The Cell Theory, Cell Cycle & Mitosis

The Cell Theory

1. All living things are composed of cells.2. Cells are the basic units of living organisms.3. All cells come from pre-existing cells.

Why are Cells so Tiny?• Your body contains 60 trillion cells.• Cells take in nutrients and dispose of waste through

the cell membrane.

Why are Cells so Tiny?• As cells grow larger, the volume grows faster than the

membranes’ surface area.• When the cell becomes too large to absorb enough

nutrients it must divide.

Why do Cells Divide?

• Growth: To increase the number of cells in the body.

• Repair: To replace dead, damaged or old cells.

• Reproduction: Can make exact copies (clones).

The Cell Cycle

• Every hour, about 1 billion of your cells die and another 1 billion are made.

• Body cells go through several stages in their lifetime called the Cell Cycle.

The Cell Cycle

• The Cell Cycle has 2 parts:1. Interphase:

Cell repairs any damages, re-energizes, grows and prepares for division.2. Mitosis & Cytokinesis:

Division of the cell into 2 identical “daughter” cells.

Interphase

• Longest stage of the cell cycle.• Divided into three phases.

Interphase

• First Gap Phase (G1):

Cells grow in size. New proteins and organelles are made.

• Synthesis Phase (S): DNA is replicated (known as chromatin).

• Second Gap Phase (G2):

Cells prepare for division.

Cell Division

1. Mitosis (division of the nucleus)- Prophase- Metaphase- Anaphase- Telophase

2. Cytokinesis (division of the cytoplasm)

The Nucleus

Chromosomes

• Hereditary information.• Contained within the nucleus.• Human cells contain 23 pairs of chromosomes.• When a cell divides, each

daughter cell gets the same 23 pairs of chromosomes.

Chromosomes

• Composed of DNA and proteins.• A gene is a segment of DNA that codes for a

particular trait.

Prophase

• Chromatin condenses to form paired chromosomes (sister chromatids).

• Centrioles move to each pole.

• Nucleolus disappears.• Nuclear membrane

breaks down.

Metaphase

• Spindle fibers attach to centromere.

• Chromosomes line up at metaphase plate.

Anaphase

• Spindle fibers shorten.• Sister chromatids

separate and move to opposite poles (pulled by spindle fibres).

Telophase & Cytokinesis

• Cell membrane pinches inwards.• Cytoplasm splits in half.• Cells return to interphase.

Plant vs. Animal Cell

Visualizing the Cell Cycle

• The stages of the cell cycle are used to understand the process.

• In reality it is a continuous process.• These animations and video depict this:- http://www.johnkyrk.com/mitosis.html- http://www.cellsalive.com/mitosis.htm- http://www.youtube.com/watch?v=VlN7K1-9QB0

Go Phase

• Cells can enter a resting phase called Go.

• e.g., nerve cells in the brain stop dividing when the brain is developed and cannot reenter the cell cycle.

Cell Death

• Necrosis:– Cells may die due to external factors.– e.g., Toxins, infections, trauma.

• Apoptosis:– The controlled death of old cells. – e.g., White blood cells divide to fight viral

infections. When they are no longer required, they undergo apoptosis.

Cancer• Genetic mutations can cause cells to undergo

abnormal cell division.• Instead of apoptosis, these cells divide

uncontrollably.

Cancer

Cancer

• Cell division occurs so fast that cells pile up on top of one another, forming a tumor.

• Reduces the effectiveness of the surrounding tissue.

Cancer• Changes to these abnormal cells in the tumor can

create cancer.• May invade and destroy neighbouring cells.• May stimulate other cells to start replicating

uncontrollably, thus spreading cancer and causing different types of cancer.

Skin Cancer