Meiosis A reduction division Meiosis n n Multicelled

Meiosis A reduction division

Meiosis n n 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?

n n 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, in which the cell contains only 1 set of chromosomes, is called haploid. Cells with the original number of chromosomes are called diploid. Diploid cells contain 2 sets of chromosomes. 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. n In each pair, one chromosome comes from mom and one comes from dad. n Both members of the chromosome pair are the same size, shape, and carry the same genetic information. We call these homologous chromosomes. n

n There are two main stages of meiosis: Meiosis I and Meiosis II. n Each of these is divided into four stages: Prophase, Metaphase, Anaphase and Telophase (sound familiar? ) n Before the cell starts dividing, it has to complete Interphase – What happens during this very important time? (be specific-write

Meiosis We refer to the 23 pairs n We refer to the 23 pairs as homologous pairs. Why? n 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 n n n Very similar to Prophase of mitosis. What happens during Prophase? n Chromatin does what? n Spindle fibers? n Chromosomes (tetrads) do what? n Centrioles do what? There a few extras to add! n The tetrads actually exchange some of their DNA in a process

Metaphase I _______, Tetrads not individual chromosomes, line up on the equator of the cell. Spindle fibers n _______ attach to the centromeres of chromosomes. n

Anaphase I Spindle fibers contracttetrads and pull _____ apart. n Entire double stranded chromosome pulled to opposite ends of the cell. n

Telophase I & Cytokinesis n n Cell plate (plant) or cleavage furrow (animal) divides the cytoplasm _____. Chromosome number is now haploid, half of original. 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 sca

Summary of Meiosis II n The identical mechanics of Meiosis II are _______ to Mitosis. n Each haploid daughter cell produced by Meiosis I divides again, so that four haploid cells are produced altogether. males n In _______, all four cells survive to produce viable (living) sperm. females n In _____, only one cell survives to produce a viable ovum (egg).

Compare n n n n Critical differences between Mitosis and Meiosis: Tetrads form (Pro I) Crossing over can occur (Pro I) Tetrads line up together (Meta I) and are separated (Ana 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!

Independent Assortment

Chromosome Mutations n Can occur during meiosis n Some harmful, some not n Bigger effect than gene mutations n 2 types: Structural mutations (coming later) and Numerical Mutations

Numerical Mutations n n changes in the number of chromosomes in a zygote Usually results from nondisjunction- failure of chromosomes to separate Monosomy- zygote receives only 1 of a particular chromosome EX: female being XO or getting 1 # 6 chromosome

Numerical Mutations n Trisomy- zygote receives 3 chromosomes of a particular kind n Down’s Syndrome - extra 21 st chromosome

Klinefelter’s Syndrome n XXY - 47 Males n The most common sex chromosome disorder. n ~1/1000 males n Males are usually infertile.

Numerical Changes Polyploidy- none of the chromosomes separate and the zygote gets 3 or more sets of chromosomes n EX: instead of 23 pairs, you get 46 pairs of chromosomes…this means 92 chromosomes!!! n

How many chromosomes does a human have? 46 (23 pairs) n What are pairs 1 -22 called? n Autosomes n What is the 23 rd pair called? n Sex chromosomes n Male? Female? n XY XX n

Can you find the mistake?

Can you find the mistake?

Can you find the mistake?