1. How Cells Divide
Chapter 11

2. Outline
Cell Division in Prokaryotes
Discovery of Chromosomes
Structure of Chromosomes
Phases of the Cell Cycle
Interphase
Mitosis
Cytokinesis
Cell Cycle Control
Cancer

3. Cell Division in Prokaryotes
Prokaryotic cell division occurs as binary fission in which cell divides into two halves.
Genetic information exists as a single, circular double-stranded DNA molecule.
Copying begins at replication origin, and proceeds bi-directionally.
One genome ends up in each daughter cell.

4. Binary Fission

5. Discovery of Chromosomes
All eukaryotic cells store genetic information in chromosomes.
Most eukaryotes have between 10 and 50 chromosomes in their body cells.
Human cells have 46 chromosomes.
23 nearly-identical pairs

6. Structure of Chromosomes
Chromosomes are composed of a complex of DNA and protein, chromatin.
heterochromatin - not expressed
euchromatin - expressed
DNA exists as a single, long, double-stranded fiber extending chromosome’s entire length.
forms nucleosome every 200 nucleotides
DNA coiled around histone proteins

7. Eukaryotic Chromosomal Organization

8. Structure of Chromosomes
Karyotype - Individual’s particular array of chromosomes.
diploid - A cell possessing two copies of each chromosome (human body cells).
Homologous chromosomes are made up of sister chromatids joined at the centromere.
haploid - A cell possessing a single copy of each chromosome (human sex cells).

9. Chromosomes

10. Phases of the Cell Cycle
Five phases of cell division:
G1 - primary growth phase
S - genome replicated
G2 - secondary growth phase
collectively called interphase
M - mitosis
C - cytokinesis

11. Cell Cycle

12. Interphase
G1 - cells undergo majority of growth
S - each chromosome replicates to produce sister chromatids
attached at centromere
contains attachment site (kinetochore)
G2 - chromosomes condense
assemble machinery such as centrioles

14. Mitosis
Prophase
spindle apparatus assembled
Microtubules connect kinetochores on each pair of sister chromatids to the spindle poles.
nuclear envelope breaks

16. Mitosis
Metaphase
chromosomes align in cell’s center
metaphase plate
spindle

18. Mitosis
Anaphase
sister chromatids pulled toward poles
poles move apart
centromeres move toward poles
microtubules shorten
Telophase
spindle disassembles
nuclear envelope forms around each set of sister chromatids

20. Cytokinesis
Cleavage of cell into two halves
animal cells
constriction belt of actin filaments
plant cells
cell plate
fungi and protists
mitosis occurs within the nucleus

21. Cytokinesis

23. Cell Cycle Control
Two irreversible points in cell cycle:
replication of genetic material
separation of sister chromatids
Cell can be put on hold at specific checkpoints.

24. Cell Control Cycle
G1 / S - primary division decision point
G2 / M - commitment to mitosis
Spindle checkpoint - all chromosomes are attached to spindle

26. Growth Factors and the Cell Cycle
Each growing cell binds minute amounts of positive regulatory signals (growth factors) that stimulate cell division.
If neighboring cells use up too much growth factor, there is not enough left to trigger cell division.
Growth factors trigger intercellular signaling systems.

28. Cancer and Cell Proliferation
Cancer is essentially a failure of cell division control (unrestrained, uncontrolled cellular growth).
gene p53 plays a role in G1 checkpoint
halts cell division if detects damaged DNA and stimulates activity of repair enzymes
appears to be absent in many cancerous cells

29. Growth Factors and Cancer
Growth factors influence cell cycle
proto-oncogenes - normal cellular genes that become oncogenes when mutated
positive approach that stimulates cell growth
tumor-suppressor genes - negative approach that inhibits cell division

31. Summary
Cell Division in Prokaryotes
Discovery of Chromosomes
Structure of Chromosomes
Phases of the Cell Cycle
Interphase
Mitosis
Cytokinesis
Cell Cycle Control
Cancer