1. Cell Division

2. Overview: Cell Division
Cell Division: New cells arise from the division of pre-existing cells
Most cell division results in daughter cells with identical genetic information, DNA
Integral part of the cell cycle, the life of a cell from formation to its own division
Three types of cell division:
Binary fission (observed in asexually reproducing organisms)
Mitosis (refer to next slide)
Meiosis (observed in sexually reproducing organisms)

3. Mitosis is seen in: Unicellular (definition: one –cell) organism
Reproduction of an entire organism
Multicellular (definition: more than one/multiple cell) organisms
Development from a fertilized cell
Growth
Repair
100 µm
200 µm
20 µm
(a) Reproduction
(b) Growth and
development
(c) Tissue renewal

4. Mitosis and the Cell cycle
Interphase
G1: growth phase, making cell organelles
S phase: DNA synthesis (replication)
G2 phase: preparation for cell division, mitosis and cytokinesis
Mitotic phase: creates two identical daughter cells
Mitosis: Division of the nucleus
Cytokinesis: Division of the cytoplasm

5. Cellular Organization of the Genetic Material
IN EUKARYOTES…..
Chromatin: complex of DNA and associated proteins (histones)
-Condenses during the cell division

6. chromosomes VERSUS chromatid
During DNA synthesis, one chromosome duplicates to result in two sister chromatids
During cell division, the two sister chromatids separate into two daughter cells
The centromere is the narrow “waist” of the duplicated chromosome, where the two chromatids are most closely attached

7. Cellular Organization of the Genetic Material
In prokaryotes (example: bacteria): One chromosome made of a single circular molecule of DNA (a plasmid).
In eukaryotes (example: humans): Variable number of chromosomes (characteristic of species) made of molecules of long linear DNA.
Example: humans have 46 chromosomes

8. Types of cell division Eukaryotic cells:
Two types of cell division for Two types of cells
Mitosis occurs in somatic cells (body cells)
-Mitosis results in genetically identical cells
Meiosis occurs in reproductive cells called gametes (eggs and sperm)
-Meiosis is a way to combine genetic traits during reproduction
Prokaryotic cells: Binary fission

9. MITOSIS Mitosis is conventionally divided into the phases (P-M-A-T):
Prophase
Metaphase
Anaphase
Telophase
Cytokinesis is well underway by late telophase
NOTE: In some textbooks, interphase is considered part of mitosis….hence in that case it is IPMAT

10. MITOSIS

11. Cytokinesis
Accumulation of vesicles from Golgi apparatus, carrying cell wall materials.
→ Cell plate
Use a network of microtubules (tubulin) to align vesicles
Formation of cleavage furrow
Contractile ring made of microfilaments (actin) associated with myosin

12. Does mitosis occur in all cells?
No…
-Sex cells (gametes) do not undergo mitosis
Why?
reproduction involves the combining of traits from two organisms
- cannot combine/fuse any two random
cells

13. Sexual Reproduction
Meiosis = specialized cell division so you have only one of each chromosome, called
Gametes: made only in gonad (testis, ovary)
n (23)
n (23) +
2n = 46

14. Why cannot we have mitosis for sex cells (gametes)?
Sex cells (gametes) allow traits to be combined from two organisms.
Can’t just fuse any two random cells.
2n (46)
2n (46) + A1 A2 A3
A26
B35
B44
B41
B35
4n = 92
too many

15. Chromosome numbers
Diploid: possess two sets of each chromosome; one from the father and one from the mother.
Human: 46 chromosomes or 23 pairs of homologous chromosomes (2 sets; 2n).
In somatic cells, during the cell cycle:
46 chromosomes are duplicated during the S phase
Each daughter cell receives an identical copy of each of the 46 chromosomes.
Gametes, obtained via meiosis, are haploid
Yield non-identical cells containing 23 chromosomes (1 set; n)
Fusion of two gametes (fertilization) returns the number of chromosomes to 46.

16. Mitosis versus Meiosis
Diploid
somatic
cell
gamete
precursor
MITOSIS
division
duplication
Diploid
Haploid
division
Meiosis has 2 divisions: Meiosis I and Meiosis II

17. The Cell Cycle Control System
The sequential events of the cell cycle are directed by a distinct cell cycle control system, which is similar to a clock
The cell cycle control system is regulated by both internal and external controls
The clock has specific checkpoints where the cell cycle stops until a go-ahead signal is received
Monitor integrity of genome
Prevent the cell from progressing through the cell cycle if abnormalities are detected
Correct size
Type and amount of specific proteins

19. For many cells, the G1 checkpoint seems to be the most important one
If a cell receives a go-ahead signal at the G1 checkpoint, it will usually complete the S, G2, and M phases and divide
If the cell does not receive the go-ahead signal, it will exit the cycle, switching into a nondividing state called the G0 phase

20. Loss of Cell Cycle Controls in Cancer Cells
Do not respond normally to the body’s control mechanisms
Exhibit neither density-dependent inhibition nor anchorage dependence
May not need external growth factors to grow and divide:
They may make their own growth factor
They may convey a growth factor’s signal without the presence of the growth factor
They may have an abnormal cell cycle control system

21. Development of cancer
Transformation: a normal cell is converted to cancer cell in a tissue; normally degraded by immune system.
Tumor: mass of abnormal cells within a tissue
Benign tumor: abnormal cells remain in the tissue
Malignant tumor: abnormal cells; become invasive; impair the function of one or more organs → Cancer
Metastasis: spreading of cancer cells to location(s) distant from original site where they may proliferate and create secondary tumors.
Through fluids such as blood and lymph
STUDENTS WERE TOLD THAT THEY ARE NOT RESPONSIBLE FOR THIS SLIDE