1. The Cell Cycle
Modified with permission from a PPT originally courtesy of Mrs. Jennifer Boyd, WHS

2. I. Refresher: Cell Theory
A. What are the three parts to the cell theory?
All cells come from existing cells
All living things are made of cells
Cells are the basic unit of structure and function

3. II. Types of Cellular Reproduction
In unicellular organisms, division of one cell reproduces the entire organism: asexual reproduction in bacteria is called…
Multicellular organisms depend on cell division for:
Development from a fertilized cell
Growth
Repair
Cell division is an integral part of the cell cycle and probably evolved from binary fission in prokaryotes

4. III. Asexual Cellular Reproduction
Cell wall
Origin of
replication
Plasma
membrane
E. coli cell
Bacterial
chromosome
Chromosome replication begins.
Soon thereafter,
one copy of the origin moves rapidly toward the other end of the cell.
Two copies
of origin
Origin
Origin
Replication continues. One copy of the origin is now at each end of the cell.
III. Asexual Cellular Reproduction
Replication finishes.
The plasma membrane grows inward, and
new cell wall is deposited.
Two daughter
cells result.

5. III. Asexual Cellular Reproduction
G0: non-dividing
Result: Genetically identical daughter cells.
Most of the time the cell is in what stage?
G1? S? G2?
Cell divides about 1x every 24 hrs.
10-12 hrs S phase
Less than 1 hour Mitosis
INTERPHASE S G1
Cytokinesis
Mitosis G2
MITOTIC
(M) PHASE
about 90% of the cell cycle

6. III. Asexual Cellular Reproduction

7. III. Asexual Cellular Reproduction D. How often do cells divide?
Some cells divide constantly (e.g. skin) while others rarely or never divide (e.g. neurons)
Constantly dividing cells cycle though: Interphase – Mitosis – Interphase – Mitosis
Interphase = period between 2 cell divisions.
3 stages: G1 , S, G2

8. III. Asexual Cellular Reproduction E. Checkpoints
LE 12-14
G1 checkpoint
The G1 checkpoint seems to be the most important one
Control
system S G1 G2 M
M checkpoint
G2 checkpoint

9. IV. Storing DNA Chromosome Number
Every eukaryotic species has a characteristic number of chromosomes in each cell nucleus
Humans have ___ total chromosomes in their somatic cells
Types of Cells:
Somatic (non-reproductive, “body” cells), 2 sets of chromosomes (2n), made by mitosis
Gametes (sperm, egg) 1/2 the # of chromosomes , (1n), made by meiosis

10. IV. Storing DNA C. Chromosomes
When cells are about to divide, DNA coils into compact _________.

11. IV. Storing DNA Draw this Chromatids
Chromosomes are made of two identical halves called chromatids.
Chromatids are made when DNA copies itself before cell division.
When the cell divides, the 2 new cells will contain one chromatid, now considered to be a chromosome
Centromere: structure that holds the two chromatids together
Draw this

12. QUICK CHECK! Tightly coiled DNA is called ____________.
When DNA is copied, it forms two identical _________________ that are connected at the center, which is called the _____________.

13. IV. Storing DNA D. Chromosome Numbers
Different species have different #’s of chromosomes (some have the same #).
Examples:
Fruit fly= 8 chromosomes
Cat= 32 chromosomes
Human= 46 chromosomes
Sand dollar= 52 chromosomes
Dog=78 chromosomes

14. V. Nuclear Division: Mitosis
What is it?
Division of a cell’s nucleus
4 phases:
1. Prophase
2. Metaphase
3. Anaphase
4. Telophase

15. QUICK CHECK! After mitosis, how many cells should you have?
If you start with 46 chromosomes, how many chromosomes should be in each daughter cell?