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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 – Cells differentiate • Cell division → growth The chromosomes are copied The nucleus divides Result: Each new cell receives a complete set of chromosomes. Cell division → gametes Producing gametes is different. Mitosis Two kinds of cell division Mitosis For growth and repairing or replacing cells Occurs in body (somatic) cells Only used for reproduction–for making sperm and eggs Occurs only in specialized cells in the ovary or testes The egg and sperm are produced by a special type of cell division called meiosis. The “daughter” cells only receive half the normal number of chromosomes Human Chromosomes Our cells have 46 chromosomes Meiosis mitosis Chromosomes in pairs called diploid (2n) Sex cells (gametes) have only 23 chromosomes called haploid (n) Homologous Chromosomes Chromosomes come in matching pairs Humans have Chromosomes 22 pairs of matching chromosomes Plus two sex chromosomes, X and Y. So we have 46 chromosomes Typically bears thousands of genes This is a human karyotype What happens to chromosomes when a cell divides? Chromosomes 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 DNA replication Chromosomes duplicate (4n) Each new cell receives a complete set of chromosomes How does this happen? 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 What’s happening in the nucleus? At the beginning of mitosis: 1. Chromosomes condense 2. Nuclear membrane disintegrates 3. Mitotic spindle forms Body cell Diploid (2n) Found in the cell nucleus Genes are located on chromosomes Each chromosome contains one very long DNA molecule 1. 2. Chromosomes divide (2n) Spindle fibers connect to each chromosome They pull the chromosomes apart Early in Mitosis The DNA condenses into visible chromosomes Later in mitosis Each duplicated chromosome appears as 2 identical sister chromatids joined together The spindle fibers attach to the centromeres Two new nuclei form and the cytoplasm divides Nucleus of each daughter cell forms The nuclear membrane reappears. The chromosomes uncoil The spindle disappears Cytokinesis - the cytoplasm begins to divide A new cell membrane forms Spindle fibers pull the sister chromatids apart, towards opposite ends of the cell. Chromosomes form two new nuclei Cytokinesis 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 Cell division → growth The chromosomes are copied They move to opposite ends of the cell The nucleus divides The cytoplasm divides → 2 daughter cells The Cell Cycle Mitosis Events that take place within the cell between one cell division and the next Two distinct phases: Each daughter cell receives a complete set of chromosomes Interphase (growth phase) Interphase (cell growth) Mitosis (cell division) Interphase – Interphase – a period of normal cell activity Mitosis The cell spends 90% of its time in interphase Period of cell growth a period of normal cell activity Interphase sets the stage for cell division The cell makes new molecules and organelles Chromosomes are duplicated Note: chromosomes are not visible in the cell’s nucleus The chromosomes are replicated during interphase They are loosely packed fibers Interphase (growth phase) Programmed cell death Cell cycle control G1 checkpoint G2 checkpoint = Cell cycle checkpoints Are proper growth factors present? Is DNA damaged? Has DNA replicated properly? M checkpoint Has the spindle assembled properly? Meiosis: What is cancer? a specialized kind of cell division Humans are diploid organisms. 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 Cells growing out of control They lose control over cell division Our cells contain two of each chromosome (2n). Our gametes are haploid, only have one of each chromosome How does this happen? Meiosis – it only happens in sex cells Cells divide by meiosis only to produce gametes (eggs and sperm). Meiosis DNA is replicated But there are two cell divisions The result? Takes place only in the ovaries or testes Gametes are haploid Have half the number of chromosomes Mitosis Formation of gametes Occurs in body cells Occurs in sex cells 1 cell division 2 divisions Results in 2 diploid (2n) genetically identical cells Germ-line cell Results in 4 haploid (n) genetically different cells Germ-line cell Meiosis plays a key role in sexual reproduction End up with just one egg with all the cytoplasm, plus small polar bodies. In males, meiosis typically produces 4 sperm DNA replication Meiosis → egg production One diploid cell forms 4 haploid cells Meiosis → sperm production Meiosis Growth and repair of cells not one as in mitosis n Comparing Mitosis and Meiosis 2n Meiosis halves the number of chromosomes each gamete is haploid (1n) In fertilization, the gametes fuse to form a zygote, which has two copies of each chromosome diploid or 2n 1n Gametes 2n 1n Fertilized cells carry chromosomes from BOTH parents What is crossing over? Why is SEX important? sister chromatids 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 chromosomes during meiosis Random fertilization 1000s of sperm to 1 egg Crossing-over Synapsis of homologues Random Alignment of Chromosomes Chromatids after exchange Recombinant daughter chromosomes Homologous chromosomes exchange parts This produces hybrid chromosomes and Increases genetic variability Random Fertilization Chromosome pairs align randomly during meiosis. The chromosomes are “shuffled” The human egg is fertilized by one sperm, leading to genetic differences in the offspring. 1000s of sperm to 1 egg Accidents During Meiosis Down Syndrome: An Extra Chromosome 21 Change in chromosome number In nondisjunction, The members of a chromosome pair fail to separate Produces gametes with an incorrect number of chromosomes. trisomy Down Syndrome is also called trisomy 21.