Meiosis and Sexual Reproduction

Meiosis Meiosis specialized cell division process produces haploid gametes Each gamete receives 1 member of each pair of homologous chromosomes

Homologous Chromosomes hopes.stanford.edu

Meiosis Separates Homologues Meiosis consists of 1 round of DNA replication, followed by 2 rounds of nuclear divisions These events occur in 2 stages: Meiosis I Meiosis II

Replication Before Meiosis Both members of each homologous chromosome pair are replicated prior to meiosis After replication, each chromosome consists of 2 sister chromatids

Meiosis I Each daughter cell receives one member of each pair of homologous chromosomes

Meiosis II Sister chromatids separate into independent chromosomes Each daughter cell receives one of these independent chromosomes

Fusion of Haploid Gametes Meiosis reduces chromosome number by half, producing 1n gametes (eggs and sperm) Fusion of gametes (fertilization) combines two chromosome sets to produce diploid (2N) zygote

FIGURE 11-20 (part 2) Meiotic cell division is essential for sexual reproduction

Overview of Meiosis I & II Phases of meiosis have same names as the phases in mitosis, followed by I or II to distinguish the two nuclear divisions that occur in meiosis

Overview of Meiosis I Meiosis I separates homologous chromosomes into 2 haploid daughter nuclei

FIGURE 11-21 (part 1) Meiotic cell division in an animal cell In meiotic cell division (meiosis and cytokinesis), the homologous chromosomes of a diploid cell are separated, producing four haploid daughter cells. Each daughter cell contains one member of each pair of parental homologous chromosomes. In these diagrams, two pairs of homologous chromosomes are shown, large and small. The yellow chromosomes are from one parent (for example, the father), and the violet chromosomes are from the other parent (for example, the mother).

Overview of Meiosis II Meiosis II separates sister chromatids into 4 gametes

FIGURE 11-21 (part 2) Meiotic cell division in an animal cell In meiotic cell division (meiosis and cytokinesis), the homologous chromosomes of a diploid cell are separated, producing four haploid daughter cells. Each daughter cell contains one member of each pair of parental homologous chromosomes. In these diagrams, two pairs of homologous chromosomes are shown, large and small. The yellow chromosomes are from one parent (for example, the father), and the violet chromosomes are from the other parent (for example, the mother).

Meiotic Prophase I 1. Homologous chromosomes pair up 2. Crossing over (genetic recombination) occurs between homologues Enzymes facilitate exchange of DNA between arms of adjacent chromatids, producing chiasmata 3. Spindle microtubules assemble, nuclear envelope breaks down, and microtubules capture chromosomes

FIGURE 11-21a Meiotic cell division in an animal cell In meiotic cell division (meiosis and cytokinesis), the homologous chromosomes of a diploid cell are separated, producing four haploid daughter cells. Each daughter cell contains one member of each pair of parental homologous chromosomes. In these diagrams, two pairs of homologous chromosomes are shown, large and small. The yellow chromosomes are from one parent (for example, the father), and the violet chromosomes are from the other parent (for example, the mother).

FIGURE 11-22a The mechanism of crossing over

FIGURE 11-22c,d,e The mechanism of crossing over

Meiotic Metaphase I Duplicated homologous chromosomes are pulled into a line perpendicular to the spindle Chromosomes line up as pairs of replicated homologous chromosomes

FIGURE 11-21b Meiotic cell division in an animal cell In meiotic cell division (meiosis and cytokinesis), the homologous chromosomes of a diploid cell are separated, producing four haploid daughter cells. Each daughter cell contains one member of each pair of parental homologous chromosomes. In these diagrams, two pairs of homologous chromosomes are shown, large and small. The yellow chromosomes are from one parent (for example, the father), and the violet chromosomes are from the other parent (for example, the mother).

FIGURE 11-23 Chromosome attachment to the spindle in mitosis

Meiotic Anaphase I Meiotic Anaphase I Homologous chromosome pairs separate Each homologous chromosome pair moves to a pole, pulled by microtubules

Meiotic Telophase I Meiotic Telophase I Spindle microtubules disappear Cytokinesis occurs Nuclear envelopes may reappear Chromosomes usually remain condensed

Meiosis II Meiotic Prophase II Meiotic Metaphase II Spindle microtubules reform and capture duplicated chromosomes Each chromatid contains a kinetochore Meiotic Metaphase II Duplicated chromosomes line up singly, perpendicular to the spindle Meiotic Anaphase II Chromatids separate Meiotic Telophase II Cytokinesis occurs, nuclear membranes reform, chromosomes relax

FIGURE 11-21 (part 2) Meiotic cell division in an animal cell In meiotic cell division (meiosis and cytokinesis), the homologous chromosomes of a diploid cell are separated, producing four haploid daughter cells. Each daughter cell contains one member of each pair of parental homologous chromosomes. In these diagrams, two pairs of homologous chromosomes are shown, large and small. The yellow chromosomes are from one parent (for example, the father), and the violet chromosomes are from the other parent (for example, the mother).

Table 11-1a A Comparison of Mitotic and Meiotic Cell Divisions in Animal Cells

When Do Mitotic and Meiotic Cell Divisions Occur in the Life Cycles of Eukaryotes?

FIGURE 11-25 The three major types of eukaryotic life cycles The lengths of the arrows correspond roughly to the proportion of the life cycle spent in each stage.

Haploid Life Cycles Fungi and unicellular algae Most of life cycle is haploid Asexual reproduction by mitotic cell division produces a population of identical, haploid cells Life cycle - Chlamydomonas

Diploid Life Cycles Most animals Most of cycle is in diploid state Haploid gametes are formed by meiosis Gametes fuse to form a diploid zygote Zygote develops into adult through mitotic cell divisions

Alternation-of-Generation Cycles Plants Includes both multicellular diploid and multicellular haploid body forms Multicellular diploid body gives rise to haploid spores, through meiosis Spores undergo mitosis to produce a multicellular haploid generation

Alternation-of-Generation Cycles Eventually, certain haploid cells differentiate into haploid gametes 2 gametes fuse to form a diploid zygote Zygote grows by mitotic cell division into a diploid multicellular diploid generation

How Do Meiosis and Sexual Reproduction Produce Genetic Variability?

Novel Chromosome Combinations Genetic variability among organisms is essential in a changing environment Mutations produce new variation but are relatively rare occurrences

Novel Chromosome Combinations Randomized line up and separation of homologous chromosomes in Meiotic Metaphase I and Anaphase I increase variation # of possible combinations is 2n, where n = number of homologous pairs Metaphase I Anaphase I

Crossing Over Variation enhanced by genetic recombination Crossing over creates chromosomes with new allele combinations Combined with homologue shuffling in Metaphase/Anaphase I, each gamete produced in meiosis is virtually unique

Fusion of Gametes Fusion of gametes from 2 individuals further increases possible 2n combinations Gametes from 2 humans could produce about 64 trillion different 2n combinations Taken together with crossing over, each human individual is absolutely genetically unique

The End