1. Unit 2: Molecules and Cells
Cell Growth and Division

2. Cell Growth Cells cannot continue to grow indefinitely because:
The genetic information in a single cell cannot serve the needs of a giant cell due to its complexity and many needs.
A giant cell has too great a volume compared to its surface area for material exchange.

3. Cell Growth
In order for a cell to continue living, it must form 2 “daughter cells” from itself in the process of Cell Division.
During Cell Division, the cell replicates its entire DNA sequence so that each daughter cell will get a complete set of genetic information.

4. Cell Division in Prokaryotes
Most Prokaryotic Cells divide through binary fission, during which 2 identical cells are created from 1 cell.

5. Cell Division in Eukaryotes
Eukaryotes divide by a process called MITOSIS.
A eukaryotic cell lives through a Cell Cycle with different phases:
Interphase = the time between cell divisions
G1 Phase – cell growth
S Phase – DNA is copied
G2 Phase – growth and preparation for cell division
M Phase =
Mitosis – division of nucleus and contents
Cytokinesis – division of the cytoplasm and cell

6. The Cell Cycle
A Pie Chart of the Cell Cycle with times for each phase in a typical animal cell:
Complete Cell Cycle = about 24 hours (in typical animal cells)
12 hr
0.5 hr
M Phase
Mitosis
6 hr
6 hr

7. Cell Cycle Control
The Cell Cycle is regulated by certain genes coding for proteins called cyclins, which monitor “checkpoints” during the cycle.
There are 3 checkpoints: the G1, G2 and Mitosis checkpoints
If conditions are not favorable for the cell to undergo the following phase, cell division will stop.
Cancer is the result of uncontrolled cell growth/division from speeding up the cycle or inactivating the proteins that slow or stop the cycle.

8. Chromosomes
Genetic Information in the form of DNA is held in Chromatin inside the nucleus.
When the DNA needs to replicate and divide during mitosis, this chromatin condenses to form Chromosomes.
Chromosomes are composed of
2 sister chromatids (contains DNA) and
a centromere, which holds the 2 chromatids together

9. Mitosis
After Interphase (cell growth, DNA replication), a eukaryotic cell undergoes Mitosis.
There are 4 Steps of Mitosis –
Prophase
Metaphase
Anaphase
Telophase

10. Prophase Chromatin condenses into chromosomes.
Nuclear Envelope breaks down.
The Centrioles (microtubule “organizing centers” in animal cells) move to opposite “poles” of the cell and begin sprouting spindle fibers (microtubules) that will later help separate chromosomes.

11. Metaphase
Spindle Fibers from centrioles either form a cage around the chromosomes or attach directly to the centromeres.
Microtubules pull chromosomes to line up along cell’s “equator,” known as the Metaphase Plate.

12. Anaphase
Sister chromatids are separated into individual chromosomes and transported by spindle fibers to opposite poles.

13. Telophase
Chromosomes at each end begin to unwind and lose distinct shape.
Spindle Fibers dissolve.
2 new nuclear membranes are formed.

14. Cytokinesis
After Mitosis, the cell must complete the division with the dividing of cytoplasm during Cytokinesis.
Cell membrane creates
a cleavage furrow.
Cell membrane pinches
together completely and
fuses
One cell becomes two,
and each new cell begins
the cycle over starting
with G1.

15. New Topic: Meiosis
All animal somatic (body) cells divide through Mitosis.
However, reproductive cells, called gametes, are created through Meiosis by specialized cells.

16. Some Prerequisites for Meiosis
Normal body cells contain 2 of each type of chromosome; a pair of these similar chromosomes are called homologous chromosomes.
A cell having 2 copies of each chromosome contains a Diploid (2n) Number of chromosomes (in humans, this number is 46).
Gametes contain only 1 type of each chromosome; this is considered a Haploid (n) Number of chromosomes (in humans, this number is 23).

17. Meiosis
Meiosis consists of 2 separate cell divisions, Meiosis I and Meiosis II.
Meiosis I
Prophase I
Metaphase I
Anaphase I
Telophase I/Cytokinesis
Meiosis II
Prophase II
Metaphase II
Anaphase II
Telophase II/Cytokinesis

18. Meiosis I
Prophase I – homologous chromosomes form a tetrad where they can exchange genetic information in a process known as “crossing over.”
Metaphase I – Homologous chromosomes line up at metaphase plate
Anaphase I – Homologous chromosomes separate, leaving neither daughter cell with a complete set of genetic information
Telophase I/Cytokinesis – 2 daughter cells are formed, each with a Haploid (n) Number of chromosomes.

19. Meiosis I

20. Meiosis II
Prophase II – The haploid daughter cells do not replicate their DNA
Metaphase II – The chromosomes line up at the metaphase plate.
Anaphase II – The paired chromatids separate and move to opposite poles
Telophase II/Cytokinesis – a total of 4 new cells are formed each containing a Haploid (n) Number of chromosomes.

21. Meiosis II
(Haploid cell)
(Haploid cell)

22. Formation of Gametes
The production of Sperm, called Spermatogenesis, produces 4 sperm cells.
The production of Eggs, called Oogenesis, produces 1 Egg cell and 3 polar bodies (which degenerate) due to unequal cytoplasm division during meiosis.

23. Mitosis versus Meiosis
Begins with 1 DIPLOID cell
Produces 2 genetically identical DIPLOID cells
Occurs in Somatic (body) cells
MEIOSIS
Begins with 1 DIPLOID cell
Produces 4 genetically different HAPLOID cells
Occurs in Reproductive “precursor” cells