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How does a single cell become a human being? Cell division 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 – Cell division → growth Cell division plays a role in: mitosis Cells differentiate • Growth and replacing cells Producing gametes (egg and sperm) How are chromosomes passed on from cell to cell? 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 chromosomes A parent cell duplicates its chromosomes The nucleus divides The cytoplasm divides → 2 daughter cells Each daughter cell receives a complete set of chromosomes. Two kinds of cell division Mitosis For growth and repairing or replacing cells Occurs in body (somatic) cells Meiosis Only used for reproduction–for making sperm and eggs Occurs only in germ-line cells in the ovary or testes Human Chromosomes Our cells have 46 chromosomes 23 pairs called diploid (2n) Homologous Chromosomes Sex cells (gametes) have only 23 chromosomes called haploid (n) Chromosomes Chromosomes come in matching pairs Humans have 22 pairs of matching chromosomes Plus two sex chromosomes, X and Y. So we have 46 chromosomes This is a human karyotype Chromosomes Found in the cell nucleus Genes are located on chromosomes Each chromosome contains one very long DNA molecule Typically bears thousands of genes What happens to chromosomes when a cell divides? Are a combination of DNA and protein molecules. Are not visible in the nucleus until they condense before cell division. Mitosis: growth and replacing cells Body cell DNA replication Diploid (2n) A copy is made of each chromosome before cell division begins During cell division, the chromosomes are distributed to the daughter cells Each daughter cell ends up with a complete set of chromosomes Chromosomes duplicate (4n) Each new cell receives a complete set of chromosomes How does this happen? Chromosomes divide (2n) 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 Chromosomes 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 chromosomes apart 1. The DNA condenses into visible chromosomes At the beginning of mitosis: 1. Chromosomes 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 http://www.youtube.com/watch?v=rgLJrvoX_qo&NR=1 The nuclear membrane reappears. The chromosomes uncoil The spindle disappears Cytokinesis - the cytoplasm begins to divide A new cell membrane forms Examples of mitosis 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 – Interphase – a period of normal cell activity Mitosis The cell spends 90% of its time in interphase Period of cell growth The cell makes new molecules and organelles The chromosomes are replicated during interphase a period of normal cell activity Interphase sets the stage for cell division Chromosomes are duplicated Note: chromosomes are not visible in the cell’s nucleus They are loosely packed fibers Interphase (growth phase) Cell cycle control G1 checkpoint = Cell cycle checkpoints Are proper growth factors present? Is DNA damaged? G2 checkpoint Apoptosis Has DNA replicated properly? M checkpoint Has the spindle assembled properly? 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 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 chromosomes (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 chromosomes Meiosis: making gametes Comparing Mitosis and Meiosis n 2n Mitosis 4n In males, meiosis typically produces 4 sperm Formation of gametes Occurs in body cells Occurs in sex cells 1 cell division 2 divisions Growth and repair of cells Chromosomes duplicate once but divide two times Meiosis plays a key role in sexual reproduction Meiosis Results in 2 diploid (2n) genetically identical cells Results in 4 haploid (n) genetically different cells In females, Germ-line cell DNA replication Germ-line cell Meiosis and Fertilization Why is SEX important? Meiosis halves the number of chromosomes each gamete is haploid (1n) In fertilization, the gametes fuse to form a zygote, which contains two copies of each chromosome diploid or 2n Meiosis produces just one egg with all the cytoplasm, plus small polar bodies. 1n Sexual reproduction has an enormous impact on how species evolve – it generates new genetic combinations Gametes 2n This genetic variability is produced by 1n Fertilized cells carry chromosomes from BOTH parents Crossing over Random alignment of chromosomes during meiosis Random fertilization 1000s of sperm to 1 egg Random Alignment of Chromosomes What is crossing over? sister chromatids Chromosome pairs align randomly during meiosis. The chromosomes are “shuffled” Crossing-over Synapsis of homologues Chromatids after exchange Recombinant daughter chromosomes Homologous chromosomes exchange parts This produces hybrid chromosomes 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 chromosomes. 1000s of sperm to 1 egg Down Syndrome: An Extra Chromosome 21 Obtaining fetal chromosomes 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