Mitosis Recap  Small part of the cell cycle  During mitosis, the nucleus of the cell divides (nucleus contains DNA)  Purpose identical  to create two identical daughter cells that:  grow and develop  repair damaged tissue

Mitosis Phases  Prophase  Nuclear envelope disintegrates  chromatin coil and chromosomes become visible  Metaphase  Chromosomes (sister chromatids) line up in middle of cell  Anaphase  Sister chromatids (chromosomes) are pulled apart by spindle fibers  Telophase  Nuclear membrane forms again  Cell contains two nuclei

Remember!  The result of mitosis is the creation of two identical daughter cells  Mitosis is Asexual Reproduction  Asexual Reproduction – one parent cell or organism makes an exact copy of itself

Chromosomes Remember:  All eukaryotic cells have a nucleus  The nucleus contains DNA  DNA coils up to form chromatin then chromosomes  Chromosomes contain genes  Genes are made up of DNA  Gene: segment of DNA that codes for proteins

Autosomes and Sex Chromosomes autosomes  Chromosomes #1-22 are autosomes  chromosomes that do not control gender sex chromosomes  Chromosome #23 are the sex chromosomes  (males XY, females XX)

92 Ponder This…  If a new human is created from the union of two cells (one from the father and mother)…  How then, do you avoid doubling your chromosome number each time? 46 egg (mother) sperm (father) zygote (cell)

Ponder This…  If each of your cells contains two alleles (alternative forms of genes – like brown and blond hair) for each trait…  How then, do you only inherit one from each parent? BbBb Bb two alleles two alleles four alleles???

Ponder This…  If all the cells made by your mother are the same, &  If all the cells made by your father are the same…  Why don’t you and your siblings look exactly the same?

Explanation  The cells used to pass on genes from parent to offspring are significantly different than every other cell in your body.  They contain half the normal number of chromosomes than a normal cell  they are called sex cells (gametes)  they are produced by meiosis, not mitosis.

Somatic Cells vs. Gametes  Two kinds of cells in your body:  Somatic Cell – non-sex cells; all cells except egg and sperm cells  Gametes – sex cells  Females have egg cells  Males have sperm cells  Different #s of chromosomes in these cells:  Somatic cells – diploid (2N) – 46 chromosomes  Gametes – haploid (1N) – 23 chromosomes

Diploid  Cells that contain both sets of homologous chromosomes are said to be diploid (2n). The diploid number in…  Drosophilia melanogaster is 8. (2n=8)  Homo sapiens is 46. (2n=46)  Cambarus clarkii (crayfish) is 200! (2n=200)  All the cell types in your body (except for one type) are diploid cells.

Haploid  Cells that contain one set of homologous chromosomes are said to be haploid (n). Therefore, the haploid number in…  Drosophilia melanogaster is 4. (n=4)  Homo sapiens is 23. (n=23)  Cambarus clarkii (crayfish) is 100! (n=100) Notice the haploid numbers are exactly ½ the diploid number  Only one type of cells are haploid cells: the sex cells or gametes (sperm & egg)

46 Ponder This…  How can you avoid doubling your chromosome number each time?  Each sperm & egg contain half the number of chromosomes as before. 23 egg (mother) sperm (father) zygote (cell)

Ponder This…  How can you only inherit one from each parent?  Each gamete only contains 1 allele from each parent. Bb b B it works!

Ponder This…  Why don’t you and your siblings look exactly the same?  Because not all of the cells produced are identical.

Meiosis  Haploid cells are produced through a process called meiosis  Meiosis is the process of cell division in which the number of chromosomes per cell is cut in half. Chromosomes in a sex cell. Notice there are ½ as many as in a body cell

Meiosis vs. Mitosis Meiosis  Occurs in two phases:  Meiosis I  Meiosis II  Genetic information is shuffled around.  Results in four different daughter cellsMitosis  Occurs in one phase.  Genetic information remains the same.  Results in two identical daughter cells.

Phases of Meiosis  Meiosis I:  prophase I  metaphase I  anaphase I  telophase I  cytokinesis

Phases of Meiosis  Meiosis I:  prophase I  metaphase I  anaphase I  telophase I  cytokinesis  Meiosis II:  prophase II  metaphase II  anaphase II  telophase II  cytokinesis

Prophase I  Chromosomes become visible (they coil)  Nuclear membrane breaks down (disintegrates)  Homologous chromosomes pair up in what is called a tetrad  They then exchange parts in a process is called crossing over

Metaphase I  Homologous chromosomes line up with each other in the middle of the cell.  Notice the difference between this stage in meiosis & mitosis. What do you notice?Meiosis:Mitosis:

Anaphase I  Homologous chromosomes move to opposite poles of the cell

Telophase I  Chromosomes gather at the poles of the cell.

Cytokinesis  The cytoplasm divides  Both cells contain one chromosome from each pair of homologous chromosomes. * Chromosomes do not replicate between meiosis I and meiosis II.

Anaphase I, Telophase I & Cytokinesis  For the most part these processes are the same as mitosis.  However, in Meiosis, because of the way the chromosomes were lined up during metaphase, neither of the daughter cells are identical. This is good! Meiosis: Mitosis:

Meiosis II  The second stage of meiosis is exactly like mitosis.  Chromosomes line up, travel to opposite sides of the cell, and the cell splits.

Prophase II  A new spindle forms around the chromosomes.

Metaphase II  Chromosomes line up at the equator (middle)  Chromosomes are attached at their centromere to the spindle fibers

Anaphase II  Centromeres divide  Chromatids move to opposite poles of the cell

Telophase II  Nuclear envelope forms around each set of chromatids  Spindle breaks down

Cytokinesis  Cell undergoes cytokinesis  Result of meiosis is four haploid cells genetically different  Each cell is genetically different from parent cell and other daughter cells!

Importance of Meiosis  The products of these two processes are very different:  Mitosis: 2 identical daughter cells.  Meiosis: 4 different daughter cells.  Why are these differences in the cells important?  Increases genetic variation in the offspring. THIS is the big idea. Remember this!!

Recap! Meiosis

Prophase I Prophase II

Metaphase I Metaphase II

Anaphase I Anaphase II

Telophase I Telophase II