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

3. 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

4. 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)

5. 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

6. 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.

7. 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.

8. Molecular Genetics DNA Functions
Replication

9. 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

10. 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.

11. 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.

12. 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

13. 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

14. 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.

15. Molecular Genetics DNA Functions
Central Dogma of Molecular Genetics

16. 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.

17. 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.

18. 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

19. 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

20. Two generations of offspring
Mendelian Genetics
Two generations of offspring

21. 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.

22. 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

23. 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.

24. Mendelian Genetics
Dihybrid cross

25. 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.

26. 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

27. 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.

28. 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.

29. 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.

30. 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

31. 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