AP Biology : Understanding the Cell Cycle

Study concepts, example questions & explanations for AP Biology

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Example Questions

Example Question #11 : Understanding The Cell Cycle

During which of the following phases will a human somatic cell contain the least amount of DNA?

Possible Answers:

G2 phase

Metaphase

G1 phase

Prophase

Correct answer:

G1 phase

Explanation:

Remember that DNA is replicated during the S phase of the cell cycle. Immediately after mitosis, the cell contains a single chromatid for each chromosome, for a total of 46 chromatids. This period corresponds to the G1 phase, during which the cell produces proteins and grows.

The S phase follows the G1 phase and doubles the amount of DNA to 92 chromatids, composing 46 complete chromosomes with two chromatids each. The S phase is followed by the G2 phase, which is followed by the M phase (or mitosis). Through this remaining period, until the conclusion of mitosis, the cell contains 92 chromatids.

Only G1 phase precedes the S phase and has half the amount of DNA as the rest of the cell cycle; therefore, cells in the G1 phase have the least amount of DNA.

Example Question #12 : Understanding The Cell Cycle

Histones, molecules that package DNA into nucleosomes, are mostly made up of __________.

Possible Answers:

fatty acids

monosaccharides

amino acids

nucleotides

Correct answer:

amino acids

Explanation:

Histones are nuclear proteins that organize and compact DNA. Histones act as core proteins around which DNA molecules can wrap and form nucleosomes. Since histones are proteins, they are mostly made up of amino acids.

Recall that amino acids are the building blocks of proteins, nucleotides are the monomers of nucleic acids, and monosaccharides are the monomers of carbohydrates. Fatty acids are a common component in lipids.

Example Question #11 : Cellular Division

The cell cycle has many checkpoints to ensure the cell is ready for division. Which of the following can occur if a cell skips cell cycle checkpoints?

Possible Answers:

The cell can have inhibited growth

The cell can divide into three daughter cells instead of two

The cell can become cancerous

The cell can grow in size, but never divide

None of these could occur

Correct answer:

The cell can become cancerous

Explanation:

The checkpoints in the cell cycle are necessary to make sure the cell is ready to divide. Certain checkpoints will check the size of the cell, the integrity of the DNA, and the proper attachment of spindle fibers. If a cell fails to use these checkpoints, it can continue to divide even when it is not ready.

This unregulated division is a hallmark of cancer, in which a cell continuously divides without the necessary checkpoints. Cancerous cells often display genetic defects due to bypassing checkpoints.

Example Question #14 : Understanding The Cell Cycle

During cellular interphase, DNA appears to have regions of tight winding within its strands. This gives the appearance of "beads" within the DNA. Which of the following forms the "beads" seen during interphase?

Possible Answers:

Heterochromatin

Nucleoids

Histone tails

Looped domains

Nucleosomes

Correct answer:

Nucleosomes

Explanation:

Nucleosomes are complexes of DNA wrapped around eight histone molecules, often resembling "beads on a string" during interphase. The tight wrapping of DNA around the histones prevents it from being transcribed, but also condenses it. Nucleosomes can be loosened to regulate the transcription of associate DNA and genes.

Looped domains are higher order structures in which the DNA is more tightly packed. Heterochromatin refers to clumps of tightly packed chromatin. Histone tails are extensions of the histone molecules. Nucleoids are the region in prokaryotic cells that houses the cell's DNA.

Example Question #15 : Understanding The Cell Cycle

In the cell cycle, which of the following steps is not a component of interphase?

Possible Answers:

G2 phase

G1 phase

M phase

S phase

Correct answer:

M phase

Explanation:

Interphase alternates with the mitotic phase and accounts for approximately 90% of the cell cycle. During interphase, the cell grows (G1 phase), replicates/synthesizes its chromosomes (S phase), and completes preparation for cell division (G2 phase).

The M phase occurs after the G2 phase and corresponds with mitosis, which is not considered part of interphase.

Example Question #16 : Understanding The Cell Cycle

The accumulation of cyclin protein results in which of the following?

Possible Answers:

Degradation of cyclin-dependent kinases

Growth factor synthesis

Progression through cell cycle checkpoints

Completion of DNA synthesis

Correct answer:

Progression through cell cycle checkpoints

Explanation:

Cyclin proteins bind to cyclin-dependent kinases and drive the cell division cycle. One mechanism to control this process is through the amount of cyclin protein present and available to bind and activate cyclin-dependent kinases (CDKs). The cyclin-CDK complexes provide signals for the cell to progress through cell cycle checkpoints. After completion of a stage of the cell cycle, the cyclin protein is rapidly degraded to prevent inappropriate signaling. Overproduction or accumulation of cyclin protein can cause inappropriate continuation through the cell cycle and unregulated cell division.

Example Question #17 : Understanding The Cell Cycle

A developing cell can pass the G1 checkpoint in the cell cycle if sufficient conditions are present for its further growth. This process is regulated by a number of proteins, including cyclins and their associated kinases. Imagine that a cell is arrested at the G1 checkpoint and never proceeds further. Which of these statements properly describes the situation?

Possible Answers:

The cell has already undergone prophase of mitosis, but will not enter metaphase

The cell has not yet replicated its chromosomes, and will not be able to do so because it cannot enter the S phase

The cell has already replicated its chromosomes, but will not proceed into the S phase

The cell has already replicated its chromosomes, but will not enter mitosis

Correct answer:

The cell has not yet replicated its chromosomes, and will not be able to do so because it cannot enter the S phase

Explanation:

The stages of the cell cycle begin with G1, during which the cell grows and prepares for chromosome replication by synthesizing proteins and cellular structures. The chromosomes do not actually duplicate themselves until the next step, the S phase. After the S phase comes G2, another resting/growth period, and if the cell is properly developed it can then finally enter mitosis. If the cell stops at the G1 checkpoint, it has been arrested before it can enter the S phase and its chromosomes will be unable to replicate.

The three key checkpoints in the cell cycle occur after the G1 phase, before chromosome replication, after the G2 phase, before mitosis, and after metaphase, before chromatids are separated.

Example Question #18 : Understanding The Cell Cycle

What is the restriction point?

Possible Answers:

A decision point in mitosis; if the restriction point is passed mitosis will not progress

A decision point in mitosis; if the restriction point is passed mitosis will progress

A decision point in the cell cycle; if the restriction point is passed the cell cycle will progress to the next phase

A decision point in the cell cycle; if the restriction point is passed the cell cycle will not progress to the next phase

Correct answer:

A decision point in the cell cycle; if the restriction point is passed the cell cycle will progress to the next phase

Explanation:

The restriction point is a decision point in G1 of the cell cycle. If the cell passes this point, the cell cycle will progress to the S phase. If the cell does not pass this point, there is likely a lack of stimulation from nutrients or growth factors, or some type of internal damage that must be corrected before the cell cycle can progress. Prolonged lack of progression past the restriction point can result in a quiescent cell, which enters the G0 phase and does not readily divide.

The other two cell cycle checkpoints occur after the G2 phase (before mitosis) and after metaphase (before anaphase).

Example Question #11 : Understanding The Cell Cycle

A scientist takes a sample of human epithelial cells, injects it with a dye that highlights the genetic information, and places the sample under the microscope. What will the genetic information for the majority of the cells look like?

Possible Answers:

Condensed into 23 discrete chromosomes and floating in the cytoplasm

Condensed into 46 discrete chromosomes and restricted to the nucleus

Condensed into a single circular chromosome and floating in the cytoplasm

Uncondensed and restricted to the nucleus

Condensed into 23 discrete chromosomes and restricted to the nucleus

Correct answer:

Uncondensed and restricted to the nucleus

Explanation:

The DNA will appear uncondensed and restricted to the nucleus. This is because a majority of the cells found in the sample will be in interphase, the longest stage of the cell cycle. Since it is the longest phase in the cell cycle, a majority of cells will be undergoing this phase at any given sample time. It is a point when the cell is undergoing normal cell processes and preparing for replication. During this phase, the DNA is uncondensed, allowing for transcription and giving DNA replication enzymes easy access to the nucleotide sequence.

DNA is condensed into 46 discrete chromosomes and restricted to the nucleus during early prophase, which makes up only a short period of the cell cycle. DNA is never condensed into 23 discrete chromosomes in somatic cells (such as epithelium), since 23 chromosomes would indicate a haploid cell. In humans, haploid cells only exist in the gametes, or sex cells. While prokaryotes have a single circular chromosome floating in the cytoplasm, such structure for a eukaryotic genome is not possible.

Example Question #20 : Understanding The Cell Cycle

In the cell cycle, what happens if the cell contains cyclin proteins at the G1 checkpoint? 

Possible Answers:

The cell goes into the G0 phase

The cell goes into the G1 phase

The cell stays in the G1 phase

The cell goes into the S phase

The cell goes into the M phase

Correct answer:

The cell goes into the S phase

Explanation:

The G1 phase is the first stage of interphase. During this period, the cell grows and develops. In order to proceed to the next phase of interphase, the cell must pass the G1 checkpoint. In order for this to happen, cyclin proteins must be present and the proper cellular conditions must occur. If the cell passes the checkpoint it will proceed to the S phase, during which DNA replication occurs. If the cell fails to pass the checkpoint it can enter the G0 phase and become quiescent.

The G2 phase follows the S phase and ends with the G2 checkpoint. This checkpoint determines if the cell will enter mitosis, the M phase.

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