How Cells Divide – Mitosis and Meiosis Chapters 11&12

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).

Karyotype & 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.

Sexual Reproduction and Meiosis Chapter 12

Reduction Division In sexual reproduction, gametes fuse (fertilization) to produce a zygote. Gamete formation involves a mechanism (meiosis) that reduces the number of chromosomes to half that found in other cells. Adult body cells are diploid. Gamete cells are haploid. alternation of generations

Sexual Life Cycle Diploid cells carry chromosomes from two parents 2 haploid cells join to form diploid cell

Sexual Life Cycle Three types of sexual life cycles. In sexual reproduction, haploid cells or organisms alternate with diploid cells or organisms

Sexual Life Cycle

Meiosis Synapsis Homologues pair along their length. Homologous recombination Genetic exchange (crossing over) occurs between homologous chromosomes. Reduction division Meiosis involves two successive divisions, with no replication of genetic material between them.

Unique Features of Meiosis

Prophase I Homologous chromosomes become closely associated in synapsis, exchange segments via crossing over, and then separate. Presence of a chiasma indicates crossing over has occurred.

Metaphase I Terminal chiasmata holds homologous pair together. Spindle microtubules attach to kinetochore proteins on the outside of each centromere. Joined pairs of homologues lines up on metaphase plate. orientation of each pair is random

Completing Meiosis Anaphase I Spindle fibers begin to shorten and pull whole centromeres toward poles. Each pole receives a member of each homologous pair. complete set of haploid chromosomes random orientation results in independent assortment

Completing Meiosis Telophase I Chromosomes are segregated into two clusters; one at each pole. Nuclear membrane re-forms around each daughter cell. Sister chromatids are no longer identical due to crossing over.

Second Meiotic Division Meiosis II resembles normal mitotic division. prophase II - nuclear envelope breaks down and second meiotic division begins metaphase II - spindle fibers bind to both sides of centromere anaphase II - spindle fibers contract and sister chromatids move to opposite poles telophase II - nuclear envelope re-forms Final result - four haploid cells

Sex Asexual reproduction - individual inherits all its chromosomes from a single parent parthenogenesis - development of an adult from an unfertilized egg Sexual reproduction - produces genetic variability. Segregation of chromosomes tends to disrupt advantageous combinations. Only some progeny maintain advantages.

Origin and Maintenance Of Sex Theories DNA repair hypothesis Only diploid cells can effectively repair certain kinds of chromosomal damage. Contagion hypothesis A secondary consequence of the infection of eukaryotes by mobile genetic elements.

Origin and Maintenance Of Sex Red Queen hypothesis Current recessive alleles can be stored in reserve for future use. Miller’s Ratchet Sexual reproduction may be a method of keeping the mutational load low.

Evolutionary Consequences of Sex Evolutionary process is revolutionary and conservative. pace of evolutionary change is accelerated by genetic recombination evolutionary change not always favored by selection may act to preserve existing gene combinations

Independent Assortment