Cell Division (review) Compare and contrast mitosis and meiosis. How does a cell reproduce? Some internal or external signal initiates the process. The DNA is replicated and supplies of other cell components are built up. The DNA is distributed to each of the two new cells. Two unique cell membranes (and cell walls, if present) grow during cytokinesis. The process in prokaryotes is called (binary) fission. The cell responds to signals from the environment, such as food supplies. The single chromosome is replicated starting at the “ori” point and ending at the “ter” point. The two DNA strands stick to the cell membrane and as the membrane grows between them they are pulled apart. The plasma membrane pinches in due to the action of a tubulin-like protein and forms the two new cells. Eukaryotes have a more complex process called mitosis or meiosis. Once they are differentiated and mature some eukaryote cells divide rarely or not at all. Initiation signals are more complex and involve CDK (an enzyme) and cyclin proteins. Eukaryotes have multiple chromosomes and the replication process is more complex. The process of chromosome movement involves an intricate mechanism of microtubules. The presence of a distinct nuclear membrane also complicates things in eukaryotes. Cytokinesis is different in plants and animals. Mitosis produces two genetically identical cells. The cell typically grows in size. The DNA is replicated. Copies of all cellular organelles are made. The nuclear membrane dissolves. The DNA divides into two equal parts. (There is only one nuclear division.) Cytokinesis divides the cytoplasm and organelles. Meiosis produces four genetically unique cells. “Sex cells” undergo meiosis to make haploid gametes. The DNA is replicated only once but is divided twice, thus reducing the chromosome number in half. The homologous chromosomes (copies from each parent) are randomly assorted and segregated into the new cells at each division. Synapsis (crossing-over) often occurs. Genetically diverse gametes are produced. Division of the cytoplasm may be uneven. The Cell Cycle Most of the cell’s life is spent in interphase. Some cells never leave the G1 stage. They live and die without dividing. The signal to divide is during the G1 stage. DNA replicates in the S. G2 prepares for mitosis. The M or mitotic phase consists of four (or five in some books) steps. The Signal to Divide happens in G1. Cyclin-dependent kinases (enzymes) are activated by cyclin proteins to start the S stage using internal signals. Division only takes place if conditions in the cell are right to make both CDKs and cyclins. Some division is stimulated from outside the cell by growth factors (chemicals that bind on the cell surface to trigger cell division). Synthesis of DNA makes nearly exact copies of the chromosomes. Chromosomes are made of chromatin: DNA and proteins called histones. The replicated chromosomes consist of two chromatids attached at a centromere (an X shape once the chromatids condense during prophase). As the DNA is replicated a pair of centrosomes (and in some organisms, centrioles) are also formed from a single one near the nucleus. The centrosomes determine the cell “equator” and orient the plane of cell division. G2 prepares the cytoplasm and organelles for division. After the DNA is replicated the cell makes copies of some of its organelles and stockpiles the molecules it will need to produce new membranes and other parts. The microtubules that will form the cell spindle apparatus (the “ropes” that will move the chromosomes during division) are made at this time. Mitosis Begins! Prophase: DNA supercoils around histones; centrosomes or centrioles move to opposite ends of cell; micotubules form. Late prophase(prometaphase): chromosomes attach to micotubules at kinetochores; nuclear membrane dissolves. Metaphase: chromosomes are all lined up at the cell equator. Anaphase: dyneins and shortening microtubules pull chromatids apart. Telophase: 2 new nuclei form Cytokinesis – division of cytoplasm Once nuclear division (mitosis) is complete the cytoplasm and cell membranes divide. In animals a cleavage furrow pinches in the cell membrane to form two new cells. Actin and myosin microfilaments do this. In plants a cell plate forms between the two new nuclei and new cell wall and membrane grow there. Vesicles from the golgi apparatus start this. Each new daughter cell contains an exact copy of the DNA and copies of the organelles that were in the cytoplasm. Meiosis is different in several ways. There are two nuclear divisions (meiosis I and II) The homologous chromosomes inherited from each of the parents are separated to reduce the total number of chromosomes by half. Synapsis occurs during prophase of the first division (prophase I). The homologous chromosomes separate during anaphase I, BUT the chromatids do NOT separate at their centromeres. The chromatids separate during anaphase II. Simplified Meiosis DNA is replicated prior to prophase I and sister chromatids are attached at centromeres. Homologous chromosomes pair up during prophase I. Synapsis may occur to recombine some genes. Homologous chromosomes segregate during anaphase I. Meiosis II begins immediately after cytokinesis of the first division. Sister chromatids separate into daughter chromatids during anaphase II. Four 1n (haploid) cells are made. Unequal division of the cytoplasm occurs in some animal eggs (ova). During cytokinesis of meiosis I and II, one of the new cells gets nearly all of the cytoplasm. The other cells are called polar bodies. They do not survive. This ensures that the egg cell will have a large supply of stored materials in its cytoplasm for the mitotic divisions that will follow fertilization and formation of the zygote. Errors of Meiosis Nondisjunction leads to aneuploidy. The homologous chromosomes do not segregate properly during meiosis I. One new cell gets both of that particular chromosome and the other gets none. If an egg cell with the extra chromosome is fertilized by a normal sperm a condition called trisomy will occur. That zygote will have three copies of that chromosome instead of two. Down Syndrome is caused by trisomy of chromosome 21 in humans. There are other examples of aneuploidy - trisomy and monosomy - that you can look up on the internet.
Pages to are hidden for
"Cell Division (review)"Please download to view full document