If parents have two copies of each chromosome, how do they pass on only one to their offspring? MEIOSIS is the process of cell division that decreases the number of chromosomes to make sperm or egg cells Original cell Sperm or egg cells

The purposes of Meiosis: Split the total number of chromosomes in half so that only ONE chromosome from each pair is inherited from each parent. Produce variation in offspring by randomly sorting the chromosomes.

Meiosis Diploid (2n) (n = # of pairs of chromosomes) Haploid (n) Original cell Diploid (2n) (n = # of pairs of chromosomes) Sperm or egg cells (GAMETES) Haploid (n) (single chromosomes)

Fertilization Gametes: Zygote: (haploid sex cells) (diploid embryo) Sperm Egg Gametes: (haploid sex cells) Zygote: (diploid embryo) Fertilized egg

The Cell Cycle – “life” of the cell 4 stages in cell life: G1 – Growth, making proteins, doing work S – Synthesis – making a copy of DNA G2 – Growth (more) getting ready for cell division M – Mitosis or Meiosis – cell division

Mitosis Produces two identical daughter cells Each daughter cell has the same kind and number of chromosomes as the original parent cell Mitosis takes place AFTER DNA replication

Interphase (Resting stage) Chromosomes cannot be seen Chromosomes duplicate and double in number How do the chromosomes duplicate?

nuclear membrane disappearing Prophase nuclear membrane disappearing chromatids centromere Chromosomes become visible Each chromosome consists of two identical chromatids homologous chromosomes

Metaphase Centrosome (made of 2 centrioles) Chromosomes line up at the equator (centre of the cell) Spindles are formed to attach to the centromere of each chromosome

Anaphase Sister chromatids separate as individual chromosomes They move apart towards the opposite poles

nuclear membrane forming Telophase nuclear membrane forming Chromosomes gradually disappear Nuclear membrane is formed around each set of chromosomes

Can you identify the different stages of mitosis from the diagram below? anaphase interphase telophase metaphase prophase

Meiosis Same steps as Mitosis: Prophase, Metaphase, Anaphase, Telophase Meiosis happens TWICE!!! Metaphase I, Anaphase II, etc. http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter28/animation__how_meiosis_works.html

Some Vocab Homologous Chromosomes: chromosomes that contain the same genes, but may have different alleles (one is from mom, one is from dad) Sister Chromatids: identical copies of a single chromosome connected by a centromere

Meiosis 1

Close-up of Metaphase I Crossing over: segments of DNA are exchanged between homologous chromosomes

Law of Independent Assortment It is random where the chromosomes line up! Mom can be either side, Dad either side.

Meiosis II

Close up of Metaphase II

Summary of Meiosis Segregation of alleles: one chromosome in each of the 4 cells: only one copy of the allele Creates 4 haploid daughter cells Meiosis Video

Comparison of Mitosis and Meiosis

Mitosis vs. Meiosis Mitosis Meiosis Purpose To produce new cells for 1) growth 2) replacement of old or damaged cells To produce new gametes for sexual reproduction Start with 1 Diploid somatic cell (i.e., skin cells, liver cells, muscle cells, lung cells, etc.) 1 Diploid germ cell End with 2 identical Diploid somatic cells 4 haploid gametes Number of cell divisions that occur 1 2

Demonstration of mitosis and meiosis

Mendel’s Laws related to Meiosis Law of Segregation Law of Independent Assortment

Pop bead modeling of meiosis Follow directions from teacher Use pop beads to model each step in animation with your teacher Draw each step on your worksheet Meiosis step 1 Meiosis step 2 Meiosis step 3