Presentation on theme: "THE CELL CYCLE Karyokinesis & cytokinesis can overlap in time, depending upon the species."— Presentation transcript:
1THE CELL CYCLEKaryokinesis & cytokinesis can overlap in time, depending upon the species.
2Events that occur in the life of a cell. Includes 3 major stages: The Cell CycleEvents that occur in the life of a cell.Includes 3 major stages:InterphaseMitosisCytokinesisKaryokinesis & cytokinesis can overlap in time, depending upon the species.
41. Interphase (Cell is not dividing) G1 Phase – carries out basic functions & performs specialized activities.duration is extremely variablecontains restriction checkpoint ~ cell “decides” to:divideenter a quiescent phase (G0)die“G” stands for “gap” or “growth”
12Mitosis (M phase) – Equal distribution of replicated genetic material. Five steps:ProphasePrometaphaseMetaphaseAnaphaseTelophase“G” stands for “gap” or “growth”
13replicated chromosomes condense Mitosis – Prophasereplicated chromosomes condensecentrosomes separate & migrate toward opposite sides of cellmitotic spindle forms (microtubules grow out from centrosomes)nucleolus disappears“G” stands for “gap” or “growth”
21mitotic spindle breaks down chromosomes decondense Mitosis – Telophasemitotic spindle breaks downchromosomes decondensenuclear membranes reform around two nucleinucleoli reappear“G” stands for “gap” or “growth”
25Cytokinesis in animal cells Cleavage furrow (slight indentation) forms around equator of cellActin & myosin microfilaments act like a drawstring to pinch the cell in twoUsually an equal division“G” stands for “gap” or “growth”
26Cytokinesis in plant cells phragmoplast (microtubule structure) forms in cytoplasm & traps vesicles containing cell wall materialvesicles fuse, forming a cell plate across midline of cellcell plate gives rise to two primary cell walls“G” stands for “gap” or “growth”
39Does cytokinesis always accompany karyokinesis? Karyokinesis in the absence of cytokinesis results in a syncytium (mass of multinucleated cells).Usually, but not always.Examples:human skeletal muscle tissue (pictured)endosperm tissue in some plants (nourishes the developing embryo in a seed).
40Control of the Cell Cycle Checkpoints - groups of interacting proteins that ensure cell cycle events occur in the correct sequence.Various mechanisms interact to regulate the cell cycle.Are numerous checkpoints - 4 are described in figure.Restriction checkpoint - cell “decides” whether to divide, enter G0 phase, or die. Most important checkpoint.DNA damage checkpoint - turns on genes that manufacture proteins that repair damaged DNA.Apoptosis checkpoint - necessary to override a signal for the cell to die.Spindle assembly checkpoint - necessary for construction of spindle & binding of chromosomes to it.
41Shortening of telomeres - loss of telomere DNA signals cell to stop dividing. Some cells produce telomerase (enzyme that continually adds telomere DNA).Telomeres are repeating nucleotide sequences found at the tips of chromosomes. Every time a cell divides, some of its telomere DNA is lost. After about 50 divisions (Hayflick limit), a key amount of lost telomere DNA signals cell division to cease.NOTE: Cells that produce telomerase are able to divide beyond the 50-division limit. They include:plant cellscells lining small intestinebone marrow cellscertain immune system cellssperm-forming cellscancer cells
42Contact Inhibition - healthy cells stop dividing when they come in contact with other cells.
43Hormones - stimulate cell division. Ex. Estrogen stimulates uterine cell divisionGrowth factors - proteins that stimulate local cell division.Ex. Epidermal growth factor (EGF) stimulates epithelial cell division filling in new skin underneath a scabInteraction of kinases & cyclins - activate genes that stimulate cell division.
44B. Apoptosis Programmed cell death; part of normal development. Example of apoptosis:Feet of embryonic chickens & ducks have webbing between the toes.Webbing vanishes as the chicken’s foot develops (due to apoptosis).Webbing is retained as duck’s foot develops.Apoptosis fine-tunes the human immune system - in the fetus, it destroys T cells that do not recognize self. If they were not destroyed, these T cell would begin attacking the body (autoimmune disease).
45Steps of Apoptosis: Death receptors activate enzymes called capases. Capases destroy proteins & various cell components & ready the cell for phagocyte destruction.From outside, cell rounds up, forms bulges called blebs and fragments (fragments are surrounded by cell membrane, so they don’t initiate an inflammatory response).Cell death in response to an injury is called necrosis. The cell swells & bursts, initiating an inflammatory response.
46C. Cancer (loss of cell cycle control) Condition resulting from excess cell division or deficient apoptosis.Characteristics of Cancer Cells:can divide uncontrollably & eternallyare heritable & transplantablelack contact inhibitionreadily metastasizeexhibit angiogenesisexhibit genetic mutabilityCancer cells can divide eternally because they produce telomerase. Cervical cancer cells (HeLa) of Henrietta Lacks who died in 1951 are used extensively in cancer research all over the world today.Heritability - when a cancer cell becomes cancerous, it passes its loss ov cell cycle control to its descendants.Transplatability - cancer cells can be injected into a healthy animal; animal will develop cancer because those cancer cells divide to form more cancerous cells.Cancer cells lack contact inhibition - they tend to form masses called tumors.Metastasize = ability to spreadAngiogenesis = ability to induce local blood vessel formation.Cancer cells mutate - a treatment that shrank the original tumor may have no effect on its subsequent growth.
47Causes of Cancer: Over-expression of oncogenes Oncogenes are genes that trigger limited cell division.Inactivation of tumor suppressor genesTumor suppressor genes prevent a cell from dividing or promote apoptosis.Oncogenes are normally switched off in most cells. They are activated only under certain circumstances (ex. in cells at wound site).Tumor suppressor genes are normally switched on in most cells. If they are inactivated or removed cells will divide continually or apoptosis does not occur (ex. a childhood kidney cancer is caused by absence of a tumor suppressor gene that normally halts mitosis in kidney tubule cells in the fetus)Factors that turn on oncogenes or turn off tumor suppressor genes at inappropriate times would lead to development of tumors.
48Normal functioning of oncogenes & tumor suppressor genes may be affected by environmental factors: carcinogensradiationvirusesdietexercise habitsCarcinogens = chemicals that cause cancer (tobacco, asbestos, insecticides, saccharine)Radiation - UV & x-ray photons have enough energy to cause DNA damage.
49Meiosis - formation of gametes Somatic cells – body cellsIn contrast to mitosis (occurs in somatic cells), gametes (eggs or sperm) are produced only in gonads (ovaries or testes).In the gonads, cells undergo a variation of cell division (meiosis) which yields four daughter cells, each with half the chromosomes of the parent.In humans, meiosis reduces the number of chromosomes from 46 to 23Chromosomes #1 through 22 – autosomalChromosome #23 – sex
50Meiosis - formation of gametes Fertilization fuses two gametes together and doubles the number of chromosomes to 46 again.Organisms inherit single copy of each gene from each parentThese copies are segregated from each other during formation of the gametesHomologous – corresponding male and female chromosomes
51Meiosis - formation of gametes A cell that contains both sets of chromosomes (1 from each parent ) is said to be diploid (2n)Cells containing 1 set of chromosomes are said to be haploid (n)
52MeiosisIt produces 4 haploid cells that are genetically different from each other and from the diploid parent2 parts:Meiosis I – separation of homologuesMeiosis II – separation of sister chromatids
53Prophase IEverything that happens in Prophase of mitosis also happens in Prophase I of meiosisChromosomes find their pairs to form a tetrad (process called synapsis)They can exchange genetic info (crossing over)Site of crossing over is the chiasmata
54Metaphase I Same as Metaphase of mitosis Tetrads line up at the equator
55Anaphase I Same as Anaphase of mitosis Homologous chromosomes separate and move to the poles
56Telophase I Same as Telophase of mitosis Instead of having two genetically identical cells, the chromosomal number has been halved (2n to n)Chromosomes are still double stranded (sister chromatids still attached)
57Meiosis II No replication occurs Mitosis resembles meiosis II more than meiosis ISister chromatids are separated to make daughter cells that have a single set (n) of single stranded chromosomes
59Metaphase II and Anaphase II Double stranded (not homologous) chromosomes align along the equator in Metaphase II
60Telophase II and cytokinesis At the end of meiosis, there are four haploid daughter cells
61Mitosis and meiosis have several key differences. The chromosome number is reduced by half in meiosis, but not in mitosis.Mitosis produces daughter cells that are genetically identical to the parent and to each other.Meiosis produces cells that differ from the parent and each other.
62Mitosis produces two identical daughter cells, but meiosis produces 4 genetically different cells.
64Sexual vs. Asexual Reproduction In asexual reproduction, a single individual passes along copies of all its genes to its offspringSingle-celled eukaryotes reproduce asexually by mitotic cell division to produce two identical daughter cellsEven some multicellular eukaryotes, like hydra, can reproduce by budding cells produced by mitosis
65Sexual vs. Asexual Reproduction Sexual reproduction results in greater variation among offspring than does asexual reproductionOffspring of sexual reproduction vary genetically from their siblings and from both parents
66Sexual vs. Asexual Reproduction Three mechanisms contribute to genetic variation:independent assortmentcrossing over (Prophase I)random fertilization – each zygote is the result of 1 of 70 trillion possible chromosomal combos (223 x 223)