The Cell Cycle and Cell Division

7 The Cell Cycle and Cell Division

Chapter 7 The Cell Cycle and Cell Division • Key Concepts • 7.1 Different Life Cycles Use Different Modes of Cell Reproduction • 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells • 7.3 Cell Reproduction Is Under Precise Control

Chapter 7 The Cell Cycle and Cell Division • Key Concepts • 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity • 7.5 Programmed Cell Death Is a Necessary Process in Living Organisms

Chapter 7 Opening Question How does infection with HPV result in uncontrolled cell reproduction?

Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction • The lifespan of an organism is linked to cell reproduction—usually called cell division. • Organisms have two basic strategies for reproducing themselves: • Asexual reproduction • Sexual reproduction • Cell division is also important in growth and repair of tissues.

Figure 7.1 The Importance of Cell Division (Part 1)

Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction In asexual reproduction the offspring are clones—genetically identical to the parent. Any genetic variations are due to mutations. A unicellular prokaryote may reproduce itself by binary fission. Single-cell eukaryotes can reproduce by mitosis. Other eukaryotes are also able to reproduce through asexual or sexual means.

Figure 7.2 Asexual Reproduction on a Large Scale

Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction Sexual reproduction requires gametes—two parents each contribute one gamete to an offspring. Gametes form by meiosis—a process of cell division. Gametes—and offspring—differ genetically from each other and from the parents.

Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction DNA in eukaryotic cells is organized into chromosomes. A chromosome consists of a single molecule of DNA and proteins. Somatic cells—body cells not specialized for reproduction Each somatic cell contains two sets of chromosomes (homologs) that occur in homologous pairs.

Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction Gametes contain only one set of chromosomes—one homolog from each pair. Haploid cell—Number of chromosomes = n Fertilization—Two haploid gametes (female egg and male sperm) fuse to form a zygote. Chromosome number in zygote = 2n and cells are diploid.

Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction All kinds of sexual life cycles involve meiosis: Haplontic life cycle—in protists, fungi, and some algae—zygote is only diploid stage After zygote forms it undergoes meiosis to form haploid spores, which germinate to form a new organism. Organism is haploid, and produces gametes by mitosis—cells fuse to form diploid zygote.

Figure 7.3 All Sexual Life Cycles Involve Fertilization and Meiosis (Part 1)

Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction Alternation of generations—most plants, some protists; meiosis gives rise to haploid spores Spores divide by mitosis to form the haploid generation (gametophyte). Gametophyte forms gametes by mitosis. Gametes then fuse to form diploid zygote (sporophyte), which in turn produces haploid spores by meiosis.

Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction Diplontic life cycle—animals and some plants; gametes are the only haploid stage A mature organism is diploid and produces gametes by meiosis. Gametes fuse to form diploid zygote; zygote divides by mitosis to form mature organism.

Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction The essence of sexual reproduction is that it allows the random selection of half the diploid chromosome set. This forms a haploid gamete that fuses with another to make a diploid cell. Thus, no two individuals have exactly the same genetic makeup.

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Four events must occur for cell division: Reproductive signal—to initiate cell division Replication of DNA Segregation—distribution of the DNA into the two new cells Cytokinesis—division of the cytoplasm and separation of the two new cells

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells In prokaryotes, cell division results in reproduction of the entire organism. The cell: Grows in size Replicates its DNA Separates the DNA and cytoplasm into two cells through binary fission

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Most prokaryotes have one chromosome, a single molecule of DNA—usually circular. Two important regions in reproduction: ori - where replication starts ter - where replication ends

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Replication occurs as the DNA is threaded through a “replication complex” of proteins in the center of the cell. Replication begins at the ori site and moves towards the ter site.

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells As replication proceeds, the ori complexes move to opposite ends of the cell. DNA sequences adjacent to the ori region actively bind proteins for the segregation, hydrolyzing ATP for energy. An actin-like protein provides a filament along which ori and other proteins move.

Figure 7.4 Prokaryotic Cell Division

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Cytokinesis begins after chromosome segregation by a pinching in of the plasma membrane—protein fibers form a ring. As the membrane pinches in, new cell wall materials are synthesized resulting in separation of the two cells.

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Eukaryotic cells divide by mitosis followed by cytokinesis. Replication of DNA occurs as long strands are threaded through replication complexes. DNA replication only occurs during a specific stage of the cell cycle.

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells In segregation of DNA after cell division, one copy of each chromosome ends up in each of the two new cells. In eukaryotes, the chromosomes become highly condensed. Mitosis segregates them into two new nuclei— the cytoskeleton is involved in the process.

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Cytokinesis follows mitosis. The process in plant cells (which have cell walls) is different than in animal cells (which do not have cell walls).

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells The cell cycle—the period between cell divisions In eukaryotes it is divided into mitosis and cytokinesis—called the M phase—and a long interphase. During interphase, the cell nucleus is visible and cell functions including replication occur Interphase begins after cytokinesis and ends when mitosis starts.

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Interphase has three subphases: G1, S, and G2. G1 (Gap 1)—variable, a cell may spend a long time in this phase carrying out its functions S phase (Synthesis)—DNA is replicated G2 (Gap 2)—the cell prepares for mitosis, synthesizes microtubules for segregating chromosomes

Figure 7.5 The Phases of the Eukaryotic Cell Cycle (Part 1)

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells In mitosis, one nucleus produces two daughter nuclei each containing the same number of chromosomes as the parent nucleus. Mitosis is continuous, but can be can be divided into phases—prophase, prometaphase, metaphase, anaphase, and telophase.

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells During interphase, only the nuclear envelope and and the nucleolus are visible. The chromatin (DNA) is not yet condensed. Three structures appear in prophase: The condensed chromosomes Centrosome Spindle

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Condensed chromosomes appear during prophase. Sister chromatids—two DNA molecules on each chromosome after replication Centromere—region where chromatids are joined Kinetochores are protein structures on the centromeres, and are important for chromosome movement.

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells The karyotype of an organism reflects the number and sizes of its condensed chromosomes. Karyotype analysis can be used to identify organisms, but DNA sequence is more commonly used.

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Segregation is aided by other structures: The centrosome determines the orientation of the spindle apparatus. Each centrosome can consist of two centrioles—hollow tubes formed by microtubules. Centrosome is duplicated during S phase and each moves towards opposite sides of the nucleus.

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Centrosomes serve as mitotic centers or poles; the spindle forms between the poles from two types of microtubules: Polar microtubules form a spindle and overlap in the center. Kinetochore microtubules—attach to kinetochores on the chromatids. Sister chromatids attach to opposite halves of the spindle.

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Chromosome separation and movement is highly organized. During prometaphase, the nuclear envelope breaks down. Chromosomes consisting of two chromatids attach to the kinetochore mictotubules.

Figure 7.6 The Phases of Mitosis (1)

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Durin metaphase, chromosomes line up at the midline of the cell. During anaphase, the separation of sister chromatids is controlled by M phase cyclin-Cdk; cohesin is hydrolyzed by separase. After separation, they move to opposite ends of the spindle and are referred to as daughter chromosomes.

Figure 7.6 The Phases of Mitosis (2)

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells A protein at the kinetochores—cytoplasmic dynein—hydrolyzes ATP for energy to move chromosomes along the microtubules towards the poles. Microtubules also shorten, drawing chromosomes toward poles.

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Telophase occurs after chromosomes have separated: Spindle breaks down Chromosomes uncoil Nuclear envelope and nucleoli appear Two daughter nuclei are formed with identical genetic information

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Cytokinesis: Division of the cytoplasm differs in plant and animals In animal cells, plasma membrane pinches between the nuclei because of a contractile ring of microfilaments of actin and myosin.

Figure 7.7 Cytokinesis Differs in Animal and Plant Cells (Part 1)

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Plant cells: Vesicles from the Golgi apparatus appear along the plane of cell division These fuse to form a new plasma membrane. Contents of vesicles form the cell plate—the beginning of the new cell wall.

The Cell Cycle and Cell Division