MEIOSIS … making gametes….

Meiosis is… Making the cells that make babies! (egg and sperm) The process of nuclear division that reduces the number of chromosomes by half Start with somatic cell End with gamete

What type of cells produce egg/sperm? Ovary and testes cells!

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

Cells with 1 set of chromosomes (gametes) are said to be HAPLOID (n) Each of your body cells has 2 sets of chromosomes – one from mom, one from dad. Any cell that has 2 complete sets of chromosomes is said to be DIPLOID (2n) Cells with 1 set of chromosomes (gametes) are said to be HAPLOID (n) When 2 cells come together in fertilization to make a zygote, each can only have 1 set of chromosomes. That way, the zygote will have 46 chromosomes. (23 + 23 = 46) (Instead of 46 +46 = 92!)

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

Homologous pairs. In each body cell you have 23 homologous pairs. 2 chromosomes are homologous if: - one is from mom, one is from dad - they have the same gene sequence on them.

Homologous Chromosomes have: 1. the same gene sequences. 2. The same banding pattern 3. The same position of the centromere.

Karyotype is used to determine If there is an abnormality in number or structure of the chromosomes (eg. Down’s syndrome) 2. The gender

HOW TO MAKE A KARYOTYPE Photograph a cell in metaphase. Cut out the chromosomes from the picture. Paste homologous pairs together according to size, banding pattern and centromere. Lay pairs out largest to smallest, sex chromosomes at end.

You Spermatogonial cell Oogonial cell 2n 2n egg (n) sperm (n) fertilization zygote MEIOSIS MEIOSIS MITOSIS You

Products of MITOSIS: Products of MEIOSIS: 4 unique haploid (n) cells 2 identical diploid (2n) cells Products of MEIOSIS: 4 unique haploid (n) cells   2n 2n 2n 2n n n n n

Meiosis Vocabulary Crossing Over: Genetic Recombination: Exchange of genetic material from homologous chromosomes during prophase I of Meiosis Genetic Recombination: Major source of genetic variation resulting from crossing over OR independent assortment

Synapsis increases genetic variability – and that’s a good thing… MEIOSIS I Interphase I – Period of cell growth and preparation for division (G1, S, G2) PROPHASE I - TETRADS APPEAR. The homologous chromosomes (mom’s and dad’s) come together with another homologous pair to form a tetrad -Crossing over occurs (SYNAPSIS) Synapsis increases genetic variability – and that’s a good thing…

Metaphase I: Tetrads (2 homologous pairs) line up on metaphase plate and lock on to spindle fibers  Anaphase I: Spindle fibers pull tetrad apart; one homologous chromosome goes to each side of the cell. Independent assortment occurs  

Telophase I: nuclear membranes form around the two new nuclei; Cytokinesis I: cell splits in two.

Interphase between the two divisions: NO replication!!! - so nothing changes with the chromosomes between telophase I and prophase II

MEIOSIS II

Prophase II: When chromosomes show up you will see sister chromatids. Metaphase II: Sister chromatids line up on equator Anaphase II: Sister chromatids are pulled apart. Telophase II: Nuclear membranes form.

Cytokinesis: cytoplasm divides Cytokinesis: cytoplasm divides. Total product is 4 cells, each with only a mom or Dad’s chromosome, in other words, half the normal chromosome number, or 4 monoploid (n) cells, or 4 gametes. Uneven cytokinesis in females: In oogenesis, get one big egg and 3 non-functional polar bodies: ( who die off) and 1 big functional gamete

OOGENESIS First division: results in one big primary oocyte and a little polar body. Second division: One secondary oocyte and 3 polar bodies. (polar bodies die)

Equal divisions producing four useable sperm Spermatogenesis Equal divisions producing four useable sperm

Meiosis vs. Mitosis (Make a chart) Used for Sexual Reproduction Produces Gametes (Egg & Sperm) Have 23 chromosomes 2 sets of divisions Begins with a diploid cell Homologous chromosomes pair up Result: 4 genetically different HAPLOID SEX cells Used for Growth and Asexual Reproduction Produces body cells Have 46 chromosomes Somatic cells One round of division Begins with diploid cells No crossing over Homologous chromosomes don’t pair up Results in 2 genetically identical DIPLOID SOMATIC cells

Compared to the number of chromosomes contained in a body cell of a parent, how many chromosomes would normally be contained in a gamete? F. the same number G. twice as many H. one-fourth as many J. half as many During mitosis, the chromosomes A. are located at the cell equator during prophase B. are located at the cell equator during telophase C. move toward the poles of the cell during anaphase D. move toward the poles of the cell during metaphase

http://www.cellsalive.com/meiosis.htm Other amazing videos!

. Which structures normally have a haploid number of chromosomes? A. Seed plant spores and animal zygotes B. Seed plant spores and animal gametes C. Seed plant zygotes and animal zygotes D. Seed plant gametes and animal zygotes  

Non-disjunction = Non-separation of chromosomes resulting in diploid or empty gametes.

Conditions caused by non-disjunction Downs syndrome = Trisomy 21 - 3 copies of chromosomes 21 equalling a total of 47 chromosomes. = 2n + 1 (chance of occurring in oogenesis increases with maternal age) Turners syndrome = Monosomy X - has only one X chromosomes totalling only 45 chromosomes in her body cells. (monosomy X) = 2n-1 Klinefelter’s syndrome = XXY - male which has an extra X chromosome = 2n + 1

Down’s syndrome – trisomy 21 A simple check for the presence of a bone in the nose could more accurately test unborn babies for Down's syndrome, scientists say. Researchers say combining the nose test with existing screening methods (amniocentesis, CVS) could lead to a five-fold reduction in the number of miscarriages linked to an invasive procedure used to confirm Down's syndrome.

Turner’s syndrome X -Caused by missing or incomplete X chromosome -Genes affected are involved with growth and sexual development -May fail to start puberty -Shorter than normal -Treatment = Hormone Replacement Therapy

QUIZ TIME… In streptomycin fungus n = 11. What is the diploid number for this species? If a horse egg has 98 chromosomes how many does a horse egg cell have? If n = 16 for goldfish how many chromosome in a fin cell? If 2n = 108 for black spruce trees then what is the haploid number? 2n = 14 for this species. Fill in the chromosome numbers for each cell.