1.2 Understanding the Cell Cycle
(Introduction)
P. 17
The cell is the basic unit of all living things, as we have seen. It
spends most of it’s lifetime carrying on much the same functions
as all living things- growth, respiration, digestion, excretion,
absorption, circulation, etc.
But, at some point the cell must
divide because all living things have a certain limited lifespan. So,
if the cell didn’t reproduce, the organism would die as the cells
aged and died.
Most living things consist of huge numbers (hundreds of
millions or trillions) of cells. Yet, even in the case of humans,
living things begin life as one cell which splits to form two
identical cells. As this process continues over and over, these two
cells split further to form four, then eight, sixteen, thirty -two
and so on. As you can see on P. 25, the rate of reproduction varies
from cell type to cell type. Brain and nerve cells rarely reproduce
while the lining of the stomach and intestine last only a few days
and so must be replaced rapidly.
Thus, in order for an organism to grow and to replace dying
and damaged cells, regeneration of cells is necessary. Before a
cell can split, the DNA in the nucleus must be reproduced so each
cell has a complete set of DNA. Imagine a set of blueprints with
half the information missing. The structure built would not be
what was intended. In the same way, a cell must have a complete
set of DNA in order to function properly. So, the first step in
cell division is the reproduction of another complete set of the
DNA strands (chromosomes) in the cell. This is what we will look
at next.
Questions: Introduction & Mitosis P.17
1. The average adult consists of 10 trillion cells. Yet you began
life as only one fertilized cell. What happened to produce
the trillions?
2. A) Does a cell live forever?
B) What, then, must happen if the entire organism is to go on
living?
3. A) What vital material does every cell need in order to control
its normal functioning?
B) This material forms long strands called what?
4. Before mitosis can begin, what must happen to this item
mentioned in the previous question?
5. What is the name of the process which provides the cell with
these double strands?
Answers to introductory questions above:
1. The original cell divides many times.
2. A) No, every cell has a definite lifespan.
B) All cells must reproduce or the organism dies.
3. A) A complete set of DNA.
B) Chromosomes.
4. The DNA must make a double of itself.
5. This is termed replication.
For each of the 4 stages, the text provides a short write up, a photo
of an actual cell and a drawing of each step.
-
For each stage, please emphasize the following:
1. Prophase:
Strands of DNA now visible under microscope.
Nuclear membrane (not cell membrane) disappears.
Centrioles (not visible in first photo but are in the diagrams) move to
each end of cell.
Spindle fibres begin to grow outwards form centrioles.
2. Metaphase:
- The spindle fibres have stretched from one centriole to the other
and have pulled the chromosomes into a line across the middle of the
cell.
3. Anaphase:
- Spindle fibres shorten or shrink and pull the DNA strands toward the
ends of the cell.
4. Telophase:
- A complete set of chromosomes at each end of cell.
- Spindle fibres disappearing.
- Nuclear membrane reforming around each set of DNA.
Cell Division
Plant Cell – A cell plate develops across the center of the cell forming a
new cell wall between the two new cells.
Animal Cell- the new membrane pinches in new the middle of the cell
dividing the cytoplasm into two new cells.
After Mitosis is finished, the cell continues to split into two new cells.
(MITOSIS IS THE SEPARATING OF THE DUPLICATED DNA AND
IS COMPLETED BEFORE THE CELL DIVIDES.)
Interphase:
- most of a cell’s life is in interphase
- “inter” means between
- during interphase the cell grows, replicates its DNA and becomes
prepared for the first phase of mitosis
Interphase P. 22-23
1.
From the discussion on P. 22, what do you think the term
“interphase” refers to?
2.
What activities does a cell carry on during interphase?
3.
What does the term “cell cycle” refer to?
Answers to above questions:
1. This is the period between mitosis that takes up most of a cell’s life.
2. The cell grows, replicates and does other functions of any living thing.
3. Mitosis, cell division & interphase make up the cell cycle- the life
cycle of a cell.
Mitosis P. 17-19:
1.A) What is meant by the term “replication”?
B) When does replication take place in a cell?
2. What happens to this replicated chromatin (now called
“chromosomes”) before mitosis occurs?
3. List, in the order in which they occur, the four stages of
mitosis.
4. A) What happens to the organelle known as the centriole in the
first stage of mitosis (prophase)?
B) What happens to the nuclear membrane at the same time?
C) What begins to form or grow from each centriole?
5. Describe where the DNA is by the second phase of mitosis
(metaphase).
6.A) What happens to these spindle fibres in the third phase
of mitosis (anaphase)?
B) What does this do to the DNA lined up across the middle of
the cell?
7. By the fourth stage (telophase), where is the cell’s DNA?
8. What now reappears around this DNA?
9. After mitosis is complete, there is still just the one original
cell. Explain how the next generation of cells appears.
Answers to the above:
1.
Replication is the copying or duplicating of the DNA in the cell.
2.
They coil up to form double strands of DNA which started out too
small to be visible with a light microscope but increase in size enough to be
visible with the microscope as mitosis begins.
3.
4.
The four stages are: prophase, metaphase, anaphase and telophase.
A) The centrioles move to opposite ends of the cell.
B) The nuclear membrane disappears.
C) Spindle fibres grow outward from each centriole to eventually join at the
middle in the next stage of mitosis.
5.
The DNA is lined up across the middle of the cell on the spindle
fibres.
6.
A) The spindle fibres begin to contract or shrink back toward each
centriole.
B) The DNA is pulled toward each centriole at the ends of the cell.
7.
A complete set of DNA is present at each pole or end of the cell (at
the centriole).
8.
A new nucleus appears and grows around this DNA.
9.
With DNA and a nucleus at each end of the cell, the cell begins to
divide into two daughter cells as the cell membrane divides.