SNC 2D
Unit: Tissues
Name: _____________________________
Date: _____________________________
The Cell Cycle
When taken together, the continuous events in a cell’s life are called the cell cycle. The cell cycle is made up of two parts:
(1) cell growth (interphase)
The cell spends most of its life time
in interphase. During this time it is
taking in nutrients, getting rid of
wastes, doing its designated job
and growing. Toward the end of
interphase, the cell creates a copy
of each of its chromosomes to get
ready for cell division.
(2)
(2) Cell Division
- mitosis
- cytokinesis
(1) Cell Growth
- Interphase
cell division
Cell Division involves two stages:
(a) mitosis – when the duplicated
chromosomes are separated from
one another to form two nuclei and
(b) cytokinesis – when the rest of
the cytoplasm contents are divided.
Cell division begins with one
“parent” cell and ends with two
identical “daughter” cells.
The Cell Cycle in More Detail
1. Cell Growth - Interphase

This is the period between cell divisions.

Generally, the cell spends more of its time in this stage than in any other. The
cell grows and carries on basic life functions like cellular respiration and
photosynthesis (if it is a green plant cell) during this stage of its life cycle.
Nutrients are transported around the cell and wastes are eliminated.

The cell is primarily in a stage of its life cycle called “interphase”. In the latter
part of interphase, the cell will make a copy of each of its chromosomes. The
two copies are now called “sister chromatids” and they are held together by a
centromere. This copying process is called DNA replication.
centromere
This animal cell is in interphase.
A – cytoplasm
B – nucleus containing chromatin
C – nucleolus
D – nuclear membrane
E – centrioles (also called
centrosomes) These
organelles are only found in
animal cells. They “anchor” the
spindle fibers.
Sister chromatids

In interphase, the sister chromatids are not visible under a light microscope.
Instead, they look like threads in the nucleus. When the DNA material is
thread-like rather than visible as distinct chromosomes or sister chromatids, it
is called chromatin. In interphase, DNA appears as chromatin in the nucleus.
2. Cell Division (mitosis and cytokinesis)



When cell division begins, ONE “parent cell” is present with its replicated DNA. When cell division ends, TWO
“daughter” cells are present. The daughter cells are genetically identical to each other and to the “parent cell”.
Cell division is important for the following reasons:
a) to produce more cells so that a multi-cellular organism can grow
b) to produce new cells for the replacement of older ones that are now dead or dying
c) to regenerate damaged tissue (e.g. after a burn or a cut)
Cell division occurs in two parts:
i) mitosis – the division of the duplicated chromosomes from inside the nucleus into two equal parts
ii) cytokinesis – the division of the rest of the cell (the cytoplasm and the organelles) into two equal parts
i)
Mitosis – division of the genetic material (see page 34)
Mitosis consists of 4 stages. These stages are continuous, but we break them down into distinct steps or stages
in order to be able to more clearly understand the changes that are taking place. The next page shows the
stages.
Stage 1 of Mitosis: Prophase
Spindle fibres (F)
Notice that the nuclear membrane is
form between
disappearing.
centrioles (E)
Early prophase
Later prophase
 The chromosomes shorten and thicken. Remember that each piece of a replicated chromosomes is called a sister
chromatid. Sister chromatids and are attached by the centromere. The sister chromatids “condense” or pull
together so that now you can see them in a light microscope.
 In animal cells (only), the centrioles or centrosomes separate and move to opposite poles of the cell. They provide attachment
for the spindle fibres, which serve as guides for the attachment of chromosomes during cell division.
 The nuclear membrane begins to fade. It dissolves to allow for separation of chromosomes and movement of cell organelles.
Stage 2 of Mitosis: Metaphase
The copied chromosomes, (sister chromatids) move toward the center of the
cell, midway between the poles of the cell.
Each chromosome attaches to a spindle fibre (H). Chromatids can become
intertwined during metaphase.
A – cytoplasm
E – cenrioles
G – sister chromatids lined up along the centre of the cell
H – spindle fibers anchored at centrioles
I – centromere holding together sister chromatids
Stage 3 of Mitosis: Anaphase
The centromeres holding the sister chromatids together, divide.
The sister chromatids, which are now called single-stranded
chromosomes (G2), move to opposite poles of the cell.
The same number and type of chromosomes will be found at each
pole of the cell.
Stage 4 of Mitosis: Telophase
The chromosomes reach the opposite poles of the cell and they begin to
lengthen.
The spindle fibres (H) dissolve.
The nuclear membrane (D) begins to form around each mass of chromatin
(G).
ii)
Cytokinesis – division of the rest of the cell

Once the chromosomes have moved to opposite poles of the cell, the cytoplasm begins to divide. This often begins
before telophase looks like it is over.

In animal cells, a furrow develops, pinching the cell into two parts from the outside inwards. (See page 37.)

In plant cells, the separation is accomplished by a cell plate that forms between the two new nuclei. The cell plate
divides the material into two cells by growing from the inside outward. It develops into a new cell wall and seals off
the contents of the new cells from each other. (See page 38.)