Cell and Molecular Biology Final Exam
19-5 Which of the following statements about the benefits of sexual reproduction is false? (a) Sexual reproduction permits enhanced survival because the gametes that carry alleles enhancing survival in harsh environments are used preferentially during fertilization. (b) Unicellular organisms that can undergo sexual reproduction have an increased ability to adapt to harsh environments. (c) Sexual reproduction reshuffles genes, which is thought to help species survive in novel or varying environments. (d) Sexual reproduction can speed the elimination of deleterious alleles.
(a) Sexual reproduction permits enhanced survival because the gametes that carry alleles enhancing survival in harsh environments are used preferentially during fertilization.
18-57 Which of the following precede the re-formation of the nuclear envelope during M phase in animal cells? (a) assembly of the contractile ring (b) decondensation of chromosomes (c) reassembly of the nuclear lamina (d) transcription of nuclear genes
(a) assembly of the contractile ring
19-9 Both budding yeast and the bacteria E. coli are unicellular life. Which of the following statements explains why budding yeast can undergo sexual reproduction while E. coli cannot. (a)Unlike E. coli, budding yeast can alternate between a diploid state and a haploid state. (b) Unlike E. coli, budding yeast cannot multiply by undergoing cell division. (c)Unlike E. coli, haploid budding yeast cells can undergo meiosis to produce the gametes necessary for sexual reproduction. (d) E. coli DNA is unable to undergo homologous recombination, making it incapable of producing gametes.
(a) Unlike E. coli , budding yeast can alternate between a diploid state and a haploid state.
18-50 Which of the following statements about kinetochores is true? (a) Kinetochores assemble onto chromosomes during late prophase. (b) Kinetochores contain DNA-binding proteins that recognize sequences at the telomere of the chromosome. (c) Kinetochore proteins bind to the tubulin molecules at the minus end of microtubules. (d) Kinetochores assemble on chromosomes that lack centromeres.
(a) Kinetochores assemble onto chromosomes during late prophase.
Which of the following statements about the cell cycle is false? (a) Once a cell decides to enter the cell cycle, the time from start to finish is the same in all eukaryotic cells. (b) An unfavorable environment can cause cells to arrest in G. (c) A cell has more DNA during G than it did in G. (d) The cleavage divisions that occur in an early embryo have short G and G phases.
(a) Once a cell decides to enter the cell cycle, the time from start to finish is the same in all eukaryotic cells.
18-65 Programmed cell death occurs ________________. (a) by means of an intracellular suicide program. (b) rarely and selectively only during animal development. (c) only in unhealthy or abnormal cells. (d) only during embryonic development.
(a) by means of an intracellular suicide program.
18-40 The principal microtubule-organizing center in animal cells is the ____________. (a) centrosome. (b) centromere. (c) kinetochore. (d) cell cortex.
(a) centrosome.
18-26 Cells in the G state ________________. (a) do not divide. (b) cannot re-enter the cell cycle. (c) have entered this arrest state from either G or G. (d) have duplicated their DNA.
(a) do not divide.
18-22 Mitogens are _____. (a) extracellular signals that stimulate cell division. (b) transcription factors important for cyclin production. (c) kinases that cause cells to grow in size. (d) produced by mitotic cells to keep nearby neighboring cells from dividing.
(a) extracellular signals that stimulate cell division.
18-33 Which of the following does not occur during M phase in animal cells? (a) growth of the cell (b) condensation of chromosomes (c) breakdown of nuclear envelope (d) attachment of chromosomes to microtubules
(a) growth of the cell
18-61 Cytokinesis in animal cells ________________. (a) requires ATP. (b) leaves a small circular "scar" of actin filaments on the inner surface of the plasma membrane. (c) is often followed by phosphorylation of integrins in the plasma membrane. (d) is assisted by motor proteins that pull on microtubules attached to the cell cortex.
(a) requires ATP.
18-52 A friend declares that chromosomes are held at the metaphase plate by microtubules that push on each chromosome from opposite sides. Which of the following observations does not support your belief that the microtubules are pulling on the chromosomes? (a) the jiggling movement of chromosomes at the metaphase plate (b) the way in which chromosomes behave when the attachment between sister chromatids is severed (c) the way in which chromosomes behave when the attachment to one kinetochore is severed (d) the shape of chromosomes as they move toward the spindle poles at anaphase
(a) the jiggling movement of chromosomes at the metaphase plate
19-3 Which of the following statements is false ? (a) Asexual reproduction typically gives rise to offspring that are genetically identical. (b) Mutations in somatic cells are passed on to individuals of the next generation. (c) Sexual reproduction allows for a wide variety of gene combinations. (d) Gametes are specialized sex cells
(b) Mutations in somatic cells are passed on to individuals of the next generation.
18-30 You create cells with a version of Cdc6 that cannot be phosphorylated and thus cannot be degraded. likely of this. Which of the following statements describes the consequence change in Cdc6? (a) Cells will enter S phase prematurely. (b) Cells will be unable to complete DNA synthesis. (c) The origin recognition complex (ORC) will be unable to bind to DNA. (d) Cdc6 will be produced inappropriately during M phase.
(b) Cells will be unable to complete DNA synthesis.
18-64 Which organelle fragments during mitosis? (a) endoplasmic reticulum (b) Golgi apparatus (c) mitochondrion (d) chloroplast
(b) Golgi apparatus
18-48 Disassembly of the nuclear envelope ________________. (a) causes the inner nuclear membrane to separate from the outer nuclear membrane. (b) results in the conversion of the nuclear envelope into protein-free membrane vesicles. (c) is triggered by the phosphorylation of integrins. (d) must occur for kinetochore microtubules to form in animal cells.
(d) must occur for kinetochore microtubules to form in animal cells.
18-28 Which of the following statements is false? (a) DNA synthesis begins at origins of replication. (b) The loading of the origin recognition complexes (ORCs) is triggered by S-Cdk. (c) The phosphorylation and degradation of Cdc6 help to ensure that DNA is replicated only once in each cell cycle. (d) DNA synthesis can only begin after prereplicative complexes assemble on the ORCs.
(b) The loading of the origin recognition complexes (ORCs) is triggered by S-Cdk.
Which of the following events does not usually occur during interphase? (a) Cells grow in size. (b) The nuclear envelope breaks down. (c) DNA is replicated. (d) The centrosomes are duplicated.
(b) The nuclear envelope breaks down.
18-34 Condensins ________________. (a) are degraded when cells enter Mphase. (b) assemble into complexes on the DNA when phosphorylated by M-Cdk. (c) are involved in holding sister chromatids together. (d) bind to DNA before DNA replication begins.
(b) assemble into complexes on the DNA when phosphorylated by M-Cdk.
18-58 A cell with nuclear lamins that cannot be phosphorylated in M phase will be unable to _______________. (a) reassemble its nuclear envelope at telophase. (b) disassemble its nuclear lamina at prometaphase. (c) begin to assemble a mitotic spindle. (d) condense its chromosomes at prophase.
(b) disassemble its nuclear lamina at prometaphase.
A mutant yeast strain stops proliferating when shifted from 25°C to 37°C. When these cells are analyzed at the two different temperatures, using a machine that sorts cells according to the amount of DNA they contain, the graphs in Figure Q18-3 are obtained. Which of the following would not explain the results with the mutant? (a) inability to initiate DNA replication (b) inability to begin M phase (c) inability to activate proteins needed to enter S phase (d) inappropriate production of a signal that causes the cells to remain in G
(b) inability to begin M phase
You have isolated a strain of mutant yeast cells that divides normally at 30°C but cannot enter M phase at 37°C. You have isolated its mitotic cyclin and mitotic Cdk and find that both proteins are produced and can form a normal M-Cdk complex at both temperatures. Which of the following temperature-sensitive mutations could not be responsible for the behavior of this strain of yeast? (a) inactivation of a protein kinase that acts on the mitotic Cdk kinase (b) inactivation of an enzyme that ubiquitylates M cyclin (c) inactivation of a phosphatase that acts on the mitotic Cdk kinase (d) a decrease in the levels of a transcriptional regulator required for producing sufficient amounts of M cyclin
(b) inactivation of an enzyme that ubiquitylates M cyclin
Progression through the cell cycle requires a cyclin to bind to a Cdk because _________. (a) the cyclins are the molecules with the enzymatic activity in the complex. (b) the binding of a cyclin to Cdk is required for Cdk enzymatic activity. (c) cyclin binding inhibits Cdk activity until the appropriate time in the cell cycle. (d)without cyclin binding, a cell-cycle checkpoint will be activated.
(b) the binding of a cyclin to Cdk is required for Cdk enzymatic activity.
19-8 Which of the following does not describe a situation of asexual reproduction? (a) A bacterium multiplying by simple cell division. (b) Using a part of a plant to create a new independent plant. (c) Using in vitro fertilization to combine a sperm and an egg to create an embryo. (d) The parthenogenetic development of eggs produced by some species of lizards.
(c) Using in vitro fertilization to combine a sperm and an egg to create an embryo.
19-4 Somatic cells ___________________________. (a) are not necessary for sexual reproduction in all eukaryotic organisms. (b) are used to produce germ-line cells when organisms reach sexual maturity (c) leave no progeny. (d) do not contain sex chromosomes
(c) leave no progeny.
You engineer yeast cells that express the M cyclin during S phase by replacing the promoter sequence of the M cyclin gene with that of S cyclin. Keeping in mind that yeast cells have one common Cdk that binds to all cyclins, which of the following outcomes is least likely during this experiment? (a) There will be both M cyclin-Cdk and S cyclin-Cdk complexes in the cell during S phase. (b) Some substrates that are normally phosphorylated in M phase will now be phosphorylated in S phase. (c) G cyclins will be expressed during S phase. (d) S-Cdk targets will be phosphorylated during S phase.
(c) G cyclins will be expressed during S phase.
18-54 Which of the following statements about the anaphase-promoting complex (APC) is false? (a) It promotes the degradation of proteins that regulate M phase. (b) It inhibits M-Cdk activity. (c) It is continuously active throughout the cell cycle. (d) M-Cdk stimulates its activity.
(c) It is continuously active throughout the cell cycle.
18-29 How does S-Cdk help guarantee that replication occurs only once during each cell cycle? (a) It blocks the rise of Cdc6 concentrations early in G. (b) It phosphorylates and inactivates DNA helicase. (c) It phosphorylates the Cdc6 protein, marking it for destruction. (d) It promotes the assembly of a prereplicative complex.
(c) It phosphorylates the Cdc6 protein, marking it for destruction.
Which of the following descriptions is consistent with the behavior of a cell that lacks a protein required for a checkpoint mechanism that operates in G? (a) The cell would be unable to enter M phase. (b) The cell would be unable to enter G. (c) The cell would enter M phase under conditions when normal cells would not. (d) The cell would pass through M phase more slowly than normal cells
(c) The cell would enter M phase under conditions when normal cells would not.
18-66 Apoptosis differs from necrosis in that necrosis ________________. (a) requires the reception of an extracellular signal. (b) causes DNA to fragment. (c) causes cells to swell and burst, whereas apoptotic cells shrink and condense. (d) involves a caspase cascade.
(c) causes cells to swell and burst, whereas apoptotic cells shrink and condense.
18-35 At the end of DNA replication, the sister chromatids are held together by the ___________. (a) kinetochores. (b) securins. (c) cohesins. (d) histones.
(c) cohesins.
The concentration of mitotic cyclin (M cyclin) ________________. (a) rises markedly during M phase. (b) is activated by phosphorylation. (c) falls toward the end of M phase as a result of ubiquitylation and degradation. (d)is highest in G phase.
(c) falls toward the end of M phase as a result of ubiquitylation and degradation.
18-23 The Retinoblastoma (Rb) protein blocks cells from entering the cell cycle by ______. (a) phosphorylating Cdk. (b) marking cyclins for destruction by proteolysis. (c) inhibiting cyclin transcription. (d) activating apoptosis.
(c) inhibiting cyclin transcription.
19-7 During sexual reproduction, novel mixtures of alleles are generated. This is because ______. (a) in all diploid species, two alleles exist for every gene. (b) a diploid individual has two different alleles for every gene. (c) every gamete produced by a diploid individual has several different alleles of a single gene. (d) during meiosis, the segregation of homologs is random such that different gametes end up with different alleles of each gene.
(d) during meiosis, the segregation of homologs is random such that different gametes end up with different alleles of each gene.
18-67 Which of the following statements about apoptosis is true? (a) Cells that constitutively express Bcl2 will be more prone to undergo apoptosis. (b) The prodomain of procaspases contains catalytic activity necessary for procaspase activation. (c) Bax and Bak promote apoptosis by binding to procaspases in the apoptosome. (d) Apoptosis is promoted by the release of cytochrome c into the cytosol from mitochondria.
(d) Apoptosis is promoted by the release of cytochrome c into the cytosol from mitochondria.
MPF activity was discovered when cytoplasm from a Xenopus M-phase cell was injected into Xenopus oocytes, inducing the oocytes to form a mitotic spindle. In a control experiment, Xenopus interphase cytoplasm was injected into oocytes and shown not to induce the formation of a mitotic spindle. Which of the following statements is not a legitimate conclusion from the control experiment? (a) The piercing of the oocyte membrane by a needle is insufficient to cause mitotic spindle formation. (b) An increased volume of cytoplasm is insufficient to cause mitotic spindle formation. (c) Injection of extra RNA molecules is insufficient to cause mitotic spindle formation. (d) Components of an interphase nucleus are insufficient to cause mitotic spindle formation.
(d) Components of an interphase nucleus are insufficient to cause mitotic spindle formation.
18-55 Which of the following statements is true? (a) Anaphase A must be completed before anaphase B can take place. (b) In cells in which anaphase B predominates, the spindle will elongate much less than in cells in which anaphase A dominates. (c) In anaphase A, both kinetochore and interpolar microtubules shorten. (d) In anaphase B, microtubules associated with the cell cortex shorten.
(d) In anaphase B, microtubules associated with the cell cortex shorten.
8-63 Which of the following statements is false? (a) Cytokinesis in plant cells is mediated by the microtubule cytoskeleton. (b) Small membrane vesicles derived from the Golgi apparatus deliver new cell- wall material for the new wall of the dividing cell. (c) The phragmoplast forms from the remains of interpolar microtubules of the mitotic spindle. (d) Motor proteins walking along the cytoskeleton are important for the contractile ring that guides formation of the new cell wall.
(d) Motor proteins walking along the cytoskeleton are important for the contractile ring that guides formation of the new cell wall
18-17 Which of the following statements is false? (a) Mitotic Cdk must be phosphorylated by an activating kinase (Cak) before it is active. (b) Phosphorylation of mitotic Cdk by the inhibitory kinase (Wee1) makes the Cdk inactive, even if it is phosphorylated by the activating kinase. (c) Active M-Cdk phosphorylates the activating phosphatase (Cdc25) in a positive feedback loop. (d) The activating phosphatase (Cdc25) removes all phosphates from mitotic Cdk so that M-Cdk will be active.
(d) The activating phosphatase (Cdc25) removes all phosphates from mitotic Cdk so that M-Cdk will be active.
What would be the most obvious outcome of repeated cell cycles consisting of S phase and M phase only? (a) Cells would not be able to replicate their DNA. (b) The mitotic spindle could not assemble. (c) Cells would get larger and larger. (d) The cells produced would get smaller and smaller.
(d) The cells produced would get smaller and smaller.
18-62 Which of the following statements is false? (a) The cleavage furrow is a puckering of the plasma membrane caused by the constriction of a ring of filaments attached to the plasma membrane. (b) The cleavage furrow will not begin to form in the absence of a mitotic spindle. (c) The cleavage furrow always forms perpendicular to the interpolar microtubules. (d) The cleavage furrow always forms in the middle of the cell.
(d) The cleavage furrow always forms in the middle of the cell.
18-36 Which of the following statements is true? (a) The mitotic spindle is largely made of intermediate filaments. (b) The contractile ring is made largely of microtubules and actin filaments. (c) The contractile ring divides the nucleus in two. (d) The mitotic spindle helps segregate the chromosomes to the two daughter
(d) The mitotic spindle helps segregate the chromosomes to the two daughter
Imagine meiosis in a diploid organism that only has a single chromosome. Like most diploid organisms, it received one copy of this chromosome from each of its parents and the two homologs are genetically distinct. If only a single homologous recombination event occurs during meiosis, which of the following choices below correctly describes the four gametes formed. (a)None of the gametes will contain chromosomes identical to the chromosomes found in the original diploid cell. (b)All four of the gametes will have chromosomes identical to the chromosomes found in the original diploid cell. (c) Three of the gametes will have chromosomes identical to the chromosomes found in the original diploid cell, while one of the gametes will have chromosomes that are different. (d) Two of the gametes will have chromosomes identical to the chromosomes found in the original diploid cell, while two of the gametes will have chromosomes
(d) Two of the gametes will have chromosomes identical to the chromosomes found in the original diploid cell, while two of the gametes will have chromosomes that are different.
18-20 Which of the following is not good direct evidence that the cell-cycle control system is conserved through billions of years of divergent evolution? (a) A yeast cell lacking a Cdk function can use the human Cdk to substitute for its missing Cdk during the cell cycle. (b) The amino acid sequences of cyclins in plants are similar to the amino acid sequences of cyclins in humans. (c) The Cdk proteins in humans share conserved phosphorylation sites with the Cdk proteins in yeast. (d) Yeast cells have only one Cdk, whereas humans have many Cdks.
(d) Yeast cells have only one Cdk, whereas humans have many Cdks.
18-56 When introduced into mitotic cells, which of the following is expected to impair anaphase B but not anaphase A? (a) an antibody against myosin (b) ATPγS, a nonhydrolyzable ATP analog that binds to and inhibits ATPases (c) an antibody against the motor proteins that move from the plus end of microtubules to the minus end (d) an antibody against the motor proteins that move from the minus end of microtubules toward the plus end
(d) an antibody against the motor proteins that move from the minus end of microtubules toward the plus end
In which phase of the cell cycle do cells check to determine whether the DNA is fully and correctly replicated? (a) at the transition between G and S (b) when cells enter G (c) during M (d) at the end of G
(d) at the end of G
Levels of Cdk activity change during the cell cycle, in part because ________________. (a) the Cdks phosphorylate each other. (b) the Cdks activate the cyclins. (c) Cdk degradation precedes entry into the next phase of the cell cycle. (d) cyclin levels change during the cycle.
(d) cyclin levels change during the cycle.
18-25 The G DNA damage checkpoint ________________. (a) causes cells to proceed through S phase more quickly. (b) involves the degradation of p53. (c) is activated by errors caused during DNA replication. (d) involves the inhibition of cyclin-Cdk complexes by p21.
(d) involves the inhibition of cyclin-Cdk complexes by p21.
Meiosis is a highly specialized cell division in which several events occur in a precisely defined order. Please order the meiotic events listed below. 1. loss of cohesins near centromeres 2. chromatid pairing 3. chromosome condensation 4. chromosome replication 5. degradation of cohesins bound to chromosome arms 6. formation of chiasmata (chiasmata = plural of chiasma) 7. homolog pairing 8. alignment of chromosomes at the metaphase plate
4, 2, 7, 6, 3, 8, 5, 1.
Match the following labels to the numbered label lines on Figure Q18-10. Figure Q18-10 A. G phase B. mitotic cyclin C. mitotic Cdk D. S phase E. S-phase cyclin F. S-phase Cdk G. MPF H. G phase
A) 5 B) 6 C) 3 D) 2 E) 4 F) 1 G) 7 H) 8
18-39 Name the stage of M phase in which the following events occur. Place the numbers 1-8 next to the letter headings to indicate the normal order of events. A. alignment of the chromosomes at the spindle equator B. attachment of spindle microtubules to chromosomes C. breakdown of nuclear envelope D. pinching of cell in two E. separation of two centrosomes and initiation of mitotic spindle assembly F. re-formation of the nuclear envelope G. condensation of the chromosomes H. separation of sister chromatids
A) 5 metaphase B) 4 prometaphase C) 3 prometaphase D) 8 cytokinesis E) 2 prophase F) 7 telophase G) 1 prophase H) 6 anaphase
19-29 Meiosis includes a recombination checkpoint that is analogous to the checkpoints in cell-cycle progression. Double-strand breaks in the DNA initiate recombination in meiosis. The broken end of a DNA molecule finds the corresponding sequence on a homologous chromosome and exchanges a chromosomal segment with its homolog, thereby repairing the break. Ongoing recombination sends a negative regulatory signal that prevents cells from entering meiotic division I. A. Mutations in several genes inactivate the recombination checkpoint. What do you predict will happen if a cell proceeds through meiotic division I before completing recombination? B. What will happen if a cell fails to initiate recombination and proceeds through meiotic division I? Meiotic division II?
A. If a cell proceeds through meiotic division I when its chromosomes are broken and incompletely repaired, segregation will be disastrous. Some recombination intermediates will be pulled to opposite spindle poles, thus breaking the DNA. Other chromosome fragments lacking centromeres will not be attached to microtubules and thus will segregate randomly, causing some meiotic products to have too little DNA and others to have too much. B. In the absence of recombination, the homologs will not segregate from each other properly in meiotic division I but the sister chromatids will segregate normally in meiotic division II. In meiotic division I, the unrecombined homologous chromosomes will not be held together by chiasmata. Thus, the homologous chromosomes will line up independently on the metaphase plate and segregate randomly, causing chromosome nondisjunction. Some products will have both homologs of a given chromosome and others will have none. In meiotic division II, the events will proceed normally and sister chromatids will be properly segregated to opposite poles. Nonetheless, because the chromosome sets at the start of meiotic division II were distributed unevenly, the gametes produced after meiotic division II will be aneuploid (that is, they will have an incorrect number of chromosomes).
18-49 Consider an animal cell that has eight chromosomes (four pairs of homologous chromosomes) in G phase. How many of each of the following structures will the cell have at mitotic prophase? A. sister chromatids B. centromeres C. kinetochores D. centrosomes E. centrioles
A. 16 B. 16 C. 16 D. 2 E. 4
18-31 Cells can pause in G when DNA is damaged, and can pause in S when there are replication errors. Indicate whether the mechanism below applies to a G arrest, an S-phase arrest, both types of arrest, or neither. A. p53 activates the transcription of a Cdk inhibitor. B. Cyclins are phosphorylated and destroyed. C. Cdk is unable to phosphorylate its substrates. D. The Cdc25 phosphatase is inhibited.
A. G arrest B. neither C. both D. S-phase arrest
Are the statements below true or false? Explain your answer. A. Statement 1: Generally, in a given organism, the S, G, and M phases of the cell cycle take a defined and stereotyped amount of time in most cells. B. Statement 2: Therefore, the cell-cycle control system operates primarily by a timing mechanism, in which the entry into one phase starts a timer set for sufficient time to complete the required tasks. After a given amount of time has elapsed, a molecular "alarm" triggers movement to the next phase.
A. True, in nearly all cells in an organism, the S, G, and M phases of the cell cycle take the same amount of time. B. False, the cell cycle control system does use a timing mechanism of sorts, but it also employs various surveillance and feedback mechanisms.
19-13 Indicate whether each of the following is true for meiosis, mitosis, both, or neither. A. formation of a bivalent B. genetically identical products C. condensation of chromosomes D. segregation of all paternal chromosomes to one cell E. involvement of DNA replication
A. meiosis B. mitosis C. both D. neither E. both
19-2 Which of the following statements is true ? (a) Another name for the fertilized egg cell is the zygote. (b) Diploid organisms reproduce only sexually. (c) All sexually reproducing organisms must have two copies of every chromosome. (d) Gametes have only one chromosome
Another name for the fertilized egg cell is the zygote.
18-72 Of the following mutations, which are likely to cause cell-cycle arrest? If you predict a cell-cycle arrest, indicate whether the cell will arrest in early G, late G, or G. Explain your answers. A. a mutation in a gene encoding a cell-surface mitogen receptor that makes the receptor active even in the absence of the mitogen B. a mutation that destroyed the kinase activity of S-Cdk C. a mutation that allowed G-Cdk to be active independently of its phosphorylation status D. a mutation that removed the phosphorylation sites on the Rb protein E. a mutation that inhibited the activity of Rb
B and D
18-24 Irradiated mammalian cells usually stop dividing and arrest at a G checkpoint. Place the following events in the order in which they occur. A. production of p21 B. DNA damage C. inhibition of cyclin-Cdk complexes D. accumulation and activation of p53
B, D, A, C
19-36 Do you agree or disagree with the following statement? Explain your answer If a diploid organism has 16 chromosomes (and thus 8 pairs of homologous chromosomes), that organism can produce only 2 8 genetically different gametes
Disagree. There are two distinct mechanisms for genetic variation in gametes. One method, the reassortment of chromosomes during meiosis, would result in a diploid organism with 16 chromosomes producing 2 genetically different gametes. However, a 8 much greater number of genetically different gametes can be produced as a result of the recombination between chromosomes that occurs during every meiosis
19-22 Imagine a diploid sexually reproducing organism, Diploidus sexualis , that contains three pairs of chromosomes. This organism is unusual in that no recombination between homologous chromosomes occurs during meiosis. A. Assuming that the chromosomes are distributed independently during meiosis, how many different types of sperm or egg cells can a single individual of this species produce? B. What is the likelihood that two siblings of this species will be genetically identical? You can assume that the homologous chromosomes of each parent are different from one another and from their counterparts in the other parent.
Eight different types. With respect to each of the three chromosomes, an individual can produce two kinds of gamete. The gamete can receive the copy that the individual received from "mom" or the copy from "dad." Thus, with three chromosomes, there are 2 × 2 × 2 = 8 possible gametes. B. 1/64. The mother and father together can produce 8 × 8 = 64 different genetic combinations
18-27 What is the main molecular difference between cells in a G state and cells that have simply paused in G?
In G, the cell cycle control system is partly dismantled, so that some of the Cdks and cyclins are not present. Cells paused in G still contain all of the components of the cell cycle control system. Whereas the latter cells can rapidly progress through the cycle when conditions are right, G cells need to synthesize the missing cell-cycle control proteins so as to re-enter the cycle.
19-23 You have received exactly half of your genetic material from your mother, who received exactly half of her genetic material from her mother (your grandmother). A. Explain why it is unlikely that you share exactly one-quarter of your genetic material with your grandmother, and instead it is more accurate to say that in general people receive an average of one-quarter of their genetic endowment from each grandparent. B. Consider a gene on Chromosome 3 that you received from your grandmother. Is it likely you received an entire Chromosome 3 from your grandmother? Why or why not? C. What portion of your genetic material do you share with your sibling? Your aunt? Your cousin?
It is unlikely that you share exactly one-quarter of your genetic material with your grandmother, but it is true that organisms receive an average of one-quarter of their genes from each grandparent. When cells in your mother's germ line were undergoing meiosis, the chromosomes that she received from your grandmother and your grandfather were shuffled by recombination and then randomly assorted into the gametes. Each of these gametes probably received slightly less or slightly more than half of its genetic material from your grandmother. Thus, fusion of your mother's gamete with your father's created a zygote that shared approximately, but not exactly, one-quarter of its genes with your grandmother; this zygote divided repeatedly to form all the cells in your body. B. It is unlikely that you received the entire Chromosome 3 from your grandmother, because each chromosome undergoes at least one recombinational crossover with its homolog to ensure proper chromosome segregation in meiotic division I. C. You share an average of one-half of your genetic material with your sibling, one- quarter with your aunt, and one-eighth with your cousin. Because you and your sibling each get half of each parent's DNA, you have approximately half of your genetic material in common with your sibling. As your mother and her sister share approximately half of their genetic material, you and your aunt share 1/2 × 1/2 = 1/4. As your cousin has half of your aunt's genetic material, you share 1/2 × 1/4 = 1/8`
The four phases of the cell cycle are G, ___, ____, and ______. A cell contains the most DNA after __________ phase of the cell cycle. A cell is the smallest in size after ______ phase of the cell cycle. Growth occurs in ______, _______, and _______ phases of the cell cycle. A cell does not enter mitosis until it has completed ______ synthesis
S,G,M S M G,S,G DNA
What would happen to the progeny of a cell that proceeded to mitosis and cell division after entering S phase but had not completed S phase? Keep in mind that highly condensed chromatin, including the centromere region, is replicated late in S phase. Explain your answer.
The daughter cells would probably die. Those chromosomes that had not completed replication in S phase would have only one centromere, because the centromere is the last part of chromosome to be replicated, the chromosome would therefore be segregated to only one of the two daughter cells at random. At least one, and probably both, of the daughter cells would thus receive an incomplete set of chromosomes and would be unlikely be viable
19-10 Why is sexual reproduction more beneficial to a species living in an unpredictable environment than to one living in a constant environment?
The real benefit in sexual reproduction seems to be that parents produce children that are genetically unlike either parent and that are not genetically identical to each other. Sexual reproduction provides more variation in the population than asexual reproduction could provide, and is an advantage if the environment is variable, because any one combination of the parents' characteristics, however well adapted to the prevailing conditions, may or may not be the best in a new situation.
19-11. Is the following statement true or false? Explain. Somatic cells leave no progeny and thus, in an evolutionary sense, exist only to help create, sustain, and propagate the germ cells
True. Only germ cells contribute genetic material to the next generation of organisms, and thus only germ cells leave an evolutionary legacy in the gene pool of the species. The only contribution of somatic cells to subsequent generations arises through the assistance they provide to dissemination of the genetic material in the germ cells. In accordance with this, mutations that arise in somatic cells are not passed along to offspring.
19-35 During fertilization in humans, _______________________. (a) a wave of Ca ions is released in the fertilized egg's cytoplasm. (b) only one sperm binds to the unfertilized egg. (c) a sperm moves in a random fashion until it encounters an egg. (d) several sperm pronuclei compete in the cytoplasm to fuse with the egg nucleus
a) a wave of Ca ions is released in the fertilized egg's cytoplasm.
19-25 Which of the following statements about meiosis is true? (a)During meiosis, the paternal chromosomes pair with the maternal chromosomes before lining up at the metaphase plate. (b) Unicellular organisms that have a haploid state undergo meiosis instead of mitosis during cell division. (c) Meiosis produces four genetically identical cells. (d)In general, meiosis is faster than mitosis.
a)During meiosis, the paternal chromosomes pair with the maternal chromosomes
19-14 The formation of a bivalent during meiosis ensures that _______. (a) one chromatid from the mother and one chromatid from the father will segregate together during meiosis I. (b) all four sister chromatids remain together until the cell is ready to divide. (c)recombination will occur between identical sister chromatids. (d) the sex chromosomes, which are not identical, will line up separately at the metaphase plate during meiosis I
b) all four sister chromatids remain together until the cell is ready to divide.
19-21 A diploid cell containing 32 chromosomes will make a haploid cell containing ___ chromosomes. (a) 8 (b) 16 (c) 30 (d) 64
b) 16
19-28 After the first meiotic cell division _____________________ (a) two haploid gametes are produced. (b) cells are produced that contain the same number of chromosomes as somatic cells. (c) the number of chromosomes will vary depending on how the paternal and maternal chromosomes align at the metaphase plate. (d)DNA replication occurs.
b) cells are produced that contain the same number of chromosomes as somatic cells.
19-27 During recombination ________________________. (a) sister chromatids undergo crossing-over with each other. (b) chiasmata hold chromosomes together. (c) one crossover event occurs for each pair of human chromosomes. (d) the synaptonemal complex keeps the sister chromatids together until anaphase II.
b) chiasmata hold chromosomes together.
19-31 In mammals, there are two sex chromosomes, X and Y, which behave like homologous chromosomes during meiosis. Normal males have one X chromosome and one Y chromosome, and normal females have two X chromosomes. Males with an extra Y chromosome (XYY) are found occasionally. Which of the following could give rise to such an XYY male? Explain your answer. (a) nondisjunction in the first meiotic division of spermatogenesis; normal meiosis in the mother (b) nondisjunction in the second meiotic division of spermatogenesis; normal meiosis in the mother (c)nondisjunction in the first meiotic division of oogenesis; normal meiosis in the father (d) nondisjunction in the second meiotic division of oogenesis; normal meiosis in the father
b) nondisjunction in the second meiotic division of spermatogenesis; normal meiosis in the mother
You examine a worm that has two genders: males that produce sperm and hermaphrodites that produce both sperm and eggs. The diploid adult has four homologous pairs of chromosomes that undergo very little recombination. Given a choice, the hermaphrodites prefer to mate with males, but just to annoy the worm, you pluck a hermaphrodite out of the wild and fertilize its eggs with its own sperm. Assuming that all the resulting offspring are viable, what fraction do you expect to be genetically identical to the parent worm? Assume that each chromosome in the original hermaphrodite is genetically distinct from its homolog. worm? Assume that each chromosome in the original hermaphrodite is genetically distinct from its homolog. (a) all (b) none (c) 1/16 (d) 1/256
c) 1/16
19-19 In the absence of recombination, how many genetically different types of gamete can an organism with five homologous chromosome pairs produce? (a) 5 (b) 10 (c) 32 (d) 64
c) 32
19-32 Which of the following would not lead to aneuploidy during meiosis? (a) sister chromatids segregating inappropriately (b) non-sister chromatids segregating inappropriately (c) a reciprocal rearrangement of parts between nonhomologous chromosomes (for example, the left arm of Chromosome 2 exchanging places with the right arm of Chromosome 3) (d) an extra set of chromosomes produced during S phase (for example, if paternal Chromosome 3 were replicated twice)
c) a reciprocal rearrangement of parts between nonhomologous chromosomes (for example, the left arm of Chromosome 2 exchanging places with the right arm of Chromosome 3)
19-16 There are organisms that go through meiosis but do not undergo recombination when forming haploid gametes. Which of the following statements correctly describes the gametes produced by such an organism. (Assume that these organisms are diploid, that each of the two homologous chromosomes are genetically distinct as typically found in the wild, and that these organisms have more than one chromosome.) (a)All gametes formed during a single meiosis will be identical. (b) Due to the random assortment of homologs, each of the gametes formed during a single meiosis will be different. (c)This organism could potentially produce 2 genetically distinct gametes, where is its haploid number of chromosomes. (d) The fusion of any two gametes produced by such an organism that does not undergo recombination during meiosis will create a cell that is genetically identical to that individual.
c)This organism could potentially produce 2 genetically distinct gametes, where is its haploid number of chromosomes.
19-33 A single nondisjunction event during meiosis ___________________. (a) will block recombination. (b) will occur only during meiosis II. (c) cannot occur with sex chromosomes. (d) will involve the production of two normal gametes if it occurs during meiosis II
d) will involve the production of two normal gametes if it occurs during meiosis II
19-20 Which of the following statements most correctly describes meiosis? (a)Meiosis involves two rounds of DNA replication followed by a single cell division. (b)Meiosis involves a single round of DNA replication followed by four successive cell divisions. (c)Meiosis involves four rounds of DNA replication followed by two successive cell divisions. (d)Meiosis involves a single round of DNA replication followed by two successive cell divisions
d)Meiosis involves a single round of DNA replication followed by two successive cell divisions
18-69 The number of cells in an adult tissue or animal depends on cell proliferation. What else does it depend on?
programmed cell death also influences cell numbers. Most animal cells require survival signals from other cells to avoid programmed cell death, so that the levels of such signals can help determine how many cells live and how many die
18-51 Is the following statement true or false? After the nuclear envelope breaks down, microtubules gain access to the chromosomes and, every so often, a randomly probing microtubule captures a chromosome and ultimately connects to the kinetochore to become a kinetochore microtubule of the spindle.
true
Organisms that reproduce sexually ________________________. (a) must be haploid, unlike organisms that reproduce asexually. (b) can reproduce only with a partner that carries the same alleles. (c) create zygotes that are genetically identical to each other. (d) undergo a sexual reproductive cycle that involves an alternation of haploid cells with the generation of diploid cells.
undergo a sexual reproductive cycle that involves an alternation of haploid cells with the generation of diploid cells.
