M E I O S I S

Mitosis and Meiosis

M E I O S I S

Why dont cells get bigger forever??Lets do a lab!With some math!Surface area = 6 (l x w)Volume = l x w x hCalculationsCell 2Sides: 1 cmSA=

Volume =

Cell 1Sides: .5 cmSA=

Volume =

Cell 3Sides: 2 cmSA=

Volume =

Limits to cell growthThe larger a cell becomes, the more demands the cell places on its DNA.In addition, the cell has more trouble moving enough nutrients and wastes across its cell membraneActivity

Surface Area to volume ratioWe want surface area to be bigger then volumeWHY? (Think about the lab)

So how do cells solve this problem?Before they get too largeThey divide!Into two daughter cellsCalled CELL DIVISIONWhat steps may be involved?

Two stagesStage 1: MITOSISDividing nucleusStage 2: CYTOKINESISDividing cytoplasm

ChromosomeWe have 46!Super condensed DNAIn mitosis, each chromosome gets copiedNow called two SISTER CHROMATIDS held together by a centromere

Animation 1Animation 2Sketch it out!

The Cell CycleSeries of events that cells go through as they grow and divideGrows, prepares to divide, and dividesTwo main partsInterphaseMitosis

Divided into 2 parts InterphaseG1 - Gap - Cell grows from last cell cycleS Synthesis of DNA (DNA makes a copy of itself)G2 Cell grows and gets ready for cell divisionG0 No cell division typical of cells like nerve cells that stop dividing at maturityMitosis M phaseProphaseMetaphaseAnaphaseTelophaseCytokinesis

Interphase Longest phase in the Cell Cycle over half the time is spent in this phase

MitosisProphase longest phase in MitosisCopied chromosomes pair up. Held together by Centromere (each is called a chromatid)Shortening up of chromosomes, makes them visibleReplicated Centrioles (of animal cells) migrate to opposite polesNuclear envelope & nucleolus start to break downSpindle fibers (microtubules) extend from centrioles to centromeres

Metaphase shortest part of MitosisChromosomes line up on the equator (middle)

Anaphase And away they go! Chromatids separate and go to opposite poles. Now called chromosomes!! Spindle fibers shorten up

Telophase the end Final phaseChromosomes reach opposite endsCell membrane pinches in (cleavage furrow) or Cell plate forms (if plants)Chromosomes uncoilNuclear membrane & nucleolus reappearSpindle fibers disassembles

Cytokinesis Dividing cellular contents. Occurs simultaneously with TelophaseMITOSIS OVERVIEW

Difference between Plant and Animal mitosisNo centrioles in plant cells

A cell plate forms in plant cells

Difference between Plant and Animal mitosisAnimal cells have a cleavage furrow thatsplits the two cells

How do cellsKnow when toFound important factorsContact prevents growinggrow?

Cyclin is a protein that regulates the timing of the cell cycle. Amount of cyclin in a cell rises and falls with the steps in the cell cycle.There are other internal and external proteins that are also involved the cell cycle.Cancer uncontrolled cell growth. Cells do not respond to normal signals and grow out of control. Cause masses called tumors.

CancerCancer occurs when cells do not respond to signals that tell them to stop growing

Count those chromosomesWe have 46If we made a baby, how many would they have?46 from mom4646 from dad + 46

92 for babyNo longer human!!92

What do we do?We go through MEIOSISProcess that takes our two sets of chromosomes (diploid number{2N}) and cuts them into one set (haploid number{1N})462323 from mom + 23 from dad = 46!Normal baby!

Meiosis StepsUnlike mitosis, meiosis has two stages of divisionMeiosis IMeiosis II

Meiosis I:Interphase I: Replication of chromosomes. Like chromosomes are called homologous chromosomes

Prophase I:The homologous chromosomes come together (Synapsis) and form a group of 4 called a tetrad. Twisting & exchange parts of adjacent chromatids will occur Crossing over. This results in genetic recombination and unlinks genes that are located on the same chromosome.HUGE GENETIC VARIETY!!!!!

Anaphase I & Telophase IChromosomes separate independently of each other. This is known as Independent assortment. Results in Random separation of chromosomes Genetic recombination!!!!!

At the end of Meiosis I, there are 2 - 1N cells

Meiosis IResults in 2 - 1N cellsMeiosis IIResults in 4-1N cellsNo interphaseMeiosis II Occurs immediately following Telophase I. NO INTERPHASE II !! 1N cells immediately go into Prophase II, then Metaphase II, Anaphase II, and Telophase IISplit into four 1N cells, each with unpaired chromosomesHaploid At the end of Meiosis II, there are four 1N cells.

Meiosis Animationshttp://www.sumanasinc.com/webcontent/animations/content/meiosis.htmlJohn KrykCells AliveYou Tubehttp://www.csuchico.edu/~jbell/Biol207/animations/meiosis.htmlMcGrawArizonaHow Mitosis and Meiosis Differ

GametogenesisCreation of gametes (sex cells) Spermatogenesis (testes) Makes 4 haploid sperm cells from 1 - 2N primary sperm cell. Looks like normal MeiosisOogenesis (ovaries)Produces ovum (eggs) from 1 diploid primary egg cell. Difference occurs during cytokinesis during meiosis I and meiosis II.Unequal distribution of the cytoplasm resulting in 1 large ovum and 3 small polar bodies. Polar bodies serve no function in animals but are needed in to plant embryos.

1st Meiotic Division2nd Meiotic DivisionComparing mitosis and meiosisBe sure to knowHow many divisions for each?How many starting cells?How many cells at the end?What N are those cells?