# Cell Growth & Division Question: Why do cells divide?

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Cell Growth & Division Question: Why do cells divide?

Why do we need to make more cells?
We start out as one cell! We need to make more and more cells until we have about one hundred trillion!

Q: Who has bigger cells? A: Same cell size, RonRon has MORE!

Sea Urchin Cell Division
From One Cell to Many Sea Urchin Cell Division

Why do we need to make more cells?

Why are we one hundred trillion SMALL cells and not one hundred LARGE cells?
Differentiation, replaceable 100,000,000,000,000 cells because....

They need to be small!

I. Why do Cells Divide?

A. Cells need to stay small because:
The larger a cell becomes, the more demands on its DNA Trouble moving enough nutrients and wastes across the cell membrane Organization!

DNA “Overload” DNA is the cell’s “library” of information.
Imagine a very large city using one local library for all materials

A big bag is weaker, harder to find things
Large cell, difficult to maneuver organelles

More volume = bigger need
The larger the volume of the balloon, the weaker it is. The balloon skin stays the same.

Large surface area SPEEDS UP the movement of materials
B. What is Surface area? The total area of the surface of a three- dimensional object 2 cm What is the surface area of this cube? 2 cm 24 cm2 The villi of small intestines and the cristae of mitochondria are examples of maximizing surface area for chemical reaction and diffusion to take place Large surface area SPEEDS UP the movement of materials

200 cm3 C. What is Volume? The amount of 3-dimensional space
that an object occupies, “capacity” Large volume SLOWS down movement of materials What is the VOLUME of the shape here? 200 cm3 cm

D. Ratio of Surface Area to Volume
As the length of cell increases, volume increases faster surface area (cm3 compared to cm2) HIGH ratio desired: quick movement of materials Ex: 6000/1 is better than 2/1

It’s better to have lots
of small cells instead of fewer large cells!

II. Chromosomes All genes located in DNA in nucleus of eukaryotic cell
B. Chromosomes are condensed forms of DNA

Chromosomes Chromosome number is unique to every species
Humans: 46 chr. Chimpanzees: 48 chr. Yeast: 32 chr. Adders-Tongue Fern: 1440 chr.!

Anatomy of a Chromosome
Chromatids attached at the centromere Copied during Interphase D. After duplication phase, each chromosome consists of two identical “sister” chromatids

II. Cell Division A. 46 chr 46 chromosomes 46 chr B. Chromosome # stays the same [Cells growths, doubles chromosomes, then splits, forming two daughter cells with original # of chromosomes]

C. Common Locations for Cell Division
Intestinal lining- every 24 hours Skin Blood cells/bone marrow- 120 days Liver- sometimes

D. Cells that Rarely Divide (In G0 phase)
Muscle Cells Cardiac cells Kidney Nerve cells

III. Cell Cycle

Gap0 “resting phase”, cell is not growing
A. Interphase: “I-ball” 90% of the time! Gap “resting phase”, cell is not growing Gap cell grows, doubles organelles Synthesis duplication of the DNA in the cell's chromosomes Gap cell grows, microtubules assembled G0- is “quiescent” phase.. Some cells stay here after terminal differentiation, while others are here and can move into G1

C. Checkpoints G1 Checkpoint: DNA to be replicated is healthy; cell size G2 Checkpoint: Checks that DNA that was replicated is healthy; rest of cell ready for division M checkpoint: Chromosomes are properly attached to the spindle fibers.

What happens if the cell cannot pass through the checkpoint?
Repair the damage OR B) Self-destruct: APOPTOSIS (Programmed Cell Death) There are proteins in the cell that regulate these processes and determine which way the cell will go.

Now entering “M Phase”.. First stop, Mitosis!

Prophase- “pasta” Chromatin fibers condense
Nuclear membrane breaks down Spindle of microtubules forms from centrioles [animals only] Attach to chromatids on centromere

Metaphase- “middle” Chromosomes line up in the middle
Spindle fibers attach centrioles to centromeres Every sister chromatid has fiber attached to it Centriole Spindle

Anaphase: “away phase”, form “A’s”
Spindle fibers contract Pull sister chromatids apart The chromosomes continue to move until they are in two groups Each side has own copy of DNA Individual chromosomes

Telophase- “end phase”
Nuclear membranes reform at each pole Chromosomes unwind Spindle disappears

Last part of “M Phase”.. Cytokinesis!

During cytokinesis, the cytoplasm cuts in half

Cytokinesis in Plants In plants, a structure known as the CELL PLATE forms midway between the divided nuclei.

Cytokinesis in Animals
Animal cells contract an actin fiber across middle of cell and “pinch” into 2 new cells- called a “cleavage furrow”.

Twilight STUDIES IT TOO!

MITOSIS Video Daughter Parent Cell Cells
Why cant a cell just split into two? Parent Cell to daughter cells- Note daughter cells look genetically identical to parent cells DNA replication

V. Terms A. Spindle: network of microtubules that move chromosomes during mitosis and meiosis B. Equator: center line of cell where chromosomes line up during metaphase C. Poles: the opposite ends of cell D. Centrioles: animal cells only, move the spindle and chromosomes during division

E. Cleavage Furrow: the pinching in of animal cells during cytokinesis
F. cell plate: disk in plant cells that divide the cell into two daughter cells during cytokinesis G. Centromere: region where two sister chromatids are joined tightly together

“Apoptosis”: programmed cell death

VI. Results of Mitosis A. Production of 2 new daughter cells
B. Daughter cells are exactly the same as original parent cell C. Cell --> Tissue --> Organ --> Organ System --> Organism

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