The Cell Cycle

Cell Division

• Cell Division– Cells divide in order for us to grow– As a cell grows, its volume increases more rapidly

than its surface area– When the surface area-to-volume ratio is too small,

the cell cannot move materials in and out of the cell at a sufficient rate or in sufficient quantities.

– Why are we made of millions of small cells instead of just one big cell?

Surface Area to Volume Ratio

Length of side

Surface Area

Volume SA:Volume Ratio

100 cm

10 cm

1 cm

0.1 cm

3 Phases of the Cell Cycle• Interphase

– G1 – S phase– G2

• Mitosis– Prophase– Metaphase– Anaphase– Telophase

• Cytokinesis

Cell Life Span

CELL TYPE APPROXIMATE LIFE SPAN (Time Spent in

Interphase)

Skin Cell 2 weeks

Red Blood Cell 4 months

Liver Cell 300-500 days

Intestine—internal lining 4-5 days

Intestine—muscle and other tissues

16 years

Purpose of the Cell Cycle:

• To make 2 genetically identical daughter cells from a single parent cell

Interphase

• The "holding" stage or the stage between two successive cell divisions.

• Some 90 % of a cell's time in the normal cell cycle may be spent in this phase

C = chromatinI = nucleolus

Cell Cycle

G1 phase S phase G2 phaseThe cell prepares to

divideThe DNA replicatesThe cell grows

INTERPHASE

InterphaseGap 1 (G1)

• Cell carries out its normal cell functions

• Cell increases in size (grows)

• Organelles increase in number

• A cell spends most of its time in this stage, although the length of time varies by cell type.

InterphaseSynthesis (S)

• Synthesis—combining of parts to make a whole

• Cell makes a copy of its DNA (nucleus)

• By the end of the S stage, the cell nucleus contains two complete sets of DNA

InterphaseGap 2 (G2)

• Additional growth occurs

• Checkpoint—everything must be in order before the cell can move on and go through mitosis and then cell division

Mitosis (M Stage)

• Mitosis—division of the cell nucleus and its contents

• Nuclear membrane dissolves

• The duplicated DNA condenses

• Two new nuclei form

MitosisProphase

• Loose chromatin condenses into tightly coiled chromosomes

• The nuclear envelope breaks down

• Centrioles begin to move to the opposite poles and spindle fibers form

Condensing of Chromosomes• One chromosome = one continuous strand of

DNA• DNA wraps around proteins called histones,

forming chromatin• Chromatin compacts further, forming a chromatid• Two identical chromatids are called sister

chromatids• Sister chromatids are held together at the

centromere• Telomeres are located at the ends of the

chromatids• Humans have 23 pairs of chromosomes (46 total)

Sister Chromatids

Telomeres—regions of repetitive DNA at the end of a chromosome, which protects the end of the chromosome from destruction

Progeria

• Spindle fibers attach to each chromosome

• Sister chromatids are aligned along the equator by the spindle fibers

• In humans, 46 sister chromatids line up in the middle of the cell

MitosisMetaphase

• Chromatids separate to opposite sides of the cell

• Pulled by spindle fibers

MitosisAnaphase

• A complete set of identical chromosomes is positioned at each pole of the cell

• The nuclear membranes start to form

• Chromosomes begin to uncoil

• Spindle fibers fall apart

MitosisTelophase

Cytokinesis

• Cytokinesis—division of the cytoplasm

• Results in two daughter cells

• Both new cells are genetically identical to the original parent cell

Cytokinesis Differs in Animal and Plant Cells

• In Animal cells, the membrane forms a furrow and pinches closed

• In Plant cells, the membrane cannot pinch inward because of the cell wall. Instead, a cell plate forms between the two new nuclei

At the End of the Cell Cycle:

• Final Products are two identical cells– Smaller in size (larger surface area to volume

ratio)– Each new cell has 46 chromosomes– Organelles get divided up between the two

cells during cytokinesis• Not always exactly evenly divided• The cell can make more organelles if needed