Cell Division Mitosis & Meiosis

Learning Target I can describe the basic process of mitosis. (Quality Core Standard B.1.j)

Why is it important to me? Cell Division What is it? Why do Cells do it? Why is it important to me?

Takes place in Regular Body Cells Cell Division Also known as Mitosis Takes place in Regular Body Cells Keeps Cells Living and Growing

Why Do Cells Divide? Two main reasons: 1. The larger a cell becomes, the more demands the cell places on it's DNA.

Food goes in 2. The bigger the cell gets the harder it becomes to move food and waste across the membrane Waste goes out

Protein Pumps This happens because the surface area and volume ratio does not stay the same as the cell size increases.

The cell's ability to either get substances from the outside or eliminate waste from the inside is related to the surface area of the cell membrane. (outside)

How much food and other material is required, and how much waste the cell produces and has to get rid of, is related to the volume of the cell. (inside)

As a cell gets bigger there comes a time when its surface area is not large enough to meet the demands of the cell's volume and the cell stops growing.

So, once cells reach a certain size they must divide in order to continue to function – or they will no longer be able to take in nutrients and eliminate waste.

Why Is Cell Division Important? 1. All Living Things are made of Cells

2. The Cell is the basic unit of Structure and Function in Living Things.

3. All Cells come from pre existing Cells These three statements are part of what?

You are a living organism, made of cells. In order to keep living, your cells must stay alive. In order for cells to keep living, they must divide and multiply

Phases of Mitosis

The Basic Phases of a Cell’s Life Cycle: Interphase G1 S G2 Mitosis Prophase Metaphase Anaphase Telophase Cytokinesis

The longest stage of a Cell’s life Interphase The longest stage of a Cell’s life The time spent between divisions Produces all materials required for growth Preparation for division

Interphase: G1 Growth Phase Cells do most of their growing and synthesize new proteins and organelles.

Interphase: S Phase (Synthesis Phase) Chromosomes are duplicated and DNA is synthesized If the cell gets to this phase, division will be completed.

Mitosis Interphase: G2 Shortest phase Many organelles and other molecules are made Next step – Mitosis

Mitosis: Cell Division The last part of the Cell Cycle is called Mitosis and has 4 phases during which the cell divides into 2 cells

The Cell begins the division process Prophase The Cell begins the division process 1.The nucleolus disappears 2.The nuclear membrane breaks apart

3. The chromosomes become visible 4. The spindle apparatus forms and attaches to the centromeres of the chromosomes

The Second Phase of Mitosis Metaphase The Second Phase of Mitosis The Nuclear Membrane is completely gone 2. The duplicated chromosomes line up along the cell's equator.

Anaphase The third phase of Mitosis Daughter chromosomes separate They are pushed and pulled toward opposite poles of the cell by the spindle fibers

Spindle Fibers

Telophase The nuclear membrane and nucleoli and nucleus reform. Cytokinesis is nearly complete,

The Cell Plate begins to form The Cell prepares for final division

Cytokinesis – The final stage of the cell cycle The cytoplasm, organelles, and nuclear material are evenly split and two new cells are formed. Cell Plate

The two new cells – each exactly like the other – are called Daughter Cells

Learning Target I can describe the basic process of mitosis (Quality Core Standard B.1.j)

Meiosis Why We Are Who We Are

Learning Target I can describe the basic process of meiosis. (Quality Core Standard C.1.f)

Meiosis Process of reduction division. Number of chromosomes per cell is cut in half. Occurs when two organisms join gametes to form the first cell of a new individual Gametes are sperm and egg; or pollen and ovule All of our cells are diploid meaning we have two copies of each chromosome

Meiosis One chromosome comes from the male parent; the other from the female: homologous Our sex cells: sperm and egg are haploid meaning these special cells only have half the chromosomal complement Involves two divisions of the nucleus called Meiosis I and Meiosis II

Prophase I Chromosomes condense Nuclear envelope breaks down Homologous chromosomes pair all along their length forming tetrads (4 chromatids) Crossing over occurs when parts of one chromosome are broken and exchanged with the same parts on its corresponding chromosome

Metaphase I Pairs of homologous chromosomes are moved by spindle fibers to the equator of the cell Homologous chromosomes, each made of two chromatids stay together

Anaphase I Homologous chromosomes separate Chromosomes of each pair are pulled to opposite poles by spindle fibers Chromatids do not separate at the centromeres—each is still made of two chromatids

Telophase I & Cytokinesis Individual chromosomes gather at each pole Cytokinesis occurs forming two new cells Both cells contain one chromosome from each pair of homologous chromosomes The chromosomes do not replicate between Meiosis I and II

Prophase II New spindle forms around chromosomes

Metaphase II Chromosomes line up at equator Attached at centromeres to spindle fibers

Anaphase II Centromeres divide Chromatids (chromosomes) move to opposite poles

Telophase II Nuclear envelope forms around each set of chromosomes Spindle breaks down Cytokinesis occurs Four haploid cells are created, each carrying one set of chromosomes

Meiosis Contributes to genetic variation by way of three mechanisms: Independent assortment Crossing over Random fertilization

Independent Assortment Each gamete receives one chromosome from each of the 23 pairs of homologous chromosomes The random distribution of homologous chromosomes is independent assortment Each of the 23 pairs independently separates so that 223 (about 8 million gametes are possible from just one cell)

Crossing over An exchange of DNA segments between two homologous chromosomes In Prophase I, the chromosomes have made a tetrad with each pair of chromatids joined at centromeres Crossing over occurs at chiasma When recombined offspring may have one trait from each parent: dad’s eyes and mom’s hair

Crossing over

Random Fertilization The zygote that forms a new individual is the result of random joining of gametes Each new sperm or new egg is vastly different from one another due to the different combination

Meiosis in Males

Meiosis in Males Process is called spermatogenesis Occurs in testes Can occur throughout males life once hits puberty At end of meiosis, four complete sperm cells are made

Meiosis in Females

Meiosis in Females Called oogenesis Occurs in ovaries Cytokinesis causes uneven splitting of cytoplasm in Meiosis I One cell gets most of cytoplasm and is much larger; small one is called polar body Meiosis II; same process again. One large cell is made; three small polar bodies die Large ovum is made that has enough nutrients for zygote during its initial formation

Learning Target I can describe the basic process of meiosis. (Quality Core Standard C.1.f)