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Chapter 12 The Cell Cycle
1) Continuity of life is based upon the reproduction of cells, or cell division
a) Unicellular organisms reproduce by cell division
b) Multicellular organisms depend on cell division for:
i) development from a fertilized cell
ii) growth
iii) repair
c) The cell division process is a part of the cell cycle
2) Concept 12.1: Cell division results in genetically identical daughter cells
a) Cells duplicate their genetic material before they divide, ensuring that each daughter cell receives an
exact copy of the genetic material, DNA
b) A cell’s endowment of DNA (its genetic information) is called its genome
c) The DNA molecules in a cell are packaged into chromosomes in each cell’s nucleus
i) Eukaryotic chromosomes consist of chromatin, a complex of DNA and protein that condenses during
cell division
ii) In humans
(1) somatic cells have two sets of chromosomes (2n=46)
(2) gametes have one set of chromosomes (n=23)
3) Distribution of Chromosomes During Cell Division
a) In preparation for cell division DNA is replicated and the chromosomes condense
b) Each duplicated chromosome has two sister chromatids, which separate during cell division
c) Eukaryotic cell division consists of:
i) mitosis, the division of the nucleus
ii) cytokinesis, the division of the cytoplasm
4) In meiosis, sex cells are produced after a reduction in chromosome number (to be discussed later)
5) Concept 12.2: The mitotic phase alternates with interphase in the cell cycle
a) Phases of the Cell Cycle
i) Interphase can be divided into subphases
(1) G1 phase
(2) S phase: chromosomes replicate
(3) G2 phase
b) The mitotic phase (M) is made up of mitosis and cytokinesis
i) Mitosis consists of five distinct phases
(1) Prophase
(a) chromatin fibers condense & can be seen with a light microscope
(b) nucleoli disappear
(c) the mitotic spindle begins to form
(i) The mitotic spindle is an apparatus of microtubules that controls chromosome movement
during mitosis
(ii) the spindle arises from the centrosomes and includes the microtubules and asters
(d) the centrosomes move a way from each other
(2) Prometaphase
(a) the nuclear envelope fragments
(b) Some spindle microtubules attach to the kinetochores of chromosomes and move the
chromosomes towards the metaphase plate
(c) nonkinetocore microtubules interact with those from the opposite pole of the spindle
(3) Metaphase
(a) centrosomes are at opposite ends of the cell
(b) chromosomes’ centromeres are on metaphase plate
(4) Anaphase: sister chromatids separate; each chromatid is now a chromosome
(a) the chromosomes move toward opposite ends of the cell.
(b) nonkinetechore microtubules from opposite poles overlap and push against each other,
elongating the cell
(5) Telophase: genetically identical daughter nuclei form at opposite ends of the cell Mitosis is now
complete
ii) Cytokinesis:
(a) In animal cells: cytokinesis occurs by a process known as cleavage, forming a cleavage
furrow (contractile ring of actin and myosin pulls until cell pinched in 2)
(b) In plant cells, a cell plate forms (vesicles coalesce to form plate, then cell wall forms)
6) Binary Fission
a) Prokaryotes (bacteria and archaea) reproduce by a type of cell division called binary fission
i) In binary fission the bacterial chromosome replicates at a specific place called the origin of
replication
ii) The two daughter chromosomes actively move apart
7) The Evolution of Mitosis
a) Since prokaryotes preceded eukaryotes by billions of years it is likely that mitosis evolved from
bacterial cell division
b) Certain protists exhibit types of cell division that seem intermediate between binary fission and mitosis
carried out by most eukaryotic cells
8) Concept 12.3: The cell cycle is regulated by a molecular control system
a) The frequency of cell division varies with the type of cell
i) These cell cycle differences result from regulation at the molecular level
ii) there is evidence for cytoplasmic signals; molecules present in the cytoplasm that regulate progress
through the cell cycle
b) The Cell Cycle Control System
i) Similar to a clock
(1) The clock has specific checkpoints where cell cycle stops until a go-ahead signal is received
(2) G1 checkpoint (“restriction point” in mammal cells) most important; cell can go to G0 if no goahead signal received. Most human body cells are in G0. (special cases: liver cells; mature
muscle & nerve cells)
ii) The Cell Cycle Clock: cyclins and cyclin-dependent kinases (Cdks)
(1) Two types of regulatory proteins involved in cell cycle control
(2) The activity of cyclins and Cdks fluctuates during the cell cycle (see p229-230)
iii) Stop and Go Signs: Internal and External Signals at the Checkpoints
(1) internal example: M phase checkpoint
(2) external examples:
(a) Growth factors stimulate other cells to divide. example: platelet-derived growth factor
(b) density-dependent inhibition: crowded cells stop dividing
(c) Most animal cells exhibit anchorage dependence in which they must be attached to a
substratum to divide
(3) Loss of Cell Cycle Controls in Cancer Cells
(a) Cancer cells: exhibit neither density-dependent inhibition nor anchorage dependence
(b) Cancer cells do not respond normally to the body’s control mechanisms
(i) hypotheses: they make required growth factors themselves, or have abnormal signaling
pathways, or abnormal cycle control systems?
(ii) Form tumors by process called transformation (conversion of a normal cell to a cancer
cell)
1. benign tumor: remains at original site
2. malignant tumors: invade surrounding tissues and can metastasize exporting cancer
cells to other parts of the body where they may form secondary tumors