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Anatomy and Function of a Gene. Dissection through mutation. Outline of Chapter 7. What mutations are How often mutations occur What events cause mutations How mutations affect survival and evolution Mutations and gene structure

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anatomy and function of a gene

Anatomy and Function of a Gene

Dissection through mutation

outline of chapter 7
Outline of Chapter 7
  • What mutations are
    • How often mutations occur
    • What events cause mutations
    • How mutations affect survival and evolution
  • Mutations and gene structure
    • Experiments using mutations demonstrate a gene is a discrete region of DNA
  • Mutations and gene function
    • Genes encode proteins by directing assembly of amino acids
  • How do genotypes correlate with phenotypes?
    • Phenotype depends on structure and amount of protein
    • Mutations alter genes instructions for producing proteins structure and function, and consequently phenotype
mutations primary tools of genetic analysis
Mutations: Primary tools of genetic analysis
  • Mutations are heritable changes in base sequences that modify the information content of DNA
    • Forward mutation – changes wild-type to different allele
    • Reverse mutation – causes novel mutation to revert back to wild-type (reversion)
classification of mutations by affect on dna molecule
Classification of mutations by affect on DNA molecule
  • Substitution – base is replaced by one of the other three bases
  • Deletion – block of one or more DNA pairs is lost
  • Insertion – block of one or more DNA pairs is added
  • Inversion 180o rotation of piece of DNA
  • Reciprocal translocation – parts of nonhomologous chromosomes change places
  • Chromosomal rearrangements – affect many genes at one time
spontaneous mutations influencing phenotype occur at a very low rate
Spontaneous mutations influencing phenotype occur at a very low rate

Mutation rates from wild-type to recessive alleles for five coat color genes in mice

Fig. 7.3 b

general observations of mutation rates
General observations of mutation rates
  • Mutations affecting phenotype occur very rarely
  • Different genes mutate at different rates
  • Rate of forward mutation is almost always higher than rate of reverse mutation
are mutations spontaneous or induced
Are mutations spontaneous or induced?
  • Most mutations are spontaneous.
  • Luria and Delbruck experiments - a simple way to tell if mutations are spontaneous or if they are induced by a mutagenic agent
interpretation of luria delbruck fluctuation experiment and replica plating
Interpretation of Luria-Delbruck fluctuation experiment and replica plating
  • Bacterial resistance arises from mutations that exist before exposure to bactericide
  • After exposure to bactericide, the bactericide becomes a selective agent killing the nonresistant cells, allowing only the preexisting resistant cells to survive.
  • Mutations do not arise in particular genes as a direct response to environmental change
  • Mutations occur randomly at any time
chemical and physical agents cause mutations
Chemical and Physical agents cause mutations
  • Deamination removes –NH2 group. Can change C to U, inducing a substitution to and A-T base pair after replication
  • Hydrolysis of a purine base, A or G occurs 1000 times an hour in every cell

Fig. 7.6 a,b

slide14
X rays break the DNA backbone
  • UV light produces thymine dimers

Fig. 7.6 c, d

repair enzymes fix errors created by mutation
Repair enzymes fix errors created by mutation

Excision repair enzymes release damaged regions of DNA. Repair is then completed by DNA polymerase and DNA ligase

Fig. 7.7a

mistakes during replication alter genetic information
Mistakes during replication alter genetic information
  • Errors during replication are exceedingly rare, less than once in 109 base pairs
  • Proofreading enzymes correct errors made during replication
    • DNA polymerase has 3’ – 5’ exonuclease activity which recognizes mismatched bases and excises it
    • In bacteria, methyl-directed mismatch repair finds errors on newly synthesized strands and corrects them
slide20
Unequal crossing over creates one homologous chromosome with a duplication and the other with a deletion

7.10 a

transposable elements move around the genome and are not susceptible to excision or mismatch repair
Transposable elements move around the genome and are not susceptible to excision or mismatch repair

Fig. 7.10 e

trinucleotide instability causes mutations
Trinucleotide instability causes mutations
  • FMR-1 genes in unaffected people have fewer than 50 CGG repeats.
  • Unstable premutation alleles have between 50 and 200 repeats.
  • Disease causing alleles have > 200 CGG repeats.

Fig. B(1) Genetics and Society

mutagens induce mutations
Mutagens induce mutations
  • Mutagens can be used to increase mutation rates
  • H. J. Muller – first discovered that X rays increase mutation rate in fruit flies
    • Exposed male Drosophila to large doses of X rays
    • Mated males to females with balancer X chromosome (dominant Bar eyed mutation and multiple inversions)
    • Could assay more than 1000 genes at once on the X chromosome
consequences of mutations
Consequences of mutations
  • Germ line mutations – passed on to next generation and affect the evolution of species
  • Somatic mutations – affect the survival of an individual
    • Cell cycle mutations may lead to cancer
  • Because of potential harmful affects of mutagens to individuals, tests have been developed to identify carcinogens
what mutations tell us about gene structure
What mutations tell us about gene structure
  • Complementation testing tells us whether two mutations are in the same or different genes
  • Benzer’s experiments demonstrate that a gene is a linear sequence of nucleotide pairs that mutate independently and recombine with each other
  • Some regions of chromosomes mutate at a higher rate than others – hot spots
slide34

Fig. 7.15 b,c

Five complementation groups (different genes) for eye color.

Recombination mapping demonstrates distance between genes and alleles.

a gene is a linear sequence of nucleotide pairs
A gene is a linear sequence of nucleotide pairs
  • Seymour Benzer mid 1950s – 1960s
    • If a gene is a linear set of nucleotides, recombination between homologous chromosomes carrying different mutations within the same gene should generate wild-type
    • T4 phage as an experimental system
      • Can examine a large number of progeny to detect rare mutation events
      • Could allow only recombinant phage to proliferate while parental phages died
benzer s experimental procedure
Benzer’s experimental procedure
  • Generated 1612 spontaneous point mutations and some deletions
  • Mapped location of deletions relative to one another using recombination
  • Found approximate location of individual point mutations by deletion mapping
  • Then performed recombination tests between all point mutations known to lie in the same small region of the chromosome
  • Result – fine structure map of the rII gene locus
recombination mapping to identify the location of each point mutation within a small region
Recombination mapping to identify the location of each point mutation within a small region

Fig. 7.18 b

what mutations tell us about gene function
What mutations tell us about gene function
  • One gene, one enzyme hypothesis: a gene contains the information for producing a specific enzyme
    • Beadle and Tatum use auxotrophic and prototrophic strains of Neurospora to test hypothesis
  • Genes specify the identity and order of amino acids in a polypeptide chain
  • The sequence of amino acids in a protein determines its three-dimensional shape and function
  • Some proteins contain more than one polypeptide coded for by different genes
beadle and tatum one gene one enzyme
Beadle and Tatum – One gene, one enzyme
  • 1940s – isolated mutagen-induced mutants that disrupted synthesis of arginine, an amino acid required for Neurospora growth
    • Auxotroph – needs supplement to grow on minimal media
    • Prototroph – wild-type that needs no supplement; can synthesize all required growth factors
  • Recombination analysis located mutations in four distinct regions of genome
  • Complementation tests showed each of four regions correlated with different complementation group (each was a different gene)
interpretation of beadle and tatum experiments
Interpretation of Beadle and Tatum experiments
  • Each gene controls the synthesis of an enzyme involved in catalyzing the conversion of an intermediate into arginine
genes specify the identity and order of amino acids in a polypeptide chain
Genes specify the identity and order of amino acids in a polypeptide chain
  • Proteins are linear polymers of amino acids linked by peptide bonds
    • 20 different amino acids are building blocks of proteins
    • NH2-CHR-COOH – carboxylic acid is acidic, amino group is basic
    • R is the side chain that distinguishes each amino acid

Fig. 7.21 a

slide54

N terminus of a protein contains a free amino group

C terminus of protein contains a free carboxylic acid group

Fig. 7.21 c

genes specify the amino acid sequence of a polypeptide example sickle cell anemia
Genes specify the amino acid sequence of a polypeptide – example, sickle cell anemia

Mutant b chain of hemoglobin form aggregates that cause red blood cells to sickle

Fig. 7.22 a

sequence of amino acids determine a proteins primary secondary and tertiary structure
Sequence of amino acids determine a proteins primary, secondary, and tertiary structure

Fig. 7.23

how do genotypes and phenotypes correlate
How do genotypes and phenotypes correlate?
  • Alteration of amino acid composition of a protein
  • Alteration of the amount of normal protein produced
  • Changes in different amino acids at different positions have different effects
    • Proteins have active sites and sites involved in shape or structure
dominance relations between alleles depend on the relation between protein function and phenotype
Dominance relations between alleles depend on the relation between protein function and phenotype
  • Alleles that produce nonfunctional proteins are usually recessive
    • Null mutations – prevent synthesis of protein or promote synthesis of protein incapable of carrying out any function
    • Hypomorphic mutations – produce much less of a protein or a protein with weak but detectable function; usually detectable only in homozygotes
  • Incomplete dominance – phenotype varies in proportion to amount of protein
    • Hypermorphic mutations – produces more protein or same amount of a more effective protein
    • Dominant negative – produces a subunit of a protein that blocks the activity of other subunits
    • Neomorphic mutations – generate a novel phenotype; example is ectopic expression where protein is produced outside of its normal place or time