Transcript Cell cycle
CELL CYCLE How Do Cells Divide? What you will learn… 1. Why Do Cells Divide? 2. Chromosome structure 3. Cell Division in Prokaryotes 4. Cell Cycle 5. Mitosis 6. Cytokinesis 7. Control of Cell Division and Cancer 8. Meiosis 9.Why do cells need two types of cell division? 10. Gamete Formation 1. Why Do Cells Divide? Virchow: Cells can only come from preexisting cells In unicellular organisms, can reproduce an entire organism Allows multicellular organisms to reproduce asexually Basis of sexual reproduction sperm and egg Allows fertilized egg, or zygote, to develop into an adult organism Replaces worn-out or damaged cells Enables multicellular organism to grow to adult size http://www.pbs.org/wgbh/nova/miracle/progra m.html 2. Chromosome Structure Human cells carry about 20,000 genes to make 100,000 proteins. Almost all genes are located in the nucleus Genes are found on DNA DNA can be in the form of Chromatin: Very small amount found in mitochondria Diffuse mass of long, thin fibers, not seen under the microscope, less tightly coiled Combination of DNA and protein DNA must be tightly packaged before cell division, so it can be evenly divided between the two new cells. DNA will now be in the form of Chromosome! http://www.dnalc.org/resources/3d/07-how-dna-is-packaged-basic.html 2. Chromosome Structure Chromosomes Rod-shaped structure Coiled up, compact forms of chromatin Contains one long DNA molecule bearing hundreds or thousands of genes. DNA is attached to protein molecules called histones and other non-histone proteins DNA wraps with protein like wrapping paper on a present giving it the X-shape Only found in eukaryotic cells (prokaryotes have naked, circular shaped chromosomes) 2. Chromosome Structure 2. Chromosome Structure Sister chromatids Each duplicated chromosome contains two identical copies. Centromere The point by which two chromatids are joined. Chromatin Diffuse mass of long, thin fibers, not seen under the microscope, less tightly coiled Combination of DNA and protein 2. Chromosome Structure 3. Cell Division in Prokaryotes Binary fission Process by prokaryotes reproduce by cell division. Steps: Duplication of chromosomes and separation of copies. Cell elongates Divides into two daughter cells 3. Cell Division in Prokaryotes 4. Cell Cycle In your own body, millions of cells must divide every second to maintain the total number of about 100 trillion cells. Some cells divide once a day, and some do not at all (mature muscle cells, brain cells) 4. Cell Cycle Starts out with Interphase Occurs when the cell is between cell division Interphase stages: G1: Cells grow to mature size S: DNA is copied G2: Cell prepares for division Cells exit the cell cycle via… G0: Cells do not copy DNA or prepare for mitosis, but are still alive (e.g. nervous system) 5. Mitosis The last stage of the cell cycle when the nucleus of a cell divides to produce two new daughter cells (after cytokinesis) each with the same amount and type of chromosomes as the parent cells. Mitosis is divided into four phases: A.Prophase B. Metaphase C. Anaphase D. Telophase 5. Mitosis A.Prophase: What does the cell look like? Centrioles and spindle fibers appear Nuclear envelope disappears, and chromosomes are visible What happens to the DNA and nucleus? Chromosomes form when chromatin tightens and coils Nuclear membrane breaks down and disappears What two things appear near where the nucleus was? Centrioles and spindle fibers 5. Mitosis A. Prophase 5. Mitosis B. Metaphase What does the cell look like? Chromosomes Where move to the middle are the chromosomes during metaphase? Middle of the cell 5. Mitosis B. Metaphase 5. Mitosis C. Anaphase: What does the cell look like? Chromosomes What move to the end of cell happens to the chromosomes? Chromosome splits at centromere into 2 chromatids and moves to end of cell 5. Mitosis C. Anaphase 5. Mitosis D. Telophase What does the cell look like? Cell starts to pinch in Nucleus starts to reform Chromosomes are at opposite ends What happens to the chromosomes and nucleus? Nucleus forms back around single chromatids 5. Mitosis D. Telophase 6. Cytokinesis What is cytokinesis? Cytoplasm What’s special about cytokinesis in plants? Cell and contents (other organelles) divide wall also divides with new cell plate in middle What’s special about cytokinesis in animals? Takes place when the cell membrane pinches in until the cytoplasm is pinched into two equal halfs 7. Control of Cell Division and Cancer Cell division is a complex process that needs to be regulated. These regulators determine when and how the cell should divide. External Regulators Internal Regulators 7. Control of Cell Division and Cancer External regulators: Various proteins produced by other cells that speed up or slow down the cycle. If the cell touches other cells, than cell division slows down. If enough space between cells and nutrients are available, growth factors and other proteins make cells divide or speed up their cell cycle. 7. Control of Cell Division and Cancer Internal regulators: Cyclins – proteins that regulate the timing of the cell cycle in eukaryotic cells. Other regulator proteins (checkpoints)– they make sure that certain things happen in the cell before the cell moves to the next phase of the cell cycle 3 major checkpoints in the cell cycle. The age of the cell. 7. Control of Cell Division and Cancer Cancer cells lack normal checkpoints and continue to grow without inhibition do not respond to normal signals within the cell are not inhibited by other cells will divide indefinitely 7. Control of Cell Division and Cancer Mutations in the genes of these checkpoint proteins may lead to cancer: The uncontrolled growth of cells. Tumor: an abnormally growing mass of body cells Benign tumor If abnormal cells remain at original site Can be problematic if disrupt certain organs, but usually easily removed by surgery Malignant If tumor abnormal cells spread into other tissues and body parts, interrupting organ function 7. Control of Cell Division and Cancer Tumor Progression 1. Tumor growth 2. Blood vessels feed tumor 3. Tumor cells enter blood and lymph vessels 4. Secondary tumors form in other parts of the body Movie clips on cancer, its nature and experiments to treat it (Parts 2 and 6) http://www.pbs.org/wgbh/nova/cancer/program.html http://www.youtube.com/watch?v=HonoQ6mE6dY&feature=rela ted 7. Control of Cell Division and Cancer Treatment of Cancer: Surgical removal of tumor – Most effective when tumor is in a defined area Chemotherapy – Medicines that disrupt the process of mitosis in rapidly growing cells Radiation Therapy - High energy gamma radiation is aimed at the growing tumour. This damages the DNA in rapidly dividing cells and helps to destroy the tumor. 8. Meiosis Many of the stages of meiosis closely resemble corresponding stages in mitosis. Type of cell division that produces haploid gametes in diploid organisms. 8. Meiosis 8. Meiosis Like mitosis, is preceded by the replication of chromosomes. However, this single replication is followed by two consecutive cell divisions, called Meiosis I and Meiosis II. These divisions result in four daughter cells, each with a single haploid set of chromosomes. Produces daughter cells with only half as many chromosomes as the parent cell. 8. Meiosis We will be looking at chromosome pairing up: called homologous chromosomes (or homologues) because they both carry genes controlling the same inherited characteristics. 8. Meiosis Any cell with two homologous (the same) sets of chromosomes is called a diploid cell the total number of chromosomes is called the diploid number (abbreviated 2n) For humans, the diploid number is 46; that is 2n=46 Almost all human cells are diploid 8. Meiosis The exception are the egg and sperm cells, collectively known as gametes. A cell with a single chromosome set is called a haploid cell. For humans, the haploid number (abbreviated n) is 23; that is n=23 8. Meiosis Prophase I – Each chromosome pairs with its corresponding homologous chromosome to form a tetrad. The tetrads overlap and exchange some of their genetic material – crossing-over. 8. Meiosis Crossing over in Prophase I results in great diversity because new genetic variations can result from it. 8. Meiosis Metaphase I – Spindle fibers attach to the chromosomes. 8. Meiosis Anaphase I – The fibers pull the homologous chromosomes toward opposite ends of the cell. The cells are now containing half of the genetic information from the original parent cell and are thus considered HAPLOID! 8. Meiosis Telophase I and cytokinesis – Nuclear cells. membranes reforms, the cell separates into two 8. Meiosis Prophase II – Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original cell. 8. Meiosis Metaphase II – The chromosomes line up in a similar way to the metaphase stage of mitosis. Anaphase II – The sister chromatids separate and move toward opposite ends of the cell. 8. Meiosis Telophase II and cytokinesis – Meiosis II results in four haploid (N) daughter cells. http://www.sumanasinc.com/webcontent/animations/content/meiosis.html http://www.pbs.org/wgbh/nova/baby/divi_flash.html 9.Why do cells need two types of cell division? Mitosis Provides growth, tissue repair, and asexual reproduction Produces daughter cells genetically identical to the parent cell Involves one division of the nucleus, and is usually accompanied by cytokinesis, producing two diploid daughter cells. Meiosis Need for sexual reproduction human egg and sperm cells Entails two nuclear and cytoplasmic divisions Yields four haploid daughter cells, with one member of each homologous chromosome pair. Form tetrads; crossing over occurs. 10. Gamete Formation In females: 10. Gamete Formation In males: 11. Karyotype The term karyotype refers to the chromosome complement of a cell or a whole organism. A karyotype is an ordered display of magnified images of an individual’s chromosomes arranged in pairs, starting with the longest. In particular, it shows the number, size, and shape of the chromosomes as seen during metaphase of mitosis. Chromosome numbers vary considerably among organisms and may differ between closely related species. 11. Karytype Karyotypes are prepared from the nuclei of cultured white blood cells that are ‘frozen’ at the metaphase stage of mitosis. Shows the chromosomes condensed and doubled A photograph of the chromosomes is then cut up and the chromosomes are rearranged on a grid so that the homologous pairs are placed together. Homologous pairs are identified by their general shape, length, and the pattern of banding produced by a special staining technique. 11. Karyotype Male karyotype Has 44 autosomes, a single X chromosome, and a Y chromosome (written as 44 + XY) Female karyotype Shows two X chromosomes (written as 44 + XX) 11. Karyotype- Normal 11. Karyotype- Abnormal 12a. Chromosome mutations Can either be with… Chromosome number Chromosome structure 12a. Mutations- Chromosome number Aneuploidy- involves a single set of chromosomes Polyploidy- involves a whole set of chromosomes in 12a. Mutations- Chromosome number Aneuploidy If there is an extra or missing chromosome Caused by nondisjunctions: members of a chromosome fail to separate. For example, trisomy 21 (Down Syndrome) if there is a nondisjunction affecting human chromosome 21 during meiosis, the resulting gametes will carry an extra chromosome 21. 12a. Mutations- Chromosome Number 12a. Mutations- Chromosome number Polyploidy Cells containing more than two paired homologous sets of chromosomes. For example- triploid cell (3n) or a tertroploid cell (4n) More common in plants, very rare in humans 12a. Mutations- Chromosome number 12b. Mutations- Chromosome Structure Abnormalities in chromosome structure: Breakage of a chromosome can lead to a variety of rearrangements affecting the genes of that chromosome: 1. deletion: if a fragment of a chromosome is lost. Usually cause serious physical and mental problems. Deletion of chromosome 5 causes cri du chat syndrome: child is mentally retarded, has a small head with unusual facial features, and has a cry that sounds like the mewing of a distressed cats. Usually die in infancy or early childhood. 12b. Mutations- Chromosome Structure 2.duplication: if a fragment from one chromosome joins to a sister chromatid or homologous chromosome. 3.inversion: if a fragment reattaches to the original chromosome but in the reverse direction. Less likely than deletions or duplications to produce harmful effects, because all genes are still present in normal number 4. translocation: moves a segment from one chromosome to another nonhomologous chromosome Crossing over between nonhomologous chromosomes! 12b. Mutations- Chromosome Structure