THE CELL CYCLE AND CELL DIVISION

LS Chapter 3

The Cell Cycle

• Every organism begins as a single cell• In animals, including humans: sperm fertilizes egg

• Fertilized egg is called a zygote

• How, then, does the zygote become millions or trillions of cells?• Cells divide

• 1 cell becomes 2, which become 4, which become 8 and so on

• Cells have a life of their own• They grow, develop, then divide

• This is called the cell cycle

Some Terms That May Prove Confusing• Centriole-The cell structure that assists in cell division

• Made up of microtubules

• Chromatin- Thin, unwound strands of DNA• Present in Interphase

• Chromatids-DNA that is wound into a tight structure• There are 2, and they are called _______________________________

• Chromosomes-Sister chromatids that are attached to one another

• Centromere-The place at which the sister chromatids attach to form the chromosome

Phases of the Cell Cycle

• IPMAT

• Interphase

• Mitosis• Prophase• Metaphase• Anaphase• Telophase

• Cytokinesis

Interphase (Intermission)• The resting phase of the cell

• Cells spend a majority of their lives in interphase

• The growth and development stage

• 3 parts of interphase• G1 Phase (Gap phase 1)

• Cell grows and functions as normal• S phase

• DNA duplicates• The cell then has 2x as much DNA as

it normally does• DNA is in the form of chromatin

(thin strands)• G2 Phase (Gap phase 2)

• Cell grows again, and finishes duplicating organelles

Mitosis

• The period of nuclear division• Results in 2 identical nuclei

• 4 parts (PMAT)• Prophase• Metaphase• Anaphase• Telophase

Prophase (Prologue)

• Chromatin coils into X-shaped chromosomes • Chromosomes are made of 2

identical sides called sister chromatids

• Nucleolus disappears

• Nuclear membrane disappears

• Centrioles begin to move to pole of the cell

• Spindle begins to form• Spindle is made of microtubules

Metaphase (Middle)

• Spindles attach to centromere of the chromosome and pull chromosomes into the middle (equator) of the cell

• Very important because it ensures that, when the chromosomes split, exactly ½ of each chromosomes (1 sister chromatid) will move to each end of the cell• When the cell splits, it will then

have exactly the same DNA as the parent cell (the one that split into the 2 new ones)

Anaphase (Apart)

• Spindle fibers shorten

• Sister chromatids are pulled apart at the centromere

• One sister chromatid moves to each pole (end) of the cell

Telophase (Two)

• Spindle breaks down

• Chromatids uncoil into chromatin

• A nucleus forms around each of the 2 sets of chromatin • One forms on each side of the

cell• This results in 2 identical nuclei

• The cell begins to split

Cytokinesis

• Occurs immediately after mitosis

• Cyto=Cell

• Kinesis=movement

• The cytoplasm splits• Results in 2 identical cells

• Called the daughter cells

• Happens differently in plant and animal cells• In animals, the cytoplasm pinches in until it splits• In plants, this cannot happen due to the cell wall

• Instead, a structure called a cell plate forms, splitting the cells

• After cytokinesis, cells are back in G1 and the cycle is ready to repeat.

Results of Cell Division

• Cytokinesis results in 2 new, identical cells

• Some unicellular organisms use a similar method as their reproduction• Called binary fission, the parent cell splits into 2 daughter cells, each being an

organism

• Multicellular organisms use cell division to make new cells for several reasons• Growth

• As you grow, you must add new cells• Zygote splits into 2, which split into 4, which split into 8…etc.

• Repair• Damage to the body kills cells

• They need to be replaced• This could be a scrape, a cut, a broken bone, etc.

• Replacement• You lose thousands of cells a day, and they need to be replaced with new ones

Not in your Book!• Why is it important to understand the cell cycle?

• Good to know how you get your cells…• More important is what happens when the cell cycle goes wrong

• The cell cycle results in the production of new cells• It regulates when cells are formed and makes sure that there are no

issues• However, sometimes DNA can be damaged and the cell begins to

divide out of control• Leads to a mass of cells in an area

• Called a tumor

• The cells in the tumor crowd out and steal nutrients from healthy cells• Sometimes they are also able to break free and move throughout the body

• This results in one of the most deadly diseases…• Cancer

• Cancer can be defined as uncontrolled cell division

Levels of Organization

• AtomsMolecules

• MoleculesOrganelles

• OrganellesCells

• CellsTissues

• TissuesOrgans

• OrgansOrgan systems

• Organ systemsOrganisms

Prokaryotes vs. Eukaryotes

• Prokaryotes and some unicellular eukaryotes only get to the cell level

• Some eukaryotes, such as sponges, only get to the tissue level

• Others have organs, but not organ systems

Cell differentiation

• The process by which cells become different kinds of cells, doing a specific job

• All cells have identical DNA• However, different cells need different DNA instructions to do their job

• Different genes turn on in different cells to make them different • A gene is a segment of DNA that contains instructions

• Stem cells are unspecialized cells that are able to become many different types of cells• Many are present in embryos, and are able to become any number of

cells for the developing organism• Harvested stem cells may have huge potential for treatment of a

number of diseases