Cell Reproduction
Chromosome Structure _DNA_ is a long, thin molecule that stores the information needed to direct the activities of cells. Genes are pieces of DNA that code information for particular activities. Genes are transmitted from parent cell to daughter cells. When a cell begins to divide, its DNA and associated proteins coil together into thick rod-shaped structures called _chromosomes_. Your chromosomes are about 60_% DNA and40% protein. During interphase, before cell division begins, the chromosomes double. The two copies of each chromosome are called _chromatids, and are held together by a protein disk called a centromere. Humans have 23_ sets of two chromosomes in each of their body (somatic) cells. _Gametes_ (egg and sperm) have only one of each of the 23 different kinds of chromosomes. Each set of two chromosomes is called a homologous pair. Homologous chromosomes are similar in shape and size, and contain genes for the same_ traits. Two chromatids of a doubled chromosome Chromatid Centromere ·
Chromosomes Affect Development Each human chromosome holds __________________ of genes. _________ of these genes must be present to form a complete human. Extra or missing chromosomes cause severe problems. ____________________, an extra chromosome #21, causes Down syndrome. ________________________, either by amniocentesis or by chorionic villi sampling, can determine if a baby has Down syndrome or other chromosomal abnormalities. Although rare, other changes can happen to the genes in chromosomes. These changes are called ___________________. Pieces of chromosomes can be missing (________________) or copied (____________________). Chromosome fragments can also rejoin the chromosome backwards (_________________) or join a non-homologous chromosome (_____________________). Chromosomes determine your sex. If you have two X chromosomes, you’re ______________, but an X and a Y make you a __________. ·
Mitosis and Cell Division WWhen bacteria reproduce, they simply split in half. This process is called _binary fission_. DDuring binary fission, the bacterium copies its _DNA_, forms a new cell membrane and a new cell wall. Afterwards, the cell splits in two. EEukaryotic cells undergo nuclear division, called mitosis_. TThe life cycle of eukaryotic cells is a repeating series of steps called the cell cycle_____________. G 1 Phase - cell growth The cell carries out its normal activities. This is most of the time between cell divisions. S Phase - DNA is copied. Chromosomes exist as chromatids. G 2 Phase - more growth and preparation for mitosis Mitosis Cyto- kinesis Interphase ·
What Happens In Mitosis? ________________ cells spend most of their lives in the G 1, S, and G2 G2 phases, which together are called___________________. At the end of interphase, the cell begins to build the structures needed to move chromosomes to opposite ends of the cell. In animal cells, a pair of organelles called __________________ move to opposite ends of the cell. As the centrioles separate, protein strands called ____________________ form between them. Other spindle fibers extend outwards from centromeres binding the______________________ together. When the spindle fibers are complete, each chromatid is attached to _________________ poles of the cell. Next, the ________________ holding together the chromatids separate, and the ___________________________, pulling the chromatids apart, and toward the poles of the cell. Each pole of the cell will receive a complete set of chromosomes. The last step of cell division is formation of two new __________________________ around the two sets of chromosomes, and a pinching inwards of the cell membrane to split the cell in half, called ________________________. ·
Stages of Mitosis Metaphase - Chromosomes move to the center of the cell and are aligned along the cell’s “equator.” Prophase - Doubled chromosomes become visible, nuclear envelope disappears, spindle fibers begin to appear. Anaphase - Chromatids separate while centromeres divide, separated chromatids move towards opposite poles of the cell Telophase - Chromosomes uncoil and nuclear envelopes form while spindle fibers disappear. Mitosis is complete. Cytokinesis now begins. ·
What Happens in Meiosis? In organisms that reproduce sexually, the sex cells (_______________) fuse together to produce the offspring or _____________. Gametes must have ________ of each type of chromosome, not two. In the sex organs of animals, gametes are produced by ____________, which is also called reduction division. In plants, meiosis is usually used to produce _________________. Meiosis consists of ________ successive nuclear divisions, but the DNA is only copied in ________ of them. Thus, one diploid cell produces four haploid cells. During meiosis, ______________________ can happen. This is when the arms of paired chromosomes cross over each other and pieces of the chromosomes break off, then reattach to the “wrong” chromosome in the pair. The result of cross-over is _______________________________ __________________________. Meiosis I Meiosis II ·
Meiosis I The two nuclear divisions of meiosis are divided into eight stages. Although they have the same names as the stages of mitosis, what happens in meiosis is different. Prophase I - Chromosomes condense and nuclear envelope breaks down, just like in mitosis. But homologous chromosomes also pair up and crossover can occur! Metaphase I - Pairs of homologous chromosomes (called tetrads) are moved by spindle fibers to the cell’s equator. Anaphase I - The homologues separate and are pulled by spindle fibers to opposite poles, however, chromatids do not separate. Each chromosome still consists of two joined chromatids. Nondisjunction can occur!. Telophase I - Individual chromosomes gather at each pole, and in most organisms, the cytoplasm divides. Daughter cells are haploid! ·
Meiosis II Prophase II - As in mitosis Chromosomes condense, nuclear envelope disappears, and spindle apparatus appears. Chromosomes do not double in meiosis II! Each of two dividing daughter cells from meiosis I is haploid. Metaphase II - Chromosomes are at the cell’s equator. Anaphase II - Centromeres divide, and chromatids (now called chromosomes) move towards opposite poles of the cell. Telophase II - Chromosomes uncoil at cell’s poles. Nuclear membranes and organelles reappear. Spindle apparatus disappears. The four daughter cells produced are haploid. ·