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Biology I
Adapted by H. Shere
Name: _____________________________
Period: ________
Cell Cycle Overview
THE CELL CYCLE
The cell cycle is the series of events that take place in a eukaryotic cell between its
formation and the moment it replicates itself. The cell cycle is an essential process that allows
cells to grow and replicate—in fact, the cell cycle is how you grew from a single cell (a fertilized
egg) into a human with organs and tissues! By learning about the cell cycle, you will gain an
understanding of the development of every living organism on planet Earth.
The events of the cell cycle can be divided in two broad categories: interphase (in between
divisions phase), during which the cell is growing and carries on with its normal metabolic
functions, and the mitotic phase (or M phase), during which the cell divides to replicate itself.
1. In what type of cells (prokaryotes or eukaryotes) does the cell cycle occur?
2. In your own words, why do you think that the cell cycle is important?
3. Name the two main phases of the cell cycle.
4. What happens in a cell during interphase?
5. What happens in a cell during the mitotic phase?
6. Looking at the cell cycle diagram above, which phase of the cell cycle do you think takes
longer (interphase or M phase)?
INTERPHASE
Interphase is a phase of the cell cycle defined only by the absence of cell division. During
interphase, the cell obtains nutrients, grows, and duplicates (copies) its genetic material.
Eukaryotic cells spend most of their time in interphase. For example, human skin cells, which
divide about once every 24 hours, spend roughly 22 hours in interphase! Some cells, such as
nerve cells, can stay in interphase for decades.
There are 3 parts of interphase: G1 (short for growth 1), in which the cell creates organelles and
begins metabolism, S phase (short for synthesis), in which the cell’s DNA is copied, and G2
(short for growth 2), in which the cell grows in preparation for cell division.
7. What process NEVER occurs in interphase?
8. Name the 3 stages of interphase.
9. What does G1 stand for and what occurs in this stage?
10. What does S stand for and what occurs in this stage?
11. What does G2 stand for and what occurs in this stage?
Sometimes, a cell exits the cell cycle (usually from G1 phase) and enters the G0 phase. In the G0
phase, cells are alive and metabolically active, but do not divide. In this phase, cells do not copy
their DNA and do not prepare for cell division.
Many cells in the human body, including those in the heart, eyes, and brain are in the G0 phase.
If these cells are damaged they cannot be replaced.
12. What happens to cells that enter the G0 stage?
13. Are cells in the G0 stage alive?
14. Name 3 types of cells that enter the G0 phase when they are mature.
The G1 phase is the first period in the cell cycle during interphase, after mitosis (cell division)
and before the S phase. For many cells, this phase is the major period of growth during its
lifespan. New organelles are being synthesized (made). To do this, the cell requires both
structural proteins and enzymes, resulting in great amount of protein synthesis.
15. What major event happens in a cell during G1?
16. What cell structures are made in G1?
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The S phase (short for synthesis phase) is the next step of interphase. Following G1, the cell
enters the S phase, when DNA replication occurs. Before we discuss S phase, we need to learn a
bit about DNA structure.
DNA is the genetic material of the cell. It is found in the nucleus
of the cell, arranged into structures called chromatids.
At the beginning of S phase, each chromatid is replicated to make
an identical copy. These chromatids are arranged into
chromosomes, which consist of two identical chromatids (called
sister chromatids) that are connected by a centromere. The
image to the left shows a duplicated chromosome (after S phase).
17. What does the S phase stand for?
18. What happens during the S phase?
19. What is a chromatid?
20. What structure holds the duplicated chromatids together?
G2 phase is the third, final, and usually shortest part of interphase, in which the cell undergoes a
period of rapid growth to prepare for mitosis. It follows successful completion of chromosomal
replication during the S phase, and usually takes about 4-5 hours. The G2 phase continues growth
of the cell and prepares the cell for mitosis (M phase) by producing all the enzymes that the cell
will need in order to divide.
After the G2 phase of interphase, the cell is ready to enter the mitotic phase (or M phase) and
start dividing. The nucleus and genetic material (chromosomes) divide first during mitosis.
Mitosis is also called karyokinesis (karyon means nucleus) because only the nucleus is dividing.
Then, the rest of the cell (the cytoplasm and its contents) will divide during cytokinesis to
produce two new, separate cells.
21. How is the cell prepared for mitosis during the G2 phase?
22. What phase follows the G2 phase?
23. What part of the cell is dividing in mitosis?
24. What part of the cell divides in cytokinesis?
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