Presentation on theme: "THE CELL CYCLE. The Cell Cycle Events that occur in the life of a cell. Includes 3 major stages: 1.Interphase 2.Mitosis 3.Cytokinesis."— Presentation transcript:
THE CELL CYCLE
The Cell Cycle Events that occur in the life of a cell. Includes 3 major stages: 1.Interphase 2.Mitosis 3.Cytokinesis
1. Interphase (Cell is not dividing) G 1 Phase – carries out basic functions & performs specialized activities. duration is extremely variable contains restriction checkpoint ~ cell decides to: divide enter a quiescent phase (G 0 ) die
1. Interphase (Cell is not dividing) G 0 Phase – cell maintains specialized characteristics, but does not divide Ex. neurons & muscle cells
1. Interphase (Cell is not dividing) S Phase – cell replicates chromosomes & synthesizes proteins animal cells replicate centrioles as well
1. Interphase (Cell is not dividing) G 2 Phase - cell synthesizes additional proteins (ex. tubulin) & assembles/stores membrane material
2.Mitosis (M phase) – Equal distribution of replicated genetic material. Five steps: Prophase Prometaphase Metaphase Anaphase Telophase
2.Mitosis – Prophase replicated chromosomes condense centrosomes separate & migrate toward opposite sides of cell mitotic spindle forms (microtubules grow out from centrosomes) nucleolus disappears
2.Mitosis – Prometaphase nuclear membrane breaks down spindle fibers attach to centromeres of chromosomes
2.Mitosis – Metaphase chromosomes are lined up single-file along equator of mitotic spindle
2.Mitosis – Anaphase Centromeres part, sister chromatids (now called chromosomes) separate chromosomes move toward opposite poles
2.Mitosis – Telophase mitotic spindle breaks down chromosomes decondense nuclear membranes reform around two nuclei nucleoli reappear
3.Cytokinesis Distribution of cytoplasm to daughter cells begins during anaphase or telophase differs in animal & plant cells
3.Cytokinesis in animal cells Cleavage furrow (slight indentation) forms around equator of cell Actin & myosin microfilaments act like a drawstring to pinch the cell in two Usually an equal division
3.Cytokinesis in plant cells phragmoplast (microtubule structure) forms in cytoplasm & traps vesicles containing cell wall material vesicles fuse, forming a cell plate across midline of cell cell plate gives rise to two primary cell walls
Review of the M-phase
Does cytokinesis always accompany karyokinesis? Karyokinesis in the absence of cytokinesis results in a syncytium (mass of multinucleated cells).
Control of the Cell Cycle Checkpoints - groups of interacting proteins that ensure cell cycle events occur in the correct sequence.
Shortening of telomeres - loss of telomere DNA signals cell to stop dividing. Some cells produce telomerase (enzyme that continually adds telomere DNA).
Contact Inhibition - healthy cells stop dividing when they come in contact with other cells.
Hormones - stimulate cell division. Ex. Estrogen stimulates uterine cell division Growth factors - proteins that stimulate local cell division. Ex. Epidermal growth factor (EGF) stimulates epithelial cell division filling in new skin underneath a scab Interaction of kinases & cyclins - activate genes that stimulate cell division.
B. Apoptosis Programmed cell death; part of normal development.
Steps of Apoptosis:
C. Cancer (loss of cell cycle control) Condition resulting from excess cell division or deficient apoptosis. Characteristics of Cancer Cells: can divide uncontrollably & eternally are heritable & transplantable lack contact inhibition readily metastasize exhibit angiogenesis exhibit genetic mutability
Causes of Cancer: Over-expression of oncogenes Oncogenes are genes that trigger limited cell division. Inactivation of tumor suppressor genes Tumor suppressor genes prevent a cell from dividing or promote apoptosis.
Normal functioning of oncogenes & tumor suppressor genes may be affected by environmental factors: carcinogens radiation viruses diet exercise habits
Somatic cells – body cells In 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 23 Chromosomes #1 through 22 – autosomal Chromosome #23 – sex Meiosis - 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 parent These copies are segregated from each other during formation of the gametes Homologous – corresponding male and female chromosomes
Meiosis - formation of gametes diploid A cell that contains both sets of chromosomes (1 from each parent ) is said to be diploid (2n) haploid Cells containing 1 set of chromosomes are said to be haploid (n)
It produces 4 haploid cells that are genetically different from each other and from the diploid parent 2 parts: Meiosis I – separation of homologues Meiosis II – separation of sister chromatids Meiosis
Prophase I Everything that happens in Prophase of mitosis also happens in Prophase I of meiosis Chromosomes find their pairs to form a tetrad (process called synapsis) They can exchange genetic info (crossing over) Site of crossing over is the chiasmata
Metaphase I Same as Metaphase of mitosis Tetrads line up at the equator
Anaphase I Same as Anaphase of mitosis Homologous chromosomes separate and move to the poles
Telophase 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)
Meiosis II No replication occurs Mitosis resembles meiosis II more than meiosis I Sister chromatids are separated to make daughter cells that have a single set (n) of single stranded chromosomes
Prophase II Same as prophase of mitosis
Metaphase II and Anaphase II Double stranded (not homologous) chromosomes align along the equator in Metaphase II
Telophase II and cytokinesis At the end of meiosis, there are four haploid daughter cells
Mitosis 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.
Mitosis produces two identical daughter cells, but meiosis produces 4 genetically different cells.
Sexual vs. Asexual Reproduction In asexual reproduction, a single individual passes along copies of all its genes to its offspring Single-celled eukaryotes reproduce asexually by mitotic cell division to produce two identical daughter cells Even some multicellular eukaryotes, like hydra, can reproduce by budding cells produced by mitosis
Sexual vs. Asexual Reproduction Sexual reproduction results in greater variation among offspring than does asexual reproduction Offspring of sexual reproduction vary genetically from their siblings and from both parents
Sexual vs. Asexual Reproduction Three mechanisms contribute to genetic variation: independent assortment crossing over (Prophase I) random fertilization – each zygote is the result of 1 of 70 trillion possible chromosomal combos (2 23 x 2 23 )