Cell Growth and Division:

Mitosis vs. Meiosis and What’s going on the rest of the time

Why does a cell need to divide?• A little geometry: As an object grows, the volume

increases at a faster rate than the surface area

The same happens with a cell• What is the surface area?

• What is the volume?

• The more cytoplasm there is the more materials are needed.

• How do the materials enter?

Oh no… diffusion is back!• By limiting the ratio of membrane to cytoplasm you limit the “doorways” into the cell.

• Cell can’t get enough materials to support its large size

• Cell dies, unless it divides in half!

What do our cells need to do before they can divide?

• Get bigger• Make another copy of DNA• Make more organelles

Cell Cycle

Cell CycleG1 – growth and protein synthesis

S – DNA replication (copying the DNA)

G2 – Make organelles

M – Mitosis (Nuclear division) and

Cytokinesis – division of cytoplasm and membrane

What is DNA again?

• A Chain of nucleotides

• Twisted into a double helix (spiral)

• VERY LONG

• Contains ALL the recipes for every protein our body needs

• Recipes are called genes

Understanding DNA structure

• Most of the time out DNA is in the form of chromatin: strings of DNA wrapped around proteins called histones

Understanding DNA structure

• DNA is in chromatin form through G1.

• In S phase, each strand of chromatin is duplicated and the duplicated copies remain attached together at the centromere.

Understanding DNA structure

• During M phase the chromatin is folded into chromosomes

• DNA remains in chromosome form until cell division is over

Understanding DNA structure

Remember DNA contains the information needed to build an

organism

• Each chromosome contains some of the information.

• Each organism has a specific number of chromosomes.

Humans have 23 types of chromosomes and 2 of each type = total of 46 chromosomes

• One of each type came from your mom the other from your dad.

• Every cell in your body has all 46 chromosomes with the exception of egg/sperm cells

• Cells that contain two of each chromosome are called Diploid cells

• Cells that contain one of each chromosome are called Haploid cells

Chromosome Analogy

• Think of the Information in a cell as an Encyclopedia - Each chromosome is one book– Haploid = one set of info / one encyclopedia

A BCD E

Chromosome Analogy

Diploid = two different sets of info/ two different encyclopedias!

A B CD E A B CD E

Chromosome Analogy• Genome = total information in cell

– If a cell is haploid, the genome consists of all the information in one encyclopedia

–If Diploid, all the info in both encyclopedias

A B C D E ABCDE

Chromosome Analogy• Chromosome = One volume

– Genes: segments of DNA; each contains a specific message

–Genes are like Articles in encyclopedia

AF jkasdkfjh

Jadlfl he;

Kjadh fchw

Laksjdfh

Kasjdf;aj

Skdjfa;ie

F jkasdkfjh

Jadlfl he;

Kjadh fchw

Laksjdfh

Kasjdf;aj

Skdjfa;ie

Chromosome Analogy• Genes can have different

variations. The variations are called Alleles.

• Think of the alleles as two different articles on the aardvark!

AF jkasdkfjh

Jadlfl he;

Kjadh fchw

Laksjdfh

Kasjdf;aj

Skdjfa;ie

F jkasdkfjh

Jadlfl he;

Kjadh fchw

Laksjdfh

Kasjdf;aj

Skdjfa;ie

Chromosome Analogy• Homologous Chromosomes :

–Contain same genes, but may contain different alleles

–Example: both might contain Hair color gene but one might have brown hair info the other blonde

–One from Mom, one from Dad

Chromosome Analogy• Homologous Chromosomes =

A AHomologousChromosomes

Volume “A” from each encycl.

Chromosome Analogy

When DNA duplicates itself before cell division it makes two identical copies of each of chromosome

Identical Copies are Sister Chromatids

A AA A

SisterChromatids

SisterChromatids

Single, unduplicated Chromosome

1 duplicated chromosome, 2 sister chromatids

2 separated chromatids become individual chromosomes

Telomeres

Centromere

Mitosis

• One Fluid Event; no stopping and starting.

• BUT: for ease of study, we break it into 4 stages

• REMEMBER: all phases are continuous and may, in part, overlap

1. Prophase• Longest phase of Mitosis

• Chromatin folds up into Chromosomes which can now be seen

1. Prophase• Centrosomes separate and move

toward opposite poles

• These are parts of the cell that make microtubules

1. Prophase• Centrosomes start to form the

Mitotic spindle

–Made of microtubules

1. Prophase• Nuclear membrane breaks down

• Nucleolus disappears

2. Metaphase• Chromosomes line up along

equator• Spindle fibers attach to the

centromeres of chromosomes

3. Anaphase• Centromeres connecting sister

chromatids separate and one chromatid of each chromosome moves toward each poles

4. Telophase• Chromosomes begin to

unwind back into chromatin

• Nuclear Envelope reforms around chromosomes

• Spindle breaks down

• Nucleolus reappears

Cell Division is not yet Done!!

• What have we made?

– One Cell with Two complete Nuclei

What is left to do?

- Cytokinesis: Divide the cytoplasm and separate the cells

Cytokinesis

• Animal cells: Membrane pinches inward forming a Cleavage Furrow until it divides the cytoplasm into two equal parts

Cytokinesis• Plant cell: Cell

plate forms in the middle of the cytoplasm and extends toward the edges. Cell wall forms from this cell plate

Cell division is done!

• Now you have Two Identical daughter cells

Controlling Cell Division

• Cells know when they need to divide

–When?

•During growth

•Repair – injury

•Replacement (cells are not immortal!)

Controlling Cell Division• Cells know when they don’t need

to divide– Cells stop when they reach other cells

How do cells “know”?• Cells communicate by releasing

molecules

• To control cell division cells release proteins called “cyclins”

• Some cyclins are “Go” signals

–Trigger the cell division process

• Some cyclins are “Stop” signals

– shut down the cell division process

Loss of control

• What happens if the control signals don’t work?!?

– cells divide uncontrollably

–Pile up on top of each other

–Form big balls of cells called?????

»TUMORS!!!

Tumors

• Tumor cells do not respond to (or do not have) the body’s control signals

–missing a “stop” signal so cell division doesn’t stop

–Hyperactive “go” signal so cell is constantly dividing

Tumors Vs. Cancer

• Tumor = uncontrolled but isolated growth of cells

• Tumor cells become cancer when they start to invade healthy tissue–What if 1 cancer cell breaks off and enters the blood stream?

–Where ever it “lands” = new tumor = metastasis

That’s how Somatic (or body) cells divide!

• What would happen if we made Egg and Sperm cells this way?

–Way too much DNA

• What do we have to do when forming these cells?

–Reduce the amount of DNA

Meiosis

• Happens ONLY in sex cells

• Reduces information by ½

• Requires two different divisions

–How many cells at the end??

Meiosis begins the same as Mitosis

• Cell in G1 enters S phase.

• ALL DNA is copied

• Chromatin folds up to form 46 duplicated chromosomes

Meiosis IProphase I

- homologous chromosomes pair up forming tetrad; 4 chromatids together;

Meiosis IProphase I

- Centrosomes separate to poles

- Nuclear envelope breaks down

Meiosis IProphase I

- Crossing occurs between homologous chromosomes

Meiosis IMetaphase I

- Spindle fibers attach to chromosomes at metaphase plate

Meiosis IAnaphase I

- Fibers pull apart homologous chromosomes toward opposite poles

Meiosis IAnaphase I

- Fibers pull apart homologous chromosomes toward opposite poles

Meiosis ITelophase I

- Two cells are formed

- only half of the info of the Original cell

- 2N (diploid at start) N (haploid at end of meiosis I)

Meiosis IIProphase II

- Spindle reforms in each haploid daughter cell

Meiosis IIMetaphase II

- Chromosomes line up at the equator

Meiosis IIAnaphase II

- Sister chromatids are pulled to opposite poles

Meiosis IITelophase II

- Four haploid (N) cells created

Compare and Contrast Mitosis and Meiosis