The Cell Cycle

CELL SIZE IS LIMITED● Too great an increase in size could result in surface area too small for exchange of gases, nutrients and wastes

● Therefore…cells grow to a certain size and then they DIVIDE!

● Cell size, growth and division must be coordinated.

● Before it becomes too large, a cell divides creating 2 new, identical “daughter” cells

MULTICELLULAR VS. UNICELLULAR

LIMITS TO CELL GROWTH●Two reasons why cells divide

rather than continually grow larger and larger:

● DNA “overload”●cell’s DNA can’t serve the increasing needs of a growing cell

● Exchanging materials●exchange depends on the surface area of the cell●difficulty moving enough nutrients and wastes across the cell membrane

CHROMOSOMES

● Genetic information is carried by chromosomes which are made up of DNA

● Chromosomes are not visible in most cells except during cell division● Humans have 46 chromosomes

● Two identical “sister” chromatids are connected by the centromere and have a long arm and a short arm.

THE CELL CYCLE

● The cell cycle, is the series of events that take place in a eukaryotic cell between its formation and the moment it replicates itself.

TWO PARTS● These events can be divided in TWO main

parts: 1) Interphase (in between divisions)

- includes G1, S, and G2 phases- cell is forming and carrying

on with its normal metabolic functions

2) Mitotic phase (M phase)- cell replicates itself

INTERPHASE**Cellular Growth and

Replication**

INTERPHASE

● Interphase is a phase of the cell cycle, defined only by the absence of cell division.

● During interphase, the cell obtains nutrients, and duplicates (copies) its chromatids (genetic material).

● The genetic material or chromatids are located in the nucleus of the cell and are made of the molecule DNA.

INTERPHASE

● Most eukaryotic cells spend most of their time in interphase.

● For example, human skin cells, which divide about once a day, spend roughly 22 hours in interphase.

●About 90 percent of cells are in interphase.

● Some cells, such as nerve cells, can stay in interphase for decades.

●Includes G1, S, and G2 phases

G1 PHASE

● After cytokinesis and before the S phase.

● G = Gap…think “Growth”● Major period of cell growth ● During this stage new

organelles are being synthesized (made)● cell requires both structural

proteins and enzymes, resulting in great amount of protein synthesis

S PHASE● Synthesis phase: DNA

synthesis or replication occurs

● Between G1 phase and the G2 phase.

● At the BEGINNING of the S stage, each chromosome is composed of one coiled DNA double helix molecule, which is called a chromatid.

● At the END of this stage, each chromosome has two identical DNA double helix molecules, and therefore is composed of two sister chromatids.

G2 PHASE● Third, final, and usually the shortest subphase. ● Cell undergoes a period of rapid growth to prepare for mitosis. ● Although chromosomes have been replicated they cannot yet be distinguished individually because they are still in the form of loosely packed chromatin fibers. ● The G2 phase continues growth of the cell and prepares the cell for mitosis (M phase) by producing all of the enzymes that the cell will need in order to divide.

READY FOR DIVISION

● After the G2 phase of interphase, the cell is ready to start dividing.

● The nucleus and nuclear material (chromosomes made of DNA) divide first during the stage known as MITOSIS (M phase).

CELLS DIVIDE AT DIFFERENT RATES

CELL TYPE APPROXIMATE LIFE SPAN

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

● Rate at which cells divide is linked to body’s need for those cells. ● Human cells: S, G2 and M stages take approx. 12 hours; G1 stage differs

??? Do you think a skin cell would have a long or short G1 stage???