Meiosis and Sexual Reproduction Chapter 10 Meiosis and Sexual Reproduction

Bozeman Video—Cell Cycle, Mitosis, & Meiosis http://www.youtube.com/watch?v=2aVnN4RePyI

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

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

Sexual Reproduction Involves Meiosis Gamete production Fertilization Produces genetic variation among offspring

SOMATIC VS GAMETE CELLS

AUTOSOMES VS. SEX CHROMOSOMES

Homologous Chromosomes Carry Different Alleles Cell has two of each chromosome One chromosome in each pair from mother, other from father Paternal and maternal chromosomes carry different alleles

Homologous Chromosomes 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

Gamete Formation Gametes are sex cells (sperm, eggs) Arise from germ cells ovaries anther testes ovary Figure 10-3 Page 156

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

Human Karyotype 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) Fig. 10-4, p.157

Meiosis: Two Divisions 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

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

Prophase I Each duplicated chromosome pairs with homologue Homologues swap segments(THIS IS KNOWN AS CROSSING OVER WHICH OCCURS AT A SITE CALLED THE CHIASMATA) Each chromosome becomes attached to spindle Longest phase of meiosis Fig. 10-5, p. 158

Metaphase I Tetrads are aligned on the metaphase plate 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 to opposite poles The sister chromatids remain attached Fig. 10-5, p. 158

Telophase I The chromosomes arrive at opposite poles Usually followed by cytoplasmic division Interkinesis (reforming of the nuclear membrane)may occur before Meiosis II but no more DNA duplication Fig. 10-5, p. 158

Prophase II Microtubules attach to the kinetochores of the duplicated chromosomes If interkinesis happened, the nuclear membrane redisappears Fig. 10-5, p. 158

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

Anaphase II Sister chromatids and their centromeres separate to become independent chromosomes at opposite poles of each cell Fig. 10-5, p. 158

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

Metaphase I Prophase I Anaphase I Telophase I spindle equator one pair of homologous chromosomes newly forming microtubules Prophase I Anaphase I Telophase I Meiosis I Stepped Art Fig. 10-5a, p.158

Anaphase II Telophase II Prophase II Metaphase II Meiosis II Stepped Art Fig. 10-5b, p.159

Crossing Over Each chromosome becomes zippered to its homologue All four chromatids are closely aligned Nonsister chromosomes exchange segments

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

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

Possible Chromosome Combinations As a result of random alignment, the number of possible combinations of chromosomes in a gamete is: 2n (n is number of chromosome types)

Bozeman Video--Meiosis http://www.youtube.com/watch?v=rB_8dTuh73c

ROLES OF MITOSIS/MEIOSIS IN LIFE CYCLES

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

FUNGAL AND ALGAL LIFE CYCLE

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 (AKA Law of Independent Assortment of Chromosomes) Random combination of gametes at fertilization (1 in 8 million possible egg combinations x 1 in 8 million posssible sperm combinations = 1 in 64 trillion possible zygote Natural Selection-increases frequency of reproductively favorable traits

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

Bozeman –Mitosis/Meiosis Bead Simulation http://www.youtube.com/watch?v=zGVBAHAsjJM&feature=c4-overview&playnext=1&list=TLZldufdv0wDU

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

Comparison of Mitosis and Meiosis

Meiosis Square Dance Video http://www.youtube.com/watch?v=eaf4j19_3Zg