MEIOSIS making sperm and egg cells DNA Passes

MEIOSIS (making sperm and egg cells…)

DNA Passes from Parent to Offspring • Two types of reproduction (making more of you): 1. Asexual reproduction → a single parent reproduces by itself http: //artfiles. art. com/images/-/Oxford-Scientific/Green-Hydra-Budding-Photographic-Print. C 12940880. jpeg – Parent and offspring (child) are genetically identical – e. g. , bacteria, many plants and fungi http: //lifeboat. com/images/bacteria. jpg

DNA Passes from Parent to Offspring 2. Sexual reproduction → 2 cells (different parents) unite to produce the first cell of the new offspring – parents and offspring are genetically unique

Review of Chromosomes • Chromosome → condensed DNA, containing all genes • Normal cells = diploid (2 n) → 2 copies of each chromosome (good idea! backup copy!) • In diploid cells, 1 copy came from each parent (people: 1 from mom, 1 from dad) Offspring have the same amount of DNA as their parents…so how can parents only pass on HALF their genetic material?

What is meiosis? • Meiosis → special cell division for sexual reproduction • Produces haploid (1 n) cells → 1 copy of each chromosome • Haploid cells = gametes (sperm/eggs) • Human gametes: – In ♂, meiosis produces (in testes) 4 sperm cells – In ♀, meiosis produces (in ovaries) 1 egg cell and 3 polar bodies (that later disintegrate) Mitosis: cells divide once Meiosis: cells divide twice

Why do ovary and testes cells go through meiosis? Meiosis is “reduction division. ” It reduces the chromosome Why do meiosis? number in the gametes by one half. From diploid (2 n) to haploid (n).

Chromosome numbers… In humans, 2 n = 46 A normal body cell has 46 chromosomes → 23 came from mom, 23 from dad; each set of 23 from mom is similar to the set of 23 from dad In humans, n = 23 A normal gamete has half the normal chromosome number (n) → just one set

Chromosome number questions 1. If a horse retina cell has 36 chromosomes, how many chromosomes does a horse sperm cell have? 2. If n= 4 for fruit flies, how many chromosomes does a wing cell have? 3. If the 2 n number of a pine tree is 86, how many chromosomes would be found in a pine ovum? 4. If n = 16 for goldfish, how many chromosomes are in a fin cell?

Homologous Pairs • Homologous pairs → Pairs of chromosomes with the same sequence of genes • One is from mom, one from dad • Genes are in the same order, but identical! – e. g. , blood type not

Phases of Meiosis • Remember mitosis? (I)PMAT… • In meiosis, each phase occurs twice – Meiosis I: Prophase I, Metaphase I, Anaphase I, Telophase I, Cytokinesis I – Meiosis II: Prophase II, Metaphase II, Anaphase II, and Telophase II, Cytokinesis II

Don’t freak out – I changed the order of the next slides…

Meiosis I • In Prophase I, homologous chromosomes pair up in a tetrad – same gene sequences, same banding pattern, same position of centromere • Crossing-over takes place

A Sketch of Chromosomes Crossing Over (Prophase I) • During crossingover, homologous chromosomes exchange pieces of DNA • Allows for a reshuffling of genetic material http: //images. google. com/imgres? imgurl=http: //www. phschool. com/science/biology_place/labbench/lab 3/images/crossovr. gif&imgrefurl=http: //www. phschool. com/science/biology_place/labbench/lab 3/crossovr. html&h=320&w=240&sz=3&hl=en&start=11&tbnid=Gq. FKVxw 1 ZWYop. M: &tbnh=118&tbnw=89&prev=/images%3 Fq%3 Dcrossing%2 Bover%2 Bchromosomes%26 gbv%3 D 2%26 svnum%3 D 10%26 hl%3 Den%26 safe%3 Dactive%26 sa%3 DG

Meiosis I • Meiosis I results in 2 haploid daughter cells

Meiosis II Meiosis • Each of the haploid daughter cells from meiosis I divide again to make 4 haploid cells.

Comparing Mitosis and Meiosis • The processes of mitosis and meiosis are very similar, but with some important differences: Mitosis l l l Includes only 1 cycle of cell division Occurs in body cells (somatic cells) No crossing over of chromosomes Results in two genetically identical diploid cells Metaphase: 1 duplicated chromosome per spindle fiber Meiosis l l l Includes 2 cycles of cell division (meiosis I and meiosis II) Occurs only in sex cells (gametes) Chromosomes cross over in Prophase I Results in four genetically different haploid cells Metaphase I: 2 duplicated chromosomes per spindle fiber

Now, let’s look at our flipbooks and compare!!

Interphase • DNA replication occurs • DNA is in chromatin form (unwound, like spaghetti)

Prophase I • DNA condenses to form chromosomes • Homologous chromosomes pair up to form tetrads • Crossing over takes place

Metaphase I • Homologous pairs line up along center of cell • 1 homologous chromosome and its duplicate are attached to each spindle fiber

Anaphase I • Homologous pairs separate, move to opposite ends of the cell

Telophase I • Nuclear membrane reappears • Spindle fibers disappear

Cytokinesis • Cytoplasm of the two cells separate • Meiosis I results in 2 haploid cells that are genetically different

Prophase II • The 2 haploid cells from Meiosis I will divide again

Metaphase II • Chromosomes (set of sister chromatids) line up along the center of the cell • 1 sister chromatid attaches to each spindle fiber

Anaphase II • Centromeres split, sister chromatids are pulled apart

Telophase II and Cytokinesis • Cytoplasm divides, resulting in 4 genetically different haploid cells