1. Cellular Division

2. Cell Division Why do cells divide?
Cell division functions in reproduction, growth and repair.
The division of unicellular organisms reproduces the entire population.
In multicellular organisms, division is used to repair and for growth. Also, division is needed to produce egg or sperm used for reproduction.

3. Chromosomes
Chromosomes consist of tightly packed DNA coiled around proteins (histones) that support its structure. DNA contains genes which are segments of DNA that code for a protein.
(Humans ~20,000 genes)

4. Eukaryotic Cells Somatic (body) cells Sex cells (gametes)
46 chromosomes
Diploid (2n)
Sex cells (gametes)
23 chromosomes
Haploid (n)

5. Karyotype-A picture of the chromosomes from a human cell arranged in pairs by size
In Humans, there are 23 pairs of chromosomes. The first 22 pairs are called autosomes. The 23rd pair, the sex chromosome differs between males and females. Females have two X chromosomes, and males have one X and one Y chromosome

6. Chromosomes in Dividing Cells
Chromatid – half of a chromosome
Centromere – the constriction region that divides the chromosome into two chromatids
Sister chromatids – two identical DNA molecules attached at the centromere
Homologous Chromosomes – a chromosome pair, one inherited from the mother and one from the father, containing genes for the SAME trait.

7. Cell Cycle CELL CYCLE – series of events of cell growth and division
Steps of Cell Cycle
1. Interphase – G1, S, G2
2. M phase –
mitosis or meiosis
3. Cytokinesis

8. Sometimes the cells exit the cell cycle and enter the G0 phase
Sometimes the cells exit the cell cycle and enter the G0 phase. In the G0 phase, cells are alive and metabolically active, but do not divide. Many cells in the human body, including those in heart muscle, eyes, and brain are in the G0 phase. If these cells are damaged they cannot be replaced.

9. INTERPHASE – accounts for 90% of cycle
Cell Cycle: Creating Somatic cells 1. Interphase
INTERPHASE – accounts for 90% of cycle
G1 (Gap) phase – cell undergoes growth & normal functions
S phase – DNA replication
G2 (Gap) phase – cell prepares to divide
Two identical copies of DNA
Original DNA

10. Cell Cycle: Creating Somatic cells 2. Mitosis
MITOSIS – nucleus divides into 2 identical nuclei with identical genetic information
In Eukaryotes --- It is the main process of growth and repair. It is the process by which unicellular and multicellular organisms asexually reproduce

11. Steps of Mitosis
PROPHASE – chromosomes condense and become visible; centrioles move to opposite sides of cell and form spindle fibers (microtubules); nuclear membrane disappears
METAPHASE – spindle fibers attach to centromeres and align chromosomes midway
ANAPHASE – spindle fibers move sister chromatids of each chromosome apart to opposite sides of the cell
TELOPHASE – chromosomes decondense; nuclear membrane reforms; 2 new nuclei are formed

12. Cell Cycle: Creating Somatic cells 3. Cytokinesis
CYTOKINESIS – cytoplasm divides; cell pinches in half to form 2 new cells. In plant cells, a cell plate forms to divide the cytoplasm.

13. Identical Daughter Cells
Mitosis Yields: 2 diploid daughter cells - genetically identical to parent

14. Mitosis Animation

15. Mitosis Review
Prophase
Metaphase
Anaphase
Telophase
Cytokinesis

16. Cell Cycle: Creating Gametes
Cell Cycle: Creating Gametes – Interphase, Meiosis, Cytokinesis
1. INTERPHASE – growth, DNA replication, preparation
2. MEIOSIS – nucleus divides twice into 4 different nuclei with different genetic information
In Eukaryotes --- It is the formation of gametes for sexual reproduction.

17. Meiosis: Two Part Cell Division
Sister
chromatids
separate
Meiosis I
Meiosis II
Homologs
separate
Diploid
Diploid
Haploid

18. Meiosis I: PMAT I
A single cell divides into 2 diploid daughter cells that are not genetically identical.
The homologous chromosomes come together and literally swap parts of themselves with each other. This process is called Crossing Over and ensures that the daughter cells produced after the first cytokinesis will not be genetically identical.

19. Meiosis II: PMAT 2
Two diploid cells divide to form four haploid daughter cells.
Independent Assortment – each homologous chromosome is randomly assorted into different gametes

20. Cell Cycle: Creating Gametes 3. Cytokinesis
CYTOKINESIS – division of the cytoplasm
Overall Meiosis Yields: 4 haploid daughter cells – genetically different from parent

21. Control Mechanisms
CYCLE – control mechanisms govern the rate of cell division:
Molecular control system - Several checkpoints act as built-in stop signs that halt the cell until they are over-ridden by go ahead signals. Three checkpoints exists in G1, G2, and M. Cells also have a predetermined lifespan.
Timing is controlled by regulatory proteins – cyclins and kinases. These proteins selectively access, activate and silence information in DNA.
Contact Inhibition – Cells release chemicals to inhibit growth when they become too crowded.

22. Uncontrolled Mitosis Cancer cells When things go wrong?
Uncontrolled Cell Growth -- Mutant genes cause tumors to form by disrupting normal cell cycle controls. Altered cells grow and divide abnormally. Tumors may be cancerous.
Cancer
cells

23. Cell Division in Prokaryotes
Most bacterial genes are located on a single bacterial chromosome (~4million base pairs) which consists of a circular DNA molecule and associated proteins.
Bacteria do not have as many genes or DNA molecules as long as those in eukaryotes (Humans ~2.3 billion base pairs), their circular chromosome is still highly folded and coiled in the cell.
Prokaryotes reproduce by binary fission, not mitosis. In binary fission, chromosome replication begins at one point in the circular chromosome, the origin of replication site.
2 identical daughter cells

24. Comparison Chart Mitosis Meiosis Number of divisions 1 2
Number of daughter cells 4
Genetically identical?
Yes No
Chromosome #
Same as parent
Half of parent
Where
Somatic cells
Germ cells
When
Throughout life
At sexual maturity
Role
Growth and repair
Sexual reproduction