meiosis

Sexual reproduction Cell division that produces gametes Vocabulary Allows for variation through recombination and crossing over Vocabulary Genes- instructions for specific traits located on DNA Gametes – sex cells (sperm and egg) Most DNA is in sperm and egg (some may be located in mitochondria or chloroplasts) Somatic cells- body cells Chromosomes are long strands of coiled DNA which have hundreds to thousands of genes (each with a specific nucleotide sequence) Loci – the gene location on the chromosome

More vocabulary Karyotype- a picture of paired condensed chromosomes (chromosomes are paired based on length, centromere location and pattern) Homologous- matching chromosomes (genes will be at the same loci of each chromosome) Alleles – genes located at the same loci (but they may have variation; ex. Brown eye color and blue eye color) Sex Chromosomes – X and Y chromosomes (in humans that is chromosome 23) Autosome – non sex chromosomes (1-22)

Alternative life cycles Fungi/some algae meiosis produces 1N cells that divide by mitosis to produce 1N adults (gametes by mitosis) Plants/some algae Alternation of generations: 2N sporophyte (diploid) produces 1N (haploid) spores, by meiosis spore divides by mitosis to generate a 1N gametophyte gametes then made by Gametophyte mitosis which then fertilize into 2N sporophyte

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Meiosis stages https://biologydictionary.net/wp-content/uploads/2016/12/Meiosis-Stages.jpg

Prophase I (1 cell ) Nucleus disappears, spindle fibers form, and the chromatid pairs visible. Chromatid pairs find their homologous pair, forming a tetrad (bivalent). Homologous chromosomes are 2 of the same type of chromosomes. Crossing over - 2 homologous chromosomes switch pieces of DNA. Chiasma site of crossing over

A little more on prophase 1 Cohesins hold sister chromatids together At specific points along non sister chromatids, DNA is broken A synaptonemal complex holds the homologous chromatids tight to one another (synapsis) DNA is reattached but to the non- sister chromatid Chiasmata are visible https://study.com/cimages/multimages/16/duplicate4.png

Metaphase I (1 cell ) Tetrads meet at the middle of the cell. One pair faces each pole

Anaphase I (1 cell ) The tetrads split apart Homologous chromosomes split moving toward opposite poles Chromatids move as a unit

Telophase I and Cytokinesis (2 cells ) Two cells form Each have a full set of chromosomes as chromatid pairs. Two haploid daughter cells Cleavage furrow or Cell Plate form

Prophase II (2 cells ) Spindle Fiber forms Chromatid pairs continue moving to the center of the cell.

Metaphase II (2 cells ) The chromatid pairs meet at the middle. (Similar to Mitosis) Chromosomes are not genetically identical (due to crossing over in Meiosis 1)

Anaphase II (2 cells ) The chromatid pairs split apart.

Telophase II and Cytokinesis (4 cells) Each of the two cells split making a total of 4 cells. Each cell contains 1/2 the normal number of chromosomes. Humans- Spermatogenesis – 4 sperm cells Oogenesis- 1 egg and 3 polar bodies

Meiosis benefits Genetic Variation DNA Backup and Repair Increased Biodiversity Disease Resistance DNA Backup and Repair Backup copy of instructions in case one is not correct Recombination process can repair DNA

Origins of Genetic Variation, I Independent assortment: homologous pairs of chromosomes position and orient randomly (metaphase I) and nonidentical sister chromatids during meiosis II Combinations possible: 2n (n the haploid number of the organism)

Origins of Genetic Variation, II Crossing over (prophase I): the reciprocal exchange of genetic material between nonsister chromatids during synapsis of meiosis I (recombinant chromosomes) Random fertilization: 1 sperm (1 of 8 million possible chromosome combinations) x 1 ovum (1 of 8 million different possibilities) = 64 trillion diploid combinations!

Meiosis mistakes Non disjunction Translocation/Inversion/Deletion Some forms of leukemia Non disjunction Chromatids fail to separate in either anaphase 1 or 2 Resulting in an extra chromosome Tuner Syndrome XXX Kleinfelder's Syndrome XXY Downs Syndrome (Trisomy 21) https://en.wikipedia.org/wiki/File:Translocation-4-20.png

https://upload. wikimedia https://upload.wikimedia.org/wikipedia/commons/thumb/4/4a/Chromosomal_translocations.svg/525px-Chromosomal_translocations.svg.png

Cyclin – protein that activates enzymes to initiate replication Growth Factors – proteins that bind to plasma membrane receptors and trigger mitosis (can be hormones) Hormones – Steroid hormones bind to DNA via nuclear hormone receptor proteins Cyclin – protein that activates enzymes to initiate replication Stimulates transcription and replication factors Ras Cyclin – verifies cell is proper size Ensures cells have time to grow and differentiate P53 – Tumor Suppressor Protein checks DNA for damage before replication Stops cell cycle if DNA is damaged Many cancers associated with a defective p53 gene MAD1 – Verifies spindles attach to kinetochore Ensures proper distribution of chromosomes to daughter cells