Mitosis, Meiosis, and Life Cycles Chapter 12

LEARNING OBJECTIVE 1 Distinguish between a haploid cell and a diploid cell Define homologous chromosomes

KEY TERMS DIPLOID (2N) HAPLOID (N) The condition of having two sets of chromosomes per nucleus In somatic cells of diploid organisms, chromosomes are present in pairs HAPLOID (N) The condition of having one set of chromosomes per nucleus

KEY TERMS HOMOLOGOUS CHROMOSOMES Members of a chromosome pair that are similar in size, shape, and genetic constitution

LEARNING OBJECTIVE 2 Identify the phases in the cell cycle Describe the main events of each phase

KEY TERMS CELL CYCLE Cyclic series of events in the life of a dividing eukaryotic cell

KEY TERMS INTERPHASE Stage of the cell cycle between successive mitotic divisions

Interphase During interphase Interphase is divided into cell grows and prepares for next division DNA replicates Interphase is divided into first gap phase (G1) synthesis phase (S) second gap phase (G2)

KEY TERMS MITOSIS Division of the cell nucleus resulting in two daughter nuclei, each with the same number of chromosomes as the parent nucleus

KEY TERMS CYTOKINESIS Stage of cell division in which the cytoplasm divides to form two daughter cells

Cell Cycle

(Mitosis and cytokinesis) INTERPHASE G1 (First gap phase) S (Synthesis phase) G2 (Second gap phase) Figure 12.1: The cell cycle. The cell cycle includes interphase (G1, S, and G2) and M phase (mitosis and cytokinesis). The time required for each stage varies with cell type and species. Most cells spend about 90 percent of their cell cycle in interphase. M PHASE (Mitosis and cytokinesis) Fig. 12-1, p. 242

LEARNING OBJECTIVE 3 Describe the events that occur in each stage of mitosis

Stages of Mitosis Prophase Metaphase Anaphase Telophase

Prophase Chromatin condenses into chromosomes Nucleolus disappears Nuclear envelope breaks down Mitotic spindle begins to form At the end of prophase, each duplicated chromosome is composed of two sister chromatids

A Duplicated Chromosome

Centromere region Microtubules Kinetochore Sister chromatids Figure 12.3: A duplicated chromosome. Each duplicated chromosome is composed of two identical sister chromatids that are tightly associated at their centromere regions. Associated with each centromere is a kinetochore, which serves as a microtubule attachment site. Kinetochore Sister chromatids Fig. 12-3, p. 245

KEY TERMS SPINDLE Structure consisting mainly of microtubules that provides the framework for chromosome movement during cell division

Metaphase Duplicated chromosomes line up along midplane of cell

Anaphase Sister chromatids separate and move to opposite poles of the cell Each chromatid is now a separate chromosome

Telophase Nuclear envelope forms around each set of chromosomes Nucleoli reappear Chromosomes lengthen and become chromatin Spindle disappears

KEY TERMS CELL PLATE Cytokinesis generally takes place in telophase Structure that forms during cytokinesis in plants, separating two daughter cells produced by mitosis Cytokinesis generally takes place in telophase

Cytokinesis

Cell plate forming Nucleus Vesicles gather on cell's midplane Small vesicles fuse, forming larger vesicles Eventually one large vesicle exists New cell walls (from vesicle contents) Figure 12.4: Cytokinesis. Stages in the formation of plasma membranes and cell walls from the fusing vesicles of the cell plate. The electron micrograph shows cytokinesis in a maple (Acer) leaf cell. Note the cell plate, which consists of gathering vesicles. Plasma membrane Cell wall Cell plate forming New plasma membranes (from vesicle membranes) Fig. 12-4, p. 247

Interphase and Mitosis

Condensing chromosome (consists of 2 sister chromatids) (a) Interphase (b) Prophase (c) Metaphase (d) Anaphase (e) Telophase Figure 12.2: Interphase and the stages of mitosis. The micrographs are onion (Allium cepa) root tip cells prepared with stains. The diploid number for the cells shown in the diagrams is four. Condensing chromosome (consists of 2 sister chromatids) Sister chromatids (now called chromosomes) move to opposite ends of cell Cell plate New nuclei Spindle microtubules Fig. 12-2, p. 244

Condensing chromosome (consists of 2 sister chromatids) (c) Metaphase (a) Interphase (b) Prophase Condensing chromosome (consists of 2 sister chromatids) (c) Metaphase Spindle microtubules (d) Anaphase Sister chromatids (now called chromosomes) move to opposite ends of cell (e) Telophase New nuclei Cell plate Figure 12.2: Interphase and the stages of mitosis. The micrographs are onion (Allium cepa) root tip cells prepared with stains. The diploid number for the cells shown in the diagrams is four. Stepped Art Fig. 12-2, p. 244

LEARNING OBJECTIVE 4 Explain why meiosis is needed at some point in the life cycle of every sexually reproducing organism Describe the events that occur during meiosis I and meiosis II

KEY TERMS MEIOSIS Process in which a 2n cell undergoes successive nuclear divisions, potentially producing four n nuclei Leads to formation of spores in plants

Meiosis Meiosis must occur at some time in the life of a sexually reproducing organism if gametes are to be haploid Meiosis consists of two cell divisions Meiosis I Meiosis II

Meiosis I Members of each homologous pair of chromosomes separate and are distributed into separate nuclei in two daughter cells Chromosomes were duplicated prior to meiosis I, so each consists of two chromatids

Meiosis II Chromatids separate into individual chromosomes and are distributed into different haploid daughter cells Four haploid cells form

KEY TERMS SYNAPSIS Physical association of homologous chromosomes during prophase I of meiosis

Crossing Over Crossing over Exchange of segments of homologous chromosomes Synapsis and crossing over occur during prophase I of meiosis

Crossing Over

Sister chromatids Crossing over Kinetochores Fig. 12-6, p. 249 Figure 12.6: A pair of homologous chromosomes during late prophase I of meiosis. Note the four chromatids that make up the paired homologous chromosomes. Crossing over is visible at two sites. Fig. 12-6, p. 249

Meiosis

Meiosis

(a) Prophase I (b) Metaphase I (c) Anaphase I (d) Telophase I Cell plate Figure 12.5: The stages of meiosis. The micrographs are Easter lily (Lilium longiflorum) flower cells prepared with stains and flattened on slides. The diploid number for the cells shown in the diagrams is four. New nuclei Each chromosome consists of two chromatids Homologous chromosomes Spindle microtubules Fig. 12-5 (a-d), p. 248

(e) Prophase II (f) Metaphase II (g) Anaphase II (h) Telophase II Figure 12.5: The stages of meiosis. The micrographs are Easter lily (Lilium longiflorum) flower cells prepared with stains and flattened on slides. The diploid number for the cells shown in the diagrams is four. New nuclei Chromosomes (each with two chromatids) Spindle microtubules New cell walls and plasma membranes Fig. 12-5 (e-h), p. 249

LEARNING OBJECTIVE 5 Compare and contrast mitosis and meiosis

Mitosis Involves a single nuclear division in which the two daughter cells formed are genetically identical to each other and to the original cell Synapsis of homologous chromosomes does not occur during mitosis

Meiosis Involves two successive nuclear divisions and forms four haploid cells, each with a different combination of genes Synapsis of homologous chromosomes occurs during prophase I of meiosis

Mitosis and Meiosis

Mitosis Meiosis Synapsis (pairing of homologous chromosomes) Parent cell (2n = 4) Parent cell (2n = 4) Prophase I Prophase Nuclear envelope Nuclear envelope 1st meiotic division Mitosis Prophase II 2nd meiotic division Figure 12.7: Mitosis compared with meiosis. The diploid number for each cell is four. Two daughter cells (each 2n = 4) identical to parent cell Four daughter cells (each n = 2) genetically different from parent cell Mitosis Meiosis Fig. 12-7, p. 250

LEARNING OBJECTIVE 6 Define alternation of generations

KEY TERMS Alternation of generations SPOROPHYTE GAMETOPHYTE Plants alternate haploid and diploid generations SPOROPHYTE 2n, spore-producing stage in plant life cycle GAMETOPHYTE n, gamete-producing stage in plant life cycle

The Sporophyte The 2n, spore-producing stage in the life cycle of a plant A diploid sporophyte plant forms haploid spores by meiosis A spore divides mitotically to form a haploid gametophyte plant

The Gametophyte The n, gamete-producing stage in the life cycle of a plant The gametophyte produces haploid gametes by mitosis Two gametes fuse to form a diploid zygote, which divides mitotically to produce a diploid sporophyte

Plant Life Cycle

Gametophyte (n) (multicellular haploid organism) Mitosis Mitosis Spores (n) Gametes (n) Meiosis Fertilization Zygote (2n) Figure 12.8: Plant life cycle. The life cycle of plants involves an alternation of generations between diploid and haploid stages. Mitosis Sporophyte (2n) (multicellular diploid organism) Fig. 12-8, p. 251

Animation: The Cell Cycle CLICK TO PLAY

Animation: Comparing Mitosis and Meiosis CLICK TO PLAY