How Cells Divide Chapter 11

Outline Cell Division in Prokaryotes Discovery of Chromosomes Structure of Chromosomes Phases of the Cell Cycle Interphase Mitosis Cytokinesis Cell Cycle Control Cancer

Cell Division in Prokaryotes Prokaryotic cell division occurs as binary fission in which cell divides into two halves. Genetic information exists as a single, circular double-stranded DNA molecule. Copying begins at replication origin, and proceeds bi-directionally. One genome ends up in each daughter cell.

Binary Fission

Discovery of Chromosomes All eukaryotic cells store genetic information in chromosomes. Most eukaryotes have between 10 and 50 chromosomes in their body cells. Human cells have 46 chromosomes. 23 nearly-identical pairs

Structure of Chromosomes Chromosomes are composed of a complex of DNA and protein, chromatin. heterochromatin - not expressed euchromatin - expressed DNA exists as a single, long, double-stranded fiber extending chromosome’s entire length. forms nucleosome every 200 nucleotides DNA coiled around histone proteins

Eukaryotic Chromosomal Organization

Structure of Chromosomes Karyotype - Individual’s particular array of chromosomes. diploid - A cell possessing two copies of each chromosome (human body cells). Homologous chromosomes are made up of sister chromatids joined at the centromere. haploid - A cell possessing a single copy of each chromosome (human sex cells).

Chromosomes

Phases of the Cell Cycle Five phases of cell division: G1 - primary growth phase S - genome replicated G2 - secondary growth phase collectively called interphase M - mitosis C - cytokinesis

Cell Cycle

Interphase G1 - cells undergo majority of growth S - each chromosome replicates to produce sister chromatids attached at centromere contains attachment site (kinetochore) G2 - chromosomes condense assemble machinery such as centrioles

Mitosis Prophase spindle apparatus assembled Microtubules connect kinetochores on each pair of sister chromatids to the spindle poles. nuclear envelope breaks

Mitosis Metaphase chromosomes align in cell’s center metaphase plate spindle

Mitosis Anaphase sister chromatids pulled toward poles poles move apart centromeres move toward poles microtubules shorten Telophase spindle disassembles nuclear envelope forms around each set of sister chromatids

Cytokinesis Cleavage of cell into two halves animal cells constriction belt of actin filaments plant cells cell plate fungi and protists mitosis occurs within the nucleus

Cytokinesis

Cell Cycle Control Two irreversible points in cell cycle: replication of genetic material separation of sister chromatids Cell can be put on hold at specific checkpoints.

Cell Control Cycle G1 / S - primary division decision point G2 / M - commitment to mitosis Spindle checkpoint - all chromosomes are attached to spindle

Growth Factors and the Cell Cycle Each growing cell binds minute amounts of positive regulatory signals (growth factors) that stimulate cell division. If neighboring cells use up too much growth factor, there is not enough left to trigger cell division. Growth factors trigger intercellular signaling systems.

Cancer and Cell Proliferation Cancer is essentially a failure of cell division control (unrestrained, uncontrolled cellular growth). gene p53 plays a role in G1 checkpoint halts cell division if detects damaged DNA and stimulates activity of repair enzymes appears to be absent in many cancerous cells

Growth Factors and Cancer Growth factors influence cell cycle proto-oncogenes - normal cellular genes that become oncogenes when mutated positive approach that stimulates cell growth tumor-suppressor genes - negative approach that inhibits cell division

Summary Cell Division in Prokaryotes Discovery of Chromosomes Structure of Chromosomes Phases of the Cell Cycle Interphase Mitosis Cytokinesis Cell Cycle Control Cancer