CHAP. 2.3 – CELL DIVISION
Living organisms grow in size by increasing the
number of cells it has.
Cell division occurs are varying rates depending
upon age. For example, in infancy, the cells in the
head divide so rapidly as compared to the rest of the
body. As a result, the head is larger than the rest of
the body.
I. THE CELL CYCLE
When the cell divides, it divides into two equal parts.
All the organelles in the cell copy themselves so the
new cell can have the same parts and carry out the
same life functions.
The original cell is called the parent cell. When the
parent cell divides into two cells, two new cells are
called daughter cells.
Definition of cell cycle – the regular sequence of cell
growth and division
There are three stages in the cell cycle:
1
Stage 1 – Interphase
Stage 2 – Mitosis
Stage 3 – Cytokinesis
II. STAGE 1 - INTERPHASE
This is the first stage in the cell cycle and it is the
period BEFORE cell division occurs.
During this stage, the cell will
1. grow to its biggest (or mature) size,
2. make a copy of its DNA, and
3. prepare to divide into two cells.
A. GROWTH
This is the first part of interphase. The parent cell
double in size and produces all the structures needed
to carry out its functions. In other words, it matures
into its full size and structure.
For example, the endoplasmic reticulum enlarges to
make new ribosomes. Mitochondria and chloroplasts
make copies of themselves.
B. DNA REPLICATION
2
This is the second part of interphase. The cell makes
a copy of the DNA in its nucleus in a process called
replication.
The replication of DNA is important to ensure that
the two new daughter cells have a complete set to
carry out life functions.
At the end of replication, the parent cell’s nucleus
will contain two identical sets of DNA.
C. PREPARATION FOR DIVISION
This the third part of interphase. The parent cell
produces structures that it will use to divide during
the rest of the cell cycle. At this point the cell is ready
to divide.
II. STAGE 2 – MITOSIS
3
In this second stage of cell division, the parent cell’s
nucleus divides into two nuclei and one copy of the
DNA is distributed into the each of the two daughter
cells.
Mitosis is divided into four parts: prophase,
metaphase, anaphase, and telophase.
A. PROPHASE
The chromosomes in the cell’s nucleus condense and
coil from its usual threadlike structure. LOOK AT
FIGURE 10 ON PG. 63.
Spindle fibers form and the nuclear membrane
breaks down.
4
B. METAPHASE
The chromosomes line up across the center of the
cell. The spindle fibers attach to the chromosome at
its centromere.
C. ANAPHASE
The centromere splits and the two chromatids are
pulled apart toward the ends of the cell by the spindle
fibers.
This causes the cell to become stretch out.
5
D. TELOPHASE
The chromatids stretch out and lose their rod- like
appearance. A new nuclear membrane forms around
each region of chromosomes.
III. STAGE 3 – CYTOKINESIS
This the last stage of the cell division. In this stage,
the cytoplasm divides and organelles are distributed
into each of the two new daughter cells. The two new
daughter cells look exactly like the parent cell.
6
In an animal cell, cytokinesis starts when the cell
membrane squeezes together around the middle of
the cell. This causes the cytoplasm to be split in two
and ½ of all the organelles to go into the two
daughter cells.
In a plant cell, a structure called a cell plate forms
across the middle of the cell because the cell wall
cannot bend as easily as the cell membrane.
The cell plate forms into two new cell membranes.
The cell walls will form around the two new cell
membranes.
In yeast cell, cytokinesis is known as budding. In
budding, a small daughter cell forms off the large
parent cell and pinches off when ready. The bud will
grow in size to a parent yeast cell.
IV. LENGTH OF THE CELL CYCLE
The length of the cell cycle varies according the cell
you are dealing with.
Sea urchin cells – 2 hours
7
Human Liver Cells – 22 hours (21 hours in
interphase and 1 hour in mitosis & cytokinesis)
Human Brain cells – never divide
V. DNA REPLICATION
It wasn’t until the 1950’s with the work of Rosalind
Franklin, Francis Crick, and James Watson did we
learn that the DNA carries all of the cell’s instructions
and that DNA replicated.
A. THE STRUCTURE OF DNA
LOOK AT FIGURE 12 ON PAGE 67.
The shape of a DNA molecule is called a double helix
(also known as a twisted ladder).
The sides of the ladder are made up with alternating
molecules of deoxyribose (a sugar) and phosphate.
The middle of the ladder or the ‘rung’ of the ladder is
made up of 4 nitrogen bases: adenine, thymine,
guanine, and cytosine.
8
Adenine (A) only pairs up with Thymine (T).
Guanine (G) only pairs up with Cytosine (C).
VI. THE REPLICATION PROCESS
When DNA replication starts, the DNA strand
unwinds and, then, the bases separate.
Free floating nitrogen bases join up with the two
separate strands to make two new DNA molecules (4
strands in total).
The order of the new bases will match the old bases.
The two new DNA molecules will wind back up
again.
9

10