Chromosomes, Chromatids, Mitosis!

Genomes, Chromosomes, DNA, Genes Eukaryotic genomes are made up of multiple chromosomes. Each chromosome contains one double-stranded DNA molecule. Each DNA molecule contains many genes. Expression of each gene is controlled by its DNA sequence and chromosomal configuration.

Chromosome Number Haploid (N): number of chromosomes in a gamete (egg or sperm) cell Diploid (2N): number of chromosomes in a zygote (produce of fertilization) and cells derived from the zygote Value for N varies by species. –Drosophila: 4 –maize: 10 –humans: 23

Life Cycle of Sexual Organism

Two Types of Cell Division Mitosis –cells divide to produce genetically identical cells (exact copies) –chromosome number in daughter cells is same as in original cell Meiosis –cells divide to produce genetically variable cells –chromosome number in daughter cells is half the number in original cell

Two Types of Cell Division Mitosis –cells divide to produce genetically identical cells (exact copies) –chromosome number in daughter cells is same as in original cell Meiosis –cells divide to produce genetically variable cells –chromosome number in daughter cells is half the number in original cell

Mitotic Cell Cycle Phases: M: mitosis / cytokinesis G1: growth S: DNA replication G2: growth Pierce, B Genetics, a conceptual aplproach. 2nd Ed. WH Freeman.

Chromosome Anatomy and S phase Before S phase & DNA replication After S phase & DNA replication 1 DNA molecule per chromosome 2 DNA molecules: 1 per chromatid A chromatid is a chromosome held to another chromosome at the centromere. Pierce, B Genetics, a conceptual aplproach. 2nd Ed. WH Freeman.

Process of Mitosis Interphase (before mitosis): DNA replicates to form duplicated chromosomes (sister chromatids) Prophase: chromosomes condense to form X-shaped structures; nuclear membrane disintegrates Metaphase: chromosomes align in the middle of the cell; spindle fibers attach to kinetochores Anaphase: spindle fibers contract and pull chromatids apart at the centromeres; chromatids (now called chromosomes) move to opposite poles of the cell Telophase: nuclear membranes re-form around the two chromosomal clusters; new cell membrane forms in preparation for cytokinesis

Mitosis Animations utorials/cell_cycle/cells3.htmlhttp:// utorials/cell_cycle/cells3.html

Result of Mitosis Mitosis produces two cells that are genetically identical to the original cell. The daughter cells have the same number of chromosomes as the original cell. Diploid (2N) cell divides to produce two diploid (2N) cells.

Objective: Really learn mitosis. Materials: Pink and blue ribbons (chromosomes /chromatids) with velcro centers (centromeres). Numbered necklaces (chromosome labels). Silver ribbons (spindle fibers). Procedure: Enact mitosis, starting with a diploid cell with 4 chromosomes.

Regulation of Cell Division Cell division is tightly regulated. Checkpoints: –G1/S. Is the cell big enough? Is the DNA replication machinery ready? –G2/M. Is all DNA duplicated? Is the environment favorable? Pierce, B Genetics, a conceptual aplproach. 2nd Ed. WH Freeman.

Regulation of Cell Division Positive regulation –Activates cell division when conditions are favorable Negative regulation –Inhibits cell division when conditions are unfavorable Proper cell division is a careful balance of positive and negative regulation

What happens when cell division is not properly regulated? Uncontrolled (too much) cell division Formation of tumors, masses of cells that divide when they should not Tumors can become malignant (cancer)

Cancer is caused by mutations in genes that regulate cell division Oncogenes -- positive regulators –normal role is to activate cell division Growth factors: promote growth Growth factor receptors: bind growth factors, initiate signal transduction pathways Protein kinases: alter protein activity Transcription factors: turn on genes involved in cell division –mutants activate too much cell division

Cancer is caused by mutations in genes that regulate cell division Tumor suppressor genes -- negative regulators –normal role is to inhibit cell division Transcription factors: turn on genes that inhibit activity of positive regulators of cell division –mutants do not function, so cells divide too much

Tumors are Clonal in Origin Tumor cells arise through successive mitotic divisions of one original mutant cell. Additional mutations may occur, eventually leading to malignancy. Pierce, B Genetics, a conceptual aplproach. 2nd Ed. WH Freeman.

Summary The cell cycle is divided into interphase and mitosis. Mitosis gives rise to two daughter cells that are genetically identical (clones) to the original cell. Mitosis and cell division are carefully regulated. Mis-regulation can lead to too much cell division, producing tumors and / or cancer.