Bacteria are able to divide on a much faster schedule than eukaryotic cells. Some

Cell division plays many important roles in the lives of organisms. Cell division is at the heart of the reproduction of cells and organisms because cells come only from preexisting cells. Some organisms reproduce through asexual reproduction, and their offspring are all genetic copies of the parent and of each other. Others reproduce through sexual reproduction, creating a variety of offspring, each with a unique combination of traits.

Prokaryotes reproduce by binary fission. Prokaryotic cells reproduce asexually by cell division. As the cell replicates its single chromosome, the copies move apart; the growing membrane then divides the cell.

The large, complex chromosomes of eukaryotes duplicate with each cell division. A eukaryotic cell has many more genes than a prokaryotic cell, and they are grouped into multiple chromosomes in the nucleus. Each chromosome contains one very long DNA molecule associated with proteins. Individual chromosomes are visible only when the cell is in the process of dividing; otherwise, they are in the form of thin, loosely packed chromatin fibers. Before a cell starts dividing, the chromosomes duplicate, producing sister chromatids (containing identical DNA) that are joined together. Cell division involves the separation of sister chromatids and results in two daughter cells, each containing a complete and identical set of chromosomes.

The cell cycle multiplies cells.

Cell division is a continuum of dynamic changes. Mitosis distributes duplicated chromosomes into two daughter nuclei. After the chromosomes are coiled up, a mitotic spindle made of microtubules moves them to the middle of the cell. The sister chromatids then separate and move to opposite poles of the cell, where two new nuclei form. Cytokinesis, in which the cell divides in two, overlaps the end of mitosis.

Cytokinesis differs for plant and animal cells. In animals, cytokinesis occurs by a constriction of the cell (cleavage). In plants, a membranous cell plate splits the cell in two.

Anchorage, cell density, and chemical growth factors affect cell division. Most animal cells divide only when stimulated by growth factors, and some not at all. Growth factors are proteins secreted by cells that stimulate other cells to divide. In laboratory cultures, most normal cells divide only when attached to a surface. They continue dividing until they touch one another.

Growth factors signal the cell cycle control system. A set of proteins within the cell controls the cell cycle. Signals affecting critical checkpoints in the cell cycle determine whether a cell will go through the complete cycle and divide. The binding of growth factors to specific receptors on the plasma membrane is usually necessary for cell division.

Growing out of control, cancer cells produce malignant tumors. Cancer cells divide excessively to form masses called tumors. Malignant tumors can invade other tissues. Radiation and chemotherapy are effective as cancer treatments because they interfere with cell division.

Review: Mitosis provides for growth, cell replacement, and asexual reproduction.When the cell cycle operates normally, mitotic cell division functions in growth, replacement of damaged and lost cells, and asexual reproduction.

Chromosomes are matched in homologous pairs. The somatic (body) cells of each species contain a specific number of chromosomes; for example, human cells have 46, consisting of 23 pairs (two sets) of homologous chromosomes. The chromosomes of a homologous pair of autosomes carry genes for the same characteristics at the same place, or locus. X and Y chromosomes are only partly homologous.

Gametes have a single set of chromosomes. Cells with two sets of homologous chromosomes are diploid. Gameteseggs and spermare haploid cells with a single set of chromosomes. Sexual life cycles involve the alternation of haploid and diploid stages.

Meiosis reduces the chromosome number from diploid to haploid. Meiosis, like mitosis, is preceded by chromosome duplication, but in meiosis, the cell divides twice to form four daughter cells. The first division, meiosis I, starts with synapsis, the pairing of homologous chromosomes. In crossing over, homologous chromosomes exchange corresponding segments. Meiosis I separates the members of each homologous pair and produces two daughter cells, each with one set of chromosomes. Meiosis II is essentially the same as mitosis: In each of the cells, the sister chromatids of each chromosome separate. The result is a total of four haploid cells.

Mitosis and meiosis have important similarities and differences. Both mitosis and meiosis begin with diploid parent cells that have chromosomes duplicated during the previous interphase. But mitosis produces two genetically identical diploid somatic daughter cells, while meiosis produces four genetically unique haploid gametes.

Independent orientation of chromosomes in meiosis and random fertilization lead to varied offspring. Each chromosome of a homologous pair differs at many points from the other member of the pair. Random arrangements of chromosome pairs at metaphase I of meiosis leads to many different combinations of chromosomes in eggs and sperm. Random fertilization of eggs by sperm greatly increases this variation.

Homologous chromosomes may carry different versions of genes. The differences between homologous chromosomes are based on the fact that they can bear different versions of genes at corresponding loci.

Crossing over further increases genetic variability. Genetic recombination, which results from crossing over during prophase I of meiosis, increases variation still further.

A karyotype is a photographic inventory of an individuals chromosomes. To prepare a karyotype, white blood cells are isolated, stimulated to grow, arrested at metaphase, and photographed under a microscope. The chromosomes are arranged into ordered pairs so that any chromosomal abnormalities can be detected.

An extra copy of chromosome 21 causes Down syndrome. Trisomy 21, the most common chromosome number abnormality in the United States, results in a condition called Down syndrome.

Accidents during meiosis can alter chromosome number. An abnormal chromosome count is the result of nondisjunction, which can result from the failure of a pair of homologous chromosomes to separate during meiosis I or from the failure of sister chromatids to separate during meiosis II.

Abnormal numbers of sex chromosomes do not usually affect survival. Nondisjunction of the sex chromosomes during meiosis can result in individuals with a missing or extra X or Y chromosome. In some cases (such as XXY), this leads to syndromes; in other cases (such as XXX), the body is normal.

New species can arise from errors in cell division. Nondisjunction can produce polyploid organisms, ones with extra sets of chromosomes. Such errors in mitosis can be important in the evolution of new species.

Alterations of chromosome structure can cause birth defects and cancer. Chromosome breakage can lead to rearrangementsdeletions, duplications, inversions, and translocationsthat can produce genetic disorders or, if the changes occur in somatic cells, cancer.

Complete the following table to compare mitosis and meiosis

If an intestinal cell in a grasshopper contains 24 chromosomes, then a grasshopper sperm cell contains ______ chromosomes. a. 3 d. 24 b. 6 e. 48 c. 12

Which of the following phases of mitosis is essentially the opposite of prophase in terms of nuclear changes? a. telophase d. interphase b. metaphase e. anaphase c. S phase

A biochemist measured the amount of DNA in cells growing in the laboratory and found that the quantity of DNA in a cell doubled a. between prophase and anaphase of mitosis. b. between the G1 and G2 phases of the cell cycle. c. during the M phase of the cell cycle. d. between prophase I and prophase II of meiosis. e. between anaphase and telophase of mitosis.

Which of the following is not a function of mitosis in humans? a. repair of wounds b. growth c. production of gametes from diploid cells d. replacement of lost or damaged cells e. multiplication of somatic cells

A micrograph of a dividing cell from a mouse showed 19 chromosomes, each consisting of two sister chromatids. During which of the following stages of cell division could such a picture have been taken? (Explain your answer.) a. prophase of mitosis d. anaphase of mitosis b. telophase II of meiosis e. prophase II of meiosis c. prophase I of meiosis

Cytochalasin B is a chemical that disrupts microfilament formation. This chemical would interfere with a. DNA replication. b. formation of the mitotic spindle. c. cleavage. d. formation of the cell plate. e. crossing over.

It is difficult to observe individual chromosomes during interphase because a. the DNA has not been replicated yet. b. they are in the form of long, thin strands. c. they leave the nucleus and are dispersed to other parts of the cell. d. homologous chromosomes do not pair up until division starts. e. the spindle must move them to the metaphase plate before they become visible.

A fruit f ly somatic cell contains 8 chromosomes. This means that ______ different combinations of chromosomes are possible in its gametes. a. 4 d. 32 b. 8 e. 64 c. 16 1

If a fragment of a chromosome breaks off and then reattaches to the original chromosome but in the reverse direction, the resulting chromosomal abnormality is called a. a deletion. d. a nondisjunction. b. an inversion. e. a reciprocal translocation. c. a translocation. 1

Why are individuals with an extra chromosome 21, which causes Down syndrome, more numerous than individuals with an extra chromosome 3 or chromosome 16? a. There are probably more genes on chromosome 21 than on the others. b. Chromosome 21 is a sex chromosome and chromosomes 3 and 16 are not. c. Down syndrome is not more common, just more serious. d. Extra copies of the other chromosomes are probably fatal. e. Chromosome 21 is more likely to produce a nondisjunction error than other chromosomes. 1

An organism called a plasmodial slime mold is one large cytoplasmic mass with many nuclei. Explain how such a megacell could form. 1

Briefly describe how three different processes that occur during a sexual life cycle increase the genetic diversity of offspring. 1

Discuss the factors that control the division of eukaryotic cells grown in the laboratory. Cancer cells are easier to grow in the lab than other cells. Why do you suppose this is? 1

In the light micrograph below of dividing cells near the tip of an onion root, identify a cell in interphase, prophase, metaphase, anaphase, and telophase. Describe the major events occurring at each stage. 1

Compare cytokinesis in plant and animal cells. In what ways are the two processes similar? In what ways are they different? 1

Sketch a cell with three pairs of chromosomes undergoing meiosis, and show how nondisjunction can result in the production of gametes with extra or missing chromosomes. 1

Suppose you read in the newspaper that a genetic engineering laboratory has developed a procedure for fusing two gametes from the same person (two eggs or two sperm) to form a zygote. The article mentions that an early step in the procedure prevents crossing over from occurring during the formation of the gametes in the donors body. The researchers are in the process of determining the genetic makeup of one of their new zygotes. Which of the following predictions do you think they would make? Justify your choice, and explain why you rejected each of the other choices. a. The zygote would have 46 chromosomes, all of which came from the gamete donor (its one parent), so the zygote would be genetically identical to the gamete donor. b. The zygote could be genetically identical to the gamete donor, but it is much more likely that it would have an unpredictable mixture of chromosomes from the gamete donors parents. c. The zygote would not be genetically identical to the gamete donor, but it would be genetically identical to one of the donors parents. d. The zygote would not be genetically identical to the gamete donor, but it would be genetically identical to one of the donors grandparents. 1

Bacteria are able to divide on a much faster schedule than eukaryotic cells. Some bacteria can divide every 20 minutes, while the minimum time required by eukaryotic cells in a rapidly developing embryo is about once per hour, and most cells divide much less often than that. State several testable hypotheses explaining why bacteria can divide at a faster rate than eukaryotic cells. 2

Red blood cells, which carry oxygen to body tissues, live for only about 120 days. Replacement cells are produced by cell division in bone marrow. How many cell divisions must occur each second in your bone marrow just to replace red blood cells? Here is some information to use in calculating your answer: There are about 5 million red blood cells per cubic millimeter (mm3) of blood. An average adult has about 5 L (5,000 cm3) of blood. (Hint:What is the total number of red blood cells in the body? What fraction of them must be replaced each day if all are replaced in 120 days?) 2

A mule is the offspring of a horse and a donkey. A donkey sperm contains 31 chromosomes and a horse egg cell 32 chromosomes, so the zygote contains a total of 63 chromosomes. The zygote develops normally. The combined set of chromosomes is not a problem in mitosis, and the mule combines some of the best characteristics of horses and donkeys. However, a mule is sterile; meiosis cannot occur normally in its testes (or ovaries). Explain why mitosis is normal in cells containing both horse and donkey chromosomes but the mixed set of chromosomes interferes with meiosis.