Cell Cycle Lecture Notes PDF

The Cell Cycle  Cell Division Mitosis – somatic cells, body cells Meiosis – germ cells, sex cells or gametes © 2011 Pearson Education, Inc.  Somatic cells (nonreproductive cells) have two sets of chromosomes Gametes (reproductive cells: sperm...

The Cell Cycle  Cell Division Mitosis – somatic cells, body cells Meiosis – germ cells, sex cells or gametes © 2011 Pearson Education, Inc.  Somatic cells (nonreproductive cells) have two sets of chromosomes Gametes (reproductive cells: sperm and eggs) have half as many chromosomes as somatic cells © 2011 Pearson Education, Inc. Figure 12.2 100 m (a) Reproduction 200 m (b) Growth and development 20 m (c) Tissue renewal Phases of the Cell Cycle (Mitosis) The cell cycle consists of – Interphase (cell growth and copying of chromosomes in preparation for cell division) – Mitotic (M) phase (mitosis and cytokinesis) © 2011 Pearson Education, Inc. Figure 12.6 INTERPHASE G1 S (DNA synthesis) G2  Interphase (about 90% of the cell cycle) can be divided into subphases – G1 phase (“first gap”) – S phase (“synthesis”) – G2 phase (“second gap”) The cell grows during all three phases, but chromosomes are duplicated only during the S phase © 2011 Pearson Education, Inc.  Interphase – G1 phase (≈11 hrs) Accumulate chromosomal DNA and associated proteins Accumulate sufficient energy reserves – S phase (≈8 hrs) DNA replication – G2 phase (≈4 hrs) Cell replenishes its energy stores and synthesizes proteins necessary for chromosome manipulation Mitosis (≈1 hr) © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. Figure 12.3 In 1882, the German anatomist Walther Flemming developed dyes to observe chromosomes 20 m Eukaryotic chromosomes. Chromosomes (stained purple) are visible within the nucleus of this cell from an African blood lily. The thinner red threads in the surrounding cytoplasm are the cytoskeleton. The cell is preparing to divide (LM). Figure 12.4 Sister chromatids Centromere 0.5 m A highly condensed, duplicated human chromosome (SEM). Parts  Mitosis is conventionally divided into five phases – Prophase – Prometaphase – Metaphase – Anaphase – Telophase Cytokinesis overlaps the latter stages of mitosis © 2011 Pearson Education, Inc.  Nuclear envelope encloses the nucleus. The nucleus contains one or more nucleoli (singular, nucleolus). Two centrosomes have formed by duplication of a single centrosome. Each centrosome contains two centrioles. Chromosomes, duplicated during S phase, cannot be seen individually because they have not yet condensed. © 2011 Pearson Education, Inc.  Interphase © 2011 Pearson Education, Inc.  The chromatin fibers become more tightly coiled. The nucleoli disappear. Each duplicated chromosome appears as two identical sister chromatids joined at their centromeres. The mitotic spindle (named for its shape) begins to form. The centrosomes move away from each other, propelled partly by the lengthening microtubules between them. © 2011 Pearson Education, Inc.  Prophase © 2011 Pearson Education, Inc.  The nuclear envelope fragments. The microtubules extending from each centrosome can now invade the nuclear area. The chromosomes have become even more condensed. Each of the two chromatids of each chromosome now has a kinetochore. Some of the microtubules attach to the kinetochores, becoming “kinetochore microtubules,” which jerk the chromosomes back and forth. © 2011 Pearson Education, Inc.  The centrosomes are now at opposite poles of the cell. The chromosomes convene at the metaphase plate, a plane that is equidistant between the spindle’s two poles. For each chromosome, the kinetochores of the sister chromatids are attached to kinetochore microtubules coming from opposite poles. © 2011 Pearson Education, Inc.  Metaphase © 2011 Pearson Education, Inc.  Anaphase is the shortest stage. Cohesion proteins are cleaved. This allows the two sister chromatids of each pair to part. Each chromatid thus becomes a full-fledged chromosome. The two daughter chromosomes begin moving toward opposite ends of the cell. The cell elongates as the nonkinetochore microtubules lengthen. By the end of anaphase, the two ends of the cell have equivalent— and complete— collections of chromosomes. © 2011 Pearson Education, Inc.  Anaphase © 2011 Pearson Education, Inc.  Two daughter nuclei form in the cell. Nuclear envelopes arise from the fragments of the parent cell’s nuclear envelope. Nucleoli reappear. The chromosomes become less condensed. Any remaining spindle microtubules are depolymerized. © 2011 Pearson Education, Inc.  The division of the cytoplasm is usually well under way by late telophase, so the two daughter cells appear shortly after the end of mitosis. In animal cells, cytokinesis involves the formation of a cleavage furrow, which pinches the cell in two. In plant cells, cytokinesis involves the formation of cell plate. © 2011 Pearson Education, Inc.  Telophase © 2011 Pearson Education, Inc. Figure 12.10 (a) Cleavage of an animal cell (SEM) (b) Cell plate formation in a plant cell (TEM) 100 m Cleavage furrow Vesicles Wall of parent cell forming 1 m cell plate Cell plate New cell wall Contractile ring of Daughter cells microfilaments Daughter cells Figure 12.7a G2 of Interphase Prophase Prometaphase Centrosomes Fragments (with centriole Chromatin Early mitotic Aster of nuclear Nonkinetochore pairs) (duplicated) spindle envelope microtubules Centromere Plasma Nucleolus membrane Kinetochore Kinetochore Chromosome, consisting Nuclear of two sister chromatids microtubule envelope Figure 12.7b Metaphase Anaphase Telophase and Cytokinesis Metaphase Cleavage Nucleolus plate furrow forming Nuclear Spindle Centrosome at Daughter envelope one spindle pole chromosomes forming Figure 12.7c 10 m G2 of Interphase Prophase Prometaphase Figure 12.7d 10 m Metaphase Anaphase Telophase and Cytokinesis Figure 12.11 Chromatin Nucleus condensing 10 m Nucleolus Chromosomes Cell plate 1 Prophase 2 Prometaphase 3 Metaphase 4 Anaphase 5 Telophase The Cell Cycle Clock: Cyclins and Cyclin- Dependent Kinases Two types of regulatory proteins are involved in cell cycle control: cyclins and cyclin-dependent kinases (Cdks) © 2011 Pearson Education, Inc. Meiosis A gamete (sperm or egg) contains a single set of chromosomes, and is haploid (n) For humans, the haploid number is 23 (n = 23) Each set of 23 consists of 22 autosomes and a single sex chromosome In an unfertilized egg (ovum), the sex chromosome is X In a sperm cell, the sex chromosome may be either X or Y © 2011 Pearson Education, Inc. Figure 13.8a Metaphase I Anaphase I Telophase I and Prophase I Cytokinesis Centrosome (with centriole pair) Sister chromatids remain attached Sister Chiasmata Centromere chromatids (with kinetochore) Spindle Metaphase plate Cleavage furrow Homologous Homologous Fragments chromosomes chromosomes of nuclear separate envelope Microtubule Each pair of homologous Two haploid attached to chromosomes separates. cells form; each kinetochore chromosome Chromosomes line up still consists by homologous pairs. of two sister chromatids. Figure 13.8b Telophase II and Prophase II Metaphase II Anaphase II Cytokinesis During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing unduplicated chromosomes. Sister chromatids Haploid daughter separate cells forming  Three events are unique to meiosis, and all three occur in meiosis l – Synapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange genetic information – At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes – At anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate © 2011 Pearson Education, Inc. A Comparison of Mitosis and Meiosis Mitosis conserves the number of chromosome sets, producing cells that are genetically identical to the parent cell Meiosis reduces the number of chromosome sets from two (diploid) to one (haploid), producing cells that differ genetically from each other and from the parent cell © 2011 Pearson Education, Inc. Sets of Chromosomes in Human Cells Human somatic cells (any cell other than a gamete) have 23 pairs of chromosomes A karyotype is an ordered display of the pairs of chromosomes from a cell The two chromosomes in each pair are called homologous chromosomes, or homologs Chromosomes in a homologous pair are the same length and shape and carry genes controlling the same inherited characters © 2011 Pearson Education, Inc. Figure 13.3 APPLICATION TECHNIQUE Pair of homologous 5 m duplicated chromosomes Centromere Sister chromatids Metaphase chromosome Figure 13.3b Pair of homologous 5 m duplicated chromosomes Centromere Sister chromatids Metaphase chromosome Figure 13.3b