Chapter+Nine

BIOL 160: General Biology Definition Worksheet #8: Chapters 9-10 Define the following terms related to mitosis and meiosis. 1. Allele : any of several forms of a gene, usually arising through mutation, that are responsible for hereditary variation. 2. Anaphase : the stage in mitosis or meiosis following metaphase in which the daughter chromosomes move away from each other to opposite ends of the cell. 3. Centromere : a specialized structure on the chromosome, appearing during cell division as the constricted central region where the two chromatids are held together and form an X shape. 4. Chromatin : the readily stainable substance of a cell nucleus, consisting of DNA, RNA, and various proteins, that forms chromosomes during cell division. 5. Chromosome : any of several threadlike bodies, consisting of chromatin, that carry the genes in a linear order: the human species has 23 pairs, designated 1 to 22 in order of decreasing size and X and Y for the female and male sex chromosomes respectively. 6. Crossing over : at prophase I of meiosis, reciprocal exchange of segments between two nonsister chromatids of a pair of homologous chromosomes; results in novel combinations of alleles. 7. Cytokinesis : The division of the cytoplasm of a cell following the division of the nucleus. 8. Diploid number : of many sexually reproducing species, having two chromosomes of each type in body cells; the two pair as homologues at meiosis. 9. Fertilization : fusion of a sperm nucleus and an egg nucleus, the result being a single-celled zygote. 10. Gene : unit of information for a heritable trait in DNA, transmitted from parents to offspring. 11. Haploid number : total chromsome number in cells with one of each type of chromosome characterisitic of the species. 12. Homologous chromosomes : one of a pair of chromosomes in body cells of diploid organisms; except for nonidenticle sex chromosomes, they have the same size, shape and gene sequence. 13. Interphase : in a eukaryotic cell cycle, the interval between mitotic divisions; the cell grows in mass, roughly doubles the number of cytoplasmic compenents, and replicates its DNA. 14. Meiosis : of eukaryotic cells, a nuclear division process that halves the parental chromosome number, to a haplid (n) number. Prerequisite to the formation gametes and sexual spores. 15. Metaphase ; of meiosis I, stage when all pairs of homologous chromsomes have become positioned at the equator of the bipolar microtubular spindle. Of mitosis or meiosis II, stage when all duplicated chromsomes are positioned at the spindle equator. 16. Mitosis : in eukaryotic cells, a nuclear division mechanism that maintains the parental chromosome number for forthcoming daughter cells. Basis of growth, tissue repair, and often asexual reproduction. 17. Prophase : of mitosis, the stage when all duplicated chromosomes in a cell start to condense, a microtubular spindle forms, and the nuclear envelope starts to break up. Duplicated pairs of centrioles move to topposite spindle poles. In prophase I of meiosis, crossing over also occurs. 18. Sister chromatids : the two members of a duplicated chromosome, each the same DNA molecule and associated proteins; the two stay attached at the centromere until separated from each other during mitosis or meiosis. 19. Telophase : The last stage of meiosis I; two haploid clusers of chromosomes have formed. The last stage of mitosis and of meiosis II, haploid clusters of chromosomes that decondense into threadlike form, and a nuclear envelope forms around each. 20. Tetrads : a group of four chromatids formed by synapsis at the beginning of meiosis.

1. Before a cell can divide and produce a cell that is viable (able to survive) it must first make a copy of the heredity instructions (DNA) so the new cell has the instructions necessary for producing the proteins that will ultimately become structural proteins or enzymes that will guide specific chemical reactions in the cell. The cell (parent cell) must also increase the cytoplasmic machinery (organelles, cytoplasmic fluid and cell membrane) so the next generation (daughter cell) can start up its own operation. 2. There are some key terms that need to be reviewed: A. GENOME: this is the entire set of genetic information contained within a cell. B. CHROMATIN: this is a diffuse mass of very long, very thin strands of DNA & protein molecules. DNA material within the nucleus is found in this form for the majority of the time since it must be unwound in order to be “read” and used by the cell to make protein molecules. C. CHROMOSOME: this is a single DNA molecule (and associated proteins). All human cells that contain DNA have a total of 46 chromosomes. D. GENE: a section of a DNA molecule that codes for a protein. 3. There are two mechanisms used by a cell for dividing the DNA (nuclear division). A. : used by somatic cells for growth, cell repair. 1. Results in two identical cells. (clones) B. _: used by germ cells to produce gametes. 1. Results in four cells each with half of the genetic information as the parent 4. The structure of a chromosome changes as a cell prepares to divide. A. As the cell prepares to divide, each chromosome must make an identical copy by undergoing DNA __. (review from chapter 13)  B. The two resulting strands are identical to each other and remain connected to each other at a region called the__ _ __and they look like a letter “H” when viewed under a microscope. C. Together the two strands are called__ , individually each half is called a CHROMATID. D. Each “letter H” is actually ONE chromosome, that has already made a copy of itself, but the two are still attached together. Later these two halves will separate from each other. E. One more time (this is important!) Since it is only possible to see individual chromosomes when they are very condensed and ready for the cell to divide, they are only visible as what “appears” to be a “pair” of chromosomes, however, in actuality this is considered __ONE__ Chromosome with an identical copy attached to it. 5. The majority of human cells (somatic cells) have a total of _ __chromosomes. These are only visible under a microscope when a cell is in the process of dividing, the rest of the time they are unwound, mixed together and are collectively referred to as chromatin. A. For safety’s sake we have a “back-up copy” of genetic instructions (one set from each parent) so it can also be said that there are__ “23 pairs” of chromosomes__. B. These “pairs of chromosomes”, the ones from mom (egg) and dad (sperm) are called HOMOLOGOUS Chromosomes. They are similar but NOT identical to each other. (At least we hope your mom and dad don’t look identical.) C. Because somatic cells contain two sets of genetic instructions, they are also called__ cells. 6. Gamete cells (sperm and egg) have only 23 chromosomes. They have only ONE set of chromosomes because after fertilization (union of the egg and sperm) the resulting cell will have the full complement of 46 chromosomes. Germ cells therefore are also called HAPLOID cells (half the number of a diploid cell).
 * __ CHAPTER 9 __**** : Cell Division and Mitosis **

7. Choose the most appropriate answer for each term.

2. _ haploid or //n// cell 3. _ diploid or //2n// cell 4. _ chromosome 5. _ nuclear division 6. _ germ cells 7. _ somatic cells 8. _ sister chromatids 9. _ mitosis and meiosis 10. _ chromatin || A. Cells set-aside for forming gametes used for sexual reproduction B. Processes used by eukaryotic cells to divide C. Any cell having two of each type of chromosome characteristic of the species D. Possess only one of each type of chromosome characteristic of the species E. One DNA molecule with attached proteins F. Package and sort out parent DNA molecules into new cell nuclei G. A small region on a chromosome where a DNA remains connected to an identical copy H. The two attached DNA molecules of a duplicated chromosome I. Body cells that reproduce by mitosis and cytoplasmic division J. All the DNA molecules unwound and mixed with their associated protein molecules ||
 * 1. _ centromeres

8. Each cell procedes through an orderly sequence of events that occurs each time a cell prepares to divide, known as the ___, resulting in the formation of two identical cells. This process is divided into three main stages: A.__ _ is the part of the cell cycle when the cell is carrying out the normal functions of the cell and prior to cell division it increases in size, forms additional organelles and copies the DNA. B. _ is the part of the cell cycle when the cell is actually dividing of the total amount of DNA in half so each new cell will have a complete set of DNA instructions. C. is the part of the cell cycle when the cytoplasm is divided, ultimately resulting in two new daughter cells. 9. It is important to look at these three stages in more detail. A. __:__ this is the time between cell division when the cell is carrying out the normal functions of the cell or preparing for cell division by increasing in size as well as duplicating needed organelles and “supplies” (including the DNA) to permit TWO cells to survive following cell division. This is usually the longest phase of the cell cycle as there is a lot that needs to be done. 1. During this phase of the cell cycle, DNA exists in the nucleus as long thin strands all mixed together as a diffuse mass called _. Individual chromosomes are not visible under the microscope during this phase, just a dark, round, mass of “stuff” in the center of the cell. 2. When viewing the cell under a microscope it may be possible to see one or more darker regions within the nucleus. These are the __, regions where the ribosomes are produced, indicating the cell is actively producing protein molecules. 3. There will also be a darkened area near but outside the nucleus. This area is called the centrosome, which contains two pairs of centrioles that are used to help separate the chromosomes during the next phase of the cell cycle. 4. This phase is divided into three stages (phases): a. G1 Phase (__ or Growth Phase #1): at this time the cell is actively carrying on the normal functions of the cell. Many cells remain in this phase and never move forward in the process. If a cell is preparing to divide, it is in this phase that it grows in size (additional cell membrane is formed and added to the existing membrane), forms additional organelles, and increases protein synthesis. b. __Phase: at this time the cell is producing a new copy of DNA by a process called__. c. G2 Phase (Gap or Growth Phase #2): at this time there is continued synthesis of proteins and organelles needed for cell division. This is a last chance to double check if the cell is actually ready to divide. These three phases are closely controlled and regulated by key enzymes to keep the body functioning correctly. Failure of these key enzymes is the main cause of most forms of cancer in the body. B. __: this is the part of the cell cycle where the DNA is separated and divided equally so that after the cell division is completed, the two DAUGHTER CELLS produced will be IDENTICAL to each other as well as IDENTICAL to the original cell prior to the G1 phase. 1. Throughout mitosis, the chromosomes become more and more distinctly visible when the cell is viewed under a microscope and eventually move apart, separating into two groups on opposite sides of the cell. This is accomplished with the help of the__ _ (also called the spindle apparatus) which consists of a series of microtubules produced by the centrioles. 2. Mitosis is divided into four distinct stages, though at times it is difficult to tell when the cell has transitioned from one to the next. Each stage has some very distinct characteristics to look for when viewing a cell under a microscope.

a. __:__ 1. The DNA (chromatin) condenses into distinct, “threadlike” strands clearly visible under a light microscope. Remember **Early**  that the DNA was actually copied during Interphase, so what actually becomes visible at this time is more correctly called , that are now ready for cell division. 2. The nuclear membrane begins to break into fragments near the end of this phase.  3. migrate to opposite sides of **Late** the nucleus. 4. Spindle microtubules begin to elongate from the centrioles and attach to the centromeres of each chromosome. 5. Perhaps a way to help remember this phase is to know this is where the chromosomes become more PRONOUNCED (clearly visible) when viewed under a microscope b. ___:  1. Some of the spindle microtubules pull some of the chromosomes toward one side of the cell while others pull some of the chromosomes toward the other side of the cell until they all line up in the middle of the cell. 2. The remaining microtubules push against each other forcing The two centrioles farther away from each other, stretching the cell. 3. Perhaps a way to help remember this phase is to know this is where the chromosomes MEET in the MIDDLE of the cell. c.__ __: 1. Some spindle microtubles continue to elongate, pushing the centrioles even farther apart, thus stretching the cell into an oval shape 2. Other microtubules shorten, pulling the sister chromatids away from each other and the midline, toward the centrioles. 3. Perhaps a way to help remember this phase is to know this is where the chromosomes are pulled APART. d.__ :  1. The spindle microtubules dissolve (disappear). 2. The nuclear membrane reforms around each cluster of chromosomes so two nuclei are forming. 3. Chromatids begin to uncoil and become more diffuse (becoming known as chromatin again). 4. or the splitting of the cytoplasm begins as the nuclear membrane is forming. In animal cells, the cell membrane begins to pull inward forming an hour glass or figure 8 appearance. In plant cells a dark line begins to form between the two nuclei that are forming. C. __: this is when the cytoplasm is divided, ultimately resulting in two new daughter cells. 1. In plant cells a__ _ (looks like a wall) forms in the middle of the cell to create two separate cells. 2. In animal cells the cytoskeleton fibers near the equator of cell membrane contract like pulling on a draw string, forming a cleft or indention in the cell membrane called a _ FURROW. The cell looks like a “figure 8” prior to dividing

10. Identify the stage in the cell cycle indicated by each number.  1. 2. 3. 4. 5. 6. 7. 8. 9. 10.