PowerLecture: Chapter 10 Meiosis and Sexual Reproduction

What you should understand The two types of reproduction. Why each occur and how. What is a gamete and how it is made Difference between haploid and diploid and which type of cell they occur. Why sexual reproduction increases variation within a species.

Impacts, Issues: Why Sex Asexual reproduction is easier and faster Sexual reproduction can be an alternative adaption in changing environments

Differences between types of reproduction

Asexual Reproduction Single parent produces offspring All offspring are genetically identical to one another and to parent Mitosis

Sexual Reproduction Involves Meiosis Gamete production (egg and sperm) Fertilization Produces genetic variation among offspring Chromosomes are duplicated in germ cells

Germ cells undergo meiosis and cytoplasmic division Cellular descendents of germ cells (sex cells) become gametes (egg and sperm) Gametes meet at fertilization

Gamete Formation Gametes are sex cells Arise from germ cells Sperm eggs Arise from germ cells Some plants can have both parts testes ovaries anther ovary Figure 10-3 Page 156

anther (where cells that give rise to male gametes originate) FLOWERING PLANT anther (where cells that give rise to male gametes originate) ovules, inside an ovary (where cells that give rise to female gametes originate) Fig. 10-3a, p.156

Recap of chromosomes

Homologous Chromosomes Carry Different Alleles Cell has two of each chromosome (homologous) One chromosome in each pair from mother, other from father Paternal and maternal chromosomes carry different alleles Alleles are different forms of a gene Examples

Homologous Chromosomes You get one from each parent you have 23 pairs of homologous chromosomes. They carry the same genes but not necessary the same form of the gene. Fig. 10-2, p.156

Sexual Reproduction Shuffles Alleles Through sexual reproduction, offspring inherit new combinations of alleles, which leads to variations in traits This variation in traits is the basis for evolutionary change

Chromosome Number Sum total of chromosomes in a cell Germ cells are diploid (2n) Gametes are haploid (n) Meiosis halves chromosome number

Human Karyotype Explanation 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 XX (or XY) Human Karyotype Explanation Fig. 10-4, p.157

What is meiosis?

Meiosis: Two Divisions overview Two consecutive nuclear divisions Meiosis I Meiosis II DNA is not duplicated between divisions Four haploid nuclei form

Meiosis I Each homologue in the cell pairs with its partner, then the partners separate p. 158

Meiosis II The two sister chromatids of each duplicated chromosome are separated from each other two chromosomes (unduplicated) one chromosome (duplicated) p. 158

What is the difference between meiosis I and meiosis II in regards to the chromosomes.

Meiosis I - Stages Prophase I Metaphase I Anaphase I Telophase I

Prophase I Each duplicated chromosome pairs with homologue Homologues swap segments (crossing over) Each chromosome becomes attached to spindle Fig. 10-5, p. 158

Metaphase I Chromosomes are pushed and pulled into the middle of cell The spindle is fully formed Fig. 10-5, p. 158

Anaphase I Homologous chromosomes segregate The sister chromatids remain attached Fig. 10-5, p. 158

Telophase I The chromosomes arrive at opposite poles Usually followed by cytoplasmic division Fig. 10-5, p. 158

Prophase II Microtubules attach to the kinetochores of the duplicated chromosomes Fig. 10-5, p. 158

Metaphase II Duplicated chromosomes line up at the spindle equator, midway between the poles Fig. 10-5, p. 158

Anaphase II Sister chromatids separate to become independent chromosomes Fig. 10-5, p. 158

Telophase II The chromosomes arrive at opposite ends of the cell A nuclear envelope forms around each set of chromosomes Four haploid cells Fig. 10-5, p. 158

Meiosis Meiosis I and II

Meiosis Meiosis step-by-step

Crossing Over Each chromosome becomes zippered to its homologue All four chromatids are closely aligned Non-sister chromosomes exchange segments (which contain genes)

Crossing Over paternal homologue maternal homologue Stepped Art Fig. 10-6, p.160

Crossing Over a Both chromosomes shown here were duplicated during interphase, before meiosis. When prophase I is under way, sister chromatids of each chromosome are positioned so close together that they look like a single thread. Fig. 10-6a, p.160

Crossing Over b Each chromosome becomes zippered to its homologue, so all four chromatids are tightly aligned. If the two sex chromosomes have different forms, such as X paired with Y, they still get zippered together, but only in a tiny region at their ends. Fig. 10-6b, p.160

Crossing Over c We show the pair of chromosomes as if they already condensed only to give you an idea of what goes on. They really are in a tightly aligned, threadlike form during prophase I. d The intimate contact encourages one crossover (and usually more) to happen at various intervals along the length of nonsister chromatids. e Nonsister chromatids exchange segments at the crossover sites. They continue to condense into thicker, rodlike forms. By the start of metaphase I, they will be unzippered from each other. f Crossing over breaks up old combinations of alleles and puts new ones together in the cell’s pairs of homologous chromosomes. Fig. 10-6c, p.160

Effect of Crossing Over After crossing over, each chromosome contains both maternal and paternal segments Creates new allele combinations in offspring

Random Alignment During transition between prophase I and metaphase I, microtubules from spindle poles attach to kinetochores of chromosomes Initial contacts between microtubules and chromosomes are random What does this mean?

Random Alignment Either the maternal or paternal member of a homologous pair can end up at either pole The chromosomes in a gamete are a mix of chromosomes from the two parents

combinations possible Possible Chromosome Combinations 1 2 3 combinations possible or or or Fig. 10-7, p.161

combinations possible 1 2 3 combinations possible Alignment at metaphase I or or or Stepped Art Fig. 10-7, p.161

Crossing Over Crossing over

Plant Life Cycle sporophyte zygote diploid fertilization meiosis haploid gametes spores gametophytes Fig. 10-8a, p.162

Animal Life Cycle multicelled body zygote diploid fertilization meiosis haploid gametes Fig. 10-8b, p.162

Animal Life Cycle Random alignment

Oogenesis three polar bodies (haploid) first polar body (haploid) oogonium (diploid) primary oocyte (diploid) secondary oocyte (haploid) ovum (haploid) Meiosis I, Cytoplasmic Division Meiosis II, Cytoplasmic Division Growth Figure 10-10 Page 163

Generalized life cycles Oogenesis Generalized life cycles

Spermatogenesis Figure 10-9 Page 163 spermato- gonium (diploid ) primary spermatocyte (diploid) secondary spermatocytes (haploid) sperm (mature, haploid male gametes) spermatids (haploid) Spermatogenesis Meiosis I, Cytoplasmic Division Meiosis II, Cytoplasmic Division Growth cell differentiation, sperm formation Figure 10-9 Page 163

Fig. 10-10, p.163

Animal Egg Formation Sperm formation

Animal Egg Formation Egg formation

Fertilization Male and female gametes unite and nuclei fuse Fusion of two haploid nuclei produces diploid nucleus in the zygote Which two gametes unite is random Adds to variation among offspring

Factors Contributing to Variation among Offspring Crossing over during prophase I Random alignment of chromosomes at metaphase I Random combination of gametes at fertilization

Mitosis & Meiosis Compared Functions Asexual reproduction Growth, repair Occurs in somatic cells Produces clones Meiosis Function Sexual reproduction Occurs in germ cells Produces variable offspring

Each person of a pair take out a piece of paper and you will compare PMAT of mitosis to meiosis. Skip the T. How many squares do you need to do this? When you are done share with another pair. When everyone is done we will compare answers

Prophase vs. Prophase I Prophase (Mitosis) Prophase I (Meiosis) Homologous pairs do not interact with each other Prophase I (Meiosis) Homologous pairs become zippered together and crossing over occurs

Anaphase, Anaphase I, and Anaphase II Anaphase I (Meiosis) Homologous chromosomes separate from each other Anaphase/Anaphase II (Mitosis/Meiosis) Sister chromatids of a chromosome separate from each other

What are the over arching concepts for mitosis and meiosis? Focus on number of cells produced, how they are produced and chromosome number.

Results of Mitosis and Meiosis Two diploid cells produced Each identical to parent Meiosis Four haploid cells produced Differ from parent and one another

Results of Mitosis and Meiosis Comparing mitosis and meiosis