Meiosis

Heredity and Variation Is the transmission of traits from one generation to the next Variation Shows that offspring differ somewhat in appearance and other traits from parents and siblings Genetics Is the scientific study of heredity and hereditary variation

Breeding in English Shepherds Mother Father

Breeding in English Shepherds - Offspring

Genes – A simple definition Are the units of heredity Are segments of DNA Each gene in an organism’s DNA has a specific locus on a certain chromosome Sexual organisms inherit one set of chromosomes from the mother and one set from the father

The Shallow End of the Gene Pool

Asexual Reproduction For example – budding in Hydra is asexual reproduction that forms clones

Asexual Reproduction Strawberry reproducing by putting out asexual runners – also forming clones

Mitosis and development Haploid gametes (n  23) Key Haploid (n) Egg (n) Diploid (2n) Sperm (n) MEIOSIS FERTILIZATION Ovary Testis Diploid zygote (2n  46) Figure 13.5 The human life cycle. Mitosis and development Multicellular diploid adults (2n  46)

Chromosomes Homologous chromosomes Are the two chromosomes composing a pair Have the same characteristics Autosomes are the non-sex determining chromosomes Sex chromosomes Are distinct from each other in their characteristics Are represented as X and Y in mammals Determine the sex of the individual, XX being female, XY being male; X and X are homologous in female A diploid cell Has two sets of each of its chromosomes In a human has 46 chromosomes (2n = 46)

Pair of homologous duplicated chromosomes Centromere Sister chromatids Figure 13.3 RESEARCH METHOD: Preparing a karyotype Metaphase chromosome

Figure 13.x3 Human female karyotype shown by bright field G-banding of chromosomes

Figure 13.x5 Human male karyotype shown by bright field G-banding of chromosomes

Hairy Ears – a sex linked trait Square symbol = male Round symbol = female Dark square = hairy ear trait

Chromosome number in humans if there was no meiosis prior to reproduction First Generation - Sperm and Egg with 46 chromosomes each Second generation 92 chromosomes Third generation 184 chromosomes Fourth generation 368 chromosomes Etc.

Sexual life cycles Key Haploid (n) Haploid multi- cellular organism (gametophyte) Haploid unicellular or multicellular organism Diploid (2n) n Gametes n n Mitosis n Mitosis Mitosis n Mitosis n n n n n MEIOSIS FERTILIZATION Spores n n Gametes n Gametes MEIOSIS FERTILIZATION Zygote MEIOSIS FERTILIZATION 2n 2n 2n Diploid multicellular organism (sporophyte) 2n Zygote Diploid multicellular organism 2n Figure 13.6 Three types of sexual life cycles. Mitosis Mitosis Zygote (a) Animals (b) Plants and some algae (c) Most fungi and some protists

Interphase Pair of homologous chromosomes in diploid parent cell Duplicated pair of homologous chromosomes Chromosomes duplicate Sister chromatids Diploid cell with duplicated chromosomes Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number.

Interphase Pair of homologous chromosomes in diploid parent cell Duplicated pair of homologous chromosomes Chromosomes duplicate Sister chromatids Diploid cell with duplicated chromosomes Meiosis I 1 Homologous chromosomes separate Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number. Haploid cells with duplicated chromosomes

Interphase Meiosis I Meiosis II Pair of homologous chromosomes in diploid parent cell Duplicated pair of homologous chromosomes Chromosomes duplicate Sister chromatids Diploid cell with duplicated chromosomes Meiosis I 1 Homologous chromosomes separate Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number. Haploid cells with duplicated chromosomes Meiosis II 2 Sister chromatids separate Haploid cells with unduplicated chromosomes

Telophase I and Cytokinesis Telophase II and Cytokinesis MEIOSIS I: Separates homologous chromosomes MEIOSIS I: Separates sister chromatids Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis Centrosome (with centriole pair) Sister chromatids remain attached Sister chromatids Chiasmata Centromere (with kinetochore) Spindle Metaphase plate During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing unduplicated chromosomes. Cleavage furrow Homologous chromosomes separate Fragments of nuclear envelope Sister chromatids separate Haploid daughter cells forming Homologous chromosomes Microtubule attached to kinetochore Each pair of homologous chromosomes separates. Two haploid cells form; each chromosome still consists of two sister chromatids. Figure 13.8 Exploring: Meiosis in an Animal Cell Duplicated homologous chromosomes (red and blue) pair and exchange segments; 2n  6 in this example. Chromosomes line up by homologous pairs.

Telophase I and Cytokinesis Prophase I Metaphase I Anaphase I Centrosome (with centriole pair) Sister chromatids remain attached Sister chromatids Chiasmata Centromere (with kinetochore) Spindle Metaphase plate Cleavage furrow Homologous chromosomes separate Homologous chromosomes Fragments of nuclear envelope Figure 13.8 Exploring: Meiosis in an Animal Cell Microtubule attached to kinetochore Each pair of homologous chromosomes separates. Two haploid cells form; each chromosome still consists of two sister chromatids. Duplicated homologous chromosomes (red and blue) pair and exchange segments; 2n  6 in this example. Chromosomes line up by homologous pairs.

Telophase II and Cytokinesis Prophase II Metaphase II Anaphase II During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing unduplicated chromosomes. Sister chromatids separate Haploid daughter cells forming Figure 13.8 Exploring: Meiosis in an Animal Cell

Chromosome duplication Chromosome duplication Duplicated chromosome MITOSIS MEIOSIS Parent cell Chiasma MEIOSIS I Prophase Prophase I Chromosome duplication Chromosome duplication Duplicated chromosome Homologous chromosome pair 2n  6 Metaphase Metaphase I Anaphase Telophase Anaphase I Telophase I Figure 13.9 A comparison of mitosis and meiosis in diploid cells. Daughter cells of meiosis I Haploid n  3 2n 2n MEIOSIS II Daughter cells of mitosis n n n n Daughter cells of meiosis II

Figure 13.9 A comparison of mitosis and meiosis in diploid cells. SUMMARY Property Mitosis Meiosis DNA replication Occurs during interphase before mitosis begins Occurs during interphase before meiosis I begins Number of divisions One, including prophase, metaphase, anaphase, and telophase Two, each including prophase, metaphase, anaphase, and telophase Synapsis of homologous chromosomes Does not occur Occurs during prophase I along with crossing over between nonsister chromatids; resulting chiasmata hold pairs together due to sister chromatid cohesion Number of daughter cells and genetic composition Two, each diploid (2n) and genetically identical to the parent cell Four, each haploid (n), containing half as many chromosomes as the parent cell; genetically different from the parent cell and from each other Figure 13.9 A comparison of mitosis and meiosis in diploid cells. Role in the animal body Enables multicellular adult to arise from zygote; produces cells for growth, repair, and, in some species, asexual reproduction Produces gametes; reduces number of chromosomes by half and introduces genetic variability among the gametes

Independent Assortment Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Figure 13.10 The independent assortment of homologous chromosomes in meiosis.

Independent Assortment Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Figure 13.10 The independent assortment of homologous chromosomes in meiosis.

Independent Assortment Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Figure 13.10 The independent assortment of homologous chromosomes in meiosis. Daughter cells Combination 1 Combination 2 Combination 3 Combination 4

Mitosis and meiosis have several key differences. The chromosome number is reduced by half in meiosis, but not in mitosis. Mitosis produces daughter cells that are genetically identical to the parent and to each other. Meiosis produces cells that differ from the parent and each other.

Three events, unique to meiosis, occur during the first division cycle Three events, unique to meiosis, occur during the first division cycle. 1. During prophase I, homologous chromosomes pair up in a process called synapsis. A protein zipper, the synaptonemal complex, holds homologous chromosomes together tightly. Later in prophase I, the joined homologous chromosomes are visible as a tetrad. At X-shaped regions called chiasmata, sections of nonsister chromatids are exchanged. Chiasmata is the physical manifestation of crossing over, a form of genetic rearrangement. Prophase I is longest and most important phase

2. At metaphase I homologous pairs of chromosomes, not individual chromosomes are aligned along the metaphase plate. In humans, you would see 23 tetrads. 3. At anaphase I, it is homologous chromosomes, not sister chromatids, that separate and are carried to opposite poles of the cell. Sister chromatids remain attached at the centromere until anaphase II. The processes during the second meiotic division are virtually identical to those of mitosis.

The synaptonemal complex binding together four homologous chromosomes

Chiasmata and crossing over

Nonsister chromatids held together during synapsis Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Figure 13.11 The results of crossing over during meiosis.

Nonsister chromatids held together during synapsis Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma Centromere TEM Figure 13.11 The results of crossing over during meiosis.

Nonsister chromatids held together during synapsis Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma Centromere TEM Anaphase I Figure 13.11 The results of crossing over during meiosis.

Nonsister chromatids held together during synapsis Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma Centromere TEM Anaphase I Figure 13.11 The results of crossing over during meiosis. Anaphase II

Nonsister chromatids held together during synapsis Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma Centromere TEM Anaphase I Figure 13.11 The results of crossing over during meiosis. Anaphase II Daughter cells Recombinant chromosomes

Human Female vs Male Meiotic Timelines

Sexual reproduction leads to genetic variation via: Independent assortment during meiosis Crossing over during meiosis Random mixing of gametes (sperm and egg)