How does a single cell become a human being? Cell division. Cell division growth. Cell division plays a role in: Cell division gametes

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1 Cell division How does a single cell become a human being? Every time a cell divides, a copy is made of all the DNA in every chromosome Fertilized egg Blastula Many things happen Number of cells increase by cell division mitosis Cells differentiate Cell division plays a role in: Cell division growth Growth and replacing cells Producing gametes (egg and sperm) How are passed on from cell to cell? A parent cell duplicates its The nucleus divides The cytoplasm divides 2 daughter cells Each daughter cell receives a complete set of. Cell division gametes Producing gametes is different. The egg and sperm are produced by a special type of cell division called meiosis. The daughter cells receive half the normal number of Two kinds of cell division Mitosis For growth and repairing or replacing cells Occurs in body (somatic) cells Meiosis Only used for reproductionfor making sperm and eggs Occurs only in germ-line cells in the ovary or testes

2 Human Our cells have pairs called diploid (2n) Sex cells (gametes) have only 23 called haploid (n) Homologous come in matching pairs Humans have 22 pairs of matching Plus two sex, X and Y. So we have 46 This is a human karyotype Found in the cell nucleus Genes are located on Each chromosome contains one very long DNA molecule Typically bears thousands of genes Are a combination of DNA and protein molecules. Are not visible in the nucleus until they condense before cell division. What happens to when a cell divides? Mitosis: growth and replacing cells Body cell DNA replication A copy is made of each chromosome before cell division begins During cell division, the are distributed to the daughter cells Each daughter cell ends up with a complete set of Diploid (2n) duplicate (4n) Each new cell receives a complete set of How does this happen? divide (2n)

3 What s happening in the nucleus? Early in Mitosis 2. The spindle fibers attach to the centromeres Two new nuclei form and the cytoplasm divides Later in mitosis Spindle fibers pull the sister chromatids apart, towards opposite ends of the cell. Nucleus of each daughter cell forms form two new nuclei Cytokinesis Each duplicated chromosome appears as 2 identical sister chromatids joined together Spindle fibers connect to each chromosome They pull the apart 1. The DNA condenses into visible At the beginning of mitosis: 1. condense 2. Nuclear membrane disintegrates 3. Mitotic spindle forms Is the division of the cytoplasm End up with 2 new daughter cells, each with a copy of every chromosome Identical to the parent cell Nuclear division & cytokinesis 0:19 The nuclear membrane reappears. The uncoil The spindle disappears Cytokinesis - the cytoplasm begins to divide A new cell membrane forms Examples of mitosis

4 This remarkable process occurs during growth of the human fetus The Cell Cycle Mitosis Events that take place within the cell between one cell division and the next Two distinct phases: Interphase (cell growth) Mitosis (cell division) Interphase (growth phase) Interphase a period of normal cell activity Mitosis Interphase (growth phase) The cell spends 90% of its time in interphase Period of cell growth The cell makes new molecules and organelles The are replicated during interphase Interphase a period of normal cell activity Interphase sets the stage for cell division are duplicated Note: are not visible in the cell s nucleus They are loosely packed fibers Cell cycle control = Cell cycle checkpoints G 1 checkpoint Are proper growth factors present? Is DNA damaged? G 2 checkpoint Has DNA replicated properly? M checkpoint Has the spindle assembled properly? Apoptosis If a cell has an error in its DNA that cannot be repaired, it may undergo programmed cell death (apoptosis) Apoptosis removes genetically damaged cells

5 How do different kinds of cells arise? What Is Cancer? Cancer is a disease of the cell cycle. Cancer cells do not respond normally to the cell cycle checkpoints All cells contain the same DNA, so what makes them different? = Cell cycle checkpoints Different types of cells express different combinations of genes Gene activity is regulated When cells differentiate Certain genes are turned on and off. Cells become specialized Fertilized egg Blastula Muscle cells make contractile proteins (actin and myosin) Blood cells make hemoglobin Genes Glycolysis enzymes Hemoglobin Myosin Insulin Meiosis: Meiosis it only happens in sex cells a specialized kind of cell division Humans are diploid organisms. Our cells contain two sets of (2n). Our gametes are haploid, only have one of each chromosome How does this happen? Cells divide by meiosis only for the production of gametes (eggs and sperm). Takes place only in the ovaries or testes Gametes are haploid Have half the number of

6 Meiosis: making gametes n 2n 4n Comparing Mitosis and Meiosis Mitosis Growth and repair of cells Meiosis Formation of gametes Occurs in body cells Occurs in sex cells duplicate once but divide two times 1 cell division Results in 2 diploid (2n) genetically identical cells 2 divisions Results in 4 haploid (n) genetically different cells Meiosis plays a key role in sexual reproduction In males, meiosis typically produces 4 sperm In females, Germ-line cell DNA replication Germ-line cell Meiosis produces just one egg with all the cytoplasm, plus small polar bodies. Meiosis and Fertilization Why is SEX important? Meiosis halves the number of each gamete is haploid (1n) In fertilization, the gametes fuse to form a zygote, which contains two copies of each chromosome diploid or 2n 1n Gametes 1n 2n Fertilized cells carry from BOTH parents Sexual reproduction has an enormous impact on how species evolve it generates new genetic combinations This genetic variability is produced by Crossing over Random alignment of during meiosis Random fertilization 1000s of sperm to 1 egg

7 Random Alignment of What is crossing over? sister chromatids Chromosome pairs align randomly during meiosis. The are shuffled Crossing-over Synapsis of homologues Chromatids after exchange Recombinant daughter Homologous exchange parts This produces hybrid and Increases genetic variability Accidents During Meiosis Random Fertilization The human egg is fertilized by one sperm, leading to genetic differences in the offspring. Change in chromosome number In nondisjunction, The members of a chromosome pair fail to separate Produces gametes with an incorrect number of. 1000s of sperm to 1 egg Down Syndrome: An Extra Chromosome 21 Obtaining fetal Amniocentesis trisomy Down Syndrome is also called trisomy 21. Chorionic villi sampling Remove fetal cells from the amniotic fluid or placenta Check the karyotype to see if there are any chromosomal abnormalities