Bellringer Why is genetic diversity beneficial to populations? How does sexual reproduction increase genetic diversity? How does meiosis increase genetic diversity? Why don’t siblings all look almost exactly alike?

Chapter 13 Meiosis and Sexual Life Cycles

Hereditary similarity and variation Living organisms are, by definition, capable of reproducing their own kind. Offspring inherit genetic information from their parent(s) Genetic information that can be passed on is hereditary Different forms of the same gene (alleles) code for different phenotypes (gene expression)

Heredity Is the transmission of traits from one generation to the next Variation Shows that offspring differ somewhat in appearance from parents and siblings Figure 13.1

Genetics Scientific study of heredity and hereditary variation

Inheritance of Genes Offspring inherit genes, in the form of chromosomes, from their parents Chromosomes A complex of proteins and a molecule of DNA 46 chromosomes in humans Genes units of heredity segments of DNA

Chromosomes Each gene in an organism’s DNA Has a specific locus on a certain chromosome We inherit 1 set of chromosomes from mom & 1 from dad Corresponding chromosomes have the same genes, although possibly different alleles 2 copies of every gene (except for some genes on sex chromosomes)

Comparison of Asexual and Sexual Reproduction asexual reproduction 1 parent produces genetically identical offspring by mitosis sexual reproduction 2 parents have offspring with unique combinations of genes inherited from both parents Figure 13.2 Parent Bud 0.5 mm

Sexual Life Cycles Fertilization & meiosis alternate in sexual life cycles life cycle Generation to generation sequence of stages in the reproductive history of an organism

Sets of Chromosomes in Human Cells Somatic cells – all of the cells of the body except fot the sex cells 46 chromosomes in humans Gametes – sex cells 23 chromosomes in humans

5 µm Pair of homologous chromosomes Centromere Sister chromatids Figure 13.3 Karyotype Visual representation of the chromosomes in a cell

LE µm Pair of homologous chromosomes Sister chromatids Centromere

Chromosomes Homologous chromosomes 2 chromosomes, making a pair Same length, centromere position, & staining pattern also called autosomes 22 pairs in us Sex chromosomes Are distinct from each other in their characteristics X & Y Determine sex of the individual, XX female XY male

Chromosomes Haploid 1 set of chromosomes Human = 23 (n=23) diploid (2n) 2 sets of each of its chromosomes human= 46 chromo’s (2n = 46)

Chromosomes In a cell in which DNA synthesis has occurred All the chromo’s are duplicated & thus each consists of 2 identical sister chromatids Figure 13.4 Key Maternal set of chromosomes (n = 3) Paternal set of chromosomes (n = 3) 2n = 6 Two sister chromatids of one replicated chromosome Two nonsister chromatids in a homologous pair Pair of homologous chromosomes (one from each set) Centromere

Chromosomes - gametes Haploid (only 1 set of chromosomes) Contains every type of autosome Contains only one or the other of the sex chromosomes (X or Y)

Behavior of Chromosome Sets in the Human Life Cycle At sexual maturity ovaries & testes make haploid gametes by meiosis During fertilization sperm & ovum fuse, forming a diploid zygote The zygote Develops into an adult organism

Figure 13.5 Key Haploid (n) Diploid (2n) Haploid gametes (n = 23) Ovum (n) Sperm Cell (n) MEIOSIS FERTILIZATION Ovary Testis Diploid zygote (2n = 46) Mitosis and development Multicellular diploid adults (2n = 46) The Human Life Cycle

The Variety of Sexual Life Cycles 3 types of sexual life cycles Differ in timing of meiosis & fertilization

Animal Life Cycled Meiosis occurs during gamete formation Gametes are the only haploid cells Gametes Figure 13.6 A Diploid multicellular organism Key MEIOSIS FERTILIZATION n n n 2n Zygote Haploid Diploid Mitosis (a) Animals

MEIOSISFERTILIZATION n n n n n 2n Haploid multicellular organism (gametophyte) Mitosis Spores Gametes Mitosis Zygote Diploid multicellular organism (sporophyte) (b) Plants and some algae Figure 13.6 B Plant Life Cycle Plants & some algae Show an alternation of generations life cycle has both diploid & haploid multicellular stages

MEIOSIS FERTILIZATION n n n n n 2n Haploid multicellular organism Mitosis Gametes Zygote (c) Most fungi and some protists Figure 13.6 C Fungi and some protists Meiosis makes haploid cells that make a haploid multicellular adult organism haploid adult carries out mitosis, making cells that will be gametes

Sexual Life Cycles Depending on the type of life cycle, either haploid (n) or diploid (2n) cells can divide by mitosis only diploid cells can undergo meiosis In all 3 life cycles, changes in the number of chromosomes contribute to genetic variation in offspring

Meiosis Meiosis reduces the number of chromo sets from diploid to haploid Takes place in 2 sets of divisions, meiosis I & meiosis II

The Stages of Meiosis An overview of meiosis 2 cell divisions result in 4 daughter cells, rather than the two daughter cells in mitosis Each daughter cell has only ½ as many chromosomes as the parent cell Figure 13.7 Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes 1 2 Homologous chromosomes separate Haploid cells with replicated chromosomes Sister chromatids separate Haploid cells with unreplicated chromosomes Meiosis I Meiosis II

The Stages of Meiosis 1st cell division (meiosis I), homologous chromosomes separate Meiosis I results in 2 haploid daughter cells with replicated chromosomes In the 2nd cell division (meiosis II), sister chromatids separate Meiosis II results in 4 haploid daughter cells with unreplicated chromosomes

Summary Meiosis I Reduces # of chromo’s from diploid to haploid Meiosis II makes 4 haploid daughter cells

Centrosomes (with centriole pairs) Sister chromatids Chiasmata Spindle Tetrad Nuclear envelope Chromatin Centromere (with kinetochore) Microtubule attached to kinetochore Tertads line up Metaphase plate Homologous chromosomes separate Sister chromatids remain attached Pairs of homologous chromosomes split up Chromosomes duplicate Homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 in this example INTERPHASE MEIOSIS I: Separates homologous chromosomes PROPHASE I METAPHASE I ANAPHASE I Interphase and meiosis I Figure 13.8

TELOPHASE I AND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II AND CYTOKINESIS MEIOSIS II: Separates sister chromatids Cleavage furrow Sister chromatids separate Haploid daughter cells forming During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing single chromosomes Two haploid cells form; chromosomes are still double Figure 13.8 Telophase, cytokinesis, and meiosis II

A Comparison of Mitosis and Meiosis Meiosis can be distinguished from mitosis by 3 events Prophase I Synapsis & crossing over Homologous chromosomes physically connect & exchange genetic info Tetrads on the metaphase plate In metaphase I of meiosis, paired homologous chromosomes (tetrads) are positioned on the metaphase plates Separation of homologues anaphase I of meiosis- homologous pairs move toward opp poles of the cell anaphase II of meiosis- sister chromatids separate

Figure 13.9 MITOSIS MEIOSIS Prophase Duplicated chromosome (two sister chromatids) Chromosome replication Chromosome replication Parent cell (before chromosome replication) Chiasma (site of crossing over) MEIOSIS I Prophase I Tetrad formed by synapsis of homologous chromosomes Metaphase Chromosomes positioned at the metaphase plate Tetrads positioned at the metaphase plate Metaphase I Anaphase I Telophase I Haploid n = 3 MEIOSIS II Daughter cells of meiosis I Homologues separate during anaphase I; sister chromatids remain together Daughter cells of meiosis II n n nn Sister chromatids separate during anaphase II Anaphase Telophase Sister chromatids separate during anaphase 2n2n2n2n Daughter cells of mitosis 2n = 6 Mitosis and Meiosis

Sources of genetic variation Genetic variation produced in sexual life cycles contributes to evolution Reshuffling of genetic material in meiosis Produces genetic variation

Origins of Genetic Variation Among Offspring In species that produce sexually The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises each generation

Independent Assortment of Chromosomes Homologous pairs of chromosomes Orient randomly at metaphase I of meiosis In independent assortment Each pair of chromosomes sorts its maternal & paternal homologues into daughter cells independently of the other pairs

Crossing Over Crossing over Produces recombinant chromosomes that carry genes derived from 2 different parents Figure Prophase I of meiosis Nonsister chromatids Tetrad Chiasma, site of crossing over Metaphase I Metaphase II Daughter cells Recombinant chromosomes

Random Fertilization The fusion of gametes The number of possible complete genotypes for a gamete is 2^n The number of possible complete genotypes for any zygote is 2^n x 2^n In humans, n=23 How many possible gametes can one person produce? How many possible zygotes could any two people produce?

Evolutionary Significance of Genetic Variation Within Populations Genetic variation Is the raw material for evolution by natural selection Mutations Are the original source of genetic variation Sexual reproduction Produces new combinations of variant genes, adding more genetic diversity

Mutations Are the original source of genetic variation Sexual reproduction Produces new combinations of variant genes, adding more genetic diversity