1 Mitosis: nuclear division Overview: DNA goes from grainy- looking chromatin to highly condensed chromosomes. Chromosomes line up along cell equator. Duplicate chromosomes are pulled apart, divided up. DNA de-condenses.

2 Centrioles and spindle Spindle: web of microtubules that anchor at centrioles at poles of cell, attach to chromosomes and direct their movement. Nucleus

3 Mitosis Begins tidepool.st.usm.edu/ crswr/mitosis.htmltidepool.st.usm.edu/ crswr/mitosis.html ; members.tripod.com/ mrlewisclassroom/biology.htmmembers.tripod.com/ mrlewisclassroom/biology.htm Prophase: spindle forms, nucleolus disappears, nuclear membrane dissolves, chromosomes begin condensing. Cell during Interphase Cell During Prophase

4 Sister chromatids During S phase of the cell cycle, each chromosome is duplicated; these are not homologous chromosomes, but are exact duplicates = sister chromatids. Sister chromatids are attached at the centromere. Within the centromere is the kinetechore where the spindle is attached and pulls the chromosomes.

5 Spindle aligns chromosomes along equator If chromatids are not pulled on equally by the spindle so that they are all lined up, mitosis does not continue.

6 Mitosis-more ghttp:// g; plant_metaphase.htmwww.fed.cuhk.edu.hk/.../ plant_metaphase.htm Metaphase: chromosomes completely condense, line up along equator; sister chromatids visible, attached to spindle at centromeres. “Prometaphase”: describes steps up to where chromosomes are lined up along cell equator.

7 Mitosis -more Anaphase : sister chromatids lined up along equator separate from each other, moving to opposite poles of the cell. Telophase: spindle disappears, nuclear membrane reforms, and chromosomes de-condense.

8 Cytokinesis Near the end of mitosis, the cytoplasm –containing the rest of the organelles- is divided up into the two new daughter cells: cytokinesis.

9 Cytokinesis Contractile ring around cell pulls in, creates cleavage furrow, eventually pinches cell into 2 cells. Cytoplasm (including organelles) divided between the two cells.

10 Plant cells have cell walls. At the end of mitosis, vesicles containing cell wall material move to create the “cell plate” between the new cells. A new cell wall is created between the two new cells.

11 Mitosis vs. Meiosis Prior to mitosis, S phase produces identical copies of all the DNA which is distributed to the daughter cells during mitosis; the resulting cells have identical DNA, with 2 homologous sets of chromosomes. Gametes, which fuse together to create a new individual, must have only one set of DNA (1+1=2), so the cell division process that produces gametes must be different. Meiosis is the process that produces haploid gametes. Features 2 division steps. (1 cell  4 cells)

12 Meiosis: a special nuclear division process for producing haploid gametes Diploid organisms must produce haploid gametes that fuse to produce a new diploid individual. Meiosis occurs with germ line cells, a separate set of cells that produce gametes. Meiosis features two cell divisions, so that one germ line cell produces 4 gametes (in principle). Each cell division step can be divided into prophase, metaphase, anaphase, and telophase. Prophase I is subdivided because stuff happens.

13 Meiosis begins Prophase I: Sister chromatids condense, become visible. Nuclear membrane dissolves; spindle forms. Homologous chromosomes pair up! Stages of Prophase I: Leptonema Zygonema Pachynema Diplonema Diakinesis

14 Stages of Prophase I: Leptonema: threads appear, homology search begins. Zygonema: thickening continues, pairing occurs (synapsis), “bivalents” visible. Pachynema: more thickening, “tetrads” visible, crossing over starts; sister chromatids are visible. Diplonema: chiasmata are visible as pairs of sister chromatids begin to separate. Diakinesis: chromosomes repel each other, chiasmata move towards ends of chromatids.

15 Synapsis Synapsis is when homologous chromosomes are paired up and at first stick to each other along their entire length. Chromosomes cross arms, and stick to each other at a couple of places. Pieces can break and actually swap. This is called crossing over.

16 Crossing over: gene shuffling ct10/Life7e-Fig %20crossing%20over.jpg Tetrad Red: sister chromatids from one parent (identical). Blue: sister chromatids from other parent. Red and Blue are Homologous

17 Meiosis I is completed Metaphase IAnaphase I Metaphase I Paired homologues line up along cell equator. Anaphase I Homologues separate! NOT sister chromatids. Reduction division Telophase I: may be brief or incomplete, because cell will proceed to Prophase II.

18 Meiosis II In Anaphase II, sister chromatids separate, leaving only 1 of each type of chromosome in the cell. NOTE that various mixtures of pink and green are possible: This is the cellular basis for Mendel’s Independent Assortment.

19 Gamete formation in humans- sperm Through meiosis, one cell divides to produce 4 sperm cells.

Gamete formation in humans- ovum 20 Cytokinesis is unequal; in each division, one cell gets more cytoplasm than the other. The result is one ovum and 2 polar bodies which degenerate and disappear.

21 Reproduction in a flowering plant