Chromosomes Mitosis and Meiosis Chapter 10 Learning Objective

Chromosomes, Mitosis and Meiosis Chapter 10

Learning Objective 1 • What is the significance of chromosomes in terms of information?

Chromosomes

Organization • Genes • • • Chromatin • • • cell’s informational units made of DNA and protein makes up chromosomes (eukaryotes) Chromosomes • • allow DNA sorting into daughter cells

KEY CONCEPTS • In eukaryotic cells, DNA is wound around specific proteins to form chromatin, which in turn is folded and packaged to make individual chromosomes

Learning Objective 2 • How is DNA organized in prokaryotic and eukaryotic cells?

Prokaryotic Cells • Contain circular DNA molecules

Eukaryotic Chromosomes • Nucleosome • • • histone (protein) bead wrapped in DNA organized into coiled loops held together by nonhistone scaffolding proteins

Nucleosomes

DNA wound around a cluster of histone molecules Histone tails Linker DNA Nucleosome (10 nm diameter) Fig. 10 -2 a, p. 213

100 nm Fig. 10 -2 b, p. 213

Scaffolding Proteins

DNA Scaffolding proteins 2 μm Fig. 10 -3, p. 213

Chromosome Organization

1400 nm 700 nm 300 nm fiber (looped domains) 30 nm chromatin fiber DNA wound around a cluster of histone molecules Condensed Scaffolding chromosome chromatin protein Extended chromatin Packed nucleosomes Histone 10 nm 2 nm DNA double helix Nucleosomes Fig. 10 -4, p. 214

Learning Objective 3 • What are the stages in the eukaryotic cell cycle, and their principal events?

Eukaryotic Cell Cycle • Cycle of cell division • • interphase M phase

INTERPHASE G 1 (First gap phase) S (Synthesis phase) G 2 (Second gap phase) M PHASE (Mitosis and cytokinesis) Fig. 10 -5, p. 215

Interphase • First gap phase (G 1 phase) • • Synthesis phase (S phase) • • • cell grows and prepares for S phase DNA and chromosome protein synthesis chromosome duplication Second gap phase (G 2 phase) • • protein synthesis increases preparation for cell division

M Phase • Mitosis • • • nuclear division two nuclei identical to parent nucleus Cytokinesis • • cytoplasm divides two daughter cells

KEY CONCEPTS • Cell division is an important part of the cell cycle, which consists of the successive stages through which a cell passes

Animation: The Cell Cycle CLICK TO PLAY

Learning Objective 4 • What is the structure of a duplicated chromosome, including the sister chromatids, centromeres, and kinetochores?

A Duplicated Chromosome • Consists of a pair of sister chromatids • • Centromere • • • containing identical DNA sequences constricted region joins sister chromatids Kinetochore • • protein to which microtubules bind attached to centromere

Sister Chromatids

Centromere region Microtubules Kinetochore Sister chromatids 1. 0 μm Fig. 10 -7, p. 218

Learning Objective 5 • What is the process and significance of mitosis?

Mitosis • Preserves chromosome number • • in eukaryotic cell division Identical chromosomes are distributed to each pole of the cell • nuclear envelope forms around each set

Interphase

INTERPHASE Chromatin PROPHASE Nucleolus Nucleus Pieces of nuclear envelope PROMETAPHASE Sister chromatids of duplicated chromosome Spindle microtubule Nuclear envelope Centrioles Plasma membrane Kinetochore Developing mitotic spindle Fig. 10 -6 a, p. 216

Prophase Chromatin condenses into duplicated chromosomes (pair of sister chromatids) • Nuclear envelope begins to disappear • Mitotic spindle begins to form •

Mitotic Spindle

Metaphase plate (cell’s midplane) Kinetochore microtubule (spindle microtubule) Centrioles Astral microtubules Pericentriolar material Polar (nonkinetochore) microtubule Sister chromatids Fig. 10 -9 a, p. 219

10 μm Fig. 10 -9 b, p. 219

Prophase

Prometaphase Spindle microtubules attach to kinetochores of chromosomes • Chromosomes begin to move toward cell’s midplane •

Prometaphase

Metaphase Chromosomes align on cell’s midplane (metaphase plate) • Mitotic spindle is complete • Microtubules attach kinetochores of sister chromatids to opposite poles of cell •

Metaphase

Anaphase • Sister chromatids separate • • move to opposite poles Each former chromatid is now a chromosome

Anaphase

Telophase • • • Nuclear envelope re-forms Nucleoli appear Chromosomes uncoil Spindle disappears Cytokinesis begins

Telophase

ANAPHASE TELOPHASE 25 μm METAPHASE Spindle Cleavage furrow Centriole pair at spindle pole Cell’s midplane (metaphase plate) Daughter chromosomes Reforming nuclear envelope Fig. 10 -6 b, p. 217

KEY CONCEPTS • In cell division by mitosis, duplicated chromosomes separate (split apart) and are evenly distributed into two daughter nuclei

Cytokinesis

Cleavage furrow Actomyosin contractile ring 10 μm Fig. 10 -10 a, p. 220

Vesicles gather on cell’s midplane Plasma Cell membrane wall Small vesicles fuse, forming larger vesicles Cell plate forming Eventually one large vesicle exists New cell walls (from vesicle contents) New plasma membranes (from vesicle membranes) Cell plate Nucleus forming 5 μm Fig. 10 -10 b, p. 220

Learning Objective 6 • How is the cell cycle controlled?

Cell-Cycle Control • Cyclin-dependent kinases (Cdks) • • • protein kinases that control cell cycle active only when bound to cyclins Cyclins • • regulatory proteins levels fluctuate during cell cycle

Cyclins

1 5 Cdk G 1 M 1 Cyclin is synthesized and accumulates. 3 3 M-Cdk phosphorylates proteins, activating those that facilitate mitosis and inactivating those that inhibit mitosis. 4 An activated enzyme complex recognizes a specific amino acid sequence in cyclin and targets it for destruction. When cyclin is degraded, M-Cdk activity is terminated, and the cells formed by mitosis enter G 1. 5 Cdk is not degraded but is recycled and reused. S 4 G 2 Cyclin 4 Degraded cyclin 1 2 5 2 Cdk associates with cyclin, forming a cyclin–Cdk complex, M-Cdk. 2 3 M-Cdk (triggers M phase) Cdk Fig. 10 -12, p. 222

KEY CONCEPTS • An internal genetic program interacts with external signals to regulate the cell cycle

Learning Objective 7 • What is the difference between asexual and sexual reproduction?

Asexual Reproduction • Single parent • • offspring have identical hereditary traits Mitosis • basis for eukaryotic asexual reproduction

Binary Fission

Plasma membrane Cell wall Origin of replication Prokaryotic cell Bacterial DNA Two copies of bacterial DNA 1 DNA replication begins at single site on bacterial DNA. 2 Replication continues, as replication enzymes work in both directions from site where replication began. 3 Replication is completed. Cell begins to divide, as plasma membrane grows inward. 4 Binary fission is complete. Two identical prokaryotic cells result. Two identical prokaryotic cells Fig. 10 -11, p. 221

Sexual Reproduction Two haploid sex cells (gametes) fuse to form a single diploid zygote • Meiosis • • produces gametes

Learning Objective 8 What is the difference between haploid and diploid cells? • What are homologous chromosomes? •

Diploid Cell • Chromosomes are paired (homologous chromosomes) • • similar in length, shape, other features carry genes affecting the same traits

Haploid Cell • Contains only one member of each homologous chromosome pair

Fig. 10 -16, p. 229

MITOSIS PROPHASE No synapsis of homologous chromosomes ANAPHASE Sister chromatids move to opposite poles DAUGHTER CELLS Two 2 n cells with unduplicated chromosomes Fig. 10 -16 a, p. 229

MEIOSIS PROPHASE I Synapsis of homologous chromosomes to form tetrads ANAPHASE I Homologous chromosomes move to opposite poles PROPHASE II Two n cells with duplicated chromosomes ANAPHASE II Sister chromatids move to opposite poles HAPLOID CELLS Four n cells with unduplicated chromosomes Fig. 10 -16 b, p. 229

Learning Objective 9 • What is the process and significance of meiosis?

Meiosis One diploid cell divides two times, yielding four haploid cells • Sexual life cycles in eukaryotes require meiosis • • each gamete contains half the number of chromosomes in parent cell

Meiosis I • Prophase I • • Crossing-over • • • homologous chromosomes join (synapsis) between homologous (nonsister) chromatids exchanges segments of DNA strands Results in genetic recombination

Synapsis

Paternal sister chromatids Maternal sister chromatids Synaptonemal complex Chromatin Protein Chromatin Maternal sister chromatids Fig. 10 -14 a, p. 228

Chromosome Synaptonemal complex Chromosome 0. 5 μm Fig. 10 -14 b, p. 228

Meiosis I • Metaphase I • • Anaphase I • • • tetrads (homologous chromosomes joined by chiasmata) line up on metaphase plate homologous chromosomes separate distributed to different nuclei Each nucleus contains haploid number of chromosomes • each chromosome has 2 chromatids

Tetrads and Chiasmata

Sister chromatids Chiasmata Kinetochores Sister chromatids 1 μm Fig. 10 -15 a, p. 228

Sister chromatids Chiasmata Kinetochores Fig. 10 -15 b, p. 228

Meiosis II • Two chromatids of each chromosome separate • • one distributed to each daughter cell Each former chromatid is now a chromosome

Meiosis

Meiosis

INTERPHASE MEIOSIS I Mid-prophase I Nucleolus Nuclear envelope Chromatin Centrioles Interphase preceding meiosis; DNA replicates. Late prophase I Homologous chromosomes Developing meiotic spindle Homologous chromosomes synapse, forming tetrads; nuclear envelope breaks down. Fig. 10 -13 a (1), p. 226

MEIOSIS II Prophase II Metaphase II Anaphase II Daughter chromosomes Chromosomes condense again following brief period of interkinesis. DNA does not replicate again. Chromosomes line up along cell's midplane. Sister chromatids separate, and chromosomes move to opposite poles. Fig. 10 -13 a (2), p. 226

Anaphase I Metaphase I Telophase I Microtubule attached to kinetochore Sister chromatids Tetrads line up on cell's midplane. Tetrads held together at chiasmata (sites of prior crossingover). Cleavage furrow Separation of homologous chromosomes Homologous chromosomes separate and move to opposite poles. Note that sister chromatids remain attached at their centromeres. One of each pair of homologous chromosomes is at each pole. Cytokinesis occurs. Fig. 10 -13 b (1), p. 227

Telophase II Four haploid cells 25 μm Nuclei form at opposite poles of each cell. Cytokinesis occurs. Four gametes (animal) or four spores (plant) are produced. Fig. 10 -13 b (2), p. 227

Learning Objective 10 • What are the different processes and outcomes of mitosis and meiosis?

Mitosis Single nuclear division • 2 daughter cells genetically identical to each other and to original cell • No synapsis of homologous chromosomes •

Mitosis

Meiosis Two successive nuclear divisions form four haploid cells • Synapsis of homologous chromosomes occurs during prophase I •

Meiosis

KEY CONCEPTS • Meiosis, which reduces the number of chromosome sets from diploid to haploid, is necessary to maintain the normal chromosome number when two cells join during sexual reproduction

KEY CONCEPTS • Meiosis helps to increase genetic variation among offspring

Learning Objective 11 • Compare the roles of mitosis and meiosis in various generalized life cycles

Animals • Somatic cells are diploid • • produced by mitosis Gametes are haploid • produced by meiosis (gametogenesis)

Animal Life Cycle

Gametes (n) Meiosis Fertilization Zygote (2 n) Mitosis Animals Multicellular diploid organism (2 n) Fig. 10 -17 a, p. 230

Simple Eukaryotes • May be haploid • • produced by mitosis Only diploid stage is the zygote • which undergoes meiosis to restore the haploid state

Simple Eukaryote Life Cycle

Unicellular or multicellular haploid organism (n) Mitosis Gametes (n) Meiosis Fertilization Zygote (2 n) Simple eukaryotes Fig. 10 -17 b, p. 230

Plants • Alternation of generations: • • sporophyte generation gametophyte generation

Plants • Sporophyte generation • • • multicellular diploid forms haploid spores by meiosis Spore divides (mitosis) to form gametophyte generation • • multicellular haploid produces gametes by mitosis

Plants Two haploid gametes fuse to form diploid zygote • Zygote divides (mitosis) to produce new diploid sporophyte generation •

Plant Life Cycle

Gametophyte (n) (multicellular haploid organism) Mitosis Spores (n) Gametes (n) Fertilization Meiosis Zygote (2 n) Mitosis Sporophyte (2 n) (multicellular diploid organism) Plants, some algae, and some fungi Fig. 10 -17 c, p. 230

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