Chapter 10

Limits to cell size

Ability of DNA to control cell functions◦ Think of only having one set of directions and

tools to build an entire city. Would all the workers be able to get directions and enough tools to do their job?

◦ When a cell gets too big, its DNA cannot meet the needs of the entire cell

Surface Area to Volume ratio

When a cell gets bigger, the volume and surface area increase….

BUT ~ the volume increases much more quickly

This means that there is relatively LESS cell membrane for a big cell than a little one

Why do you think this might be a problem?

Look at these two “cells.” The one on the left is 5X5 and the one on the right is 8X8. Which one has the higher surface area to volume ratio?

Organisms keep growing, even though cells generally stay about the same size….How?

Cells divide!

A big organism’s cells are about the same size as a little organism’s cells, there are just more cells.

The art of making more cells

Cells constantly go through a cycle of growth and division

This cycle is divided into two parts: Interphase and Cell Division

Cell Division consists of mitosis and cytokinesis, but the cell must prepare for division during interphase

G1 (gap 1)◦ Cells increase in size and produce new proteins

and organelles S phase (synthesis)

◦ DNA is synthesized and chromosomes are duplicated in preparation for mitosis

G2 (gap 2) ◦ Organelles and molecules needed for mitosis are

produced

After G2, mitosis occurs. But first we need to know some terms!◦ Mitosis◦ Cytokinesis◦ Chromatid

It is important to realize the human cell has 46 chromosomes normally (23 pairs)

When it gets ready for mitosis, it doubles this and makes 46 pairs of homologous sister chromatids (During S phase)

•Centromere•Centriole•Spindle

Prophase Metaphase Anaphase Telophase

Cell division is not complete after mitosis….

Cytokinesis

Chromatin condenses to form chromosomes Centrioles separate and move to opposite

ends of the cell Spindle begins to form Nuclear envelope breaks down and

nucleolus disappears

Chromosomes line up at the center of the cell

Spindle fibers connect to each of the chromosomes at the centromere

Think of the boxers all meeting in the middle of the ring to touch gloves

The sister chromatids separate into single chromosomes

Chromosomes are pulled to opposite ends of the cell by the spindle

Think of Ana, the ring girl, pushing the boxers apart

Chromosomes have moved to opposite ends of the cell and start to unravel into loose DNA

Two separate nuclear envelopes start to form Spindle begins to break down Nucleolus reappears in both nuclear envelopes Cytokinesis begins here….

See the cell membrane start to pinch together?

Even though mitosis is complete, cell division is not!

The cytoplasm must now divide between the two new cells

In animal cells, the cell membrane simply “pinches off” until 2 new cells are created

In plant cells, a cell plate forms between the 2 new cells, creating a cell membrane. Then a cell wall forms.

Why is it important?

Normal cells have ways to tell when they should stop growing

What do you think will happen to the cells below when they touch each other and have no more room on the plate?

This is an example of EXTERNAL REGULATORS of the cell cycle

Cells respond to proteins on other cells and stop growing◦ This kind of regulator is helpful in wound healing

and embryo growth

This is when the cell itself regulates the cell cycle

For example, the cell will produce certain proteins when it has replicated its chromosomes.

This tells the cell that it is time to proceed with mitosis.

Cancer. Cells no longer respond to the proteins

(cyclins) and will reproduce continuously This causes uncontrolled cell growth, or

tumors. Tumors that invade the rest of the body and

cause direct damage are known as cancer