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The Cell Cycle

The Cell Cycle

Author: Eric Atkins

To gain knowledge of the cell cycle and the stages involved in mitotic division.

The cell cycle will be separated into Interphase and Mitosis.  These two stages will further be broken down into their individual components:  Interphase into G1, S, G2 and Mitosis into P, M. A. T.

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Video explaining the cell cycle

Source: Bozeman Science

Cell Cycle Notes





Newborn baby:  hard to believe 9 months ago a single cell

Small acorn grow into an Oak tree

How fast grass grows (mow)


Billions and trillions of cells in an organism

Always making new cells

Adult produces 25 million new cells every second


Every time an organism grows in size or repairs worn out/damaged tissue =  new cells are made.


Method that organisms use is very similar




Cells of a mother lion are on avg the same size as her cub.




Wide variety of sizes

Red blood cell =  8 micrometers in diameter

Micrometer =  100,000th of a meter  10–6

Nerve cells in large animal can reach up to 1meter

Yolk of an ostrich egg is largest =  8 cm in diameter


Most living cells are between 2 and 200 micrometers in diameter


Why not just one big cell?


Diffusion Limits Size

Cells require constant supply of glucose and oxygen and produce large amounts of ATP
Waste moves out
All by the process of diffusion
Into by PM and through the cytoplasm


-  Cells can metabolize only as quickly as they receive raw materials by diffusion.


Molecule of oxygen takes only a small fraction of a second to diffuse through the cytoplasm from the PM to the center of a cell.
Innermost mitochondria of a cell 20 micrometers in diameter will receive supplies of oxygen and food a fraction of a second after they pass PM.


-  If the cell is bigger?

Diffusion is fast and efficient over a short distance
Becomes slow and inefficient as distance increases
Hypothetical:  If mitochondria at center of a cell that is 20 cm in diameter would have to wait a month before receiving nutrients.
Die if one giant cell.


Cell DNA Limits Size

Most cells contain only one nucleus
If a cell doesn’t have enough DNA to program its metabolism, it cannot live.
Giant amoeba (Pelomyxa) more than one nucleus has evolved
Large amounts of DNA in many nuclei ensure that cell activities are being carried out.


Surface Area-To-Volume Ratio

As cell size increases, its volume increases much faster than its surface area
Smallest has 1 mm sides:

Surface Area =  6mm squared

Volume -  1 mm cubed

            -.  If the side of cell is doubled to 2mm

                                    Surface area increases 4 fold (6x2x2) =  24 mm squared

                                    Volume increases 8 fold =  8 mm cubed


If cell size is doubled, cell would require 8 times more nutrients and would have to expel 8 times more waste.
The surface area however would increase by a factor of only 4
PM would not have enough surface area through which oxygen, nutrients, and water could diffuse.
Cell starve to death or build up poisonous waste.


Cells divide before they reach that point.





Many cells are growing, dividing, and dying as you read this.

Old cells on feet and hands shed and replace

Cuts and bruises are healing

Intestines producing millions of cells each second

No net growth of Intestines


Tadpoles become frogs

Tomato plant grows and wraps around a stake


All organisms grow and change

Worn out tissues are repaired or replaced by new cells.





Middle of 19th Century scientists knew that cells divide

Microscopes improved =  see more detail


Most interesting discovery was just before cell division, several short, stringy structures suddenly appeared in the nucleus.

Even stranger, these structures vanished as mysteriously as the appeared after division was over.


Chromosomes =  contain DNA

Become darkly colored when stained

Chromosomes are the carriers of the genetic material DNA

Copied and passed from generation to generation of cells

Chromosomes always present, but too short to be seen until division


During most of cells life cycle =  chromosomes exist as chromatin

Chromatin =  strands of DNA wrapped around protein molecules.

Tangled up spaghetti


Before division the threads of chromatin begin to coil

The coils shorten and thicken, forming distinct chromosomes that are tightly packed with chromatin.


When DNA is uncoiled it can be actively involved in carrying out metabolic functions.






Fall follows Summer

Night follows Day

Low tide follows High Tide

Cyclical patterns in nature




Cell Cycle =  sequence of growth and division of a cell.


Two general periods =  period of growth and period of division


Interphase =  period of growth

most of cells life
grows in size and carries out metabolism
chromosomes duplicated in prep for division
chromosomes contain DNA
Important that new cells have copies of DNA


Mitosis =  period of division

nucleus then cytoplasm forms 2 daughter cells
each complete set of chromosomes





Busiest part of the Cell Cycle

Cell making ATP, repairing themselves, excreting waste

Making protein, new organelles, copying chromosomes

Carrying out Photosynthesis and Respiration


Divided into 3 parts:

cell grows in size and protein production is high
cell copies its chromosomes
mitochondria and other organelles made and parts needed for division are made





Cell division is a continuous process:  organized into 4 phases

Each phase merges into the next




first and longest phase
long stringy chromatic coils up into visible chromosomes
each duplicated chromosome made up of two halves
Sister chromatids =  DNA exact copies
Centromeres hold the sisters together


-  Nucleus begins to disappear

Nucleolus and Nuclear Envelope disintegrate (completely disappear)


-  Centrioles begin to migrate toward opposite ends (poles)

Centriole =  small, dark cylindrical structure made of microtubules and located just outside of nucleus
Only in Animal Cells


-  Spindles begin to form between centrioles

Spindles =  football shaped, cagelike structure, microtubules



Double chromosomes become attached to spindle fibers by centromeres
Chromosomes line up at the midline (equator)
One sister chromatid spindle extends to one pole and the other sister’s extends to the opposite pole.
Ensures each new cell gets an identical set of chromosomes



Separation of sister chromatids marks beginning
Centromeres split apart and chromatid sisters separate from each other
Start migration toward poles
Believed to move by the spindle fiber shortening and pulling the chromosomes.



Chromosomes reach opposite poles
Prophase changes are reversed
Chromosomes unwind to direct metabolic activities
Spindle fibers break down
Nucleolus and nuclear envelope reappear
Form around the new set of chromosomes
PM begins to separate the 2 new nuclei



Division of cytoplasm
Animal =  PM pinches along the equator

2 new cells formed =  identical

Plant =  Cell plate forms at the equator and splits the cell wall.




Genetic continuity

2 new identical cells (identical to parent cell)

Daughter cells carry out the same cellular processes and functions of the parent cell

Grow until divide


Intestines complete a cell cycle in 24-48 hours

Frog embryo less than an hour

Liver cells only once a year

Nerve cells never after they mature



Past 25 years intensive research into what causes a cell to divide

Still not fully known


Series of Enzymes

most biologist agree that it is the production of a series of enzymes that monitors cells progress from phase to phase
Some enzymes trigger progression of cell cycle (start cells)
Some enzymes inhibit progression (stop cells)


Cancer =  cell death or uncontrolled cell division
Result from a failure to produce certain enzymes or from over production or production of the wrong enzymes.



Enzyme production is directed by genes located on chromosomes (genetic given)
Gene =  segments of DNA that controls the production of a protein


A portion of Interphase just before DNA replication as being a key control period in the cell cycle.


Contact Inhibition

Cell-to-cell communication
Produce and secrete chemical signals
Normal cells allowed to grow in cultures will stop dividing when they come in contact with one another (fill up bottom).





Mistake in the cell cycle

Caused by changes in one or more of the genes controlling production of enzymes involved in the cell cycle.



Cancer cells affect normal cells

Form masses of tissue called Tumors

Tumors deprive normal cells of nutrients

In the final stage, cancer cells enter the blood stream and spread, forming new tumors in other organs.


Cancer is 2nd leading cause of death in US

Heart diseases is #1

Cancer can affect any tissue in the body

US main killers:  lung, colon, breast, and prostate

All on the rise


Environmental factors are involved

Different countries have different dominating cancers

US = breast cancer is high but low in Japan

China =  stomach cancer but low in US

When move to another country, more susceptible to that cancer

Cigarette smoke, air/water pollution, UV exposure

Source: Eric Atkins