The Cell Cycle

• Cells are limited in size because the outside (the cell membrane) must transport food and oxygen to the parts inside. • As a cell grows, the volume of the cell

increases at a faster rate than its surface area.

Why do cells need to divide?

Surface Area & Volume Changes

• As cells grow too large, they become inefficient.• Cells that grow too large do not have enough surface

area to take in nutrients and remove waste.• Cells that grow too large take longer to move

material within the cell.• Small cells have more efficient transport systems

because they have a higher surface to volume ratio, S/V.

Why would a cell want to be small?

Cell Cycle• Eukaryotic Process ONLY• Unicellular organisms: asexual reproduction•Multicellular organisms: growth and repair

• Rate of cell cycle (speed of cell division) may differ depending on:• Type of cell• Age of cell• Other factors such as• Infection by a virus• Exposure to radiation, drugs, alcohol, smoke, or

other toxins

Type of Cell• The function of the cell & type of tissue

often determine the rate of its cell cycle

• Fastest include: intestinal, skin, bone marrow, hair• Slowest: bone, liver, kidney

skin bone small intestine

External Factors• Factors outside of the cell or organism can affect the cell cycle. • Cells are exposed to many environmental factors

that can directly influence the control and regulation of the cell cycle• These factors disrupt the cell cycle and can cause

cells to divide uncontrollably, resulting in cancerous tumors. These factors are called carcinogens.

Genetic Factors• Some mutations to genes can result in

changes to the cell cycle.• The BRCA1 gene can cause breast cancer

• Progeria is a genetic disorder marked by rapid aging

Why can’t organisms live forever?• Cells cannot grow and divide indefinitely. • At the end of chromosomes are small ‘caps’ called telomeres that help keep

the DNA from uncoiling. Every time the DNA is replicated, a piece of the telomere is lost. When the telomere is gone, the DNA can no longer replicate and the cell dies.

• Many aging-related diseases are linked to shortened telomeres.• People with longer telomeres tend to live longer than people with shorter

telomeres.

The Cell Cycle•Occurs in 3 phases• Interphase: preparation for division• Mitosis: nuclear division• Cytokinesis: cell division

INTERPHASE• Longest phase of the cell cycle•Marked by 3 distinct stages• Gap 1 (G1): Cell growth & replication of organelles

(1st Checkpoint = Cell large enough & suitable environment)

• Synthesis (S): DNA replication • Gap 2 (G2): Growth & preparation for cell division.

(2nd Checkpoint = DNA correctly replicated)

What is G0?• Resting phase where certain cells

leave the cell cycle and stop dividing• Some cells, once formed, remain in G0 for the life

of the organism. These cells cannot be replaced once damaged. Examples: Nerve and Muscle

• Some cells enter an extended period of G0 , however, can be replaced overtime.

Examples: Liver and Kidney

MITOSIS

• M Phase: Process that divides the nucleus with replicated DNA• Approximately 15% of cell’s life cycle• 4 Stages• PROPHASE• METAPHASE• ANAPHASE• TELOPHASE

Prophase

•Nuclear membrane disintegrates•Nucleolus disappears• Chromatin (containing replicated DNA)

condenses into chromosomes • Centrioles begin to move apart• Spindle forms

Metaphase

• Chromosomes attach to spindle fibers• Chromosomes line up along equator

of cell• (3rd Checkpoint - Are chromosomes

aligned?)

Anaphase

• Spindle fibers shorten• Chromatids (daughter

chromosomes) separate and move to opposite poles

Telophase

• Chromatids (daughter chromosomes) reach poles• Nuclear membrane reforms• Nucleolus reappears• Chromosomes decondense back into

chromatin

Cytokenesis

• Follows mitosis• Plant Cells: Cell plate forms, dividing

daughter cells

• Animal Cells: Cleavage furrow forms & pinches cell into daughter cells

Cell Cycle

Checkpoint 1: Is cell getting too large ?

Checkpoint 2: Has DNA replicated correctly?

Checkpoint 3Are chromosomes aligned?

Cytokinesis