1. 3.4 The Cell Cycle and Cell Division
Nagwa Masad
Yusra Abuwandi
Abeer Haddad
Hour 3

2. The Cell Cycle
The series of change that the cell undergoes from the time it forms until it divides is called the cell cycle.
This cycles seems rather simple: a newly formed cell grows for a time and then divides to form two new cells, which in turn may grow and divide. Yet the phases and timing of the cycle are quite complex, and include interphase, mitosis, cytoplasmic division (cytokinesis), and differentiation.

3. Interphase
Before a cell actively divides, it must grow and duplicate much of its contents so that two “daughter” cells can form from one. This period of preparedness is called Interphase.
Interphase is a time of great synthetic activity
It obtains nutrients
Utilizes them
Manufactures new living material
Maintains routine “housekeeping functions”
The cell in interphase takes on the tremendous task of replicating its genetic material by duplicating membranes, ribosomes, lysosomes, and mitochondria.
This is important because it gives each cell a set of genetic instructions.

4. Cell Division
Stimulation from a hormone or growth factor may trigger cell division.
For example, when the breasts develop into milk-producing glands during pregnancy.
Most cells do not normally divide continually.
If grown in the laboratory, most type of human cells divide only forty to sixty times.
Some cells may divide the maximum number of times, such as the cells that line the small intestine.
Other do not divide, such as nerve cells.
A cell knows when to stop dividing because of a built in “clock” in the form of the chromosome tips.
These are called telomeres, they shorten with each cell division. When the telomeres shorten to a certain length, the cell no longer divides.

5. Meiosis, Mitosis and Cytokinesis
Two ways of cell division
Meiosis: The formation of egg cells (female) and sperm cells (male)
Through a process called reduction of division, Meiosis assures that both the male and female have 23 chromosomes to complement the egg’s need of 46 chromosomes.
Mitosis: Divides the nucleus
Can divide all cells but sperm and egg cells and red blood cells.
The new cell must have a copy of the information in the DNA get replicated during interphase and get split into two cells during mitosis
The division of the nucleus must be very precise because it contains the DNA.
The process is continuous.
Cytokinesis: Division of the cytoplasm

6. Interphase is considered in phases
Interphase is considered in phases. DNA is replicated during the S (or synthesis) phase, which is bracketed by two gap (or growth) periods, called G1 and G2, when other structures are duplicated.

7. PROPHASE
One of the first indications that a cell is going to divide is that chromosomes become visible in the nucleus when stained.
Because the cell has gone through S phase, each prophase chromosome is composed of two identical portions.
During the prophase, the two newly formed centriole pairs move to opposite ends of the cell.
A spindle-shaped array of microtubules(spindle fibers) form between the centrioles as they move apart.

8. METAPHASE The chromosomes line up about midway between the centrioles.
Spindle fibers attach to the centromeres of each chromosomes so that a fiber from one pair of centrioles contacts one centromere, and a fiber from the other pair of centrioles attaches to the other centromere.

9. ANAPHASE
Later on the centromeres are pulled apart as the chromatids separate, they become individuals chromosomes.
The separated chromosomes now move in opposite (guided by the microtubule activity).
The spindle fibers shorten and pull their attached chromosomes toward the centrioles at opposite ends of the cell.

10. TELOPHASE
The final stage of mitosis begins when the chromosomes complete their migration toward the centrioles.
It's like the prophase but reversed.
A nuclear envelope forms around each chromosome set, and a nucleoli appear within the newly formed nucleic.
Finally the microtubules disassemble into free tubulin molecules.