1. Section 3.4: The Cell Cycle
Hiba Reda, Ali Osman, Aya Nassar, Tasneem Ahmed
2nd Hour

2. The Cell Cycle
The cell cycle is: the sequence of growth and division of a cell (from the time it forms until it divides)
A new cell will grow for a period of time and then it divides into two new cells, called daughter cells, which then continue the cycle
The phases, or parts, of the cycle include: interphase, mitosis, cytoplasmic division (or cytokinesis), and differentiation
Checkpoints: groups of special proteins that interact at certain points of the cell cycle to keep the cycle under control
Restriction Checkpoints determine whether or not a cell is fit enough to continue growing and dividing, or if the cell will move into a nondividing stage as a specialized cell (ex: nerve cells), or if the cell will die

3. The Cell Cycle Cont. Disruption of the cell cycle can affect health:
If cell division is infrequent, wounds will not heal
If cell division is too frequent, a cancer grows
Not all cells normally divide continually: some cells divide times (in labs), others divide the maximum number of times (cells that line the small intestine, and others don’t divide at all (nerve cells)
The time taken to complete the cell cycle can vary greatly as well. Example: The cell cycle of a fly embryo cell takes just eight minutes while a human liver cell cycle can last longer than a year
Cells have a built-in “clock” in the form of chromosome tips called telomeres so that a cell “knows when to stop dividing
Telomeres shorten after each cell division and when they shorten to a certain length, the cell no longer divides

5. Interphase
A cell must grow and duplicate most of its components before it divides so that two daughter cells can form and this period of preparedness is called interphase
During interphase, a cell acquires nutrients, uses them to produce new living material, and maintains routine “housekeeping” functions
Cell duplicates membranes, ribosomes, lysosomes, mitochondria, and most importantly, genetic material. In the end, the two new cells will have a complete set of genetic instruction (DNA)
Broken down into 3 phases or parts:
G1 Phase: the cell begins to grow to reach the mature size
S (synthesis) Phase: DNA begins to replicate itself
G2 Phase: cell prepares for cell division, in this case, mitosis

7. Cell Division A cell can divide in two ways:
Meiosis: part of gametogenesis- forms sex cells→ in female=formation of egg cell, in male=formation of sperm cells
After the egg is fertilized by the sperm, it should have the normal number of of 46 chromosomes. Therefore, both the egg and the sperm must first halve their normal chromosome number to 23 chromosomes
The halving of the chromosomes in meiosis is accomplished by reduction division
Mitosis: more common form of cell division which increases cell number and consists of 2 processes
First part= Mitosis which is the division of the nucleus
Second part= Cytokinesis which is the division of the cytoplasm
All cells can divide by mitosis except sex cells and red blood cells

8. Cell Division Cont.
Division of the nucleus is important since each daughter cell should receive identical and precise genetic information or else the cell will not survive
DNA replicates during interphase and DNA is divided evenly to each daughter cell in mitosis
Mitosis is broken down into phases
The four stages of mitosis are:
Prophase
Metaphase
Anaphase
Telophase

9. Prophase
Chromosomes condense into tightly-coiled rods and become visible
Chromosome has two identical parts called chromatids which are attached at the centromere
One centriole is on one side of the cell and the other is at the opposite end of the cell
The nuclear envelope and nucleolus disperse and disappear
Microtubules begin to grow from centrioles
Spindle fibers form between the centrioles to equally divide the chromatids and connect to the centromere

11. Metaphase
The chromosomes line up along the middle (midline) of the cell
Spindle fibers from the centrioles attach to the centromeres of each chromosome so that each sister chromatid can go to opposite sides of the cell

12. Anaphase Centromeres are pulled apart
Sister chromatids separate and slowly move (centromere first) towards the opposite sides of the cell
After they are separated, they are considered individual chromosomes

13. Telophase Chromosomes reach opposite ends of the cell
Chromosomes unwind into threadlike and less tightly coiled chromatin form
Nuclear envelope forms around each newly formed cell and nucleolus forms in each cell
Spindle fibers disappear

14. Cytokinesis
It is the division of the cytoplasm that forms two daughter cells
Begins during anaphase and the cell membrane will begin to pinch midway between cells=cleavage furrow
Cleavage furrow pinches the cell into two cells with the help of microfilaments
This is complete at the end of telophase during mitosis
Result: two daughter cells with identical genetic material

15. Cell Differentiation
Cell differentiation: a cell that becomes specialized to perform a specific function like a liver cell, a blood cell, or a neuron
There are more than 260 general types of cells in the human body
Differentiation is why cells vary greatly from other cells like their size and shape
Stem Cell: a cell that is able to divide repeatedly without specializing
Progenitor Cell: descendants of stem cells that can differentiate to form one or more kinds of cells, but cannot divide and reproduce indefinitely
Self-Renewal: stem cell process of giving rise to at least one more stem cell by dividing
Occurs through gene expression: specific combinations of genes that are turned on or off in a cell compared to another→ controls how a cell functions

17. Cell Death A cell that does not divide or specialize can die
Apoptosis- a type of cell death that is a normal part of development
Occurs after a sunburn and apoptosis peels away the damaged skin cells. If apoptosis does not occur, the damaged cells can become cancerous

18. Cell Death Continued A.) Cell in the process of apoptosis-
Rounds up and swells
The nuclear membrane breaks down
Chromatin condenses
Enzymes cut the chromosomes into many pieces of DNA
The cell shatters into membrane enclosed fragments and scavenger cell engulfs and destroys them

19. Use What You Know A= G1 Phase B= S Phase C= G2 Phase E= Cytokinesis
D= Mitosis
E= Cytokinesis
F= Prophase
G= Metaphase
H= Anaphase
I= Telophase

20. Use What You Know Centrioles Chromatin Spindle Fibers Interphase
Prophase
Metaphase
Anaphase
Telophase
Cytokinesis

21. Thank You For Watching!