Science 9 Review Worksheet 5.1 With Answers
1. Explain why the skin cells of an adult must divide.
Skin cells must divide to make new cells to replace those that are lost continually through abrasion.
2. List the three stages of the cell cycle.
Interphase, mitosis and cytokinesis
3. What are the events that take place during interphase?
The cell grows, makes new molecules and organelles, and makes copies of each DNA strand.
4. Why is DNA replication so important?
The DNA must be exactly copies (replicated) so that each new cell from division can have an exact,
complete set of chromatin like the original cell had.
5. What does DNA look like at the end of interphase?
The DNA is loosely coiled at the end of interphase.
6. How does the cell prepare for cell division?
The cell copies each DNA strand (chromatin fiber) and they loosely coil up. The organelles are also
copied.
7. What are the phases of mitosis?
Prophase, metaphase, anaphase, telophase.
8. What do the nucleus and chromosomes look like during prophase?
During prophase, the nuclear membrane disappears and the chromatin fibers (DNA strands) coil up into
bodies called chromosomes.
9. How does cytokinesis differ in plant and animal cells?
In plant cells, a cell plate or wall forms between the two daughter cells. In animal cells, the two daughter
cells pinch apart.
10. What is the importance of checkpoints in the cell cycle?
Checkpoints help a cell to determine if it can complete cell division. Checkpoints indicate potential
problems which allow a cell to correct these or to stop cell division.
11. What may happen when checkpoint proteins no longer function?
The cell may divide out of control (cancer).
12. Outline the activities in the cell at each of the following phases of interphase:
a. Growth and preparation
The cell increases in size, making new proteins and some new organelles.
b. Replication
The cell makes a copy of each DNA strand so that there are now two DNA strands that are identical to the
original DNA strand.
c. Continued growth and preparation
The cell grows in size, new organelles are made and the chromatin (DNA) begins to loosely coil up.
Science 9 Review Worksheet 5.1 With Answers
13. List the steps in DNA replication.
The DNA molecule opens up (“unzips itself”). New nitrogen bases (connected to sugars and phosphates)
pair up with their complementary bases making two new identical strands.
14. What is the function of mitosis?
Mitosis makes new cells (“daughter cells”) identical to the original cell (“mother cell”) that divided. This
process makes an organism grow and is also used to repair body regions that are damaged.
15. Is mitosis constantly occurring in your cells? Explain.
Mitosis is NOT constantly occurring in cells. It only happens in cells that are producing growth like the
cells at the bases of hairs or cells in the stomach lining (replaced every 3 days) or lower skin layers or in
the bone marrow making new blood cells.
16. What is the function of the spindle fibers?
Spindle fibers help to pull duplicate chromosomes to opposite ends (“poles”) of the cell.
17. Use the diagrams below to answer questions below.
a.
.
b.
c.
A. Which diagram shows a cell at the beginning of anaphase?
d.
a
B. Which diagram shows a cell with sister chromatids moving to opposite poles?
a
C. Which diagram illustrates a cell where a new nuclear membrane is forming?
d
D. Write down the correct sequence of letters to show the phases of mitosis from beginning to
end.
c, b, a, d
E. Using only two of the above diagrams, explain how you could tell whether a cell has just
completed mitosis or is entering (starting) mitosis.
If a cell is at stage d, making two new nuclear membranes, mitosis has just finished.
If a cell is at stage c, losing its nuclear membrane and making a spindle and having the chromosomes
become visible, mitosis has just started.
18. How is plant cell division different from animal cell division?
Plant cells do not have asters or centrioles (centrosomes) at the end of their spindles.
Plant cells form a cell plate to separate the cells while animal cells pinch apart.
19. Why must the nuclear membrane break down for mitosis to occur?
The chromosomes need more space to separate themselves so the nuclear membrane breaks down
allowing the chromosomes the entire cell to arrange themselves along the middle (“equator”) of the cell.
Science 9 Review Worksheet 5.1 With Answers
20. In interphase, the DNA is loosely coiled. Why do you think it is important that the DNA be
compact and tightly coiled during mitosis? (Hint: Think of an unraveled spool of thread.)
If chromatin stayed in the form of threads, they would get tangled up and could not be separated to
opposite ends of the cell. When they coil up and compact themselves, chromosomes are easily separated
from each other to opposite ends of the cell.
21. What might happen if DNA replication and mitosis were not highly controlled?
If replication and mitosis were not highly controlled, irregular and perhaps cancerous growths would
occur all over in many celled organisms.
22. What are some environmental factors that can contribute to the development of cancer?
Radiation sources and mutagenic chemicals may damage DNA and if this damage occurs many times it
may lead to cells turning into cancer cells that are dividing out of control.
23. How is a cancer cell different from a normal cell?
Cancer cells do not stop dividing when they get a signal to stop dividing. Cancer cells grow in multiple
layers forming a tumour mass. Cancer cells have large, abnormal nuclei. Checkpoint proteins no longer
function in cancer cells. Cancer cells spread to other body locations through the lymph and circulatory
systems. Cancer cells have the ability to repair lost parts of chromosome telomeres which keeps them
dividing forever (past the normal 35 times). Many kinds of cancer cells have lost the apoptosis (cell
suicide) program that gets rid of defective cells.