Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
How does a single cell become a human being? Ch 18 – Cell Division Fertilized egg Blastula Many things happen • Number of cells increase by cell division – • Cell division plays a role in: mitosis Cells differentiate Cell division → more cells Cell reproduction and growth. Producing gametes (egg and sperm) Passing on genes 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. Sexual reproduction requires fertilization of an egg by a sperm. A parent cell duplicates its chromosomes before it divides. The two resulting “daughter” cells are genetically identical–they each receive a 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 Homologous Chromosomes Chromosomes come in matching pairs Humans have DNA packing in chromosomes 22 pairs of matching chromosomes Plus two sex chromosomes, X and Y. So we have 46 chromosomes DNA wraps around proteins called histones The DNA is packed into an elaborate, multilevel system of coiling and folding. The long strands of DNA are condensed This is a human karyotype Male or female? Chromosome duplication and distribution Before a cell divides, it duplicates all of its chromosomes, resulting in two copies called sister chromatids. When the cell divides, the sister chromatids separate from each other. The Cell Cycle Mitosis Events that take place within the cell between one cell division and the next Two distinct phases: Cell division Interphase (growth) Mitosis (cell division) Interphase (growth phase) Interphase – a period of normal cell activity Mitosis The cell spends 90% of its time in interphase Period of cell growth Interphase (growth phase) Chromosome are replicated in interphase Chromosome DNA replication Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. The cell makes new molecules and organelles The chromosomes are replicated during interphase Sister chromatids Centromere chromosome Duplicated chromosome Early in Mitosis 1. 2. 2. The DNA condenses into visible chromosomes Chromosomes condense Mitotic spindle forms 1. 3. Early in Mitosis The spindle microtubules attach to the centromeres Centrosomes move towards opposite ends of the cell Act to pull the chromosomes apart Each duplicated chromosome appears as 2 identical sister chromatids joined together Nuclear membrane disintegrates The mitotic spindle forms centrioles in centrosome chromosomes spindle fibers aster Later in mitosis Separation of the chromosomes Spindle fibers pull the sister chromatids apart, towards opposite ends of the cell. The spindle fibers attach to the sister chromatids Finally the nucleus reforms and cytoplasm divides Nucleus of daughter cells forms The nuclear membrane reappears. The chromosomes uncoil The spindle disappears Cytokinesis - the cytoplasm begins to divide Cytokinesis Is the division of the cytoplasm End up with 2 new daughter cells, each with a copy of the original DNA A new cell membrane forms Nuclear division & cytokinesis 0:19 http://www.youtube.com/watch?v=rgLJrvoX_qo&NR=1 Cell cycle control G1 checkpoint = Cell cycle checkpoints Has the spindle assembled properly? Cells growing out of control They lose control over cell division They lose control over mitosis Cancer is a disease of the cell cycle. M checkpoint What Is Cancer? Has DNA replicated properly? Cells growing out of control G2 checkpoint Are proper growth factors present? Is DNA damaged? What Is Cancer? = Cell cycle checkpoints Cancer cells do not respond normally to the cell cycle checkpoints What causes Cancer? Damage to genes that regulate the cell cycle Mutations damage DNA Some result from chemical or environmental exposure Cigarette smoke UV rays Some viruses also damage DNA HPV virus causes cancer of the cervix 3D Medical Animation - What is Cancer? 1:06 http://www.youtube.com/watch?v=LEpTTolebqo&NR=1&feature=fvwp 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 that could lead to cancer plays an important role in the development of the embryo Meiosis The basis of sexual reproduction Meiosis – it only happens in sex cells Meiosis: the special cell division that precedes sexual reproduction Humans are diploid organisms. Our cells contain two sets of chromosomes (2n). Our gametes are haploid, having only one of each chromosome How does this happen? Cells divide by meiosis only for the production of gametes (eggs and sperm). produced only in the gonads (ovaries or testes). Almost all cells are diploid (2n), except for gametes (haploid, n) How meiosis halves chromosome number Review: Comparing Mitosis and Meiosis Mitosis Meiosis plays a key role in sexual reproduction In males, meiosis typically produces 4 sperm Growth and repair of cells Formation of gametes Occurs in body cells Occurs in sex cells 1 cell division 2 divisions Chromosomes duplicate once, but the cell divides twice 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 produces just one ovum (egg), plus small polar bodies. Fertilization – the union of an egg and a sperm Meiosis halves the number of chromosomes each gamete is haploid (1n) In fertilization, the gametes fuse. Fertilized cells carry chromosomes from both parents Why is SEX important? 1n new genetic combinations Gametes 2n contain two copies of each chromosome Crossing-over Synapsis of homologues Chromatids after exchange Chromosome pairs align randomly during metaphase of meiosis. Recombinant daughter chromosomes Homologous chromosomes exchange parts This changes the genetic information in each chromosome and Increases genetic variability The chromosomes are shuffled Accidents During Meiosis Random Fertilization The human egg cell is fertilized randomly by one sperm, leading to genetic differences in the offspring. Change in chromosome number In nondisjunction, 1000s of sperm to 1 egg 1000s of sperm to 1 egg Random Alignment of Chromosomes Crossing over Random alignment of chromosomes during metaphase Random fertilization diploid (2n) sister chromatids This genetic variability is produced by 1n What is crossing over? Sexual reproduction has an enormous impact on how species evolve because it generates The members of a chromosome pair fail to separate during anaphase. Produces gametes with an incorrect number of chromosomes.