Concept explainers
In the 1920s, Barbara McClintock, later a Nobel laureate for her discovery of transposable elements, examined the behavior of chromosomes in wheat cells that had been subjected to X-rays. She noticed that the X-rays produced chromosomal breaks during G1 phase, and that after subsequent chromosome replication in S phase, the broken ends of the two sister chromatids could join together to make a fusion chromosome larger than the original. Even later, during mitotic metaphase and early in mitotic anaphase, the joined sister chromatids would form an unusual bridge structure in which chromatin was stretched between the two spindle poles and could then eventually break. She called this phenomenon the breakage-fusion-bridge cycle. Each of photographs (a) and (b) that follow shows a cell in early mitotic anaphase that has two such chromatin bridges.
a. | What ensures that the ends of normal chromosomes do not fuse together as do the ends of the sister chromatids after breakage? |
b. | The following figure shows a chromosome with genes A–G; the arrow indicates the location of X-ray induced breakage. Draw the resulting bridge (that is, the large fused chromosome) as it would be seen in mitotic anaphase, and label all the genes and important chromosomal structures the bridge contains. Use arrows to show the forces exerted by the spindle apparatus on this bridge. |
c. | If the sister chromatids fuse, why must the fusion chromosome behave as a bridge during mitosis? [Think about the forces pulling on the bridge described in your answer to part (b).] |
d. | What is likely to happen to the bridge during mitotic anaphase? What then is likely to happen in the two daughter cells produced by the mitosis just described, and why? (Hint: McClintock’s name for this phenomenon implies it is a cycle.) |
Want to see the full answer?
Check out a sample textbook solutionChapter 12 Solutions
ND STONY BROOK UNIVERSITY LOOSELEAF GENETICS: FROM GENES TO GENOMES
- What are sister chromatids? What is the process that forms two sister chromatids from a single chromosome? Explain how this process works.arrow_forwardMitotic nondisjunction can occur, particularly in tumor cells. Assume a single nondisjunction event involving the X chromosome occurs in a cell undergoing mitosis in a human female. First, consider the daughter cell with the smaller number of chromosomes: How many total chromosomes will this cell have? How many X chromosomes will this cell have? Next, consider the daughter cell with the larger number of chromosomes: How many total chromosomes will this cell have? How many X chromosomes will this cell have?arrow_forwardIn a turtle species, a diploid cell in the G1 phase of the cell cycle contains 22 picograms of DNA (picogram is a measure of the total mass of the DNA present). How much DNA is present in a cell of the same species at prophase II of meiosis (prophase of the second meiotic division)?arrow_forward
- For below problem, consider a diploid cell that contains three pairs of chromosomes designated AA, BB, and CC. Each pair contains a maternal and a paternal member (e.g., Am and Ap). Using these designations, demonstrate your understanding of mitosis and meiosis by drawing chromatid combinations as requested. Be sure to indicate when chromatids are paired as a result of replication and/or synapsis. You may wish to use a large piece of brown manila wrapping paper or a cut-up paper grocery bag for this project and to work in partnership with another student. We recommend cooperative learning as an efficacious way to develop the skills you will need for solving the problems presented throughout this text. Question: Assume that each gamete resulting from Problem 29 fuses, in fertilization, with a normal haploid gamete. What combinations will result? What percentage of zygotes will be diploid, containing one paternal and one maternal member of each chromosome pair?arrow_forwardHuman somatic cells have 46 chromosomes. How many chromosomes are present in human cells undergoing each of the following phases of meiosis? Interphase - Answer: 0 2 3 4 6 9 2 Prophase I - Answer: 0 2 3 4 6 9 2 Prophase II - Answer: 0 2 3 4 6 9 2 Telophase I - Answer: 0 2 3 4 6 9 2 Telophase II - Answer: 0 2 3 4 6 9 2arrow_forwardA somatic cell has the genotype DdEe. All chromosomes in this organism are metacentric. Show metaphase and the results of mitosis if the D and E locus are linked on the same chromosome in trans configuration. A sexually reproducing organism has the following phenotype DdEeAaTt: The D and E loci are on the same arm of a metacentric chromosome in cis configuration. The A locus is on the long arm of an acrocentric chromosome and the T locus is on a telocentric chromosome. 2.1. What is the haploid number of this organism? 2.2. Using diagrams show a ceii at metaphase of mitosis and show the results of mitosis.arrow_forward
- Consider a diploid organism in which the somatic cell chromosome number is 42. Assume cytokinesis to occur in mid telophase. A. How many chromosomes at the close of telophase of mitosis? B. How many chromosomes are expected at metaphase II of meiosis?arrow_forwardVariations in Chromosome NumberAneuploidy Describe the process of nondisjunction and explain when it takes place during cell division.arrow_forwardAcross 12. 9.) one of two idéntical halves of a replicated chromosomes 15 17 12.) the structure that forms during cytokinesis in plants separating the two daughter cells 8 9. 10 13.) the condition of halving more than two sets of chromosomes per. 11 12 13 |14 Down 4.) the complex of DNA and prótein that makes up eukaryotic chromosomes 15 16 17 18 5.) the condition of having one set of chromosomes (n) per nucleus 19 20 7.) the chromosome complex formed by the synapsis of a pair of homologous 4.arrow_forward
- Human somatic cells have 46 chromosomes. Are the cells different in any way from the parent cell and what is chromosome count of each of those cells once they have finished mitosis?arrow_forwardIf you expose a culture of human cells (for example,HeLa cells) to 3H-thymidine during S phase, howwould the radioactivity be distributed over a pair ofhomologous chromosomes at metaphase? Would theradioactivity be in (a) one chromatid of one homolog,(b) both chromatids of one homolog, (c) one chromatid each of both homologs, (d) both chromatids ofboth homologs, or (e) some other pattern? Choose thecorrect answer and explain your reasoning.arrow_forward● ● Illustrate the chromosome changes in interphase and mitosis using a diploid cell that is 2n=4 (two large and two small chromosomes). Label the circles as G1 and G2 stages of interphase and each phase of mitosis (in order). Draw the chromosome in each circle using the following guidelines. 1) For unreplicated chromosome draw a line (/) and for replicated chromosome draw an X. 2) For simplicity, do not indicate that the chromosomes are decondensed chromatin during interphase. 3) Skip drawing the nuclear membrane or spindle fibers, since we are focusing on the chromosomes. Label genes in the resulting daughter cells as A/a and B/b. (These letters represent genes which are at particular places along the DNA molecule. Each chromosomes can have a thousand genes but we are focusing on these two.) Ask me Qs in class if anythign on this is not clear.arrow_forward
- Human Heredity: Principles and Issues (MindTap Co...BiologyISBN:9781305251052Author:Michael CummingsPublisher:Cengage Learning