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Chapter 5:Cell Growth

and Division

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Section 5.1: The Cell Cycle• To this point we have learned what a cell is

and what parts make up a cell. • This chapter will explain how cells duplicate

themselves.• The duplication of cells is called the Cell Cycle.

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Section 5.1: The Cell Cycle• The cell cycle is a regular pattern of growth,

DNA replication, and cell division that occurs in eukaryotic cells.

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Section 5.1: The Cell Cycle• There are 4 main stages to the cell cycle:

1. Gap 1 Stage (G1)- cells grow, carry out normal functions and start replicating its organelles. – Most of a cell’s life is spent in (G1)

2. Synthesis- Cell’s start to copy the DNA code.3. Gap 2 Stage (G2)- cell continues normal growth

patterns, this is also one of the final check points to see if everything is “ok”.

4. Mitosis (M-Phase)-this is when the nucleus and the cell is actively dividing. – We will talk more about this later

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Section 5.1: The Cell Cycle• Although most all cells go through the cell cycle,

all cells go through at different rates. • The rate of cell division is dependent upon your

body’s need for certain cellsCell Type Approximate Life Span

Skin Cells 2 weeks

Red Blood Cells 4 months

Liver Cells 300-500 Days

Intestine-internal lining 4-5 Days

Intestine- muscle and other tissue 16 years

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Section 5.1: The Cell Cycle

• Remember this is just the life span we are talking about, not the amount of time spent in the cell cycle.– For most human cells (G1, S, G2 &M) takes 12 hours.

Cell Type Approximate Life Span

Skin Cells 2 weeks

Red Blood Cells 4 months

Liver Cells 300-500 Days

Intestine-internal lining 4-5 Days

Intestine- muscle and other tissue 16 years

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Section 5.1: The Cell Cycle• Cells that divide very rarely enter a stage that some scientists

call G0 State.– In G0, cells are unlikely to divide, although they continue to carry

out all the normal functions.• Some cells, such as neurons, seem to be in stage G0 their

entire lifespan and never divide. – Recently, scientists have discovered that neurons can divide, but

scientists don’t know why they don’t.– When we find how to make neurons divide, conditions like

paralysis and strokes would be temporary.

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Section 5.1: The Cell Cycle• Scientists have often wondered why cells are the size

that they are and why do they divide when they do.• Studying these two ideas have led scientist to some basic

understandings about cells:• Cells have upper and lower size limits!

-If cells were too small, then they would not be able to fit all of the necessary organelles and molecules into the cell.

-For example, a cell with too few mitochondria would not be able to function.

-If cells get to big, than the ratio of surface area to volume gets to far out of wack!

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Section 5.1: The Cell Cycle• When a cell increases in size, the volume of that cell

increases at a much faster rate than the size.• Because many substances like oxygen, nutrients, and waste

must enter and leave the cell, if the cell gets too big it will expend more energy getting materials into and out of the cell than the amount of ATP’s it can make.

• If a cell gets to big, it becomes to inefficient to survive and will eventually die. Instead, it divides to put the surface area to volume ration back in balance.

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Section 5.1: The Cell Cycle• Generally speaking, cells divide for 2 reasons:

1. Keep surface area to volume ratio.2. Repair and replace old cells.

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Section 5.2: Mitosis & Cytokinesis• In order for this whole process of the cell cycle to

work, the genetic material must be duplicated. • DNA is located on chromosomes, which are long

threads of DNA. • The amount of DNA in just one of your cells is

about 10 feet long.– So, How does it fit?

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Section 5.2: Mitosis & Cytokinesis• DNA goes through many phases to get prepared

to divide.• During interphase, DNA and the chromosome it is

on are loosely organized and it looks like spaghetti!

• At this point, the genetic material is called Chromatin.

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Section 5.2: Mitosis & Cytokinesis• Chromatin is very unorganized and is tangled

together. • Before mitosis and cell division, the DNA must

organize so it can get duplicated properly.• Chromatin starts condensing and coiling around

proteins called Histones. • These proteins organize DNA into specific

chromosomes. • Remember, each of these chromosomes have been

duplicated in the S-stage, so there are two of each.

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Section 5.2: Mitosis & Cytokinesis• It would be impossible for the cell to separate

chromatin equally, so the genetic information must organize.

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Section 5.2: Mitosis & Cytokinesis• Remember, the chromosomes

have already been duplicated so there are two of them and they form an “X” shape.

• Each “leg” is called a chromatid. • Each chromatid is held together by

a centromere.• Each end of a chromatid is called a

Telomere.

Telomere

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Section 5.2: Mitosis & Cytokinesis• Telomeres do not code for genes!– they simply keep the chromosome from unwrapping

or connecting with other chromosomes. • Once a duplicated chromosome has organized,

mitosis and cytokinesis can occur.• Mitosis has 4 phases and each can be indentified

by what the chromosomes are doing.

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Section 5.2: Mitosis & Cytokinesis1. Prophase: chromatin chromosomes, nuclear

membrane breaks down, spindle fibers start to appear, centrioles migrate to the poles.

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Section 5.2: Mitosis & Cytokinesis2. Metaphase: shortest phase… spindle attaches to

centromere as duplicated chromosomes line up on cell equator.

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Section 5.2: Mitosis & Cytokinesis3. Anaphase: sister chromatids separate… spindle

shortens…chromosomes move to the poles.

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Section 5.2: Mitosis & Cytokinesis4. Telophase: The opposite of prophase. Nuclear

membrane reforms, spindle disappears, chromosomes uncoil.

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Section 5.2: Mitosis & Cytokinesis• After telophase starts, cytokinesis

usually starts as well.• Cytokinesis is the splitting of the

cell into two daughter cells. • Cytokinesis, is an example of what

happens in animal cells. • In plant cells the cell membrane

can not pinch in because of the cell wall. Instead, a cell plate forms between the two nuclei.

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Section 5.3: Cell Cycle Regulation• Many factors affect the cell cycle. These factors

control the process of cell division. – External Factors:• Cell to cell contact:

– When cells come in contact with each other they stop dividing.

• Cells release chemicals:– This signals other cells to divide.

– Internal Factors:• Enzymes called Cyclin-Dependent Kinases:

– Carry cells through many check points to make sure the cell gets through the whole cell cycle.

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Section 5.3: Cell Cycle Regulation• When cell division is not

controlled, cancer is the end product.– Cancer is just uncontrolled

cell growth.– Cells usually grow and divide

until they come in contact with another cell and then stop.

– If the division doesn’t stop, tumors are formed.

– Tumors are big piles of rapidly dividing cells.

– There are two types of tumors

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Section 5.3: Cell Cycle Regulation• There are two types of tumors:

1. Benign: cancer cells remain clustered together.2. Malignant: cancer cells break away and spread to

other parts of the body (metastasize) • So what makes cancer cells harmful?

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Section 5.3: Cell Cycle Regulation• So what makes cancer cells harmful?– Cancer cells do not carry out the functions that

certain type of cell is supposed to.• For example, lung cells have to exchange oxygen and

carbon dioxide.– Cancer cells in the lungs don’t do this, so while these

cancer cells are taking up space, they are not functioning.

– Eventually the good cells die out and you are left with all cancer cells.

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Section 5.4: Other Types of Cell Reproduction• Not all cells reproduce by

mitosis!– Prokaryotes no not because

they have no nucleus. – Most prokaryotes reproduce

asexually by a process called binary fission.• The cell simply duplicates its DNA

and divides into roughly two equal cells.

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Section 5.4: Other Types of Cell Reproduction• Some eukaryotes also reproduce asexually.• Mitotic reproduction is common in simpler

plants and animals.– Mitotic reproduction can take several forms:• Budding- a small projection grows on the surface of the

parent organism, forming a separate new individual.• Fragmentation- a parent organism splits into pieces, each

of which can grow into a new organism.

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Section 5.5: Multicellular Life• In multicellular organisms, cells communicate

and work together in groups that form complex organisms.

Cells Tissues Organs Organ Systems Organisms• In multicellular organisms, cells go through the process

of cell differentiation.

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Section 5.5: Multicellular Life• Cell Differentiation:– Early cells of organisms are called stem cells.– Stem Cells:• These are cells that do not have a function• As the organism grows, the stem cells turn into all of the

cells your body needs.