1. Cellular Division
Lecture 4
Genetics
Dr. Heba Al-Fares

2. Cell Reproduction
For any cell to reproduce successfully, three fundamental events must take place:
its genetic information must be copied,
the copies of genetic information must be separated from one another, and
the cell must divide.
All cellular reproduction includes these three events, but the processes that lead to these events differ in prokaryotic (bacteria) and eukaryotic cells (protists, fungi, plants, & animals).

3. Cell Division All cells are derived from pre-existing cells
New cells are produced for growth and to replace damaged or old cells.

4. Types of Cell Reproduction
Asexual reproduction involves a single cell dividing to make 2 new, identical daughter cells
Mitosis & binary fission are examples of asexual reproduction
Sexual reproduction involves two cells (egg & sperm) joining to make a new cell (zygote) that is NOT identical to the original cells
Meiosis is an example

5. Prokaryotic Cell Reproduction
When prokaryotic cells reproduce, the circular chromosome of the bacterium is replicated.
The two resulting identical copies are attached to the plasma membrane, which grows and gradually separates the two chromosomes.
Finally, a new cell wall forms between the two chromosomes, producing two cells, each with an identical copy of the chromosome.
Under optimal conditions, some bacterial cells divide every 20 minutes. At this rate, a single bacterial cell could produce a billion descendants in a mere 10 hours.

6. Prokaryotic Chromosome
The DNA of prokaryotes (bacteria) is one, circular chromosome attached to the inside of the cell membrane

7. Cell Division in Prokaryotes
Prokaryotes such as bacteria divide into 2 identical cells by the process of binary fission
Binary fission: the physical process whereby a bacterial cell divides into two daughter cells
Single chromosome makes a copy of itself
Cell wall forms between the chromosomes dividing the cell by the growth of a transverse septum
Parent cell
Chromosome doubles
Cell splits
2 identical daughter cells

8. Eukaryotic Chromosomes
All eukaryotic cells store genetic information in chromosomes
Most eukaryotes have between 10 and 50 chromosomes in their body cells, potatoes have 48 chromosomes, fruit flies have 8, and humans have 46.
There appears to be no special significance between the complexity of an organism and its number of chromosomes per cell.
Each chromosome in one set has a corresponding chromosome in the other set, together constituting a homologous pair.
The two chromosomes of a homologous pair are usually alike in structure and size, and each carries genetic information for the same set of hereditary characteristics. (An exception is the sex chromosomes,

9. cell division
The process of cell division is necessary for life to function.
In this process, one cell, the parent cell, divides into two daughter cells.
This is called the cell cycle and has four distinct steps: a G1 (or gap-1) phase, an S phase, a G2 (or gap-2) phase, and mitosis.

10. Five Phases of the Cell Cycle
G1 - a cell grows in size and prepares to replicate (or copy) the chromosomes.
S – is the synthesizing phase, when the DNA molecule is replicated, doubling it.
G2 - secondary growth phase
collectively these 3 stages are called interphase
M – mitosis; is the stage in which the cell actually divides into two daughter cells, each with its own copy of the DNA molecule
C - cytokinesis

11. Cell Cycle

12. Interphase - G1 Stage 1st growth stage after cell division
Cells mature by making more cytoplasm & organelles
Cell carries on its normal metabolic activities
There is a critical point in the cell cycle, termed the G1/S checkpoint, in G1; after this checkpoint has been passed, the cell is committed to divide.
Before reaching the G1/S checkpoint, cells may exit from the active cell cycle in response to regulatory signals and pass into a nondividing phase called G0, which is a stable state during which cells usually maintain a constant size.
They can remain in G0 for an extended period of time, even indefinitely, or they can re-enter G1 and the active cell cycle.

13. Interphase – S Stage
Replication is the process of making replicas or copies (DNA synthesis)
Synthesis stage
DNA is copied or replicated
If DNA synthesis is blocked (with drugs or by a mutation), the cell will not be able to undergo mitosis.
Before S phase, each chromosome is composed of one chromatid; following S phase, each chromosome is composed of two chromatids
Two identical copies of DNA
Original DNA

14. Interphase – G2 Stage 2nd Growth Stage
Occurs after DNA has been copied
All cell structures needed for division are made (e.g. centrioles)
Both organelles & proteins are synthesized

15. Centrioles is cytoplasmic bodies contain in special region in the centrosome.
Function:
The centrioles, small organelles that will produce spindle fibers needed to allow the cell to divide, also move to the opposite ends of the cell during this phase.
Spindle fiber occurs during early stage of mitosis and meiosis composed of arrays of microtubules these fibers plays an important role in the movement of chromosome during cell division
microtubules polymers of alpha and beta subunits of the protein tubulin.
Centrosome: cellular structure from which microtubules originate.

16. What’s Happening in Interphase?
What the cell looks like
Animal Cell
What’s occurring

17. Sketch the Cell Cycle DNA Copied Cells prepare for Division
Cells Mature
Daughter Cells
Cell Divides into Identical cells

18. Mitosis Division of the nucleus Also called karyokinesis
Only occurs in eukaryotes
Has four stages
Doesn’t occur in some cells such as brain cells

19. Four Mitotic Stages
Prophase
Metaphase
Anaphase
Telophase

20. Early Prophase
Chromatin in nucleus condenses to form visible chromosomes through a process of DNA condensation and its associated histone protein in to a series of coiling.
Mitotic spindle forms from fibers in cytoskeleton or centrioles (animal)
Cytoplasm
Nucleolus
Nuclear Membrane
Chromosomes

21. Late Prophase Nuclear membrane & nucleolus are broken down
Chromosomes continue condensing & are clearly visible
Centromere replicated during the formation of sister chromatids
Multiproten complex called kinetochores attach to each sister centromere of each chromosome
Spindle fiber consists of microtubules which are a chain of protein called tubulin finishes forming between the poles of the cell and attach to one sister chromatids from one side and to the other sister chromatids in the opposite side.
The kinetochores act as the sites for attachment of microtubules

22. Late Prophase
Chromosomes
Nucleus & Nucleolus have disintegrated

23. Review of Prophase
What the cell looks like
What’s happening

24. Three Types of Spindle Fibers
The mitotic spindle forms from the microtubules in plants and centrioles in animal cells
Polar fibers extend from one pole of the cell to the opposite pole
Kinetochore fibers extend from the pole to the centromere of the chromosome to which they attach
Asters are short fibers radiating from centrioles important for positioning spindle apparatus within cell.

25. Sketch The Spindle

26. Metaphase Equator of Cell Pole of the Cell
Chromosomes, attached to the kinetochore fibers, move to the center of the cell
Spindle fibers apply tension to the chromosomes, causing them to line up in the center (the equator) of the cell.
Equator of Cell
Aster
Pole of the Cell
Chromosomes at Equator

27. Review of Metaphase
What the cell looks like
What’s occurring

28. Anaphase The chromatids are now two separate daughter chromosomes.
Occurs rapidly
The spindle fibers begin to shorten, pulling the Sister chromatids apart and toward opposite ends of the cell by kinetochore fibers
As each sister chromatid moves; its two arms appear to trail its centromeres; a set of V-shaped structures with the points of the Vs directed to the poles
The chromatids are now two separate daughter chromosomes.

29. Anaphase Review
What the cell looks like
What’s occurring

30. Removal of the tublin subunits from
microtubules at the kinetochore and perhaps molecular motors, are responsible for the pole-ward movement of chromosomes during
anaphase.
Microtubules pull sister chromatids to opposite poles; the microtubules depolymerize (release free tubulin) at kinetochores resulting in shortening of microtubules

31. Telophase
The daughter chromosomes (Sister chromatids) arrive at the opposite poles of the cell
The spindle fibers disappear (disassembles).
A new nuclear envelope also forms around the two sets of daughter chromosomes.
Chromosomes are no longer visible under a light microscope during telophase.
Nucleolus reappears
CYTOKINESIS occurs
Chromosomes reappear as chromatin

32. Comparison of Anaphase & Telophase

33. Cytokinesis
The cytoplasm of the parent cell is divided into two daughter cells, each of which contains one set of identical chromosomes inside their new nucleus
Means division of the cytoplasm
Division of cell into two, identical halves called daughter cells
In plant cells, cell plate forms at the equator to divide cell
In animal cells, cleavage furrow forms to split cell

34. Cytokinesis
Cleavage furrow in animal cell
Cell plate in plant cell

35. Daughter Cells of Mitosis
Have the same number of chromosomes as each other and as the parent cell from which they were formed
But smaller than parent cell
Must grow in size to become mature cells (G1 of Interphase)

36. Eukaryotic Cell Division
Used for growth and repair
Produce two new cells identical to the original cell
Cells are diploid (2n)
Chromosomes during Metaphase of mitosis
Cytokinesis
Anaphase
Prophase
Metaphase
Telophase

37. Uncontrolled Mitosis Cancer cells
If mitosis is not controlled, unlimited cell division occurs causing cancerous tumors
Oncogenes are special proteins that increase the chance that a normal cell develops into a tumor cell
Cancer cells