3C2

1. 3C2 Multiple processes that increase genetic variation

2. Sexual Reproduction requires Meiosis

3. Meiosis reduces the number of chromosomes • Eggs and sperm are haploid (1n) cells, which contain one set of all chromosomes, and products of meiotic division

4. Sexual life cycles have both haploid and diploid stages • During fertilization, or syngamy, the fusion of two haploid gametes results in a diploid (2n) zygote, which contains two sets of chromosomes. Meiosis and fertilization constitute a reproductive cycle in sexual organisms as the alternate between diploid and haploid chromosome numbers. Somatic cells divide by mitosis and form the body of the organism

5. Germ-line cells are set aside early in animal development • Cells that eventually will form haploid gametes by meiosis are called germ-line cells. These are set aside early in development in animals

6. Features of Meiosis

7. Homologous chromosomes pair during Meiosis • The pairing of homologous chromosomes, called synapsis, occurs during early Prophase I. Paired homologues are often joined by the synaptonemal complex. During synapsis, crossing over occurs between homologous chromosomes, exchanging chromosomal material. Because the homologues are paired, they move as a unit to the metaphase plate during metaphase I. During anaphase I, homologues of each pair are pulled to opposite poles, producing two cells that each have one complete set of chromosomes

10. Meiosis features two divisions with one round of DNA relication • Meiosis II is like mitosis but without replication of DNA. Sister chromatids are pulled to opposite poles to yield four haploid cells.

11. The Process of Meiosis

12. Prophase I sets the stage for the reductive division. • Meiotic cells have an interphase period similar to mitosis with G1, S, and G2 phases. This is followed by prophase I in which homologous chromosomes align along their entire length. The sister chromatids are held together by cohesin proteins. Homologues exchange chromosomal material by crossing over, which assists in holding the homologues together during meiosis I. The nuclear envelope disperses and the spindle apparatus forms.

13. During Metaphase I, paired homologues align • Spindle fibers attach to the kinetochores of the homologues, the kinetochores of sister chromatids behave as a single unit. Homologues of each pair become attached by kinetochore microtubules to opposite poles, and homologous pairs move to the metaphase plate as a unit. The orientation of each homologous pair on the equator is random; either the maternal or paternal homologue may be oriented toward a given pole

14. Anaphase I results from the differential loss of sister chromatid cohesion along the arms • Microtubules of the spindle shorten • Chiasmata break • Homologues are separated from each other and move to opposite poles • Sister chromatids remain attached to each other at their centromeres • Each pole has a complete haploid set of chromosomes consisting of one member of each homologous pair • Independent assortment of maternal and paternal chromosomes

15. Telophase I completes Meiosis I • Nuclear envelope re-forms around each daughter nucleus • Sister chromatids are no longer identical because of crossing over (prophase I) • Cytokinesis may or may not occur after telophase I • Meiosis II occurs after an interval of variable length

16. Achiasmate segregation of homologues is possible • Although homologues are usually held together by chiasmata, some systems are able to segregate chromosomes without this

17. Meiosis II is like a mitotic division without DNA replication • Resembles a mitotic division • Prophase II: nuclear envelopes dissolve and new spindle apparatus forms • Metaphase II: chromosomes align on metaphase plate • Anaphase II: sister chromatids are separated from each other • Telophase II: nuclear envelope re-forms around 4 sets of daughter chromosomes; cytokinesis follows

18. Errors in meiosis produce aneuploid gametes • Aneuploid gametes – gametes with missing or extra chromosomes

19. Prokaryotic Genetics

20. Prokaryotic Genetics • Prokaryotes do not reproduce sexually • 3 types of horizontal gene transfer • Conjugation – cell-to-cell contact • Transduction – by bacteriophages • Transformation – from the environment • All 3 processes also observed in archaea

21. Conjugation depends on the presence of a conjugative plasmid • DNA can be exchanged by conjugation • Depends on presence of conjugative plasmids • Example: F plasmid in E coli • F plasmid also can integrate into bacterial genome • Excision may be imprecise • F plasmid carries genetic information from the host

23. Viruses transfer DNA transduction • Generalized transduction occurs when viruses package host DNA • Transfer it on subsequent infections • Specialized transduction limited to lysogenic phage

24. Transformation is the update of DNA directly from the environment • When cells take up DNA from surrounding medium • Can be artificially induced in the laboratory

25. Antibiotic resistance can be transferred by resistance plasmids • R plasmids have played a significant role in appearance of antibiotic resistance strains of bacteria • Example: S. aureusand E. coli O157:H7

26. Variation can also arise by mutation • Can occur spontaneously by • Radiation • UV light • Various chemicals

27. Human Bacterial Disease • In the early 20th century, infectious diseases killed 20% of children before the age of five • Sanitation and antibiotics considerably improved the situation • In recent years, however, many bacterial diseases have appeared and reappeared

28. Tuberculosis (TB) • Scourge for thousands of years • Mycobacterium tuberculosis • Afflicts the respiratory system • Thwarts immune system • Easily transferred from person to person through the air • Multidrug-resistant (MDR) strains are very alarming

29. Dental caries (tooth decay) • Plaque consists of bacterial biofilms • Streptococcussobrinus ferments sugar to lactic acid • Tooth enamel degenerates • Peptic ulcers • Helicobacter pylori is the main cause • Treated with antibiotics

30. Sexually transmitted diseases (STDs) • Gonorrhea • One of the most prevalent communicable diseases in North America. • Neisseriagonorrhoeae • Transmitted through exchange of body fluids • Can pass from mom to baby via birth canal • Chlamydia • Chlamydia trachomatis • “Silent STD” – incidence has skyrocketed • Can cause PID and heart disease

31. Syphilis • Treponema pallidum • Transmitted through sex or contact with open chancre • Can pass from mom to baby via birth canal • Four distinct stages • Primary – Chancre – highly infectious • Secondary – Rash – infectious • Tertiary – Latency – no longer infectious but attacking internal organs • Quaternary – Damage now evident

33. Beneficial Prokaryotes • Decomposers release a dead organism’s atoms to the environment • Fixation • Photosynthesizers fix carbon into sugars • Ancient cyanobacteria added oxygen to air • Nitrogen fixers reduce N2 to NH3 (ammonia) • Anabaena in aquatic environments • Rhizobium in soil

34. Symbiosis refers to the ecological relationship between different species that live in direct contact with each other • Mutualism – both parties benefit • Nitrogen-fixing bacteria on plant roots • Cellulase-producing bacteria in animals • Commensalism – one organism benefits and the other is unaffected • Parasitism – one organism benefits and the other is harmed

35. Bacteria are used in genetic engineering • “Biofactories” that produce various chemicals, including insulin and antibiotics • Bacteria are used for bioremediation • Remove pollutants from water, air, and soil • Biostimulation – adds nutrients to encourage growth of naturally occurring microbes • Exxon Valdez oil spill