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Genetics

Genetics. Chapter 13. Outline. Molecular Genetics Structure of DNA DNA Functions Cytogenetics Mendelian Genetics Quantitative Traits Extranuclear DNA Linkage and Mapping The Hardy-Weinberg Law. Molecular Genetics Structure of DNA.

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Genetics

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  1. Genetics Chapter 13

  2. Outline • Molecular Genetics • Structure of DNA • DNA Functions • Cytogenetics • Mendelian Genetics • Quantitative Traits • Extranuclear DNA • Linkage and Mapping • The Hardy-Weinberg Law

  3. Molecular GeneticsStructure of DNA • Chromosomes composed 2 types of large molecules: DNA and protein • DNA molecule organized into chain of nucleotides composed of 3 parts: • Nitrogenous base • 5-carbon sugar (deoxyribose) • Phosphate group • 4 types of DNA nucleotides • 2purines - molecular structure of 2 linked rings • Adenine (A) and Guanine (G) • 2pyrimidines - molecular structure of single ring • Cytosine (C) and Thymine (T)

  4. Molecular GeneticsStructure of DNA • Nucleotides bonded to each other forming ladder twisted into double helix • Sides composed of alternating sugar and phosphate groups • H bonds hold base on one side of helix to another base on other side = rungs of ladder • Purines pair with pyrimidines • G-C • A-T DNA molecule

  5. Molecular GeneticsDNA Functions • Genetic information in DNA molecule due to sequence of nucleotides • Gene -segment of DNAdirecting protein synthesis • Genome -sum total of DNA in organism’s chromosomes Portion of DNA molecule

  6. Molecular GeneticsDNA Functions • Replication (Duplication) of Information • Occurs during S phase of cell cycle • Strands of double helix unzip • Single strands = templates for creation of new double strands • Nucleotides added by DNA polymerase in precise sequence: G-C and A-T • New DNA molecule consists of 1 strand from original molecule and 1 new strand = semi-conservative replication

  7. Molecular GeneticsDNA Functions • Expression of Information • Different subsets of genetic information read in different cell types • Cell’s environment influence set of genes expressed • Expression requires 2 processes: • Transcription -copy of gene message made from DNA template using RNA building blocks • RNA -contains ribose, instead of deoxyribose sugars; single stranded; thymine replaced by uracil • Translation - RNA translated to produce proteins • Occurs in cytoplasm

  8. Molecular GeneticsDNA Functions • Transcription • 3 types of RNA produced: • Messenger RNA (mRNA) -translated to produce proteins • Transfer RNA (tRNA) -machinery for translation • Ribosomal RNA (rRNA) -machinery for translation • RNA synthesis • Nucleotides added to single stranded DNA molecule by RNA polymerase • ~10% of genome contains genes • Remaindernoncoding DNA

  9. Molecular GeneticsDNA Functions • Transcription cont’d. • Promoter region - beginning of every gene signals transcription enzymes to begin copying gene • Terminator DNA sequence - end signals transcription enzymes to fall off • Single-stranded RNA transcript produced • Nonprotein-coding DNA fundamental to control of gene expression

  10. Molecular GeneticsDNA Functions • Transcription cont’d. • Chromosomes contain genes fortRNA • Acts as translator during translation • One end binds to mRNA • Other end binds to specific amino acid • At least one tRNA for each amino acid • Each form of tRNA has specific anticodonloop (= sequence of 3 amino acids that recognize and pair with codon on mRNA) • Genes for rRNAalso transcribed in nucleus • Used to construct ribosomes (= act as workbenches and assist with assembly of proteins during translation)

  11. Molecular GeneticsDNA Functions • Translation • mRNA transcripts code for proteins • Genetic code based on codons • Codons = 3 nucleotides • 64 possible combinations coding for 20 amino acids • Order of nucleotides on mRNA determines sequence of amino acids during translation • Genetic code universal

  12. Molecular GeneticsDNA Functions • Genetic Code

  13. Molecular GeneticsDNA Functions • Translation cont’d. • Anticodon of tRNA binds to mRNA codon • Start of translation signaled by ribosome in cytoplasm binding to mRNA • Codon AUG sets reading frame

  14. Molecular GeneticsDNA Functions • Central Dogma of Molecular Genetics

  15. Molecular GeneticsDNA Functions • Mutation -change in DNA sequence • Mutagens -agents altering DNA sequences • UV light, ionizing radiation, certain chemicals • DNA repair enzymes often find and correct damage • Somatic Mutation -occurs in body cell • Germ-line Mutation -occurs in tissues producing sex cells • Passed on to future generations • All genetic variability due to mutations

  16. Cytogenetics • Cytogenetics -study of chromosome behavior and structure from genetic point of view • Changes in chromosome structure • Inversion - chromosomal piece breaks and reinserts in opposite orientation • Inverted regions not rearranged by meiosis and inherited in blocks • Translocation -chromosomal piece breaks off and attaches to another chromosome • Inversion and translocation important in speciation

  17. Cytogenetics • Changes in chromosome # • Nondisjunction results in gametes carrying extra or missing chromosomes • Aneuploid -carries 1 or more extra chromosome(s), or missing 1 or more chromosome(s) • Polyploid - 1 or more complete extra set of chromosomes • Meiosis fails to halve chromosome #, resulting in 2n gametes • Often larger or have higher yield • Cotton, potato, peanuts, wheat, oats

  18. Mendelian Genetics • Gregor Mendel crossed tall and short pea plants (1860’s) • Parental Generation (P) • All offspring tall • 1st Filial Generation (F1) -offspring of parental generation • Crossing offspring yielded ratio of 3 tall : 1 short • 2nd Filial Generation (F2) -offspring of F1 plants

  19. Mendelian Genetics 2 generations of offspring

  20. Mendelian Genetics • Law of Unit Characters - alleles, occurring in pairs, control inheritance of various characteristics • Genes always at same position (locus) on homologous chromosomes • Law of Dominance - for any given pair of alleles, one (dominant) may mask expression of other (recessive) • Phenotype - organism’s physical appearance • Genotype -genetic information responsible for contributing to phenotype • Homozygous -both alleles identical • Heterozygous - alleles contrasting

  21. Mendelian Genetics • Start with cross between 2 true-breeding parents differing for trait • Produces F1generation • Monohybrid Cross - F1 plants intercrossed to produce F2 generation • Results in 1:2:1 genotypic ratio, and 3:1 phenotypic ratio Monohybrid cross

  22. Mendelian Genetics • Dihybrid Cross -start with parents differing in 2 traits • Law of Independent Assortment - genes controlling 2 or more traits segregate independently of each other • Linked Genes -genes on same chromosome • Do not segregate independently • Unlinked Genes -genes on different chromosomes • F1 generation composed of dihybrids • Produces 4 kinds of gametes • Dihybridcross produces 9:3:3:1 phenotypic ratio

  23. Mendelian Genetics Dihybrid cross

  24. Mendelian Genetics • Backcross -cross between hybrid and 1 of its parents • Used to test inheritance theory • Expect phenotypic ratio of 1:1 • Testcross -cross between plant having dominant phenotype with homozygous recessive plant • Determine whether plant with dominant phenotype homozygous or heterozygous • Incomplete Dominance - heterozygote intermediate in phenotype tohomozygotes

  25. Mendelian Genetics • Interaction Among Genes -more than 1 gene controls phenotype • Responsible for production of proteins that are components of biochemical pathways • How Genotype Controls Phenotype • Dominant allele codes for protein that effectively catalyzes reaction, producing phenotype • Recessive allele represents mutant form • Cannot catalyze reaction and does not produce functional product

  26. Quantitative Traits • Quantitative Traits - exhibit range of phenotypes rather than discrete phenotypes studied by Mendel • Include traits like fruit yield and days to flowering • Under identical environments phenotypes differ due to genetic differences • Genetically identical plants produce different phenotypes under different environments • Molecular geneticists identify chromosomal fragments, quantitative trait loci (QTL’s), associated with quantitative traits • QTL’s contain genes influencing trait and behave like Mendeliangenes

  27. Extranuclear DNA • Entranuclear DNA -in mitochondria and chloroplasts • Endosymbiont Hypothesis - mitochondria and chloroplasts were free-living bacteria • Established symbiotic relationship with cells of organisms that evolved into plants • DNA in mitochondria and chloroplasts similar to bacteria DNA • Sperm rarely carry mitochondria and chloroplasts, thus passed to next generation only by female = maternal inheritance

  28. Linkage and Mapping • Linked Genes -genes together on chromosome • Closer genes are to one another, more likely to be inherited together • Each gene has specific location (locus) on chromosome • Crossing-over more likely between 2 genes located far apart on chromosome than between 2 genes located closer together

  29. Linkage and Mapping • Linked Genes cont’d. • Recombinant Types -offspring in which crossing-over has occurred • Crossing-over frequency used to construct genetic mapof chromosomes • 1 map unit = 1% crossing-over between pair of genes • DNA sequence information used to explore gene function in other species

  30. Partial Genetic Map of Pea Plant

  31. The Hardy-Weinberg Law • Hardy-Weinberg Law -proportions of dominant alleles to recessive alleles in large, random mating population remain same from generation to generation in absence of forces that change proportions • Forces that can change proportions of dominant to recessive alleles: • Small populations - random loss of alleles can occur if individuals do not mate often • Selection - most significant cause of exception to H-W

  32. Review • Molecular Genetics • Structure of DNA • DNA Functions • Cytogenetics • Mendelian Genetics • Quantitative Traits • Extranuclear DNA • Linkage and Mapping • The Hardy-Weinberg Law

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