Interphase When a cell isn t dividing, the nucleus usually just looks grainy, and chromosomes are present but not visible.

Transcription

1 The dance that chromosomes go through when cells divide is known as... Interphase When a cell isn t dividing, the nucleus usually just looks grainy, and chromosomes are present but not visible. Prophase Chromosomes become visible as strands; the nuclear membrane usually disappears. At the same time, two tiny organelles (the centrosomes) produce a set of fibers called the mitotic spindle. These are hard to see with ordinary microscopy, but special fluorescence techniques show them clearly (spindle fibers in green, chromosomes in blue) 1

2 Metaphase Chromosomes line up in center, along metaphase plate, moved into place by the spindle fibers. The spindle fibers are again hard to see with regular light microscopy, but here's a fluorescent picture... Anaphase Chromosomes break apart at the centromeres, and the separate chromatids move to opposite sides of the cell and another fluorescent micrograph... 2

3 Telophase... Chromosomes cluster into new nuclei and lose individuality; new nuclear membranes form.... and Cytokinesis Usually, the cell itself divides at this time a process called cytokinesis. (Occasionally this doesn't happen, and then you have a cell with two nuclei.) And here's the spindle fibers again... Cytokinesis (division of the whole cell) is different in plant cells, which can't "pinch apart" because of the cell wall. Instead, a new cell wall (faintly visible here) forms to divide the cell. 3

4 Chromatids At the start of mitosis, each chromosome in a cell consists of two chromatids. At the end, each chromosome consists of only one chromatid. But if you watch the daughter cells until they divide again... each daughter cell will start mitosis with two chromatids per chromosome. Somewhere during interphase, each chromosome must somehow make a new chromatid. We now divide a cell's life into the cell cycle. Interphase is divided into G 1, or growth (the cell grows), S, or synthesis (the cell creates new chromatids) and G 2 (the cell grows some more and prepares for mitosis). Significance Thought mitosis was bad? It gets worse... Mitosis happens in... reproduction of single-celled eukaryotes development (from a fertilized egg to an adult) physical growth maintenance Example: Every day, million cells in your body die, and must be replaced by new cells wound repair cancer (uncontrolled mitosis) 4

5 Meiosis To understand meiosis, you need some more background on chromosomes... Chromosomes in a cell are not all identical Some are long, some are short Some have the centromere near to the actual center (centric) and some have the centromere "off-center", nearer to one end (acentric) Chromosomes differ in the pattern of bands on each one (that can be seen with special staining techniques) It turns out that in a typical eukaryotic cell, there are two of each type of chromosome. A cell with two of each type of chromosome is said to be diploid. Chromosomes from a normal human female. There are 46 chromosomes, which can be sorted by size, centromere position, and band pattern into 23 pairs, numbered 1-22 and X. (More later on what X means just know now that there are 23 pairs.) Different animal and plant species have different numbers of chromosomes mice, for example, have 40 chromosomes, making up 20 pairs per cell. But although closely related organisms usually have similar chromosome numbers, there's no simple relationship between how "complex" an organism is and how many chromosomes per cell it has. Gametes Animals, plants, and many other eukaryotes form specialized cells for sexual reproduction called gametes Usually these come in two forms: large ones (eggs) and small ones (sperm) In many protists and fungi, all the gametes are the same size, and can't be divided into eggs and sperm (they might be designated + and, or something like that) Gametes only have one of each pair of chromosomes. This is called being haploid. 5

6 Meiosis Meiosis is the specialized process of cell division that produces haploid cells from diploid cells. In animals, this is how eggs and sperm are produced. (With plants, things get a little strange from our point of view; maybe I can talk about that later.) The basics were worked out by the Belgian scientist Edouard van Beneden in Another name is reduction division because unlike mitosis, meiosis has to reduce the number of chromosomes per cell by half So here's a human egg cell being swarmed by a number of human sperm cells. Both the egg cell and each sperm cell have one of each pair of human chromosomes: 23 chromosomes each. When an egg and one sperm fuse together, they form a cell known as a zygote, which now has two of each chromosome in this case 46 chromosomes. And that zygote will divide by mitosis into 2, 4, 8, 16, 32,..., 100 trillion cells each with 46 chromosomes. 6

7 The result is that you received one of each pair of chromosomes from your mother, and the other one of each pair from your father... and you'll pass on one of each pair of your chromosomes to each of your children. An error in meiosis can cause an egg or sperm to have two copies of one chromosome, or to be missing a chromosome. One result of this is shown here: the chromosomes of someone with three copies of chromosome 21 (two from one parent and one from the other). 7