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Transcript
Meiosis A reduction division Meiosis Multi-celled organisms produce gametes (cells) through the process of meiosis. In humans and many other organisms, these cells are called ova(eggs) and sperm. When an ovum and a sperm combine, a new organism is formed with a full set of chromosomes, half of which came from the father, and half of which came from the mother. So what is a chromosome anyway? Meiosis is a type of cell division where one cell divides to form new cells, each of which have half the number of chromosomes as the original (ex – Humans have ? normally?). This condition is called haploid. Cells with the original number of chromosomes are called diploid. Another way to say this is that each new daughter cell gets half of the parent cell’s DNA. Humans have 23 pairs of chromosomes, for a total of 46. In each pair, one chromosome comes from mom and one comes from dad. Both members of the chromosome pair are the same size, shape, and carry the same genetic information. There are two main stages of meiosis: Meiosis I and Meiosis II. Each of these is divided into four stages: Prophase, Metaphase, Anaphase and Telophase Before the cell starts dividing, it has to complete Interphase – What happens during this very important time? Meiosis We refer to the 23 pairs as homologous pairs. Why? During the earliest stage of meiosis, the members of each pair will temporarily attach together to form a structure called a tetrad. Why are there four instead of two? homologous pair tetrad Prophase I Very similar to Prophase of mitosis. What happens during Prophase? Chromatin does what? N guys do what? Spindle fibers? Chromosomes (tetrads) do what? Centrioles do what? There are a few extras to add! The tetrads actually exchange some of their DNA in a process called crossing-over (Swapping spit – Disgusting!!) Metaphase I _______, Tetrads not individual chromosomes, line up on the equator of the cell. Spindle fibers attach _______ to the centromeres of chromosomes. Anaphase I Spindle fibers contract tetrads apart. and pull _____ Entire double stranded chromosome pulled to opposite ends of the cell. Chromosome number is now half of original (haploid). Telophase I Cell plate (plant) or cleavage furrow (animal) divides the _________. cytoplasm Cell usually goes directly into Prophase II, but if not, chromosomes relax, nuclei reform, and spindle fibers break down. half Each daughter cell has _____ of the number of chromosomes as the original. Chromosomes not to scale Summary of Meiosis II The mechanics of Meiosis II are identical _______ to Mitosis. Each haploid daughter cell produced by Meiosis I divides again, so that four haploid cells are produced altogether. In _____________, spermatogenesis all four cells survive to produce viable sperm. In _________, oogenesis only one cell survives to produce a viable ovum. Overview Tetrads form (Pro I) Crossing over can occur (Pro I) Tetrads line up and are separated (Meta I) Daughter cells are haploid Produces gametes (ova and sperm) Compare Critical differences between Mitosis and Meiosis: Tetrads form (Pro I) Crossing over can occur (Pro I) Tetrads line up and are separated (Meta I) Daughter cells are haploid Only occurs in germ cells Produces gametes (ova and sperm) Mitosis vs. Meiosis I Left: Metaphase of MITOSIS: no pairing!!!! Right: Metaphase I of Meiosis: homologous chromosomes Pair UP!