The Cell Cycle

Your cells are dividing…• You need new cells for:

– replacing all of the cells that are dying.– Repairing injury– New growth (getting taller/wider)

• Facts:

• You replace all of your skin every 27 days.

• 300 million cells in your body die and must be replaced every minute.

50,000 of the cells in your body will die and be replaced with new cells,

all while you have been reading this sentence!

From beginning to end…• The cell cycle begins when the cell is

formed

• Cell grows increase cytoplasm till it can’t hold anymore….or else…

• The cycle ends when the cell divides and forms new cells.

Before cells divide…1) Before a cell can divide it must make

an extra copy its DNA.

2) This is called Replication -DNA (deoxyribonucleic acid) contains all

the information that tells the cell how to make proteins.

DNA REPLICATION

• The DNA is normally long and stringy.(chromatin)

• The cell winds up the DNA into chromosomes (chromosome shape of an X).

• The cell has to an entire copy the DNA so that each new cell will have an identical copy of the DNA.

Chromosome Numbers• Number of chromosomes has nothing to do with

the complexity of organisms.

• Example: Fruit Flies have 8 chromosomes, potatoes have 48 chromosomes, humans have 46 chromosomes, a single cell protozoa 1600, chickens have 78.

• For human we have 46 chromosomes (23 pairs) During Replication the number doubles to 92.

Making More Prokaryotic Cells

• Bacteria undergo binary fission

• Binary fission is a type of asexual reproduction.

• Binary fission copies the circular DNA and results in two cells that are equal in size.

Eukaryotic Cells and DNA

• Eukaryotes are usually much larger and more complex than prokaryotic cells.

• The number of chromosomes in the cells of eukaryotes differ depending on the organism

Human Chromosomes

• Humans have.. 46 chromosomes

• Each pair are made 23 homologous chromosomes pairs.

Karyotype Chromosome Map

Making More Eukaryotic Cells• Stage 1: Starts just after division

– cytoplasm grows – toward the end the DNA condense and

makes an extra copy. (replication)

• The two copies are called sister chromatids.

• Chromosomes are held together at the centromere. (in the center) Connects all 2 sister chromatids.

Making More Eukaryotic Cells

• Stage 2: The chromatids separate in a process called mitosis.

• Mitosis ensures that each new cell receives a copy of each chromosome.

Stages of Mitosis

• Interphase

• Prophase

• Metaphase

• Anaphase

• Telophase

• Cytokinesis

Interphase

• Before mitosis begins…– The cell grows (cytoplasm) – the chromosomes are

replicated (copied).

• Each chromosome now consists of two chromatids.

Prophase• The nuclear envelope

breaks apart • Chromosomes

condense to rod-like structures (like an X).

• The centrioles move to opposite sides of the cell

• spindle fibers form between centrioles.

Metaphase

• Chromosomes line up along the equator (middle) of the cell.

• Each chromosome attaches to a spindle fiber.

Anaphase• The chromatids

separate

• Chromatids are pulled to opposite sides of the cell by the fibers attached to the centrioles.

Telophase• The nuclear

membrane forms around the two sets of chromosomes

• Chromosomes unwind, the fibers disappear.

• mitosis is complete.

Cytokinesis• In plant cells, a cell

plate forms between the two cells (this is will become the cell wall)

• In animal cells a cleavage furrow forms as the two cells pinch apart. Cell Plate

Cleavage furrow

Cytokinesis• Once mitosis is completed, the cytoplasm splits

in tow.

• This results in two identical cells that are also identical to the original cell from which they were formed.

• After cytokenesis, the cell cycle is complete and they will begin the cell cycle again.

http://www.sep.alquds.edu/biology/scripts/Biology_english/part_3_4_files/image010.jpg

Stages of Mitosis

Significance of Cell Division (mitosis and fission)

• Produces 2 cells that are genetically identical to the parent cell.

• In unicellular organisms is asexual reproduction.

• In multicellular organisms: -Primarily--growth and repair

How are Mitosis and Binary Fission Different? Binary

Fission is…• Is not mitosis (no phases).

• Has no spindle fibers or microtubules of any kind.

• The DNA does not condense into a distinct chromosome