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Celldivision

The document provides a comprehensive overview of the cell cycle, detailing interphase and mitotic phases, including processes like chromosome duplication and separation during mitosis and meiosis. Key stages such as prophase, metaphase, anaphase, and telophase are explained, highlighting differences between mitosis and meiosis, especially in genetic variation and cell division outcomes. Additionally, it covers the roles of sex cells in reproduction, illustrating how genetic diversity is achieved through these cellular processes.

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1 The Cell Cycle and How Cells Divide
2 Phases of the Cell Cycle • The cell cycle consists of – Interphase – normal cell activity – The mitotic phase – cell division INTERPHASE Growth G 1 (DNA synthesis) Growth G2 CellDivsion
3 Functions of Cell Division 20 µm100 µm 200 µm (a) Reproduction. An amoeba, a single-celled eukaryote, is dividing into two cells. Each new cell will be an individual organism (LM). (b) Growth and development. This micrograph shows a sand dollar embryo shortly after the fertilized egg divided, forming two cells (LM). (c) Tissue renewal. These dividing bone marrow cells (arrow) will give rise to new blood cells (LM).
4 Cell Division • An integral part of the cell cycle • Mitosis results in genetically identical daughter cells • Cells duplicate their genetic material – Before they divide, ensuring that each daughter cell receives an exact copy of the genetic material, DNA
5 Chromosomes • Non-homologous chromosomes – Look different – Control different traits • Sex chromosomes – Are distinct from each other in their characteristics – Are represented as X and Y – Determine the sex of the individual, In humans XX being female, XY being male • In a diploid cell, the chromosomes occur in pairs. The 2 members of each pair are called homologous chromosomes or homologues.
6 Homologues • Homologous chromosomes: • Look the same • Control the same traits • May code for different forms of each trait • Independent origin - each one was inherited from a different parent
7 Chromosome Duplication 0.5 µm Chromosome duplication (including DNA synthesis) Centromere Separation of sister chromatids Sister chromatids Centrometers Sister chromatids A eukaryotic cell has multiple chromosomes, one of which is represented here. Before duplication, each chromosome has a single DNA molecule. Once duplicated, a chromosome consists of two sister chromatids connected at the centromere. Each chromatid contains a copy of the DNA molecule. Mechanical processes separate the sister chromatids into two chromosomes and distribute them to two daughter cells. • In preparation for cell division, DNA is replicated and the chromosomes condense • Each duplicated chromosome has two sister chromatids, which separate during cell division
8 • Because of duplication, each condensed chromosome consists of 2 identical chromatids joined by a centromere. • Each duplicated chromosome contains 2 identical DNA molecules (unless a mutation occurred), one in each chromatid: Chromosome Duplication Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Two unduplicated chromosomes Centromere Sister chromatids Sister chromatids Duplication Non-sister chromatids Two duplicated chromosomes
9 Structure of Chromosomes • The centromere is a constricted region of the chromosome containing a specific DNA sequence, to which is bound 2 discs of protein called kinetochores. • Kinetochores serve as points of attachment for microtubules that move the chromosomes during cell division: Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Metaphase chromosome Kinetochore Kinetochore microtubules Centromere region of chromosome Sister Chromatids
10 Phases of the Cell Cycle • Interphase – G1 - primary growth – S - genome replicated – G2 - secondary growth • M - mitosis • C - cytokinesis
11 Interphase • G1 - Cells undergo majority of growth • S - Each chromosome replicates (Synthesizes) to produce sister chromatids – Attached at centromere – Contains attachment site (kinetochore) • G2 - Chromosomes condense - Assemble machinery for division such as centrioles
12 Mitosis  Some haploid & diploid cells divide by mitosis.  Each new cell receives one copy of every chromosome that was present in the original cell.  Produces 2 new cells that are both genetically identical to the original cell. DNA duplication during interphase Mitosis Diploid Cell
13 Mitotic Division of an Animal Cell G2 OF INTERPHASE PROPHASE PROMETAPHASE Centrosomes (with centriole pairs) Chromatin (duplicated) Early mitotic spindle Aster Centromere Fragments of nuclear envelope Kinetochore Nucleolus Nuclear envelope Plasma membrane Chromosome, consisting of two sister chromatids Kinetochore microtubule Nonkinetochore microtubules
14 METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS Spindle Metaphase plate Nucleolus forming Cleavage furrow Nuclear envelope formingCentrosome at one spindle pole Daughter chromosomes Mitotic Division of an Animal Cell
15 G2 of Interphase • A nuclear envelope bounds the nucleus. • The nucleus contains one or more nucleoli (singular, nucleolus). • Two centrosomes have formed by replication of a single centrosome. • In animal cells, each centrosome features two centrioles. • Chromosomes, duplicated during S phase, cannot be seen individually because they have not yet condensed. The light micrographs show dividing lung cells from a newt, which has 22 chromosomes in its somatic cells (chromosomes appear blue, microtubules green, intermediate filaments red). For simplicity, the drawings show only four chromosomes. G2 OF INTERPHASE Centrosomes (with centriole pairs) Chromatin (duplicated) Nucleolus Nuclear envelope Plasma membrane
16 Prophase • The chromatin fibers become more tightly coiled, condensing into discrete chromosomes observable with a light microscope. • The nucleoli disappear. • Each duplicated chromosome appears as two identical sister chromatids joined together. • The mitotic spindle begins to form. It is composed of the centrosomes and the microtubules that extend from them. The radial arrays of shorter microtubules that extend from the centrosomes are called asters (“stars”). • The centrosomes move away from each other, apparently propelled by the lengthening microtubules between them. PROPHASE Early mitotic spindle Aster Centromere Chromosome, consisting of two sister chromatids
17 Metaphase • Metaphase is the longest stage of mitosis, lasting about 20 minutes. • The centrosomes are now at opposite ends of the cell. •The chromosomes convene on the metaphase plate, an imaginary plane that is equidistant between the spindle’s two poles. The chromosomes’ centromeres lie on the metaphase plate. • For each chromosome, the kinetochores of the sister chromatids are attached to kinetochore microtubules coming from opposite poles. • The entire apparatus of microtubules is called the spindle because of its shape. METAPHASE Spindle Metaphase plate Centrosome at one spindle pole
18 • Some spindle microtubules attach to the kinetochores of chromosomes and move the chromosomes to the metaphase plate • In anaphase, sister chromatids separate and move along the kinetochore microtubules toward opposite ends of the cell Microtubules Chromosomes Sister chromatids Aster Centrosome Metaphase plate Kineto- chores Kinetochore microtubules 0.5 µm Overlapping nonkinetochore microtubules 1 µmCentrosome The Mitotic Spindle
19 Anaphase • Anaphase is the shortest stage of mitosis, lasting only a few minutes. • Anaphase begins when the two sister chromatids of each pair suddenly part. Each chromatid thus becomes a full- fledged chromosome. • The two liberated chromosomes begin moving toward opposite ends of the cell, as their kinetochore microtubules shorten. Because these microtubules are attached at the centromere region, the chromosomes move centromere first (at about 1 µm/min). • 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. ANAPHASE Daughter chromosomes
20 Telophase • Two daughter nuclei begin to form in the cell. • Nuclear envelopes arise from the fragments of the parent cell’s nuclear envelope and other portions of the endomembrane system. • The chromosomes become less condensed. • Mitosis, the division of one nucleus into two genetically identical nuclei, is now complete. TELOPHASE AND CYTOKINESIS Nucleolus forming Cleavage furrow Nuclear envelope forming
21 Cytokinesis • Cleavage of cell into two halves – Animal cells  Constriction belt of actin filaments – Plant cells  Cell plate – Fungi and protists  Mitosis occurs within the nucleus
22 Cytokinesis In Animal And Plant Cells Daughter cells Cleavage furrow Contractile ring of microfilaments Daughter cells 100 µm 1 µmVesicles forming cell plate Wall of patent cell Cell plate New cell wall (a) Cleavage of an animal cell (SEM) (b) Cell plate formation in a plant cell (SEM)
23
24 Meiosis and Sexual Life Cycles • Living organisms are distinguished by their ability to reproduce their own kind • Heredity – Is the transmission of traits from one generation to the next • Variation – Shows that offspring differ somewhat in appearance from parents and siblings
25 Sex Cells - Gametes • Unlike somatic cells, sperm and egg cells are haploid cells, containing only one set of chromosomes • At sexual maturity the ovaries and testes produce haploid gametes by meiosis
26 Sexual Reproduction - The Human Life Cycle • During fertilization, sperm and ovum fuse forming a diploid zygote • The zygote develops into an adult organism Haploid (n) Diploid (2n) Haploid gametes (n = 23) Ovum (n) Sperm Cell (n) MEIOSIS FERTILIZATION Ovary Testis Diploid zygote (2n = 46) Mitosis and development Multicellular diploid adults (2n = 46)
27 Meiosis • Reduces the chromosome number such that each daughter • Cell has a haploid set of chromosomes • Ensures that the next generation will have: – Diploid number of chromosome – Exchange of genetic information (combination of traits – that differs from that of either parent)
28 Meiosis • Only diploid cells can divide by meiosis. • Prior to meiosis I, DNA replication occurs. • During meiosis, there will be two nuclear divisions, and the result will be four haploid nuclei. • No replication of DNA occurs between meiosis I and meiosis II.
29 Meiosis • Meiosis reduces the number of chromosome sets from diploid to haploid • Meiosis takes place in two sets of divisions – Meiosis I reduces the number of chromosomes from diploid to haploid – Meiosis II produces four haploid daughter cells Figure 13.7 Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes 1 2 Homologous chromosomes separate Haploid cells with replicated chromosomes Sister chromatids separate Haploid cells with unreplicated chromosomes Meiosis I Meiosis II
30 Meiosis Phases • Meiosis involves the same four phases seen in mitosis  prophase  metaphase  anaphase  telophase • They are repeated during both meiosis I and meiosis II. • The period of time between meiosis I and meiosis II is called interkinesis. • No replication of DNA occurs during interkinesis because the DNA is already duplicated.
31 Prophase I • Prophase I occupies more than 90% of the time required for meiosis • Chromosomes begin to condense • In synapsis, the 2 members of each homologous pair of chromosomes line up side-by-side, aligned gene by gene, to form a tetrad consisting of 4 chromatids • During synapsis, sometimes there is an exchange of homologous parts between non-sister chromatids. This exchange is called crossing over • Each tetrad usually has one or more chiasmata, X-shaped regions where crossing over occurred Prophase I of meiosis Tetrad Nonsister chromatids Chiasma, site of crossing over
32 Metaphase I • At metaphase I, tetrads line up at the metaphase plate, with one chromosome facing each pole • Microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad • Microtubules from the other pole are attached to the kinetochore of the other chromosome Sister chromatids Chiasmata Spindle Centromere (with kinetochore) Metaphase plate Homologous chromosomes separate Sister chromatids remain attached Microtubule attached to kinetochore Tetrad PROPHASE I METAPHASE I ANAPHASE I Homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 Pairs of homologous chromosomes split up Tetrads line up
33 Anaphase I • In anaphase I, pairs of homologous chromosomes separate • One chromosome moves toward each pole, guided by the spindle apparatus • Sister chromatids remain attached at the centromere and move as one unit toward the pole Sister chromatids Chiasmata Spindle Centromere (with kinetochore) Metaphase plate Homologous chromosomes separate Sister chromatids remain attached Microtubule attached to kinetochore Tetrad PROPHASE I METAPHASE I ANAPHASE I Homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 Pairs of homologous chromosomes split up Tetrads line up
34 Telophase I and Cytokinesis • In the beginning of telophase I, each half of the cell has a haploid set of chromosomes; each chromosome still consists of two sister chromatids • Cytokinesis usually occurs simultaneously, forming two haploid daughter cells • In animal cells, a cleavage furrow forms; in plant cells, a cell plate forms • No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated
35 Prophase II • Meiosis II is very similar to mitosis • In prophase II, a spindle apparatus forms • In late prophase II, chromosomes (each still composed of two chromatids) move toward the metaphase plate Cleavage furrow PROPHASE II METAPHASE II ANAPHASE II TELOPHASE I AND CYTOKINESIS TELOPHASE II AND CYTOKINESIS Sister chromatids separate Haploid daughter cells forming
36 Metaphase II • At metaphase II, the sister chromatids are at the metaphase plate • Because of crossing over in meiosis I, the two sister chromatids of each chromosome are no longer genetically identical • The kinetochores of sister chromatids attach to microtubules extending from opposite poles Cleavage furrow PROPHASE II METAPHASE II ANAPHASE II TELOPHASE I AND CYTOKINESIS TELOPHASE II AND CYTOKINESIS Sister chromatids separate Haploid daughter cells forming
37 Anaphase II • At anaphase II, the sister chromatids separate • The sister chromatids of each chromosome now move as two newly individual chromosomes toward opposite poles Cleavage furrow PROPHASE II METAPHASE II ANAPHASE II TELOPHASE I AND CYTOKINESIS TELOPHASE II AND CYTOKINESIS Sister chromatids separate Haploid daughter cells forming
38 Telophase II and Cytokinesis • In telophase II, the chromosomes arrive at opposite poles • Nuclei form, and the chromosomes begin decondensing • Cytokinesis separates the cytoplasm • At the end of meiosis, there are four daughter cells, each with a haploid set of unreplicated chromosomes • Each daughter cell is genetically distinct from the others and from the parent cell Cleavage furrow PROPHASE II METAPHASE II ANAPHASE II TELOPHASE I AND CYTOKINESIS TELOPHASE II AND CYTOKINESIS Sister chromatids separate Haploid daughter cells forming
39 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 chromosomes sets from two (diploid) to one (haploid), producing cells that differ genetically from each other and from the parent cell • The mechanism for separating sister chromatids is virtually identical in meiosis II and mitosis
40 • 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 of meiosis, homologous pairs move toward opposite poles of the cell. In anaphase II of meiosis, the sister chromatids separate A Comparison of Mitosis and Meiosis
41 MITOSIS MEIOSIS Prophase Duplicated chromosome (two sister chromatids) Chromosome replication Chromosome replication Parent cell (before chromosome replication) Chiasma (site of crossing over) MEIOSIS I Prophase I Tetrad formed by synapsis of homologous chromosomes Metaphase Chromosomes positioned at the metaphase plate Tetrads positioned at the metaphase plate Metaphase I Anaphase I Telophase I Haploid n = 3 MEIOSIS II Daughter cells of meiosis I Homologues separate during anaphase I; sister chromatids remain together Daughter cells of meiosis II n n n n Sister chromatids separate during anaphase II Anaphase Telophase Sister chromatids separate during anaphase 2n 2n Daughter cells of mitosis 2n = 6 A Comparison Of Mitosis And Meiosis
42 Comparison • Meiosis • DNA duplication followed by 2 cell divisions • Sysnapsis • Crossing-over • One diploid cell produces 4 haploid cells • Each new cell has a unique combination of genes • Mitosis • Homologous chromosomes do not pair up • No genetic exchange between homologous chromosomes • One diploid cell produces 2 diploid cells or one haploid cell produces 2 haploid cells • New cells are genetically identical to original cell (except for mutation)
43 Sexual Reproduction - The Human Life Cycle • During fertilization, sperm and ovum fuse forming a diploid zygote • The zygote develops into an adult organism Haploid (n) Diploid (2n) Haploid gametes (n = 23) Ovum (n) Sperm Cell (n) MEIOSIS FERTILIZATION Ovary Testis Diploid zygote (2n = 46) Mitosis and development Multicellular diploid adults (2n = 46)
44 Regulation of cell cycle • Each cyclin is associated with a particular phase, transition, or set of phases in the cell cycle and • helps drive the events of that phase or period. For instance; – M cyclin promotes the events of M phase, such as nuclear envelope breakdown and chromosome condensation Most important core cell cycle regulators: proteins called cyclins, enzymes called Cdks, and an enzyme complex called the APC/C Cyclins are among the most important core cell cycle regulators - Cyclins are a group of related proteins and - four basic types found in humans and most other eukaryotes – - G1 cyclin, G1/S cyclins, S cyclins, and M cyclins​
45 Cyclin-dependent kinases • To drive the cell cycle forward, a cyclin must activate or inactivate different target proteins inside of the cell • In cell cycle regulation Cyclins work with a family of enzymes called the cyclin-dependent kinases (Cdks) • Binding of a cyclin activates inactive Cdks • Activation makes a functional enzyme and allowing it to modify target proteins
46 Cdks • Cdks are kinases –phosphorylate specific target proteins • The attached phosphate group acts like a switch, making the target protein more or less active • Cyclin attaching to a Cdk exert two important effects: – activates the Cdk as a kinase – directs the Cdk to a specific set of target proteins • G1/S cyclins send Cdks to S phase targets (e.g., promoting DNA replication) M cyclins send Cdks to M phase targets (e.g., making the nuclear membrane break down) also be negatively regulated by phosphorylation of other sites
47 Maturation-promoting factor (MPF) • Example of cyclins and Cdks working together to control cell cycle transitions • Researchers found that cells in M phase contained an unknown factor that could force frog egg cells (stuck in G2) to enter M phase • This mystery molecule was called MPF • Discovered in the 1980s to be a Cdk bound to its M cyclin partner • M cyclin builds up as cell division progresses to G2/M • Trigger M phase • Activation signal is the intactness of DNA
48 Action of MPF complex • The MPF complexes add phosphate tags to several different proteins in the nuclear envelope • Resulting in its breakdown (a key event of early M phase) and • Activate targets that promote chromosome condensation and other M phase events • Trigger nuclear envelope breakdown
49 The anaphase-promoting complex/cyclosome (APC/C) • MPF also triggers its own destruction by activating the anaphase-promoting complex/cyclosome(APC/C) • A protein complex that causes M cyclins to be destroyed starting in anaphase • The destruction of M cyclins pushes the cell out of mitosis • Allow the new daughter cells to enter G1 • The APC/C also causes destruction of the proteins that hold the sister chromatids together, allowing them to separate in anaphase and move to opposite poles of the cell.
50 Action of APC/C • The APC/C is an enzyme • It adds a small protein tag called ubiquitin (Ub) • When a target is tagged with ubiquitin, it is sent to the proteasome • Thought as the recycle bin of the cell, and destroyed • Eg. The APC/C attaches a ubiquitin tag to M cyclins • causing them to be chopped up by the proteasome and • Allow the newly forming daughter cells to enter G1 • The APC/C also uses ubiquitin tagging to trigger the separation of sister chromatids during mitosis • If the APC/C gets the right signals at metaphase, it sets off a chain of events that destroys cohesin, the protein glue that holds sister chromatids together
51 APC/C • The APC/C first adds a ubiquitin tag to a protein called securin and • Send it for recycling • Securin normally binds to, and inactivates, a protein called separase • When securin is sent for recycling, separase becomes active and be functional • Separase chops up the cohesin that holds sister chromatids together, allowing them to separate
52 Checkpoints and regulators • Cdks, cyclins, and the APC/C are direct regulators of cell cycle transitions • They respond to signals from inside and outside the cell • These signals determine whether the cell moves forward in the cell cycle • Positive cues, like growth factors may increase activity of Cdks and cyclins • Negative ones, like DNA damage decrease or block activity • Cells deal with DNA damage by; • Fixing it if possible and/or by • Preventing cell division • A protein called p53, a famous tumor suppressor gene involved in DNA damage response
53 Cell cycle check point in DNA damage • p53 works on multiple levels to ensure that cells do not pass on their damaged DNA through cell division First, it stops the cell cycle at the G1checkpoint by triggering production of Cdk inhibitor (CKI) proteins The CKI proteins bind to Cdk-cyclin complexes and block their activity p53's second job is to activate DNA repair enzymes If DNA damage is not fixable, p53 will play its third and final role: triggering programmed cell death so damaged DNA is not passed on

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Celldivision

  • 1.
    1 The Cell Cycleand How Cells Divide
  • 2.
    2 Phases of theCell Cycle • The cell cycle consists of – Interphase – normal cell activity – The mitotic phase – cell division INTERPHASE Growth G 1 (DNA synthesis) Growth G2 CellDivsion
  • 3.
    3 Functions of CellDivision 20 µm100 µm 200 µm (a) Reproduction. An amoeba, a single-celled eukaryote, is dividing into two cells. Each new cell will be an individual organism (LM). (b) Growth and development. This micrograph shows a sand dollar embryo shortly after the fertilized egg divided, forming two cells (LM). (c) Tissue renewal. These dividing bone marrow cells (arrow) will give rise to new blood cells (LM).
  • 4.
    4 Cell Division • Anintegral part of the cell cycle • Mitosis results in genetically identical daughter cells • Cells duplicate their genetic material – Before they divide, ensuring that each daughter cell receives an exact copy of the genetic material, DNA
  • 5.
    5 Chromosomes • Non-homologous chromosomes –Look different – Control different traits • Sex chromosomes – Are distinct from each other in their characteristics – Are represented as X and Y – Determine the sex of the individual, In humans XX being female, XY being male • In a diploid cell, the chromosomes occur in pairs. The 2 members of each pair are called homologous chromosomes or homologues.
  • 6.
    6 Homologues • Homologous chromosomes: •Look the same • Control the same traits • May code for different forms of each trait • Independent origin - each one was inherited from a different parent
  • 7.
    7 Chromosome Duplication 0.5 µm Chromosome duplication (includingDNA synthesis) Centromere Separation of sister chromatids Sister chromatids Centrometers Sister chromatids A eukaryotic cell has multiple chromosomes, one of which is represented here. Before duplication, each chromosome has a single DNA molecule. Once duplicated, a chromosome consists of two sister chromatids connected at the centromere. Each chromatid contains a copy of the DNA molecule. Mechanical processes separate the sister chromatids into two chromosomes and distribute them to two daughter cells. • In preparation for cell division, DNA is replicated and the chromosomes condense • Each duplicated chromosome has two sister chromatids, which separate during cell division
  • 8.
    8 • Because ofduplication, each condensed chromosome consists of 2 identical chromatids joined by a centromere. • Each duplicated chromosome contains 2 identical DNA molecules (unless a mutation occurred), one in each chromatid: Chromosome Duplication Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Two unduplicated chromosomes Centromere Sister chromatids Sister chromatids Duplication Non-sister chromatids Two duplicated chromosomes
  • 9.
    9 Structure of Chromosomes •The centromere is a constricted region of the chromosome containing a specific DNA sequence, to which is bound 2 discs of protein called kinetochores. • Kinetochores serve as points of attachment for microtubules that move the chromosomes during cell division: Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Metaphase chromosome Kinetochore Kinetochore microtubules Centromere region of chromosome Sister Chromatids
  • 10.
    10 Phases of theCell Cycle • Interphase – G1 - primary growth – S - genome replicated – G2 - secondary growth • M - mitosis • C - cytokinesis
  • 11.
    11 Interphase • G1 -Cells undergo majority of growth • S - Each chromosome replicates (Synthesizes) to produce sister chromatids – Attached at centromere – Contains attachment site (kinetochore) • G2 - Chromosomes condense - Assemble machinery for division such as centrioles
  • 12.
    12 Mitosis  Some haploid &diploid cells divide by mitosis.  Each new cell receives one copy of every chromosome that was present in the original cell.  Produces 2 new cells that are both genetically identical to the original cell. DNA duplication during interphase Mitosis Diploid Cell
  • 13.
    13 Mitotic Division ofan Animal Cell G2 OF INTERPHASE PROPHASE PROMETAPHASE Centrosomes (with centriole pairs) Chromatin (duplicated) Early mitotic spindle Aster Centromere Fragments of nuclear envelope Kinetochore Nucleolus Nuclear envelope Plasma membrane Chromosome, consisting of two sister chromatids Kinetochore microtubule Nonkinetochore microtubules
  • 14.
    14 METAPHASE ANAPHASE TELOPHASEAND CYTOKINESIS Spindle Metaphase plate Nucleolus forming Cleavage furrow Nuclear envelope formingCentrosome at one spindle pole Daughter chromosomes Mitotic Division of an Animal Cell
  • 15.
    15 G2 of Interphase •A nuclear envelope bounds the nucleus. • The nucleus contains one or more nucleoli (singular, nucleolus). • Two centrosomes have formed by replication of a single centrosome. • In animal cells, each centrosome features two centrioles. • Chromosomes, duplicated during S phase, cannot be seen individually because they have not yet condensed. The light micrographs show dividing lung cells from a newt, which has 22 chromosomes in its somatic cells (chromosomes appear blue, microtubules green, intermediate filaments red). For simplicity, the drawings show only four chromosomes. G2 OF INTERPHASE Centrosomes (with centriole pairs) Chromatin (duplicated) Nucleolus Nuclear envelope Plasma membrane
  • 16.
    16 Prophase • The chromatinfibers become more tightly coiled, condensing into discrete chromosomes observable with a light microscope. • The nucleoli disappear. • Each duplicated chromosome appears as two identical sister chromatids joined together. • The mitotic spindle begins to form. It is composed of the centrosomes and the microtubules that extend from them. The radial arrays of shorter microtubules that extend from the centrosomes are called asters (“stars”). • The centrosomes move away from each other, apparently propelled by the lengthening microtubules between them. PROPHASE Early mitotic spindle Aster Centromere Chromosome, consisting of two sister chromatids
  • 17.
    17 Metaphase • Metaphase isthe longest stage of mitosis, lasting about 20 minutes. • The centrosomes are now at opposite ends of the cell. •The chromosomes convene on the metaphase plate, an imaginary plane that is equidistant between the spindle’s two poles. The chromosomes’ centromeres lie on the metaphase plate. • For each chromosome, the kinetochores of the sister chromatids are attached to kinetochore microtubules coming from opposite poles. • The entire apparatus of microtubules is called the spindle because of its shape. METAPHASE Spindle Metaphase plate Centrosome at one spindle pole
  • 18.
    18 • Some spindlemicrotubules attach to the kinetochores of chromosomes and move the chromosomes to the metaphase plate • In anaphase, sister chromatids separate and move along the kinetochore microtubules toward opposite ends of the cell Microtubules Chromosomes Sister chromatids Aster Centrosome Metaphase plate Kineto- chores Kinetochore microtubules 0.5 µm Overlapping nonkinetochore microtubules 1 µmCentrosome The Mitotic Spindle
  • 19.
    19 Anaphase • Anaphase isthe shortest stage of mitosis, lasting only a few minutes. • Anaphase begins when the two sister chromatids of each pair suddenly part. Each chromatid thus becomes a full- fledged chromosome. • The two liberated chromosomes begin moving toward opposite ends of the cell, as their kinetochore microtubules shorten. Because these microtubules are attached at the centromere region, the chromosomes move centromere first (at about 1 µm/min). • 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. ANAPHASE Daughter chromosomes
  • 20.
    20 Telophase • Two daughternuclei begin to form in the cell. • Nuclear envelopes arise from the fragments of the parent cell’s nuclear envelope and other portions of the endomembrane system. • The chromosomes become less condensed. • Mitosis, the division of one nucleus into two genetically identical nuclei, is now complete. TELOPHASE AND CYTOKINESIS Nucleolus forming Cleavage furrow Nuclear envelope forming
  • 21.
    21 Cytokinesis • Cleavage ofcell into two halves – Animal cells  Constriction belt of actin filaments – Plant cells  Cell plate – Fungi and protists  Mitosis occurs within the nucleus
  • 22.
    22 Cytokinesis In AnimalAnd Plant Cells Daughter cells Cleavage furrow Contractile ring of microfilaments Daughter cells 100 µm 1 µmVesicles forming cell plate Wall of patent cell Cell plate New cell wall (a) Cleavage of an animal cell (SEM) (b) Cell plate formation in a plant cell (SEM)
  • 23.
  • 24.
    24 Meiosis and SexualLife Cycles • Living organisms are distinguished by their ability to reproduce their own kind • Heredity – Is the transmission of traits from one generation to the next • Variation – Shows that offspring differ somewhat in appearance from parents and siblings
  • 25.
    25 Sex Cells -Gametes • Unlike somatic cells, sperm and egg cells are haploid cells, containing only one set of chromosomes • At sexual maturity the ovaries and testes produce haploid gametes by meiosis
  • 26.
    26 Sexual Reproduction -The Human Life Cycle • During fertilization, sperm and ovum fuse forming a diploid zygote • The zygote develops into an adult organism Haploid (n) Diploid (2n) Haploid gametes (n = 23) Ovum (n) Sperm Cell (n) MEIOSIS FERTILIZATION Ovary Testis Diploid zygote (2n = 46) Mitosis and development Multicellular diploid adults (2n = 46)
  • 27.
    27 Meiosis • Reduces thechromosome number such that each daughter • Cell has a haploid set of chromosomes • Ensures that the next generation will have: – Diploid number of chromosome – Exchange of genetic information (combination of traits – that differs from that of either parent)
  • 28.
    28 Meiosis • Only diploidcells can divide by meiosis. • Prior to meiosis I, DNA replication occurs. • During meiosis, there will be two nuclear divisions, and the result will be four haploid nuclei. • No replication of DNA occurs between meiosis I and meiosis II.
  • 29.
    29 Meiosis • Meiosis reducesthe number of chromosome sets from diploid to haploid • Meiosis takes place in two sets of divisions – Meiosis I reduces the number of chromosomes from diploid to haploid – Meiosis II produces four haploid daughter cells Figure 13.7 Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes 1 2 Homologous chromosomes separate Haploid cells with replicated chromosomes Sister chromatids separate Haploid cells with unreplicated chromosomes Meiosis I Meiosis II
  • 30.
    30 Meiosis Phases • Meiosisinvolves the same four phases seen in mitosis  prophase  metaphase  anaphase  telophase • They are repeated during both meiosis I and meiosis II. • The period of time between meiosis I and meiosis II is called interkinesis. • No replication of DNA occurs during interkinesis because the DNA is already duplicated.
  • 31.
    31 Prophase I • ProphaseI occupies more than 90% of the time required for meiosis • Chromosomes begin to condense • In synapsis, the 2 members of each homologous pair of chromosomes line up side-by-side, aligned gene by gene, to form a tetrad consisting of 4 chromatids • During synapsis, sometimes there is an exchange of homologous parts between non-sister chromatids. This exchange is called crossing over • Each tetrad usually has one or more chiasmata, X-shaped regions where crossing over occurred Prophase I of meiosis Tetrad Nonsister chromatids Chiasma, site of crossing over
  • 32.
    32 Metaphase I • Atmetaphase I, tetrads line up at the metaphase plate, with one chromosome facing each pole • Microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad • Microtubules from the other pole are attached to the kinetochore of the other chromosome Sister chromatids Chiasmata Spindle Centromere (with kinetochore) Metaphase plate Homologous chromosomes separate Sister chromatids remain attached Microtubule attached to kinetochore Tetrad PROPHASE I METAPHASE I ANAPHASE I Homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 Pairs of homologous chromosomes split up Tetrads line up
  • 33.
    33 Anaphase I • Inanaphase I, pairs of homologous chromosomes separate • One chromosome moves toward each pole, guided by the spindle apparatus • Sister chromatids remain attached at the centromere and move as one unit toward the pole Sister chromatids Chiasmata Spindle Centromere (with kinetochore) Metaphase plate Homologous chromosomes separate Sister chromatids remain attached Microtubule attached to kinetochore Tetrad PROPHASE I METAPHASE I ANAPHASE I Homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 Pairs of homologous chromosomes split up Tetrads line up
  • 34.
    34 Telophase I andCytokinesis • In the beginning of telophase I, each half of the cell has a haploid set of chromosomes; each chromosome still consists of two sister chromatids • Cytokinesis usually occurs simultaneously, forming two haploid daughter cells • In animal cells, a cleavage furrow forms; in plant cells, a cell plate forms • No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated
  • 35.
    35 Prophase II • MeiosisII is very similar to mitosis • In prophase II, a spindle apparatus forms • In late prophase II, chromosomes (each still composed of two chromatids) move toward the metaphase plate Cleavage furrow PROPHASE II METAPHASE II ANAPHASE II TELOPHASE I AND CYTOKINESIS TELOPHASE II AND CYTOKINESIS Sister chromatids separate Haploid daughter cells forming
  • 36.
    36 Metaphase II • Atmetaphase II, the sister chromatids are at the metaphase plate • Because of crossing over in meiosis I, the two sister chromatids of each chromosome are no longer genetically identical • The kinetochores of sister chromatids attach to microtubules extending from opposite poles Cleavage furrow PROPHASE II METAPHASE II ANAPHASE II TELOPHASE I AND CYTOKINESIS TELOPHASE II AND CYTOKINESIS Sister chromatids separate Haploid daughter cells forming
  • 37.
    37 Anaphase II • Atanaphase II, the sister chromatids separate • The sister chromatids of each chromosome now move as two newly individual chromosomes toward opposite poles Cleavage furrow PROPHASE II METAPHASE II ANAPHASE II TELOPHASE I AND CYTOKINESIS TELOPHASE II AND CYTOKINESIS Sister chromatids separate Haploid daughter cells forming
  • 38.
    38 Telophase II andCytokinesis • In telophase II, the chromosomes arrive at opposite poles • Nuclei form, and the chromosomes begin decondensing • Cytokinesis separates the cytoplasm • At the end of meiosis, there are four daughter cells, each with a haploid set of unreplicated chromosomes • Each daughter cell is genetically distinct from the others and from the parent cell Cleavage furrow PROPHASE II METAPHASE II ANAPHASE II TELOPHASE I AND CYTOKINESIS TELOPHASE II AND CYTOKINESIS Sister chromatids separate Haploid daughter cells forming
  • 39.
    39 A Comparison ofMitosis and Meiosis • Mitosis conserves the number of chromosome sets, producing cells that are genetically identical to the parent cell • Meiosis reduces the number of chromosomes sets from two (diploid) to one (haploid), producing cells that differ genetically from each other and from the parent cell • The mechanism for separating sister chromatids is virtually identical in meiosis II and mitosis
  • 40.
    40 • Three eventsare 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 of meiosis, homologous pairs move toward opposite poles of the cell. In anaphase II of meiosis, the sister chromatids separate A Comparison of Mitosis and Meiosis
  • 41.
    41 MITOSIS MEIOSIS Prophase Duplicated chromosome (twosister chromatids) Chromosome replication Chromosome replication Parent cell (before chromosome replication) Chiasma (site of crossing over) MEIOSIS I Prophase I Tetrad formed by synapsis of homologous chromosomes Metaphase Chromosomes positioned at the metaphase plate Tetrads positioned at the metaphase plate Metaphase I Anaphase I Telophase I Haploid n = 3 MEIOSIS II Daughter cells of meiosis I Homologues separate during anaphase I; sister chromatids remain together Daughter cells of meiosis II n n n n Sister chromatids separate during anaphase II Anaphase Telophase Sister chromatids separate during anaphase 2n 2n Daughter cells of mitosis 2n = 6 A Comparison Of Mitosis And Meiosis
  • 42.
    42 Comparison • Meiosis • DNAduplication followed by 2 cell divisions • Sysnapsis • Crossing-over • One diploid cell produces 4 haploid cells • Each new cell has a unique combination of genes • Mitosis • Homologous chromosomes do not pair up • No genetic exchange between homologous chromosomes • One diploid cell produces 2 diploid cells or one haploid cell produces 2 haploid cells • New cells are genetically identical to original cell (except for mutation)
  • 43.
    43 Sexual Reproduction -The Human Life Cycle • During fertilization, sperm and ovum fuse forming a diploid zygote • The zygote develops into an adult organism Haploid (n) Diploid (2n) Haploid gametes (n = 23) Ovum (n) Sperm Cell (n) MEIOSIS FERTILIZATION Ovary Testis Diploid zygote (2n = 46) Mitosis and development Multicellular diploid adults (2n = 46)
  • 44.
    44 Regulation of cellcycle • Each cyclin is associated with a particular phase, transition, or set of phases in the cell cycle and • helps drive the events of that phase or period. For instance; – M cyclin promotes the events of M phase, such as nuclear envelope breakdown and chromosome condensation Most important core cell cycle regulators: proteins called cyclins, enzymes called Cdks, and an enzyme complex called the APC/C Cyclins are among the most important core cell cycle regulators - Cyclins are a group of related proteins and - four basic types found in humans and most other eukaryotes – - G1 cyclin, G1/S cyclins, S cyclins, and M cyclins​
  • 45.
    45 Cyclin-dependent kinases • Todrive the cell cycle forward, a cyclin must activate or inactivate different target proteins inside of the cell • In cell cycle regulation Cyclins work with a family of enzymes called the cyclin-dependent kinases (Cdks) • Binding of a cyclin activates inactive Cdks • Activation makes a functional enzyme and allowing it to modify target proteins
  • 46.
    46 Cdks • Cdks arekinases –phosphorylate specific target proteins • The attached phosphate group acts like a switch, making the target protein more or less active • Cyclin attaching to a Cdk exert two important effects: – activates the Cdk as a kinase – directs the Cdk to a specific set of target proteins • G1/S cyclins send Cdks to S phase targets (e.g., promoting DNA replication) M cyclins send Cdks to M phase targets (e.g., making the nuclear membrane break down) also be negatively regulated by phosphorylation of other sites
  • 47.
    47 Maturation-promoting factor (MPF) •Example of cyclins and Cdks working together to control cell cycle transitions • Researchers found that cells in M phase contained an unknown factor that could force frog egg cells (stuck in G2) to enter M phase • This mystery molecule was called MPF • Discovered in the 1980s to be a Cdk bound to its M cyclin partner • M cyclin builds up as cell division progresses to G2/M • Trigger M phase • Activation signal is the intactness of DNA
  • 48.
    48 Action of MPFcomplex • The MPF complexes add phosphate tags to several different proteins in the nuclear envelope • Resulting in its breakdown (a key event of early M phase) and • Activate targets that promote chromosome condensation and other M phase events • Trigger nuclear envelope breakdown
  • 49.
    49 The anaphase-promoting complex/cyclosome (APC/C) •MPF also triggers its own destruction by activating the anaphase-promoting complex/cyclosome(APC/C) • A protein complex that causes M cyclins to be destroyed starting in anaphase • The destruction of M cyclins pushes the cell out of mitosis • Allow the new daughter cells to enter G1 • The APC/C also causes destruction of the proteins that hold the sister chromatids together, allowing them to separate in anaphase and move to opposite poles of the cell.
  • 50.
    50 Action of APC/C •The APC/C is an enzyme • It adds a small protein tag called ubiquitin (Ub) • When a target is tagged with ubiquitin, it is sent to the proteasome • Thought as the recycle bin of the cell, and destroyed • Eg. The APC/C attaches a ubiquitin tag to M cyclins • causing them to be chopped up by the proteasome and • Allow the newly forming daughter cells to enter G1 • The APC/C also uses ubiquitin tagging to trigger the separation of sister chromatids during mitosis • If the APC/C gets the right signals at metaphase, it sets off a chain of events that destroys cohesin, the protein glue that holds sister chromatids together
  • 51.
    51 APC/C • The APC/Cfirst adds a ubiquitin tag to a protein called securin and • Send it for recycling • Securin normally binds to, and inactivates, a protein called separase • When securin is sent for recycling, separase becomes active and be functional • Separase chops up the cohesin that holds sister chromatids together, allowing them to separate
  • 52.
    52 Checkpoints and regulators •Cdks, cyclins, and the APC/C are direct regulators of cell cycle transitions • They respond to signals from inside and outside the cell • These signals determine whether the cell moves forward in the cell cycle • Positive cues, like growth factors may increase activity of Cdks and cyclins • Negative ones, like DNA damage decrease or block activity • Cells deal with DNA damage by; • Fixing it if possible and/or by • Preventing cell division • A protein called p53, a famous tumor suppressor gene involved in DNA damage response
  • 53.
    53 Cell cycle checkpoint in DNA damage • p53 works on multiple levels to ensure that cells do not pass on their damaged DNA through cell division First, it stops the cell cycle at the G1checkpoint by triggering production of Cdk inhibitor (CKI) proteins The CKI proteins bind to Cdk-cyclin complexes and block their activity p53's second job is to activate DNA repair enzymes If DNA damage is not fixable, p53 will play its third and final role: triggering programmed cell death so damaged DNA is not passed on