattern of most animals.
  • Hox genes code for transcription factors that are
    active in zones of the embryo that are in the
    same order as the genes on the chromosome

44
Eukaryote Gene Regulation
  • RNA interference can stop the mRNA from
    translating its message.

45
Mutations
  • Mistakes occur in copying the DNA during
    replication.
  • Mechanisms exist for correcting these mistakes
  • If the mistakes are permanent then a mutation
    occurs
  • If a mutation in the DNA occurs, then the protein
    that is made from this DNA instruction can be
    absent or nonfunctional.

46
Mutations
47
Mutations
48
Types of Mutations
49
Chromosomal Mutations
  • Pieces of chromosomes get deleted, duplicated,
    inverted, inserted or translocated
  • Visible on karyotype

50
Chromosomal Mutations
  • Fragile X chromosome is due about 30 extra
    repeated CGG codons near the tip of the X
    chromosome
  • Results in many mental and behavioral symptoms

51
Protein Folding and Stability
  • Incorrect amino acid sequences can lead to
    changes in the shape and thus the function of
    proteins

52
Causes of Mutations
  • Mutagens are agents that cause mutations
  • Spontaneous no know cause wrong nucleotide
  • Happens 1/100,000 base pairs
  • Goes unfixed less than one in one billion

53
Causes of Mutations
  • Chemicals like asbestos, benzene, formaldehyde,
    many agents in cigarette smoke, and many others
  • Affect DNA by changing chemical nature of the
    bases
  • May resemble nucleotides and bond in place of the
    DNA nucleotides preventing DNA replication

54
Causes of Mutations
  • Radiation high energy rays like X rays and gamma
    rays form free radicals (charged escaped
    electrons) that damages DNA
  • UV radiation can cause adjacent thymines to bind
    with each other instead of complementary
    nucleotides causing a kink in the DNA molecule
    which prevents replication

55
Body Cell versus Sex Cell Mutation
Molecular Genetics
  • Somatic cell mutations are not passed on to the
    next generation.
  • Mutations that occur in sex cells are passed on
    to the organisms offspring and will be present
    in every cell of the offspring.
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