The Cell Cycle

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 VolumeSA:Volume Ratio 100 cm 10 cm 1 cm 0.1 cm

3 Phases of the Cell Cycle Interphase –G 1 –S phase –G 2 Mitosis –Prophase –Metaphase –Anaphase –Telophase Cytokinesis

Cell Life Span CELL TYPEAPPROXIMATE LIFE SPAN (Time Spent in Interphase) Skin Cell2 weeks Red Blood Cell4 months Liver Cell days Intestine—internal lining4-5 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 = chromatin I = nucleolus

The cell prepares to divide The DNA replicates The cell grows INTERPHASE

Interphase Gap 1 (G 1 ) 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.

Interphase Synthesis (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

Interphase Gap 2 (G 2 ) 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

Mitosis Prophase 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 Mitosis Metaphase

Chromatids separate to opposite sides of the cell Pulled by spindle fibers Mitosis Anaphase

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 Mitosis Telophase

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