UCONN BIOL 1107 Exam 4 Laux

At the beginning of the day, a cell has a diameter of about 25 μm. At the end of the day, the cell has a diameter of 30 μm. What phase of the cell cycle would you expect this cell to be in?

G1

Which of the following would be true of cell in G1?
I. Increase in number of growth factors in the cytoplasm
II. Actively replicating chromosomes
III. Would be found in the largest peak in a flow cytometry graph

I & III

Which of the following scenarios would allow for the progression into S phase?

Rb is phosphorylated by Cyclin-CDK, so only E2F binds the promoter.

Suppose Cell A in G1 phase was injected with cytoplasm from Cell B. What phase would Cell B have to be in to induce DNA replication in Cell A?

S phase, because cyclins are highest

A drug competitively binds CDK in the cell. Which of the following best describes a result?

The cell cycle would halt in G1, which could ultimately trigger apoptosis (cell death).

A pair of sister chromatids contains:
I. 1 chromosome
II. 2 identical chromosomes
III. 2 non-identical chromosomes
1. 1 kinetochore
2. 2 kinetochores

II & 2

What is another name for the centrosomes?

Microtubule organizing centers

You are observing a cell with sister chromatids lined up in the
middle of the cell. What stage of the cell cycle is this?

Metaphase

You are observing an elongated cell with two distinct nuclear
membranes, each containing a diploid number of
chromosomes. What stage of the cell cycle is this?

Telophase

What is the purpose of meiosis reducing the ploidy from 2n to 1n?

So that when 1n gametes join, they form a 2n organism

In which stage of meiosis does the chromosome number go
from 2N to 1N?

Meiosis I, with the separation of homologous chromosomes

Suppose a cell has a diploid number of 10 chromosomes. Which of the
following scenarios correctly tracks its progression through meiosis?

2N = 10 → Meiosis I → N = 5 → Meiosis II → N = 5

Suppose 15N dyes DNA purple, while 14N dyes DNA red. You perform an experiment supplying 15N nucleotides for DNA replication. After the cell underwent S phase, how would the cell appear?

The cell after S phase would have an even mix of purple 15N and red 14N.

To construct a molecule of DNA, which polymerase is necessary?

DNA polymerase

What type of bonds are broken & formed during these steps?
I. Unzipping DNA
II. Adding new nucleotides to form a new DNA strand

I = breaking hydrogen bonds, II = forming phosphodiester & hydrogen bonds

You have DNA in a solution where the terminal phosphate group has been dyed with a fluorescent yellow, and the terminal hydroxyl group has been dyed with a fluorescent red. Primers, nucleotides and DNA Polymerase have been added to your solution. How would you expect replication to proceed?

The primer would first be added to the red-tagged end, followed by nucleotides in the direction of the yellow-tagged end.

What is the purpose of meiosis reducing the ploidy from 2n to 1n?

So that when 1n gametes join, they form a 2n organism

In which stage of meiosis does the chromosome number go from 2N to 1N?

Meiosis I, with the separation of homologous chromosomes

Suppose a cell has a diploid number of 10 chromosomes. Which of the following scenarios correctly tracks its progression through meiosis?

2N = 10 → Meiosis I → N = 5 → Meiosis II → N = 5

Suppose a cell in G1 phase is exposed to a drug that inhibits the binding of CDKs and cyclins. What might happen to the progression through the cell cycle?

Dephosphorylated Rb would remain bound to E2F, preventing gene transcription of S Phase proteins.

Analysis shows a sample of cells have a moderate concentration of cyclins. Some cells are 1N while others appear to be 2N. Which phase of the cell cycle are these cells likely in?

S Phase, because the replication of DNA marks the transition between 1N and 2N

Which of the following scenarios contains cells with the greatest genetic diversity relative to the parent cell?

Cells that underwent meiosis with 2 crossing over events

Suppose a par of homologous chromosomes failed to separate during meiosis. What phase of meiosis would this occur in, and what would be the result?

Meiosis I, there will be an abnormal number of chromosomes in all 4 resulting cells

A researcher is looking at cells under a microscope. There appears to be a cell in anaphase with a chromosome attached to each spindle fiber. What phase is being observed?

Mitotic anaphase and Anaphase II

Which of the following represents a scenario where crossing over is most likely to occur?

A set of homologous chromosomes are in prophase I

During the S phase of the eukaryotic cell cycle, which two major cellular events occur simultaneously to prepare the cell for division?

DNA replication forming sister chromatids and duplication of the centrosome

How do the primary physiological roles of G1 phase and G2 phase differ?

G1 phase focuses on cell growth, metabolic activity, and responding to extracellular signals; G2 phase serves as a proofreading stage to ensure DNA is fully copied and damage-free

In a standard flow cytometry experiment measuring DNA content via a fluorescent intercalating dye, what do Peak 1 and Peak 2 represent on the histogram?

Peak 1 represents cells in G1 phase (2N DNA); Peak 2 represents cells in G2/M phase (4N DNA)

A researcher treats an unstimulated culture of quiescent human fibroblast cells with Platelet-Derived Growth Factor (PDGF) and Interleukin-1 beta (IL-1β). When analyzing the sample by flow cytometry 18 hours later, what shift in the flow cytometry profile would demonstrate that these factors acted as mitogens?

The emergence and enlargement of Peak 2 (G2/M phase) as cells exit G1 and replicate their DNA.

Which cell cycle checkpoint directly verifies whether all chromosomes are properly attached to the mitotic spindle before allowing the cell to pull duplicated chromatids apart?

The M phase checkpoint (Spindle Assembly Checkpoint)

Which statement accurately describes the expression patterns and activation mechanism of Cyclin-Dependent Kinases (CDKs)?

CDK protein levels remain constant throughout the cell cycle, but CDKs are enzymatically inactive until bound by Cyclins, whose levels cyclically rise and fall

In a classic developmental biology experiment, cytoplasm extracted from a donor cell in M phase is microinjected into a recipient cell currently in G2 phase. What is the expected immediate molecular outcome for the recipient G2 cell, and why?

The G2 cell immediately enters Mitosis becasue the M-phase cytoplasm contains active M-Cyclin/CDK complexes

How does the Retinoblastoma protein (Rb) regulate entry into S phase under normal, non-dividing conditions?

Active, unphosphorylated Rb binds to the E2F transcription factor, sequestering it and inhibiting the transcription of genes required for S phase

A genetic mutation results in a mutant form of the Retinoblastoma protein (Rb) that is completely unable to bind to E2F under any circumstances (a loss-of-function mutation). What would be the downstream cellular phenotype in tissue expressing this mutant Rb?

Cells will continuously progress past the G1/S checkpoint into S phase even in the absence of extracellular growth factors

A cell line carries a knock-in mutation in the Rb gene where all target serine/threonine phosphorylation sites are substituted with alanine, creating a "non-phosphorylatable" Rb protein. What happens when these cells are treated with a powerful mitogen?

Rb will remain permanently bound to E2F, blocking E2F-mediated gene transcription and arresting the cells in G1 phase

According to the semi-conservative model of DNA replication, what is the composition of the two double-stranded DNA molecules produced after one round of replication?

Each resulting double helix consists of one original parental strand paired with one newly synthesized complementary daughter strand

In the Meselson-Stahl experiment, E. coli cells initially grown in heavy nitrogen (15N) were transferred to a medium containing only light nitrogen (14N). If these bacteria undergo TWO complete rounds of DNA replication in the 14N medium, what ratio of heavy-light (hybrid) to light-light DNA molecules will be observed upon density gradient centrifugation?

50% heavy-light hybrid DNA and 50% light-light DNA

Why is synthesis of the lagging strand discontinuous (forming Okazaki fragments), whereas synthesis of the leading strand is continuous?

Antiparallel template strands open at the replication fork, but DNA polymerase can only add nucleotides to the 3’-OH end of a growing strand (5’ > 3’ direction)

A replication bubble forms with two active replication forks expanding in opposite directions. If a specific template strand runs 3’ > 5’ toward the LEFT fork, how will the newly synthesized daughter strand on this template be built at the RIGHT fork?

Discontinuously as the lagging strand (in Okazaki fragments) away from the right fork

Which enzyme is specifically responsible for relieving torsional strain and hyper-twisting generated ahead of the advancing replication fork?

Topoisomerase (Topoisomerase II)

If a research laboratory treats actively dividing eukaryotic cells with a novel chemical compound that completely inactivates Topoisomerase, what is the most immediate cellular outcome during S phase?

DNA replication will stall rapidly due to extreme torsional strain and hyper-twisting ahead of the replication fork

Why does DNA polymerase require an RNA primer (synthesized by RNA primase) to initiate DNA synthesis?

DNA polymerase requires a pre-existing free 3’-OH group to form a phosphodiester bond with the incoming nucleotide's phosphate group

A temperature-sensitive mutant cell line has a defective DNA Ligase enzyme. When shifted to a restrictive temperature during S phase, what structural anomaly will accumulate in the cell's newly replicated genome?

Okazaki fragments on the lagging strand will remain unsealed, leaving single-stranded breaks (nicks) in the sugar-phosphate backbone

How does DNA Polymerase perform proofreading during active S-phase replication to maintain high fidelity?

It detects mispaired bases, pauses, uses its 3’ > 5’ exonuclease activity to excise the incorrect nucleotide, and replaces it with the correct match before continuing

Human DNA Polymerase copies approximately 6.4 billion base pairs in a diploid genome within ~6 hours, leaving only 1 to 5 uncorrected mutations per cell division. What would be the long-term evolutionary consequence if a biological organism evolved a "100% perfect" polymerase that made zero errors?

The species would lack new genetic variation/mutations, rendering it unable to adapt to changing environments over evolutionary time

In eukaryotic cells, what is the precise distinction between a centrosome and a centriole?

The centrosome serves as the primary Microtubule Organizing Center (MTOC) of the cell, and in animal cells it contains a pair of barrel-shaped centrioles made of microtubule triplets

Which statement best describes the structure and primary function of the kinetochore during cell division?

It is a specialized protein complex assembled at the centromeric region of each sister chromatid that acts as an anchor point for spindle microtubules

How do cohesin proteins and condensin proteins perform complementary roles during chromosome condensation in prophase?

Cohesins physically tie identical sister chromatids together along their length, whereas condensins wrap each chromatid pair into a compact structure

A human skin cell in G1 phase contains 46 unreplicated linear chromosomes. After passing through S phase and entering G2 phase, how many total DNA molecules (chromatids) are present, and how are they classified?

92 DNA molecules, organized as 46 pairs of identical sister chromatids held together by cohesins

Which key cellular event marks the transition out of prophase and the formal commencement of prometaphase?

The enzymatic breakdown of the nuclear envelope

How is the "metaphase plate" established during metaphase?

Equal and opposing pulling forces exerted by microtubules from opposite centrosomes create a "tug-of-war" that aligns bi-oriented sister chromatid pairs at the cell equator

What is the exact sequence of molecular events that triggers the destruction of cohesins and initiates anaphase?

Active APC/C ubiquitinates and degrades securin > free separase becomes active > separase cleaves cohesin proteins

How do the cellular mechanisms of telophase and cytokinesis differ in animal cells?

Telophase involves nuclear envelope reassembly around separated chromosomes, whereas cytokinesis involves a contractile ring of actin and myosin filaments pinching the cytoplasm

Taxol is a chemotherapeutic drug that binds tightly to tubulin subunits and prevents microtubule depolymerization (it stabilizes microtubules so they cannot shorten). If actively dividing cancer cells are treated with Taxol, at which specific stage of mitosis will they arrest, and why?

Metaphase/M-checkpoint, because anaphase requires kinetochore microtubules to depolymerize and shorten to pull chromatids to opposite poles

During mitosis in a mammalian cell line, a mutant form of securin is expressed that is completely resistant to degradation by APC/C (it can never be destroyed). What is the expected cellular outcome when these cells reach metaphase?

Separase will remain permanently inhibited by non-degradable securin, cohesin will never be cleaved, and sister chromatids will fail to separate, arresting the cell in metaphase

Which statement accurately distinguishes homologous chromosomes from sister chromatids?

Sister chromatids are exact duplicates of a single DNA molecule produced during S-phase replication; homologous chromosomes are equal-length chromosomes encoding the same gene loci, with one inherited from mom and one from dad

What constitutes a "bivalent" during meiotic prophase, and at which specific stage does it first assemble?

The association of two pairs of replicated sister chromatids (four chromatids total) from homologous chromosomes during synapsis in Early Prophase I

During Late Prophase I, what specific structural event occurs at a chiasma (plural: chiasmata)?

Non-sister chromatids within a bivalent cross over and physically exchange reciprocal segments of non-identical DNA

A research group isolates a mutant strain of yeast that completes synapsis in Early Prophase I but is entirely unable to form chiasmata or undergo crossing over between non-sister chromatids. What is the most direct consequence during Metaphase I and Anaphase I?

Homologous chromosomes will fail to maintain physical tension at the metaphase plate, leading to improper alignment and frequent chromosome mis-segregation (nondisjunction)

What is the fundamental mechanical difference between the chromosome segregation events of Anaphase I and Anaphase II?

Anaphase I separates homologous chromosome pairs (maternal from paternal), while Anaphase II separates identical sister chromatids

Why is Meiosis I specifically designated as a "reductional division," whereas Meiosis II is termed an "equational division"?

Meiosis I reduces the ploidy level from diploid (2n) to haploid (1n) by separating homologous chromosomes; Meiosis II separates sister chromatids without changing ploidy

In a species where the baseline somatic cell in G1 phase has 2n=4 chromosomes (4 linear DNA molecules), how many total DNA molecules (chromatids) and how many bivalents will be present in a single germ cell during Metaphase I?

8 DNA molecules and 2 bivalents

A geneticist tracks a specific gene locus with two alleles, A (maternal) and a (paternal). If a non-disjunction event occurs during MEIOSIS I during spermatogenesis, what will be the genetic makeup of the resulting gametes with respect to this locus after Meiosis II finishes?

50% of gametes will contain both A and a alleles (heterozygous extra chromosome), and 50% will lack this chromosome entirely

During meiotic division, the arm-cohesins holding homologous chromatid arms together are cleaved during Anaphase I, while centromeric cohesins holding sister chromatid centromeres are protected until Anaphase II. If a drug prematurely destroys ALL centromeric cohesins during Anaphase I, what will occur during Metaphase II?

Sister chromatids will have already separated prematurely in Anaphase I and will be unable to bi-orient at the Metaphase II plate, causing aberrant chromosome distribution

A cell culture slide shows condensed chromosomes aligned at the cell equator. Which visual feature under a high-resolution microscope would definitively identify the cell as being in METAPHASE I of Meiosis rather than Metaphase of Mitosis or Metaphase II?

Chromosomes are aligned in double rows as paired bivalents (homologous pairs linked at chiasmata) along the metaphase plate

Which of the flooring lists the phases of the cell cycle in the correct order?

G1, S, G2, M

Which of the following would be true of cell in S Phase?

I. Increase in number of deactivated Rb in the nucleus

II. Actively replicating chromosomes

III. Would be found in the largest peak in a flow cytometry graph

I & II

Which of the following are part of interphase?

I. S Phase

II. M Phase

III. G2 Phase

I and III

What type of cell might be found in G0 phase?

a neuron that does not divide

Suppose a diseased cell progresses from M phase to S phase, and then to M phase again. How might this deviation from the normal cell cycle progression affect the cell?

It will cause severe genomic instability, organelle imbalance, and protein level deviations, likely leading to cell death

What distinguishes homologous chromosomes from non-homologous chromosomes?

Homologous pairs share the same genes at corresponding loci

Suppose a cell was exposed to a drug that left cyclin-CDK complexes constitutively active. What might happen to the cell as a result?

The cell would undergo uncontrolled cell growth and division, possibly leading to cancerous proliferation

What effect might an increase in phosphorylating enzymes have on the cell?

It would hyperphosphorylate Rb, leading to the dissociation and activation of E2F, which drives unregulated progression from G1 to S phase

The cytoplasm from Cell A was injected into Cell B, immediately inducing the mitosis phase in Cell B. What phase might Cell A be in?

Anaphase

Two cells are being analyzed. They are both diploid, but one cell appears to have double the DNA content compared to the other cell. How is this possible?

one cell is in G2 phase while the other is in G1 phase

A drug decreasing the availability of cyclins may have what effect?

I. decreased activation of CDK-cyclin complexes.

II. prolonged binding of Rb to E2F.

III. prolonged DNA replication phase.

I & II

A segment of the replication bubble is being analyzed. It is found to contain higher activity of DNA polymerases compared to the adjacent strand. Which strand is being observed?

the lagging strand

Which of the following is FALSE of DNA Polymerase?

It is responsible for adding the primers to initiate replication

The new DNA strand is being synthesized in the _______ direction, while the template strand is being read in the _______ direction.

5' to 3', 3' to 5'

A cell is observed to have a severe mutation in the gene encoding topoisomerase. Which of the following effects will most likely be observed during DNA replication?

The two strands will not uncoil

When mismatched nucleotides occur, which is used to fix the abnormality?

None of the above

How would DNA replication proceed if there was no primase?

DNA replication would not proceed, as DNA Polymerase cannot synthesize a new chain on its own

Where would the enzyme ligase be most active?

in the lagging strand

Which is true of homologous chromosomes?

I. They are separated in meiosis I.

II. In humans, homologous chromosomes are tetraploid.

III. An example of a homologous pair is the maternal copy of chromosomes 7 and 11, and the paternal copy of chromosomes 7 and 11.

I only

In which phase does the nuclear membrane reform?

Telophase

Suppose a drug prevented the attachment of spindle fibers to the kinetochores. What would be the first stage of meiosis affected by this drug?

Prometaphase

Which is true of the spindle apparatus?

I. Spindle fibers are made of microtubules.

II. Spindle fibers attach to kinetochores.

III. The centrosomes retract the spindle fibers to ultimately pull chromosomes apart during anaphase.

I, II, and III

A centrosome...

I. is made of 2 centrioles

II. is made of 2 centromeres

III. is also known as a microtubule organizing center

IV. contains the kinetochores

I & III

Which of the following properly describes the ploidy of meiosis?

It begins with a diploid (2n) cell, reduces ploidy to haploid (n) during meiosis I, and maintains the haploid state through meiosis II.

In the following image, what process is occurring, and what is the purpose of this event?

Crossing over in Meiosis I, which is performed to increase genetic diversity.

What must happen for anaphase II to properly occur?

I. The cohesions connecting the sister chromatids must be broken.

II. The spindle fibers must be pulling on the kinetochores.

III. The nuclear membrane must start to reform.

I & II

A human cell contains 23 pairs of chromosomes. How many chromosomes will be present in a sex cell after meiosis has occured?

23 single chromosomes

What might occur if chromosome 7 undergoes crossing over with chromosome 2?

a chromosomal translocation leading to altered gene expression

When a cell divides, the resulting daughter cells are the same size as the original cell. Why is this the case?

During G1 phase, the cell grows and builds more organelles, which increases its size before cell division

In the following image, which phase is being depicted?

I. Metaphase of mitosis

II. Metaphase I of meiosis

III.  Metaphase II of meiosis

could be either I or III