Chapter 13: Meiosis and Sexual Life Cycles

1. Chapter 13: Meiosis and Sexual Life Cycles

2. Cell Theory • Cell Theory: Living things are made of cells and cells reproduce from other cells • Asexual Reproduction: Cells divide creating identical cells • A clone is a group of genetically identical individuals from the same parent • Sexual Reproduction: Gametes fuse to create zygote • Unique combination of chromosomes gives rise to offspring with unique set of characteristics

3. Asexual Reproduction • All prokaryotes • Some protists, invertebrates, plants • Some multicellular organisms reproduce both sexually and asexually • Budding: creation of new individuals from growths of existing individuals • Fission: splitting of one organism into 2 • Fragmentation and Regeneration: Breaking of body part, followed by growth and development

4. Asexual Reproduction • Advantages: • Sessile animals can reproduce without mate • Produce many offspring quickly, without energy invested in sperm and eggs or in mating • Produce offspring with optimal genetics for environment • Disadvantages: • Genetically uniform populations • Prevents adaptation of population in changing environment

5. Sexual Reproduction • Most animals and plants reproduce by sexual reproduction • Offspring created by fertilization= fusion of 2 haploid gametes to produce a diploid zygote • Egg, Sperm • Offspring has unique genetic characteristics

6. Sexual Reproduction • Advantages • High genetic variability among offspring • High variability= greater chance for adaption for changing environments • Evolution by natural selection • Disadvantages • Must find mate • High energy requirements for courtship, gamete production

7. Living Cells • Genetics is the scientific study of heredity and variation • Heredity is the transmission of traits from one generation to the next • Variation is demonstrated by the differences in appearance that offspring show from parents and siblings

8. Living Cells • Sexual Reproduction: Gametes fuse to create zygote • Gametes= egg and sperm cells (Haploid) • Genes are the units of heredity, and are made up of segments of DNA • Genes are passed from parent to offspring through gametes • Fertilization= Fusion of egg and sperm cell • Zygote= Fertilized egg (Diploid) • After fertilization, zygote goes from a single cell to a multicellular organism through mitosis

9. Gametes • Gametes contain haploid number of chromosomes • Single set of autosomes + one sex chromosome • Haploid gametes conserve original number of chromosomes in offspring • If two diploid cells (2n) fused, resulting offspring would be 4n • If two haploid cells (n) fused, resulting offspring would be 2n

10. Chromosomes • Sex chromosomes • Determine sex of offspring • Genes can have other functions too • Mammals= Males have XY chromosomes, Females have XX chromosomes • Female gametes= all contain X chromosome • Male gametes= 50% contain X and 50% contain Y • Autosomes • Non-sex chromosomes • In humans, 22 pairs of autosomes

11. Human Karyotype Haploid (n)=23 Diploid (2n)=46

12. Chromosome Terminology • Diploid Cell: 2n = 6 • Haploid= n= 3 • Following DNA synthesis, each of the six chromosomes consists of 2 sister chromatids, connected at the centromere Sister Chromatids Homologous Chromosome Centromere Blue: Paternal Chromosomes Pink: Maternal Chromosomes

13. Chromosome Terminology • Locus: location of gene on chromosome • Maternal and paternal chromosomes can have different versions of the same gene • Allele: alternative version of a gene

15. Plants and some algae exhibit an alternation of generations • Life cycle has a diploid and haploid multicellular stage • The diploid organism, called the sporophyte, makes haploid sporesby meiosis • Each spore grows by mitosis into a haploid organism called a gametophyte • A gametophyte makes haploid gametes by mitosis • Fertilization results in a diploid sporophyte

16. In most fungi and some protists, the only diploid stage is the single-celled zygote; there is no multicellular diploid stage • The zygote produces haploid cells by meiosis • Each haploid cell grows by mitosis into a haploid multicellular organism • The haploid adult produces gametes by mitosis

17. Life Cycles • Depending on the type of life cycle, either haploid or diploid cells can divide by mitosis • However, only diploid cells can undergo meiosis • In all three life cycles, the halving and doubling of chromosomes contributes to genetic variation in offspring

18. Meiosis • Like mitosis, meiosis is preceded by the replication of chromosomes • Meiosis takes place in two sets of cell divisions, called meiosis I and meiosis II • The two cell divisions result in four daughter cells, rather than the two daughter cells in mitosis • Each daughter cell has only half as many chromosomes as the parent cell

19. Meiosis • Cell division • Cells go through G1, S, and G2 phases prior to meiosis • All chromosomes duplicated during S phase • Two consecutive nuclear divisions + cytokinesis • Meiosis 1: separates homologous chromosomes • Cells are haploid after end of Meiosis 1 • Meiosis 2: separates sister chromatids • Similar to Mitosis • End Result: 4 haploid gametes

20. Meiosis 2 N=23 Meiosis in Humans Meiosis 1 N=23 N=23 G1 S phase/G2 N=46 N=46 N=23 N=23 N=23

21. Prophase I • Prophase I= ~90% of the time required for meiosis • Chromosomes begin to condense • In synapsis, homologous chromosomes loosely pair up, aligned gene by gene • Each pair of chromosomes forms a tetrad, a group of four chromatids • Crossing over can occur • Nonsisterchromatids exchange DNA segments • Each tetrad usually has one or more chiasmata, X-shaped regions where crossing over occurred

22. Metaphase I • Tetrads line up at the metaphase plate, with one chromosome facing each pole • Microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad • Microtubules from the other pole are attached to the kinetochore of the other chromosome

23. Anaphase I • Pairs of homologous chromosomes separate • One chromosome moves toward each pole, guided by the spindle apparatus • Sister chromatids remain attached at the centromere and move as one unit toward the pole

24. Telophase and Cytokinesis I • In the beginning of telophase I, each half of the cell has a haploid set of chromosomes; each chromosome still consists of two sister chromatids • Cytokinesis usually occurs simultaneously, forming two haploid daughter cells • Animal cells= Cleavage furrow • Plant cells= Cell plate

25. Meiosis II • Two haploid daughter cells undergo another round of cell division in Meiosis II • No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated • Meiosis II is very similar to mitosis • Separation of sister chromatids

26. Meiosis II

27. Meiosis II • At the end of meiosis, there are four daughter cells, each with a haploid set of individual chromosomes • Each daughter cell is genetically distinct from the others and from the parent cell • Crossing over causes differences in sister chromatids

28. Mitosis vs Meiosis • Mitosis conserves the number of chromosome sets, producing cells that are genetically identical to the parent cell • Meiosis reduces the number of chromosome sets from two (diploid) to one (haploid), producing cells that differ genetically from each other and from the parent cell

29. Mitosis Meiosis End Result: 2 Identical Cells End Result: 4 Unique Gametes

30. Mitosis vs Meiosis

31. Mitosis vs Meiosis • Three events are unique to meiosis, and all three occur in meiosis l • Synapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange genetic information • At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes • At anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate

32. Meiosis and Genetic Diversity Natural selection results in the accumulation of genetic variations favored by the environment Sexual reproduction contributes to the genetic variation in a population, which originates from mutations, through multiple processes: • Independent orientation of chromosomes • Random Fertilization • Homologous chromosomes can have different versions of genes • Crossing Over

33. 1. Independent orientation of chromosomes The number of combinations possible when chromosomes sort independently into gametes is 2n, where n is the haploid number

34. 2. Random Fertilization • Potential for any chromosome combination to be fused with any other chromosome combination • Fusion of 2 human gametes: 70 trillion possible combinations • Each zygote has a unique genetic identity

35. 3. Homologous chromosomes can have different versions of a gene • Mutations (changes in an organism’s DNA) are the original source of genetic diversity • Mutations create different versions of genes called alleles • Reshuffling of alleles during sexual reproduction produces genetic variation

36. 3. Homologous chromosomes can have different versions of a gene Meiosis 1 Meiosis 2 G1 S phase

37. 4. Crossing Over: Exchange of genetic information Homologous Chromosomes • Occurs during Prophase 1 in meiosis • Homologous chromosomes pair up and form tetrads • Sister chromatids from 2 chromosomes “cross-over” at chiasmata • Exchange of DNA • Genetic Recombination= production of gene combinations different from parent cell Sister Chromatids Genetic Recombination

38. What happens when meiosis goes wrong?

39. Failure to Separate • Nondisjunction= chromosome pair does not separate in meiosis 1 or 2 • Gamete created with extra chromosome • Majority of gametes with wrong number of chromosomes will not produce viable offspring • Trisomy 21= Downs Syndrome • 1 out of 700 children born • Chance increases with age of mother • Produces physical effects • Round face, flattened nose, small teeth, short stature, failure to sexually develop • Susceptible to heart defects, respiratory infections, leukemia, Alzheimer’s disease

40. What about sex cells? • Abnormal numbers of sex cells do not usually have an effect on survival • XXY= Klinefelter Syndrome • Small testes, sterile, develops female body characteristics • XYY= Normal male • Taller than normal • XXX= Normal female • No obvious effects • XO= Turner Syndrome • Short stature, web of skin between shoulders and neck, sterile and lack of sexual development • Only case where 45 chromosomes is not fatal in humans

41. Errors can give rise to new species • Plants • Polyploidy= errors in meiosis resulted in more than 2 sets of chromosomes • Plants can self-fertilize, preventing the altered chromosome number from disrupting reproduction • Creates a new species • ~50% of plants are polyploid • Wheat, potatoes, apples, cotton • Animals • Less common in animals • Fishes and amphibians • Rat= recent discovery in Chile Rainbow Trout 2n=60