Draw and label a cell with: 2 pairs of homologous chromosomes Each chromosome is attached to its sister chromatid Draw and label the 2 daughter cells produced from this cell by mitosis

                                                                   

A special case of cell division Meiosis A special case of cell division

Mendel’s “Factors” Characteristics of parents are transferred to offspring through DNA (packaged in cells as chromosomes)

Genome - Chromosome - Gene -

Your cells have 23 pairs of chromosomes: 1 - 22 plus X from Mom 1 - 22 plus X or Y from Dad TOTAL = 46 chromosomes Are you XX or XY?

Diploid = Cells with 2 copies of each chromosome (2N) Homologous chromosomes = carry the same genes (but may have different alleles for the trait because each is from a different parent: Aa)

Diploid Human Chromosomes T O S M E

Answer this! How are homologous chromosomes the same? How are they different? What is the difference between autosomes vs. sex chromosomes? homologous chromosomes vs. sister chromatids Somatic (body) cell vs. Gamete cell

Problem: If diploid cells from parents simply combine at fertilization to make offspring (that’s you), the chromosome number would double with each generation 46  92  184  etc. Why doesn’t that happen?

Diploid organisms produce specialized cells for reproduction Gamete cells = eggs and sperm Gamete cells have only ONE copy of each chromosome pair Haploid - one copy of each chromosome in cell (N)

Answer this! Explain the difference between a diploid cell and a haploid cell. If a chimpanzee has 48 chromosomes in body cells, how many chromosomes are in chimpanzee gametes (eggs or sperm)?

Meiosis Produces haploid cells with unpaired chromosomes Used only to make eggs and sperm (gametes) Occurs only in reproductive organs Ensures variability in offspring

Daughter cells: Haploid (1N - 1 copy of each chromosome) Have unique combinations of chromosomes Do not have homologous pairs 4 produced with each round of meiosis ARE NOT IDENTICAL TO PARENT!

Remember: DNA must replicate during “S” phase of cell cycle Each chromosome is attached to sister chromatid at centromere Sister chromatids separate in meiosis II.

Answer this: What is the difference between homologous chromosomes and sister chromatids? Which are produced during the “S” phase of the cell cycle? What is a centromere?

2 Divisions of Meiosis Meoisis 1 Homologs separate into 2 daughter cells Each cell gets 1 homolog (1 copy of each chromosome) Sister chromatids are still attached at centromere.

Meiosis II Sister chromatids separate into 2 daughter cells Each cell is haploid (1N) 2 X 2 = 4 cells

Stages of meiosis are similar to mitosis Prophase – chromosomes pair up Metaphase – chromsomes line up Anaphase – chromosomes separate Telophase – chromosomes re-group at opposite sides of the cell

2 Types of Cell Division have Different Goals!!!!!! Mitosis Meiosis  2 cells Daughter identical to original cell Diploid Growth & Repair MOST CELL DIVISION IS MITOSIS  4 cells Daughter cells NOT like original cell Haploid Reproduction ONLY Eggs & Sperm ONLY

Answer this! Which type of cell division produces growth? How are the cells produced in meiosis different from the cells produced in mitosis? Explain why meiosis is called a “reduction division” – what is reduced? (and why is this necessary?)

http://www.youtube.com/watch?v=D1_-mQS_FZ0

http://www.sumanasinc.com/webcontent/animations/content/meiosis.html http://biologyinmotion.com/cell_division/

Key points of Meiosis  4 daughter cells Daughter cells are haploid Daughter cells have unique combinations of chromosomes Daughter cells do not have homologous pairs Meiosis creates gametes (sperm and eggs) Meiosis ensures variability in offspring

Answer this: If a cell with 6 chromosomes (total)divides to form gametes, how many chromosomes will the resulting gametes each have?

Independent Assortment Depending on how the chromosomes arrange during metaphase I, the cells formed can have a variety of different chromosome combinations.

Why go to all this trouble? Variability! – Meiosis mixes up chromosomes in the gametes: You can have any combination of chromosomes 1 – 22 Your gametes, 1 possible combination: 1,2,3,4,5,6,7,8,9,10,11,12,13,14, 15,16,17,18,19,20,21,22

Variety is the spice of life! 223 possible combinations just by independent assortment of chromosomes > > > 8,388,608 Duggar Family

Mix it up even more! Additional variation occurs by crossing over between chromosomes Homologous chromosomes can switch pieces during prophase I. This adds to the variability of the gametes formed.

Crossing Over During Prophase I: Chromosomes partner to form tetrads Arms cross over Pieces break off and re-attach to other chromosome Result: Unique! New! Assortment of traits

Answer this: Explain two ways meiosis produces daughter cells that have different combinations of genes from each other. Why is variation important for evolution to occur?

Meiosis Hoedown

Meiosis Now do the meiosis hoedown! http://www.phschool.com/science/biology_place/biocoach/meiosis/premei.html

http://www.scienceprofonline.org/cellcyclemeiosislecturetopic