1. Cell Size and the Cell Cycle

2. Why do cells divide? The larger a cell becomes:
the more demands are placed on its DNA.
the more trouble a cell has moving materials across the cell membrane.

3. 1. DNA Demand
as cells grow larger, extra copies of DNA are not typically made
Therefore, as a cell grows, there is greater demand placed on the information contained in its DNA.
“harder to get to the instructions” as the cell gets bigger
In time the cell would not be able to keep up with the demands for information from its DNA.

4. 2. Material Transport Issues
As cells grow larger it becomes more difficult to efficiently move nutrients into the cell and wastes out of the cell by diffusion.
the smaller a cell, the larger the surface area to volume ratio
large surface area to volume ratios make cells efficient
Cell Size
1x1x1 cm
2x2x2
3x3x3
Surface Area
6cm2
24cm2
54cm2
Volume
1cm3
8cm3
27cm3
Surface area to volume ratio
6:1
3:1
2:1

5. Cell Division
when a cell becomes too large it divides to form two new “daughter” cells through cell division
before this happens the DNA is replicated
each new daughter cell gets a complete set of the genetic information
cell size is reduced

6. Why is having “new” cells helpful?
“new” cells helps us:
grow
heal
replace dead cells

7. Cell Cycle
repeating sequence of cell growth and division during an organism’s lifetime
During the cell cycle a cell grows, prepares for division, and divides to form two daughter cells, each of which then begins the cell cycle again.
the cell cycle is regulated by proteins that receive and send signals (checkpoints in the cell cycle)

8. Cell Cycle
Phases
G1 Gap 1
S Synthesis
G2 Gap 2
M Cell Division

9. Cell Cycle Phases G1 Gap 1 S Synthesis G2 Gap 2 Interphase
G2 Gap 2
M Cell Division
Interphase

10. Interphase every phase except ‘M’ Phase (cell division)
cells spend most of their lives in these phases (G1, S, and G2)

11. Interphase G1 First Gap Phase S Synthesis Phase
grow and make new proteins and organelles
S Synthesis Phase
chromosomes (DNA) are replicated

12. DNA Replication making copies of DNA
DNA strands are held together by WEAK hydrogen bonds
these WEAK hydrogen bonds are easily broken by an enzyme

13. -ase
suffix often used to identify enzymes

15. Helicase
enzyme that breaks the double helix of DNA into two single strands
“old strands” are called “parent strands”

16. Semiconservative Replication
each single strand then serves as a template (guide) for a new complementary strand

17. Semi-conservative Replication
each parent strand remains intact
Glencoe Biology, Fig. 13-7, p. 212

18. Semiconservative Replication
replicated DNA molecules are half “old” and half “new”
“new strands” are called “complementary strands”

19. Replication Fork
areas where the double helix is split

22. Replication Fork areas where the double helix is split
can occur in more than one place
speeds up replication

23. DNA Polymerases a major group of enzymes involved in DNA replication
some polymerases add complementary nucleotides to each DNA strand after the strands have been “unzipped” by helicase
A always paired with T (and vice versa)
C always paired with G (and vice versa)

25. DNA Polymerases
other polymerases will proofread the newly produced strand and correct any mistakes in base pairing

26. Base Pairing during Replication

27. Base Pairing and Semiconservative Replication
Below is a group of letters that represents the nitrogen-containing bases in a double stranded piece of DNA. What would the new pieces of DNA look like after replication?
A T
C G
G C
T A

28. Base Pairing and Semiconservative Replication
Bases from the parent strands are in white text while bases on the newly produced complementary strands are in black text.
A T
C G
G C
T A
A T A T
C G C G
G C G C
T A T A

30. Cell Cycle
Phases
G1 Gap 1
S Synthesis
G2 Gap 2
M Cell Division

31. Why does DNA need to be replicated for a cell to divide?
DNA contains “instructions” for building a cell and for completing the cell’s function
the “instructions” come in the form of genes
the genes provide instructions for how to make proteins the cell needs
each new cell will need a full copy of the instructions to produce proteins need for the cells’ survival

32. Cell Cycle
Phases
G1 Gap 1
S Synthesis
G2 Gap 2
M Cell Division

33. Interphase G1 First Gap Phase S Synthesis Phase G2 Second Gap Phase
grow and make new proteins and organelles
S Synthesis Phase
chromosomes (DNA) are replicated
G2 Second Gap Phase
shortest phase when the is preparing for mitosis
organelles and molecules required for cell division are produced

34. Cell Cycle
Phases
G1 Gap 1
S Synthesis
G2 Gap 2
M Cell Division

35. M Phase Cell Division divided into two parts Mitosis Cytokinesis
division of the cell’s nucleus
Cytokinesis
division of the cell’s cytoplasm
includes division of organelles like mitochondria and chloroplasts