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Cell Cycle ◦ Interphase – normal cell activity ◦ Mitotic phase – cell division INTERPHASE INTERPHASE Growth G1 G1 S synthesis) (DNA (DNA synthesis) Growth G2G 2 3 stages G1 - cell grows and functions normally - high amount of protein synthesis occurs - more organelles are produced 2. S - DNA replication (doubles DNA) 3. G2 - cell growth continues 1. - prepare for division Chromosome – DNA packaged around protein Diploid – 2 full sets of chromosomes (2n) Haploid – 1 set of chromosomes (n) Most human cells contain 46 chromosomes (23 sets) Humans, n = 23 2 sister chromatids joined at centromere Chromosomes Sister chromatids Homologous chromosomes: • Look the same • Control the same traits • Independent origin - each one was inherited from a different parent Chromosome Duplication Non-sister chromatids Centromere Duplication Sister chromatids Two unduplicated chromosomes Sister chromatids Two duplicated chromosomes Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. A diploid cell has two sets of each of its chromosomes A human has 46 chromosomes (2n = 46) In a cell in which DNA synthesis has occurred all the chromosomes are duplicated and thus each consists of two identical sister chromatids Maternal set of chromosomes (n = 3) 2n = 6 Paternal set of chromosomes (n = 3) Two sister chromatids of one replicated chromosome Centromere Two nonsister chromatids in a homologous pair Pair of homologous chromosomes (one from each set) Chromosomes 1-22 are autosomes Chromosome 23 is a sex chromosome INTERPHASE INTERPHASE G1 Growth G1 S S (DNA synthesis) (DNA synthesis) G2 Growth G2 100 µm 1. Reproduction. An amoeba, a single-celled eukaryote, is dividing into two cells. Each new cell will be an individual organism (LM). 200 µm 2. Growth and development. This shows a sand dollar embryo shortly after the fertilized egg divided, forming two cells (LM). 20 µm 3. Tissue renewal. These dividing bone marrow cells (arrow) will give rise to new blood cells (LM). Second part of the cell cycle Consists of : ◦ Mitosis – Internal separating cell parts and DNA ◦ Cytokinesis – cell splits Results in genetically identical daughter cells INTERPHASE G1 S (DNA synthesis) G2 1. Prophase (package & poles) ◦ DNA condenses into chromosomes ◦ Centrioles move to opposite ends of cell ◦ Nuclear envelope dissolves 2. Metaphase (middle) ◦ Spindle fibers form and attach to kinetochore connecting centrioles to the chromosomes ◦ Centrioles start to pull and chromosomes line up along the cells equator 3. Anaphase (apart) ◦ ◦ Centromeres split Sister chromatids separate and are pulled to opposite ends of cell 4. Telophase ◦ Cleavage furrow forms ◦ Reverse prophase: nucleus reforms and chromosomes unravel Contractile ring finally divides cell into two identical daughter cells with the same genetic information 100 µm Cleavage furrow Contractile ring of microfilaments Vesicles forming cell plate Wall of patent cell 1 µm Cell plate New cell wall Daughter cells Daughter cells (a) Cleavage of an animal cell (SEM) (b) Cell plate formation in a plant cell (SEM) G2 OF INTERPHASE Centrosomes (with centriole pairs) Nucleolus Chromatin (duplicated) Nuclear Plasma envelope membrane PROPHASE Early mitotic spindle Aster Centromere Chromosome, consisting of two sister chromatids PROMETAPHASE Fragments of nuclear envelope Kinetochore Nonkinetochore microtubules Kinetochore microtubule METAPHASE ANAPHASE Metaphase plate Spindle Centrosome at Daughter one spindle pole chromosomes TELOPHASE AND CYTOKINESIS Cleavage furrow Nuclear envelope forming Nucleolus forming http://www.youtube.com/watch?v=JcZQkmoo yPk Interphase ◦ G1 - primary growth ◦ S - genome replicated ◦ G2 - secondary growth Mitotic phase ◦ M – mitosis Prophase Metaphase Anaphase Telophase ◦ C - cytokinesis Meiosis and Other Factors Factors that affect cell division ◦ Anchorage ◦ cell density ◦ chemical growth factors Most animal cells divide only when stimulated, and some not at all ◦ In laboratory cultures Most normal cells divide only when attached to a surface They continue dividing until they touch Figure 8.8A Cells anchor to dish surface and divide. When cells have formed a complete single layer, they stop dividing (densitydependent inhibition). If some cells are scraped away, the remaining cells divide to fill the dish with a single layer and then stop (density-dependent inhibition). Growth factors signal the cell cycle control system ◦ A set of proteins within the cell controls the cell cycle Signals affect critical checkpoints in the cell cycle ◦ Determine whether a cell will go through the complete cycle and divide G1 checkpoint G0 Control system G1 M G2 M checkpoint G2 checkpoint S ◦ Growth factors attach to receptors to trigger cell division Growth factor Plasma membrane Receptor protein Signal transduction pathway Relay proteins G1 checkpoint Control system G1 M Figure 8.9B G2 S Cancer cells – out of control cell growth ◦ Skip checkpoints ◦ Divide excessively to form masses called tumors ◦ Crowd out normal cells and invade tissue Lymph vessels Tumor Blood vessel Glandular tissue A tumor grows from a single cancer cell. Cancer cells invade neighboring tissue. Cancer cells spread through lymph and blood vessels to other parts of the body. Mitosis – division of somatic (body) cells ◦ Produces diploid cells ◦ 2 Daughter cells identical to parent Meiosis – division which produces gametes (sex cells) ◦ Produces haploid cells ◦ 4 Daughter cells have half of parent DNA Chromosomes Homologous chromosome: one of a matching pair of chromosomes, one inherited from each parent. Sister chromatids are identical Chromosomes are matched in homologous pairs ◦ For example human cells have 46 chromosomes 23 homologous pairs Gametes have a single set of chromosomes ◦ Gametes, eggs and sperm, are haploid With a single set of chromosomes ◦ Sexual reproduction Involve the alternation of haploid and diploid stages Haploid gametes (n = 23) n Egg cell n Sperm cell Meiosis Fertilization Multicellular diploid adults (2n = 46) Diploid zygote (2n = 46) Mitosis and development Figure 8.13 2n Meiosis reduces the chromosome number from diploid to haploid ◦ Meiosis, like mitosis Is preceded by chromosome duplication ◦ But in meiosis The cell divides twice to form four daughter cells ◦ The first division, meiosis I ◦ Prophase I Starts with synapsis- the pairing of homologous chromosomes In crossing-over - Homologous chromosomes exchange segments ◦ Meiosis I separates each homologous pair And produce two daughter cells, each with one set of chromosomes MEIOSIS I: Homologous chromosomes separate INTERPHASE Centrosomes (with centriole pairs) Nuclear envelope PROPHASE I METAPHASE I Sites of crossing over Spindle Chromatin Figure 8.14 (Part 1) Sister chromatids Tetrad Microtubules Metaphase attached to plate kinetochore Centromere (with kinetochore) ANAPHASE I Sister chromatids remain attached Homologous chromosomes separate ◦ Meiosis II is essentially the same as mitosis The sister chromatids of each chromosome separate The result is a total of four haploid cells https://www.youtube.com/watch?v=rqPMp0 U0HOA MEIOSIS II: Sister chromatids separate TELOPHASE I AND CYTOKINESIS II PROPHASE II METAPHASE II ANAPHASE TELOPHASE II AND CYTOKINESIS Cleavage furrow Sister chromatids separate Figure 8.14 (Part 2) Haploid daughter cells forming 8.15 Review: A comparison of mitosis and meiosis Mitosis Meiosis Parent cell (before chromosome replication) Meiosis i Prophase I Prophase Duplicated chromosome (two sister chromatids) Figure 8.15 2n = 4 Metaphase Chromosomes align at the metaphase plate Tetrads align at the metaphase plate Anaphase Telophase Sister chromatids separate during anaphase Homologous chromosomes separate during anaphase I; sister chromatids remain together 2n Daughter cells of mitosis Tetrad formed by synapsis of homologous chromosomes Chromosome replication Chromosome replication 2n No further chromosomal replication; sister chromatids separate during anaphase II Metaphase I Anaphase I Telophase I Haploid n=2 Daughter cells of meiosis I Meiosis ii n n n n Daughter cells of meiosis II Genetic variability 1. Crossing-over 2. Independent assortment of chromosomes 3. Random fertilization of eggs by sperm All of these factors ensure genetic differences in a population ◦ Independent assortment of chromosome pairs at metaphase I of meiosis Lead to many different combinations of chromosomes in eggs and sperm Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Gametes Figure 8.16 Combination 1Combination 2 Combination 3Combination 4 Homologous chromosomes carry different versions of genes ◦ Gene – segment of DNA coding for a physical trait Coat-color genes Eye-color genes Brown Black C E Meiosis c White e Pink Tetrad in parent cell (homologous pair of duplicated chromosomes) Figure 8.17A Brown coat (C); black eyes (E) C E C E c e c e Chromosomes of the four gametes Figure 8.17B White coat (C); pink eyes (e) ◦ How crossing over leads to genetic variation Coat-color genes Eye-color genes E C e c 1 Breakage of homologous chromatids C E c e 2 Tetrad (homologous pair of chromosomes in synapsis) Joining of homologous chromatids C E c e Chiasma 3 C E C e c E e c 4 Figure 8.18B Separation of homologous chromosomes at anaphase I Separation of chromatids at anaphase II and completion of meiosis C E C e c E c e Parental type of chromosome Recombinant chromosome Recombinant chromosome Parental type of Gametes ofchromosome four genetic types 1. Deletion – part of the chromosome is lost ◦ Ex. Cri du Chat Syndrome = “Cry of the Cat” Duplication –production of extra copies of parts of the chromosome. 2. ◦ Ex. Cat eye syndrome – severe mental retardation Inversion - Reverses the direction of parts of a chromosome. 3. ◦ Least likely to have serious effects. 4. Translocation - When one part of a nonhomologous chromosome breaks off and attaches to another chromosome. ◦ All essential genetic material is present so most children appear normal ◦ Ex. Cleft palate & Chronic myelogenous leukemia (CML) http://www.youtube.com/watch?v=XAGxp9j5 rtc 5. Non-disjunction - When homologous chromosomes fail to separate properly during anaphase 1 of meiosis. Ex. Trisomy 21 (aka. Down Syndrome)