Chapter 13 Lecture Outline Genetics Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Chapter 13

Lecture Outline

Genetics

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Outline Introduction

Molecular Genetics• Structure of DNA• DNA Functions

Cytogenetics

Mendelian Genetics

Quantitative Traits

Extranuclear DNA

Linkage and Mapping

The Hardy-Weinberg Law

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 geneo Used as tool to research function

of a gene

Corn showing effects of transposable elements

• Discovered by Barbara McClintock in 1950’s

Molecular GeneticsStructure 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 GeneticsStructure 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-TDNA molecule

Molecular GeneticsDNA 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.

• Genome - Sum total of DNA in an organism’s chromosomes.

Portion of DNA molecule

Molecular GeneticsDNA 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 GeneticsDNA Functions

Replication

Molecular GeneticsDNA 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 blockso RNA - Contains ribose, instead of deoxyribose

sugars; single stranded; thymine replaced by uracil– Translation - RNA translated to produce proteins.

o Occurs in cytoplasm

Molecular GeneticsDNA 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.o Only portions of the genome transcribed.

« Remainder is noncoding DNA.

Molecular GeneticsDNA 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 GeneticsDNA Functions

Transcription• Chromosomes contain genes for building tRNA.

– Acts as translator during translationo One end binds to mRNA.o Other end binds to specific amino acid.o At least one tRNA for each amino acido 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 GeneticsDNA Functions

Translation• mRNA transcripts code for proteins.

– Genetic code based on codonso Codons = three nucleotides

« 64 possible combinations that code for 20 amino acids

o Order of nucleotides on mRNA determines sequence of amino acids during translation.

o Genetic code universal - In bacteria, protists, fungi, plants and animals

Molecular GeneticsDNA 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 GeneticsDNA Functions

Central Dogma of Molecular Genetics

Molecular GeneticsDNA 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

chromosomeso Meiosis fails to halve chromosome number, resulting in

2n gametes.« Fusion of gametes results in polyploid.

o 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

Mendelian GeneticsTwo generations of offspring

Mendelian Genetics Law of unit characters

• 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.o Linked genes - Genes on same chromosome

« Do not segregate independentlyo Unlinked genes - Genes on different chromosomes

• F1 generation composed of dihybrids.– Produces 4 kinds of gametes

o Punnett square used to determine genotypes of zygotes.

– Dihybrid cross produces 9:3:3:1 phenotypic ratio.

Mendelian GeneticsDihybrid 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.o Established a symbiotic relationship with cells of

organisms that evolved into plantso 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.o 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• Structure of DNA• DNA Functions

Cytogenetics

Mendelian Genetics

Quantitative Traits

Extranuclear DNA

Linkage and Mapping

The Hardy-Weinberg Law