MEIOSIS (making sperm and egg cells…)

DNA Passes from Parent to Offspring Two types of reproduction (making more of you): Asexual reproduction → a single parent reproduces by itself Parent and offspring (child) are genetically identical e.g., bacteria, many plants and fungi http://artfiles.art.com/images/-/Oxford-Scientific/Green-Hydra-Budding-Photographic-Print-C12940880.jpeg http://lifeboat.com/images/bacteria.jpg

DNA Passes from Parent to Offspring 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 (2n) → 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 (1n) 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

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

Chromosome numbers… In humans, 2n = 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 If a horse retina cell has 36 chromosomes, how many chromosomes does a horse sperm cell have? If n= 4 for fruit flies, how many chromosomes does a wing cell have? If the 2n number of a pine tree is 86, how many chromosomes would be found in a pine ovum? 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 not identical! e.g., blood type

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 crossing-over, 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/lab3/images/crossovr.gif&imgrefurl=http://www.phschool.com/science/biology_place/labbench/lab3/crossovr.html&h=320&w=240&sz=3&hl=en&start=11&tbnid=GqFKVxw1ZWYopM:&tbnh=118&tbnw=89&prev=/images%3Fq%3Dcrossing%2Bover%2Bchromosomes%26gbv%3D2%26svnum%3D10%26hl%3Den%26safe%3Dactive%26sa%3DG

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 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 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