Genetics Chapter 13 Copyright Mc Graw-Hill Companies

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Outline • • • Genetics--> Definition Structure of DNA Functions Transcription Translation Mutation Cytogenetics Mendelian Genetics Quantitative Traits Linkage Stern - Introductory Plant Biology: 9 th Ed. - All Rights Reserved - Mc. Graw Hill Companies

Genetics can be defined as • The biological discipline involving the study of heredity v the study of genes. v the inheritance of physical traits. v the study and inheritance of DNA. v the study and inheritance of chromosomes v all of the above. Focus on the MOLECULAR-->DNA (deoxyribonucleic acid) Stern - Introductory Plant Biology: 9 th Ed. - All Rights Reserved - Mc. Graw Hill Companies

Structure of DNA • • DNA molecule organized into chain of nucleotides composed of three parts: v Nitrogenous Base v 5 -Carbon Sugar (Deoxyribose) v Phosphate Group Nucleotides bonded to each other forming a helix. v Hydrogen bonds hold each base on one side of the helix to another base on the other side. Stern - Introductory Plant Biology: 9 th Ed. - All Rights Reserved - Mc. Graw Hill Companies

• Structure of DNA-->nucleotides Four types of DNA nucleotides: v Purines--Adenine (A) and Guanine (G) v Pyrimidines--Cytosine (C) and Thymine (T) Nitrogenous Base Part Stern - Introductory Plant Biology: 9 th Ed. - All Rights Reserved - Mc. Graw Hill Companies

DNA Functions (ACGT) • Storage of Genetic Information v Gene - Segment of DNA that directs protein synthesis. - Protein may act as an enzyme influencing cell activities. v Genome - Sum total of DNA in an organism’s chromosomes. - Codon - Groups of three nucleotides. (in a row, ex. AAT; AAA; GCT; etc…) Stern - Introductory Plant Biology: 9 th Ed. - All Rights Reserved - Mc. Graw Hill Companies

DNA Functions • Replication of Information v Occurs during S phase of cell cycle. - Strands of double helix unzip allowing DNA polymerase (enzyme) to pair individual nucleotides with the template strands. Ø Semi-Conservative Replication § Occurs simultaneously in both directions, and begins at several points simultaneously. Stern - Introductory Plant Biology: 9 th Ed. - All Rights Reserved - Mc. Graw Hill Companies

DNA Functions • Expression of Information v Different subset of the genetic information is read in each cell type. v Expression requires two processes. - Transcription - Copy of gene message using RNA (intermediate nucleic acid). - Translation Ø Messenger RNA (m. RNA) translated to produce proteins. Stern - Introductory Plant Biology: 9 th Ed. - All Rights Reserved - Mc. Graw Hill Companies

• • Transcription Three different types of RNA produced: v Messenger RNA (m. RNA)-->proteins v Transfer RNA (t. RNA)-->translation v Ribosomal RNA (r. RNA) RNA Synthesis (ACGU nucleotide parts) v Nucleotides added to single stranded DNA molecule by RNA polymerase. - Only portions of the genome replicated. Ø Remainder is noncoding DNA (includes directions for expression!). Stern - Introductory Plant Biology: 9 th Ed. - All Rights Reserved - Mc. Graw Hill Companies

Translation • • m. RNA transcripts are decoded into proteins. v Genetic code based on codons. - 64 possible combinations of ACGT/U) (43) Ø Code for 20 amino acids. t. RNA acts as a translator. v Anticodon binds to m. RNA codon. v Ribosomal RNA (r. RNA) binds to m. RNA and facilitates translation Stern - Introductory Plant Biology: 9 th Ed. - All Rights Reserved - Mc. Graw Hill Companies

Mutation • • Mutations - Changes in DNA sequence. v Mutagens - Agents that alter DNA. - Ultraviolet light - Ionizing radiation (Pasta wheat/Ruby red) v Somatic Mutation - Occurs in body cell. v Germ-Line Mutation - Occurs in tissues that will produce sex cells. - Passed on to future generations. All genetic variability due to mutations. Stern - Introductory Plant Biology: 9 th Ed. - All Rights Reserved - Mc. Graw Hill Companies

• • Cytogenetics is the study of chromosome behavior and structure, from a genetic point of view. Changes in Chromosome Structure v Inversion - Chromosomal piece is broken and reinserted in the opposite orientation. - ABCDE ABDCE v Translocation - Chromosomal piece breaks off and attaches to another chromosome. - Important in speciation (rye) Stern - Introductory Plant Biology: 9 th Ed. - All Rights Reserved - Mc. Graw Hill Companies

Cytogenetics • Changes in Chromosome Number v Mistakes during chromosome pairing and separation can result in gametes carrying extra or missing chromosomes. - Aneuploid - Carries, or is missing, an extra chromosome(s). - Polyploid - Has at least one complete extra set of chromosomes. - Many cultivated plants are polyploid because they are bigger and produce more. Stern - Introductory Plant Biology: 9 th Ed. - All Rights Reserved - Mc. Graw Hill Companies

S. tuberosum Tetraploid Diploid Copyright © Mc. Graw-Hill Companies Permission Required for Reproduction or Display Stern - Introductory Plant Biology: 9 th Ed. - All Rights Reserved - Mc. Graw Hill Companies

Genetics from Mendel to Today • • • Gregor Mendel (Austrian Monk) inheritance of traits (1850 -1865); rediscovered in 1900 s Used peas Watson & Crick model of DNA (1954) Stern - Introductory Plant Biology: 9 th Ed. - All Rights Reserved - Mc. Graw Hill Companies

Mendelian Genetics • Mendel crossed self-pollinated pea plants and recorded his observations. v Crossed a tall plant with a short plant. v Parental Generation (P) - All offspring were tall. - First Filial Generation (F 1) Ø Cross offspring yielded a ratio of three tall individuals to one short individual. Ø Second Filial Generation (F 2) Stern - Introductory Plant Biology: 9 th Ed. - All Rights Reserved - Mc. Graw Hill Companies

Mendelian Genetics • • Law of Unit Characters v 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 v For any given pair of alleles, one (Dominant) may mask the expression of the other (recessive). Stern - Introductory Plant Biology: 9 th Ed. - All Rights Reserved - Mc. Graw Hill Companies

Mendelian Genetics • • Phenotype refers to an organism’s physical appearance. Genotype refers to the genetic information responsible for contributing to the phenotype. v Homozygous - Both alleles identical. v Heterozygous - Alleles are contrasting. Stern - Introductory Plant Biology: 9 th Ed. - All Rights Reserved - Mc. Graw Hill Companies

• Monohybrid Cross (one trait) A cross is made between two true-breeding parents differing for a single trait, producing an F 1 generation. RR X rr v These plants are intercrossed to produce an F 2 generation. R r Rr Rr R RR Rr rr Stern - Introductory Plant Biology: 9 th Ed. - All Rights Reserved - Mc. Graw Hill Companies

Mendelian Inheritance Pattern • The phenotype of the F 1 resembled one parental form. • All F 1 s looked the same. • The “hidden” parental form reappeared in the F 2 generation. Stern - Introductory Plant Biology: 9 th Ed. - All Rights Reserved - Mc. Graw Hill Companies

Dihybrid Cross • Cross made for parents differing in two traits. v Mendel’s Law of Independent Assortment - Factors (genes) controlling two or more traits segregate independently of each other. Ø Linked genes do not segregate independently. - Punnett Square used to determine genotypes of the zygotes. Stern - Introductory Plant Biology: 9 th Ed. - All Rights Reserved - Mc. Graw Hill Companies

• Dihybrid Cross Parents differing in 2 unlinked genetic traits Green pod, Tall plant (GGTT) X yellow pod, dwarf plant (ggtt) Stern - Introductory Plant Biology: 9 th Ed. - All Rights Reserved - Mc. Graw Hill Companies

Mendelian Genetics • • Backcross v A cross between a hybrid and one of its parents. - Can be used to test inheritance theory. Incomplete Dominance v Some genes exhibit traits in which a heterozygote is intermediate in phenotype to the two homozygotes. Stern - Introductory Plant Biology: 9 th Ed. - All Rights Reserved - Mc. Graw Hill Companies

Quantitative Traits • Quantitative traits exhibit a range of phenotypes rather than discrete phenotypes studied by Mendel. v Molecular geneticists are able to identify chromosomal fragments, quantitative trait loci, associated with quantitative traits. Stern - Introductory Plant Biology: 9 th Ed. - All Rights Reserved - Mc. Graw Hill Companies

Inheritance of Corn height (Quantitative) • • Cross Tall corn (6 m) x short corn (2 m) F 2 are not 3: 1 tall to short but all heights between 1. 5 m and 8 m # of plants P 1 P 2 Plant height Stern - Introductory Plant Biology: 9 th Ed. - All Rights Reserved - Mc. Graw Hill Companies

Linkage • • • Occurs with 2 traits - often located close together on chromosome Often segregate like a single trait No recombination occurs Get a shortage of unique gamete genotypes in F 1 generation Get fewer genotypes in F 2 generation Stern - Introductory Plant Biology: 9 th Ed. - All Rights Reserved - Mc. Graw Hill Companies

Linkage and Mapping • Genes that are together on a chromosome are said to be linked. v Each gene has a specific location (locus) on a chromosome. - Crossing-over should be more likely between two genes located far apart on a chromosome that between two genes located closer together. Ø Crossing over frequency can be used to construct a genetic map. § 1 map unit = 1% crossing over. Stern - Introductory Plant Biology: 9 th Ed. - All Rights Reserved - Mc. Graw Hill Companies

Review • • • Structure of DNA Functions Transcription Translation Mutation Cytogenetics Mendelian Genetics Quantitative Traits Linkage Stern - Introductory Plant Biology: 9 th Ed. - All Rights Reserved - Mc. Graw Hill Companies

Extranuclear DNA • Entranuclear DNA is found in both mitochondria and chloroplasts. v Endosymbiont Hypothesis theorized mitochondria and chloroplasts were freeliving, and at some point became incorporated into cells of organisms that evolved into plants and established a symbiotic relationship. Stern - Introductory Plant Biology: 9 th Ed. - All Rights Reserved - Mc. Graw Hill Companies

Transcription • • • Promoter Region signals transcription enzymes to attach to the DNA. v Single-stranded RNA transcript produced. Chromosomes contain genes for building t. RNA. v Each form of t. RNA has a specific anticodon loop. - Base pairs with codon. Genes for r. RNA also transcribed in the nucleus - Used to construct ribosomes. Stern - Introductory Plant Biology: 9 th Ed. - All Rights Reserved - Mc. Graw Hill Companies

Hardy-Weinberg Law • Hardy-Weinberg states the proportions of dominant alleles to recessive alleles in a normally interbreeding population will remain the same from generation to generation in the absence of external forces. Stern - Introductory Plant Biology: 9 th Ed. - All Rights Reserved - Mc. Graw Hill Companies