Chapter 10: Gene & Genome Objectives

1. Chapter 10: Gene & Genome Objectives • Early genetic principles (Mendel). • Crossing over (recombination) and linkage. • Structure of the DNA double helix. • DNA supercoiling and topoisomerases • Genome complexity and repetitive DNA. • DNA denaturation, renaturation, hybridization • DNA sequence function • Mechanisms for gene duplication • Mobile DNA and consequences of transposition • Restriction Enzymes, RFLPS, Genome Maps Karp/CELL & MOLECULAR BIOLOGY 3E

2. Genetic Principles Genes, Chromosomes, Recombination, and Inheritance Karp/CELL & MOLECULAR BIOLOGY 3E

3. Figure 10.1 Karp/CELL & MOLECULAR BIOLOGY 3E

4. Mendel and Peas • Easily defined characteristics (phenotypes) • Focused on 7 clearly definable traits, height & flower color, • Each trait had 2 alternate & clearly identifiable forms • Inbred (purified) lines Karp/CELL & MOLECULAR BIOLOGY 3E

5. Mendel's conclusions • Characteristics governed by units of inheritance (genes) • Each organism has 2 copies of gene • The two genes (may be same or not - alleles) • Dominant alleles mask recessive alleles • Gametes have only 1 copy of gene • Law of Segregation - alleles separate (segregate) randomly • Law of Independent Assortment – true for 2 or more genes Karp/CELL & MOLECULAR BIOLOGY 3E

6. Discovery of chromosomes (colored bodies, 1888) • Walther Flemming, early 1880s – • During cell division, nuclear material organized into visible threads called chromosomes • Chromosomes appear as doubled structures, split, then doubled before next division Karp/CELL & MOLECULAR BIOLOGY 3E

7. Fertilization and meiosis • Sperm & egg are very different cells • Sperm & egg equally important genetically • Most obvious shared character: nuclei • Theodore Boveri (German) - sea urchin polyspermy • Results in disruptive cell division & early death • Daughter cells receive variable numbers of chromosomes • Each chromosome possesses different qualities • Edouard van Beneden (Belgian, 1883) Karp/CELL & MOLECULAR BIOLOGY 3E

8. Fertilization and meiosis • Edouard van Beneden (Belgian, 1883) • Discovered Ascaris eggs & sperm had 2 chromosomes each • Somatic cells had four chromosomes • August Weismann (German biologist, 1887) • meiosis involved reduction division where chromosome number was cut in half before forming gametes • If not, chromosome number would increase with every succeeding generation Karp/CELL & MOLECULAR BIOLOGY 3E

9. Chromosomes as genetic information • Walter Sutton (Columbia, 1903) • Studied grasshopper sperm formation • Saw 2 types of division by spermatogonia • Mitotic divisions where spermatogonia make more spermatogonia • Meiotic division where spermatogonia differentiate into sperm • 11 homologous chromosome pairs (look alike) & extra (X) • Correlated with Mendel's inheritable pairs of factors • Hypothesized chromosomes have Mendel's factors • First meiotic division separated pair members Karp/CELL & MOLECULAR BIOLOGY 3E

10. Figure 10.3 Karp/CELL & MOLECULAR BIOLOGY 3E

11. Chromosomes as linkage groups • How did Mendel's factors assort independently? • Mendel owed his results to good luck or lack of interest in traits that did not fit his predictions • All Mendel’s traits were unlinked or nearly so, but… • Flower color & pollen shape on same chromosome • Crossing over (more later) Karp/CELL & MOLECULAR BIOLOGY 3E

12. Drosophila Genetics– T.H. Morgan • Fruit flies were ideal for genetics • Short generation time (14 days) • Produce up to 1000 eggs in a lifetime • Small; easy to maintain & breed; inexpensive Karp/CELL & MOLECULAR BIOLOGY 3E

13. Figure 10.5 Karp/CELL & MOLECULAR BIOLOGY 3E

14. Drosophila Genetics– T.H. Morgan • by 1915 he found many mutants (85) • Localized to 4 linkage groups, one with few mutants • On rare occasions, mutation occurred • Mutation + Selection = Evolution • Mechanism for new species to slowly emerge • Linkage groups = chromosome pairs (4), one small Karp/CELL & MOLECULAR BIOLOGY 3E

15. Crossing over & recombination • F. A. Janssens (1909) • observed interaction of homologs • hypothesized breakage & exchange of pieces • Morgan (1911) – this could explain recombination • Crossing over makes linkage incomplete • Can separate genes on same chromosome • Can reshuffle genes • recombination percent constant for given gene pair • but different between different gene pairs Karp/CELL & MOLECULAR BIOLOGY 3E

16. Figure 10.7 Karp/CELL & MOLECULAR BIOLOGY 3E

17. Crossing over & recombination • Therefore position (locus) of genes fixed • Recombination percentage is a measure of distance • Bigger distance means more crossovers • Big distance = independent assortment • Small distance = tightly linked • Alfred Sturtevant (1911) • realized recombination could be used to map • constructed maps of the 4 fruit fly chromosomes • since used to map genes in many organisms Karp/CELL & MOLECULAR BIOLOGY 3E

18. Mutagenesis & giant chromosomes • Mutants valuable for genetic analysis • Herman Muller (Indiana U., 1927) • Genetic material can be damaged by X-rays • Sublethal dose raises mutation frequency >100 fold • Other mutagenic agents: UV irradiation, ENU Karp/CELL & MOLECULAR BIOLOGY 3E

19. Mutagenesis & giant chromosomes • Theophilus Painter (U. of Texas, 1933) • Rediscovers Drosophila polytene chromosomes • Arise in salivary gland cells; Strands stay in register • As much as 1,024 times normal copy number • Cell stops dividing, but keeps growing • Allows cell to maintain high rate of secretion • Exhibit constant banding patterns (~5000 bands) • Bands correlate with specific genes • Visualize Deletion by X-rays • Visualize species specific evolutionary change • Active genes puff out (RNA Synthesis) Karp/CELL & MOLECULAR BIOLOGY 3E

20. Figure 10.8a Karp/CELL & MOLECULAR BIOLOGY 3E