Cell Division Mitosis and Meiosis

Mitosis Asexual reproduction Purpose: Growth, maintenance, and repair

Cell Cycle 1. Interphase Cells are in interphase 90% of the time. –G1- growth –S - chromosome replication –G2- Prepare for mitosis 2. Mitosis (nuclear division) 3. Cytokinesis (cytoplasmic division)

Interphase (pre game show) Chromosomes are copied (# doubles) Chromosomes appear as threadlike coils (chromatin) at the start, but each chromosome and its copy (sister chromosome) change to sister chromatids at end of this phase

6 Synthesis Phase (S phase) S phase during interphase of the cell cycle Nucleus of eukaryotes Mitosis -prophase -metaphase -anaphase -telophase G1G1 G2G2 S phase interphase DNA replication takes place in the S phase.

7 DNA Replication Facts DNA has to be copied before a cell divides in order to make new cellsDNA has to be copied before a cell divides in order to make new cells New cells will need identical DNA strandsNew cells will need identical DNA strands

How do cells replicate their DNA? unwound by an enzyme. 1.The DNA double helix is

adds complementary nucleotides. 2.Another enzyme moves down the 2 original DNA strands and Base Pairing Rule

3.Two separate DNA molecules are formed, each with one new strand and one old strand.

Practice DNA Replication

Replication Mistakes 6 billion base pairs per cell. All are replicated in a few hours. Mistake happens one per billion nucleotides.

Mistakes = Mutations Mutations are changes in the base sequence of the DNA molecules. This can have serious affects on genes and the functions of cells. Cancer is caused by mutations in DNA.

Mutagens Physical or chemical factor that increases the frequency of mutations. Examples: UV rays, X-rays, smoking

Overview of Mitosis Parent Cell=2n (diploid) Daughter cell= 2n (diploid) Daughter cell= 2n (diploid)

PROPHASE CENTRIOLES APPEAR AND MOVE TO OPPOSITE ENDS OF THE CELL Spindle fibers form between centrioles Centriole Spindle Fibers

METAPHASE Chromosomes “meet” in the middle. Spindle fibers attach to the centromere of each chromosome.

ANAPHASE Sister chromatids separate from each other and are “pulled” to opposite ends of the cell.

Telophase Cell plate forms and cell membrane begins pinching in= “cleavage.” Chromosomes look like threads (chromatin) again. Nuclear envelope forms again.

Cytokinesis Division of the cytoplasm Cell membrane completely pinches off. Cycle starts again.

Results: 2 diploid (2n) daughter cells, genetically identical to parent cell

Meiosis

Type: Sexual Reproduction Purpose: To create haploid gametes

Meiosis Types of cells that undergo meiosis: Gametes (sex cells) –Egg and sperm

Meiosis 1 First division of meiosis Prophase 1: Each chromosome dupicates and remains closely associated. These are called sister chromatids. Crossing-over can occur during the latter part of this stage.Crossing-over Metaphase 1: Homologous chromosomes align at the equatorial plate. Anaphase 1: Homologous pairs separate with sister chromatids remaining together. Telophase 1: Two daughter cells are formed with each daughter containing only one chromosome of the homologous pair.

Meiosis II Second division of meiosis: Gamete formation Prophase 2: DNA does not replicate. Metaphase 2: Chromosomes align at the equatorial plate. Anaphase 2: Centromeres divide and sister chromatids migrate separately to each pole. Telophase 2: Cell division is complete. Four haploid daughter cells are obtained.

Meiosis: Overview Parent Cell: diploid= 2n After Meiosis I After Meiosis II Daughter Cells: Haploid= n ; and all genetically different from each other and parent cell

??? Why would it be important for the gametes to contain half as many chromosomes as a somatic cell?

Comparing Mitosis and Meiosis

Mitosis vs. meiosis

Meiosis – key differences from mitosis Meiosis reduces the number of chromosomes by half. Daughter cells differ from parent, and each other. Meiosis involves two divisions, Mitosis only one. Meiosis I involves: –Synapsis – homologous chromosomes pair up. –Crossing over of non-sister chromatids occur. –In Metaphase I, homologous pairs line up at metaphase plate. –In Anaphase I, sister chromatids do NOT separate. –Overall, separation of homologous pairs of chromosomes, rather than sister chromatids of individual chromosome.

Exit Ticket Name 3 key differences between mitosis and meiosis.