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Chapter 12:
The Cell Cycle
The Cell Cycle  A. The Role of Cell Division
Purposes of Cell Division (Or, one reason we need that
ATP from Cellular Respiration (Chapter 9):
a. Single celled organisms (e.g. the Protista) can replicate
by reproducing the entire organism, e.g. an amoeba; the
process is called mitosis
b. Allows reproduction for multicellular organisms by
producing female and male gametes (egg, sperm) in the
reproductive tissues; the process is called meiosis (Meiosis
is described in Chapter 13)
c. Growth and also replacement of aged cells for
multicellular organisms – these are the somatic cells and the
process is also called mitosis and is described here in
Chapter 12
- Process of mitosis:
The aim is to produce two identical daughter cells each
containing exact replicas of the mother cell’s chromosomes
(this means the entire genome = all genes need to be
duplicated exactly).
Thus, all the DNA must be copied so there are two
complete sets, one set for each daughter cell.
The outline of the replication process for a chromosome is
given in Figure 12.4 (p. 220): Chromosome duplication and
distribution during mitosis.
Remember the terms used in this Figure!
Note that:
- Genes are present in structures called chromosomes
- Each eukaryotic cell has a nucleus (Chapter 6)
containing a species-specific number of chromosomes. The
human nucleus contains 46 chromosomes  23 from egg, 23
from sperm that fertilize the egg. This means that there are
two of each type of chromosome in our cells and there are at
least two of each type of gene. There will be an exception to
this that we will see later (sex-related genes).
- Each chromosome contains hundreds or thousands
of genes (these make up our inherited traits).
- The DNA that makes up the chromosomes is
complexed with proteins  this is called chromatin. The
proteins maintain the shape of the chromosome.
B. Mitotic Cell Cycle exists in four phases:
1.
2.
3.
4.
G1 Phase  1st growth phase
S Phase  DNA duplicated
G2 Phase  Final growth phase
Mitosis/Cytokinesis
Figure 12.5 (p. 221) – Note that G1, S, and G2 phases are
together called interphase and represent 90% of the cells
growth cycle. During interphase, the cell has not started to
divide into two daughter cells, but is growing and preparing
for division. This is illustrated later in a short movie.
Division takes place during the mitotic phase (10% of the
cell’s growth cycle). In this phase, mitosis divides the
nucleus into two nuclei and cytokinesis (cell division) divides
the cytoplasm into two cells.
C. Mitosis itself can be divided into distinct phases as
follows (Note, this all needs to be memorized):
1.
2.
3.
4.
5.
Prophase
Prometaphase
Metaphase
Anaphase
Telophase
You need to pay attention and memorize Figure
12.5 and the information that it contains!
1. Prophase
- Nucleolus disappears
- Chromatin condenses, this causes the chromosomes
to begin to become visible as discrete units
- Centrosomes separate, moving to opposite ends of
the cell.
- The centrosomes start to form a framework used to
separate the two sister chromatids called the
mitotic spindle, that is made of microtubules
2. Prometaphase
- Nuclear envelope fragments
- Chromosomes become more condensed
- A kinetochore is formed at the centromere, the
point where the sister chromatids are
attached
- Microtubules attach at the kinetochores,
leading back to the spindle poles (where
the centrioles now reside
3. Metaphase
- Chromosomes align on an axis called the
metaphase plate. This is a very distinct
aspect of metaphase.
- Note: the spindle consists of microtubules, one
attached to each chromosome
Figure 12.7 (p. 224) – The mitotic spindle at
metaphase.
4. Anaphase
- Paired centromeres separate
- One chromatid goes toward each pole
- Cell begins to elongate, caused by
microtubules not associated with the
kinetochore
5. Telophase
- Formation of nuclear membrane and nucleolus
- Formation of the cleavage furrow = a shallow
groove in the cell near the old metaphase plate.
- Cytokinesis = division of the cytoplasm
The process of cytokinesis is presented in
Figure 12.9. Photos of mitosis for a plant cell are
presented in Figure 12.10. Please review these
Figures.
D. Binary fission in bacteria – Note bacteria have a single
chromosome, versus the 46 human have.
- Split the cell into two halves
Figure 12.11 (p. 227) – Bacterial cell division (binary
fission). Know the factors described in the Figure.