THE CELL CYCLE & MITOSIS Asexual Reproduction: Production of genetically identical offspring from a single parent. Sexual Reproduction: The fusion of two separate parent cells that produce offspring with some genetic information from each parent. Cell division functions: reproduction, growth, and repair The division of a unicellular organism reproduces an entire organism, increasing the population. Cell division is also central to the development of a multicellular organism that begins as a fertilized egg or zygote. Multicellular organisms also use cell division to repair and renew cells that die from normal wear and tear or accidents. Cell division requires the distribution of identical genetic material - DNA - to two daughter cells. What is remarkable is the accuracy with which DNA is passed along, without changing, from one generation to the next. A dividing parent cell duplicates its DNA, moves the two identical copies to opposite ends of the cell, and then splits into two daughter cells, each with the same DNA. Prokaryotic Cells Reproduce by binary fission, a type of asexual reproduction that produces identical offspring DNA is copied then the cell divides by adding a cell membrane between the DNA copies Eukaryotic Cells Chromosomes become visible and line up in the middle of the cell, then separate to send on genetic material.
Mitosis and Meiosis Genome A cell s entire set of genetic information, packaged as DNA, is called its genome. In prokaryotes, the genome is often a single, long, circular DNA molecule. In eukaryotes, the genome consists of several shorter, linear DNA molecules. How many in yours? A human cell must duplicate about 3 m of DNA and separate the two copies such that each daughter cell ends up with a complete genome. DNA molecules are packaged into chromosomes. Human Chromosomes Somatic Cell: Any cell other than a sperm or egg cell. Each cell has 23 pairs of homologous chromosomes. Diploid because it has 2 sets of chromosomes.
Human somatic cells (body cells) have 46 total chromosomes Gamete: Sex cells, sperm or egg Haploid because only has 1 set of chromosomes. Human gametes (sperm or eggs) have 23 chromosomes, half the number in a somatic cell. 22 pairs are called autosomes (not involved in determining sex) 1 pair called sex chromosomes (determines sex) X and Y chromosomes XX = girl XY = boy Chromosome Abnormalities: determined by a karyotype (photograph of chromosomes) Down syndrome : extra copy of chromosome 21 The Cell Cycle The Cell Cycle takes between 7 hours and 48 hours depending on the type of cell The longest phase of the Cell Cycle is Interphase Interphase takes up to 95 to 99% of the cell cycle time Muscle and Nerve Cells do not go through this process, considered G0, therefore if they are destroyed, they are not replaced! Interphase has three sub phases: o G1 phase ( first gap ) centered on growth (protein synthesis, respiration, etc.), Sometimes cells stop their cell cycle indefinitely during this phase (G0) o S phase ( synthesis ) when the chromosomes are copied,
o G2 phase ( second gap ) where the cell completes preparations for cell division (like microtubule formation), and divides (M). By late interphase, the chromosomes have been duplicated but are loosely packed. The centrosomes have been duplicated and begin to organize microtubules into an aster ( star ). The mitotic (M) phase of the cell cycle alternates with the much longer interphase. The M phase includes mitosis and cytokinesis Cell Cycle Checkpoints There are 3 checkpoints to control cell division and growth 1. G1 Checkpoint: Cell Growth Decides if the cell will continue to DNA replication 2. DNA synthesis checkpoint DNA checked by repair enzymes 3. Mitosis Checkpoint Signals the beginning of growth Loss of Control: Cancer Damage to genes controlling cell cell cycle goes too fast growth is uncontrolled Mitosis Produces two identical cells through asexual reproduction Used for all cells except sex cells Creates 2 diploid cells Prophase o The chromosomes are tightly coiled, with sister chromatids joined together. o The nucleoli disappear.
o The mitotic spindle begins to form and appears to push the centrosomes away from each other toward opposite ends (poles) of the cell. o During prometaphase (late prophase), the nuclear envelope fragments and microtubules from the spindle interact with the chromosomes. o Microtubules from one pole attach to each chromosome at the centromere region. Metaphase o The spindle fibers push the sister chromatids until they are all arranged at the metaphase plate or equator, an imaginary plane equidistant between the poles. Anaphase o The centromeres divide, separating the sister chromatids. o Each is now pulled toward the pole to which it is attached by spindle fibers. o By the end, the two poles have identical collections of chromosomes. Telophase o Two nuclei begin to form, surrounded by the fragments of the parent s nuclear envelope. o Chromatin becomes less tightly coiled. o Cytokinesis, division of the cytoplasm, begins. Cytokinesis Cytokinesis divides the cytoplasm Cytokinesis follows mitosis. Cytokinesis in Animals: o In animals, the first sign of cytokinesis (cleavage) is the appearance of a cleavage furrow in the cell surface near the old metaphase plate. o On the inside of the cleavage furrow, a ring of actin microfilaments and the motor protein myosin form. o Contraction of the ring pinches the cell in two. o This is often referred to as the purse-string method.
Cytokinesis in plants o Cytokinesis in plants, which have cell walls, involves a completely different mechanism. o During telophase, vesicles from the Golgi connect at the metaphase plate, forming a cell plate. o The plate enlarges until its membranes fuse with the plasma membrane at the perimeter, with the contents of the vesicles forming new wall material in between. The frequency of cell division varies Some human cells divide frequently throughout life (skin cells), others have the ability to divide, but keep it in reserve (liver cells), and mature nerve and muscle cells do not appear to divide at all after maturity, although there is current evidence that they may under certain conditions. Cancer (tumor) cells seem to have lost such controls over their division, and divide repeatedly. Any cells in an organ can become cancerous.