Chapter #12 The Cell Cycle
Overview
1. The ability of organisms to reproduce their own kind is the one characteristic
that best distinguishes living things from nonliving matter.
2. The ability of an organism to procreate has cellular basis.
3. Rudolf Virchow put it this way: “Where a cell exists, there must have been a
preexisting cell.” He summarized this concept with the Latin axiom, “Omnis
cellula e cellula,” meaning “Every cell from a cell.”
4. The continuity of life is based on the reproduction of cells, or cell division.
5. The cell division process is an integral part of the cell cycle.
I. Cell Division Results in Genetically Identical Daughter Cells
1. Most cell division involves the distribution of identical genetic material –DNA
to two daughter cells.
2. A dividing cell duplicates its DNA, allocates the two copies to opposite ends
of the cell, and only then splits into daughter cells.
A. Cellular Organization of the Genetic Material
1. A cell’s endowment of DNA, its genetic information, is called its genome.
2. DNA molecules (a typical human cell has approximately 2m of DNA,
about 250,000 times greater than the cell’s diameter) are partitioned or
packaged into chromosomes
3. In the nuclei of human somatic cells (body cells) each contain 46 chromosomes.
4. Reproductive cells (gametes, sperm and eggs) have one set of 23 chromosomes.
5. Eukaryotic cells consist of chromatin, a complex of DNA and associated
protein molecules.
B. Distribution of Chromosomes During Eukaryotic Cell Division
1. In preparation for cell division, chromosomes replicate, each one then consisting of two identical sister chromatids joined along their lengths by
sister chromatid cohesion.
2. The duplicated chromosome has a narrow “waist” at the centromere, a
Specialized region where the two chromatids are most closely attached.
3. When this cohesion is broken, the chromatids separate during cell division
becoming chromosomes of the new daughter cells.
4. Eukaryotic cell division consists of
a) mitosis (division of the nucleus)
b) cytokinesis (division of the cytoplasm)
II. The Mitotic Phase Alternates With Interphase In The Cell Cycle
A. Phases of the Cell Cycle
1. Mitosis is one part of the cell cycle.
2. Mitosis (M) phase which includes both mitosis & cytokinesis, is usually the
shortest part of the cell cycle.
3. Mitosis alternates with interphase which often accounts for about 90% of the
cycle.
a) It is during interphase that the cell grows & copies its chromosomes in
preparation for cell division.
b) Interphase is divided into subphases: the G1 phase (“first gap”), the
S phase (“synthesis”), and the G2 phase (“second gap”).
c) During G1 and G2 the cell grows by producing proteins & cytoplasmic
organelles. During the S phase the chromosomes replicate,
d) The sequence is as follows: G1---S---G2—M phase.
e) Mitosis divides the nucleus and distributes its chromosomes to the daughter
nuclei, & cytosinesis divides the cytoplasm, producing two daughter cells.
f) Mitosis is broken down into five stages: prophase, prometaphase,
metaphase, anaphase, and telophase. Cytokinesis completes the mitotic phase.
B. The Mitotic Spindle: A Closer Look
1. The mitotic spindle is an apparatus of microtubules that controls chromosome
movement during mitosis.
2. In animal cells, the assembly of spindle microtubules starts at the centrosome
(it is also called the microtubule-organizing center).
3. A pair of centrioles is located at the center of the centrosome (in animal cells),
but they are not essential for cell division.
4. Centrioles are not present in plant cells, which do not form mitotic spindles.
5. During interphase in animal cells, the single centrosome replicates, forming
two centrosomes, which remain together near the nucleus.
6. The two centrosomes move apart during prophase and prometaphase of mitosis
as spindle microtubules grow out from them.
7. An aster, a radial array of short microtubules, extends from each centrosome.
9. Each of the two sister chromatids (each duplicated chromosome) of a
replicated chromosome has a kinetochore, a structure of proteins associated
with specific sections of chromosomal DNA at the centromere.
10. During prometaphase, some of the spindle microtubules attach to the kinetochore; these are called kinetochore microtubules.
11. Microtubules from each pole interact with each other (these are called
nonkinetochore microtubules or “polar microtubules.”)
12. The kinetochore microtubules attached to the kinetochores of chromatids
move the chromosomes to the metaphase plate.
13. In anaphase, sister chromatids separate and are moved toward opposite ends
of the cell.
14. Meanwhile, motor proteins push nonkinetochore microtubules from opposite
poles away from each other, elongating the cell.
15. In telophase, genetically identical daughter nuclei form at opposite ends of
the cell.
C. Cytokinesis: A Closer Look
1. Mitosis is usually followed by cytokinesis (cytoplasm dividing).
2. Animal cells carry out cytokinesis by cleavage,
3. Plant cells carry out cytokinesis by forming a cell plate.
D. Binary Fission
1. The asexual reproduction of single-celled eukaryotes, such as the amoeba occurs
by a type of cell division called binary fission, meaning “division in half.”
2. In Bacteria (a prokaryote) , reproduce by binary fission , but the prokaryotic
process does not include mitosis. (The eukaryotic binary fission process includes
mitosis).
3. In bacteria, most genes are carried on a single bacterial chromosome that consists
of a circular DNA molecule and associated proteins.
4. During binary fission in bacteria, the chromosome replicates and the two daughter
chromosomes actively move apart.
5. The cell elongates.
6. Replication finishes. The plasma membrane grows inward, and a new cell wall is
deposited.
7. Two daughter cells result.
III. The Eukaryotic Cell Cycle Is Regulated By A Molecular Control System
A. Evidence for Cytoplasmic Signals
B. The Cell Cycle Control System
1. Cyclic changes in regulatory proteins work as a cell cycle clock.
2. The clock has specific checkpoints where the cell cycle stops until a go-ahead
signal is received.
3. The three major checkpoints are found in G1, G2 and M phases.
4. For many cells, the G1 checkpoint –dubbed the “restriction point” – seems
to be the most important.
5. If the cell receives a go-ahead signal at the G1 checkpoint, it will usually complete
the cycle.
6. If the cell does not receive a go-ahead signal at that point, it will exit the cycle,
switching into a nondividing state called the G0 phase.
C. Loss of Cell cycle Controls in Cancer Cells