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

Corn showing effects of transposable elements Introduction Transposition - Movement of a chromosome piece to another chromosome location Transposable elements (jumping genes) - Genes or small DNA fragments that can move to a new location Can disrupt the function of a gene or restore original function of a gene Used as tool to research function of a gene Corn showing effects of transposable elements Discovered by Barbara McClintock in 1950’s

Molecular Genetics Structure of DNA Chromosomes composed two types of large molecules: DNA and protein. DNA molecule organized into chain of nucleotides composed of three parts: Nitrogenous base 5-carbon sugar (deoxyribose) Phosphate group Four types of DNA nucleotides, each with unique nitrogenous base Two purines - Molecular structure of two linked rings Adenine (A) and Guanine (G) Two pyrimidines - Molecular structure of a single ring Cytosine (C) and Thymine (T)

Molecular Genetics Structure of DNA Nucleotides bonded to each other forming a ladder twisted into a helix. Sides composed of alternating sugar and phosphate groups. Hydrogen 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

Molecular Genetics DNA Functions Storage of Genetic Information Genetic information in DNA molecule resides in sequence of nucleotides. Gene - Segment of DNA that directs protein synthesis Protein used by cell as structural or storage material or may act as an enzyme influencing cell activities. Portion of DNA molecule Genome - Sum total of DNA in an organism’s chromosomes.

Molecular Genetics DNA Functions Replication (Duplication) of Information Occurs during S phase of cell cycle Strands of double helix unzip. Single strands are 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 one strand from original molecule and another built using that parental strand as a template = semi-conservative replication.

Molecular Genetics DNA Functions Replication

Molecular Genetics DNA Functions Expression of Information Different subsets of genetic information read in different cell types. Cell’s environment can influence set of genes expressed. Expression requires two 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

Molecular Genetics DNA Functions Transcription Three different 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, using complimentary base pairing. Only portions of the genome transcribed. Remainder is noncoding DNA.

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

Molecular Genetics DNA Functions Transcription Chromosomes contain genes for building tRNA. 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 anticodon loop. Anticodon - Sequence of three amino acids that recognize and pair with codon on mRNA Genes for rRNA also transcribed in nucleus. Used to construct ribosomes which act as workbenches and assist with assembly of proteins during translation

Molecular Genetics DNA Functions Translation mRNA transcripts code for proteins. Genetic code based on codons Codons = three nucleotides 64 possible combinations that code for 20 amino acids Order of nucleotides on mRNA determines sequence of amino acids during translation. Genetic code universal - In bacteria, protists, fungi, plants and animals

Molecular Genetics DNA Functions Translation Anticodon of tRNA binds to mRNA codon. Start of translation signaled by a ribosome in cytoplasm binding to mRNA. Codon AUG sets reading frame.

Molecular Genetics DNA Functions Central Dogma of Molecular Genetics

Molecular Genetics DNA Functions Mutation - Change in DNA sequence Mutagens - Agents that alter DNA sequences Ultraviolet light Ionizing radiation Certain chemicals DNA repair enzymes can often find and correct damage. Somatic mutation - Occurs in body cell Germ-line mutation - Occurs in tissues that will produce sex cells Passed on to future generations All genetic variability due to mutations.

Cytogenetics Cytogenetics - Study of chromosome behavior and structure from a 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.

Cytogenetics Changes in Chromosome Number Mistakes during chromosome pairing and separation can result in gametes carrying extra or missing chromosomes. Aneuploid - Carries one or more extra chromosome(s), or is missing one or more chromosome(s) Polyploid - Has at least one complete extra set of chromosomes Meiosis fails to halve chromosome number, resulting in 2n gametes. Fusion of gametes results in polyploid. Often larger or have higher yield Cotton, potato, peanuts, wheat, oats, strawberry, sugar cane

Mendelian Genetics Gregor Mendel crossed tall and short pea plants (1860’s). Parental generation (P) All offspring were tall. First filial generation (F1) - Offspring of parental generation Crossing offspring yielded ratio of three tall individuals to one short individual. Second filial generation (F2) - Offspring of F1 plants

Two generations of offspring Mendelian Genetics Two generations of offspring

Mendelian Genetics Law of unit characters Law of dominance Factors (alleles), which always occur in pairs, control the inheritance of various characteristics. Genes are always at the same position (locus) on homologous chromosomes. Law of dominance For any given pair of alleles, one (dominant) may mask the expression of the other (recessive). Phenotype - Organism’s physical appearance Genotype - Genetic information responsible for contributing to phenotype Homozygous - Both alleles identical. Heterozygous - Alleles are contrasting.

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

Mendelian Genetics Dihybrid cross - Start with parents differing in two traits. Law of independent assortment Factors (genes) controlling two 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 Punnett square used to determine genotypes of zygotes. Dihybrid cross produces 9:3:3:1 phenotypic ratio.

Mendelian Genetics Dihybrid cross

Mendelian Genetics Backcross - A cross between a hybrid and one of its parents Can be used to test inheritance theory Expect phenotypic ratio of 1:1. Testcross - Cross between a plant having a dominant phenotype with a homozygous recessive plant Will determine whether plant with dominant phenotype is homozygous or heterozygous Incomplete dominance (absence of dominance) Heterozygote is intermediate in phenotype to the two homozygotes.

Mendelian Genetics Interaction Among Genes - More than one 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 a mutant form. Cannot catalyze reaction and does not produce functional product

Quantitative Traits Quantitative traits exhibit range of phenotypes rather than discrete phenotypes as 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 that influence trait and behave like Mendelian genes.

Extranuclear DNA Entranuclear DNA - In mitochondria and chloroplasts Endosymbiont hypothesis Mitochondria and chloroplasts were free-living bacteria. Established a 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.

Linkage and Mapping Linked genes - Genes together on a chromosome Closer genes are to one another, more likely to be inherited together Each gene has a specific location (locus) on a chromosome. Crossing-over more likely between two genes located far apart on chromosome than between two genes located closer together. Recombinant types - Offspring in which crossing over has occurred Crossing over frequency used to construct genetic map of chromosomes. 1 map unit = 1% crossing over between pair of genes DNA sequence information used to explore gene function in other species.

The Hardy-Weinberg Law Hardy-Weinberg law - Proportions of dominant alleles to recessive alleles in a large, random mating population will remain same from generation to generation in the absence of forces that change those proportions. Forces that can change proportions of dominant to recessive alleles: Small populations - Random loss of alleles can occur if individuals do not mate as often. Selection - Most significant cause of exception to H-W

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