The Cell Cycle

Why do cells need to divide? 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.

Surface Area & Volume Changes

Why would a cell want to be small? 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.

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 3 Are chromosomes aligned? Cytokinesis Checkpoint 2: Has DNA replicated correctly? Checkpoint 1: Is cell getting too large ?