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 Cycle G1 – 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

Think of the Information in a cell as an Encyclopedia 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 B C D E

Diploid = two different sets of info/ two different encyclopedias! Chromosome Analogy Diploid = two different sets of info/ two different encyclopedias! A B C D E A B C D E

Genome = total information in cell 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 A B C D E

Chromosome = One volume Chromosome Analogy Chromosome = One volume Genes: segments of DNA; each contains a specific message Genes are like Articles in encyclopedia A F jkasdkfjh Jadlfl he; Kjadh fchw Laksjdfh Kasjdf;aj Skdjfa;ie F jkasdkfjh Jadlfl he; Kjadh fchw Laksjdfh Kasjdf;aj Skdjfa;ie

Think of the alleles as two different articles on the aardvark! Chromosome Analogy Genes can have different variations. The variations are called Alleles. Think of the alleles as two different articles on the aardvark! A F 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

Homologous Chromosomes = Chromosome Analogy Homologous Chromosomes = Volume “A” from each encycl. A A Homologous Chromosomes

Chromosome Analogy When DNA duplicates itself before cell division it makes two identical copies of each of chromosome Identical Copies are Sister Chromatids Sister Chromatids Sister Chromatids A A A A

1 duplicated chromosome, 2 sister chromatids Single, unduplicated Chromosome 1 duplicated chromosome, 2 sister chromatids 2 separated chromatids become individual chromosomes

Centromere Telomeres

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

Meiosis I Prophase I - Centrosomes separate to poles - Nuclear envelope breaks down

Meiosis I Prophase I - Crossing occurs between homologous chromosomes

Meiosis I Metaphase I - Spindle fibers attach to chromosomes at metaphase plate

- Fibers pull apart homologous chromosomes toward opposite poles Meiosis I Anaphase I - Fibers pull apart homologous chromosomes toward opposite poles

Meiosis I Anaphase I - Fibers pull apart homologous chromosomes toward opposite poles

Meiosis I Telophase I - Two cells are formed (after cytokinesis) - only half of the info of the Original cell - 2N (diploid at start)  N (haploid at end of meiosis I)

Meiosis II Prophase II - Spindle reforms in each haploid daughter cell

Meiosis II Metaphase II - Chromosomes line up at the equator

Meiosis II Anaphase II - Sister chromatids are pulled to opposite poles

Meiosis II Telophase II - Four haploid (N) cells created

Compare and Contrast Mitosis and Meiosis