The Cell Cycle

Question 1

Name the two types of eukaryotic cell division.

Answer 1

1. mitosis
2. meiosis

Mitosis produces somatic cells that are identical to the parent, while meiosis produces germ cells with only half of the parental genetic material.

Question 2

Explain the difference between a haploid and a diploid cell.

Answer 2

• Haploid cells have only one full set of chromosomes. In humans, the only haploid cells are germ cells (sperm and eggs) produced by meiosis.
• Diploid cells have two distinct sets of chromosomes. In humans, all somatic cells are diploid and are produced by mitosis.

Question 3

What is the difference between germ cells and somatic cells?

Answer 3

• Germ cells, including sperm and ova, are haploid and participate in reproduction. They are formed via meiosis.
• Somatic cells are diploid and include the rest of the cells in the body. Somatic cells only undergo mitosis.

Question 4

What features distinguish homologous chromosomes from sister chromatids?

Answer 4

• Homologous chromosomes can be genetically different; each homologous chromosome has the same loci, but can have different genes than its partner. Homologous chromosomes separate during meiosis I.
• Sister chromatids are genetically identical, as they result from the replication of a single chromosome. Sister chromatids separate during meiosis II and mitosis.

Question 5

Name the microtubule-based structure that separates chromosomes during cell division.

Answer 5

The spindle apparatus

Spindle fibers radiate from the centrosome, which contains centrioles, and bind at the kinetochore region of the centromere.

Question 6

Describe the role of the centrioles during cell division.

Answer 6

They are cylinder-shaped microtubular structures, generally found in pairs as part of the centrosome, which act to organize the spindle apparatus along which the chromosomes align.

Question 7

Describe the role of the centromere during cell division.

Answer 7

At the centromere, sister chromatids attach in pairs. This region also contains the kinetochore, which binds to a spindle fiber and allows the chromatids to be pulled to opposite poles of the cell.

The centromere is the location on the chromosome that appears “pinched.”

Question 8

Describe the role of the kinetochore during cell division.

Answer 8

It is a complex arrangement of various proteins that forms on the centromere. Here, spindle fibers bind, allowing sister chromatids to be separated to opposite poles of the cell.

Question 9

Which type of cytoskeletal fiber is associated with MTOCs?

Answer 9

Microtubules are anchored at MTOCs, or microtubule-organizing centers. The fibers attach to the MTOC at one end and radiate toward other parts of the cell.

Centrosomes are a specific type of MTOC involved in cell division. Spindle fibers attach to the centrosomes, each of which is composed of two tubulin-based centrioles.

Question 10

Label the following structures on the diagram below: centrosome, centriole, spindle fiber, aster.

Answer 10

Question 11

Name the four phases of the cell cycle.

Answer 11

1. G₁
2. S (synthesis)
3. G₂
4. M (mitosis)

An additional phase is G₀, in which the cell is not immediately preparing for cell division. G₀ can be either temporary or permanent.

Question 12

Which phases of the cell cycle are part of interphase?

Answer 12

• G₁
• S
• G₂

In these phases, the cell grows, replicates its DNA, and prepares for mitosis.

The M phase (mitosis itself) is not a part of interphase. G₀ is generally thought to be distinct when it is permanent (as in neurons, which do not divide), but that is unlikely to be tested on the AP Biology exam.

Question 13

What process most notably occurs in the cell during S phase?

Answer 13

DNA replication

More specifically, the S, or “synthesis,” phase of the cell cycle involves replication of all chromosomes to form identical copies. Together, a chromosome and its copy are termed “sister chromatids.”

Question 14

What processes most notably occur during the G₁ and G₂ phases of the cell cycle?

Answer 14

The G₁ and G₂ phases are marked by growth and the synthesis of proteins, including enzymes.

These phases help the cell prepare for cell division, or mitosis (the M phase).

Question 15

What does the cell assess at the G₁ checkpoint?

Answer 15

The DNA, to ensure that it is in proper condition to be replicated.

The G₁ checkpoint is also known as the G₁/S checkpoint or the restriction checkpoint.

Passage through this checkpoint is required to enter the S phase of the cell cycle.

Question 16

What does the cell assess at the G₂ checkpoint?

Answer 16

It makes sure that sufficient cell growth has occurred and that the DNA was replicated properly.

The G₂ checkpoint is also known as the G₂/M checkpoint.

Passage through this checkpoint is required to enter the M phase of the cell cycle.

Question 17

What is the significance of the G₀ phase?

Answer 17

It is a stage of prolonged inactivity. In this phase, the cell is not preparing for mitosis.

Mature neurons and cardiac muscle are in a permanent G₀ phase, since they do not undergo mitosis.

Question 18

During which phase of the cell cycle would DNA helicase be especially active?

Answer 18

DNA helicase, as well as the other enzymes involved in DNA replication, would be most active during the S phase.

In the S (synthesis) phase, DNA is replicated to prepare for cell division.

Question 19

Which two types of molecules determine when a cell will enter the next phase of the cell cycle?

Answer 19

• cyclins
• cyclin-dependent kinases

Cyclins are proteins that change in concentration depending on a cell’s location in the cell cycle. These proteins can bind to cyclin-dependent kinases (Cdks), which are enzymes that phosphorylate other proteins. The behavior of cyclins and Cdks is key in the regulation of the cell cycle.

Question 20

List the four phases of mitosis in the order that they occur in a dividing cell.

Answer 20

1. prophase
2. metaphase
3. anaphase
4. telophase and cytokinesis

Question 21

What cellular events characterize prophase?

Answer 21

• Loose chromatin condenses into visible chromosomes.
• The nuclear membrane disintegrates.
• The nucleolus disappears.
• The centrioles and spindle apparatus form.

Question 22

What cellular events characterize metaphase?

Answer 22

• At this point, spindle fibers are bound to the kinetochore of each chromosome.
• The chromosomes line up along the center of the cell, a region called the metaphase plate.

Question 23

What cellular events characterize anaphase?

Answer 23

• Sister chromatids are cleaved apart.
• Spindle fibers shorten, causing each sister chromatid to move to the opposite pole as its partner.

Question 24

What cellular events characterize telophase?

Answer 24

• The cell elongates.
• Each daughter nucleus forms a new nuclear membrane and nucleolus.
• The chromosomes start to become less condensed.

Question 25

Define:

cytokinesis

Answer 25

It is the actual formation of two distinct cells, characterized by the distribution of cytosol and organelles and the pinching of the cell membrane between the two nuclei.

Cytokinesis happens simultaneously with the later phases of mitosis and meiosis, but is often grouped with telophase.

Question 26

In animal cells, the first marker of cytokinesis is an indentation between the two newly-forming daughter cells. What term is given to this indentation?

Answer 26

The cleavage furrow.

This indentation acts as a contractile ring, gradually closing due to the presence of actin filaments which pinch it shut. At the end of cytokinesis, the two daughter cells are fully separated.

Question 27

Name four broad differences between mitosis and meiosis.

Answer 27

• Mitosis involves a single division, while meiosis involves two.
• Mitosis produces two identical diploid cells, while meiosis produces up to four haploid cells that are not identical.
• Mitosis is used to form somatic cells, while meiosis produces germ cells (gametes).
• Crossing over occurs in meiosis, but does not happen in mitosis.

Question 28

In animal cells, another term for meiosis is:

Answer 28

Gametogenesis

Gametes, or sex cells, are produced via meiosis in animal cells. These gametes are haploid, meaning that they have half of the chromosomes held by a typical somatic (body) cell. When two gametes join during the process of fertilization, they create a zygote, or diploid cell that can grow into a new organism.

Question 29

In animal cells, two types of gametogenesis can occur. What are these two types?

Answer 29

• Spermatogenesis
• Oogenesis

Biologically speaking, spermatogenesis occurs in male organisms and oogenesis occurs in females.

Question 30

Name the eight stages of meiosis in order.

Answer 30

1. prophase I
2. metaphase I
3. anaphase I
4. telophase I
5. prophase II
6. metaphase II
7. anaphase II
8. telophase II

This process resembles two consecutive rounds of mitosis, but has some significant differences that you should know for the AP Biology exam.

Question 31

The pairing of homologous chromosomes that occurs during meiosis is known as:

Answer 31

Synapsis

This process occurs during prophase I of meiosis and allows for crossing over, or genetic exchange between the homologous chromosomes, to occur.

Question 32

Define:

tetrad

Answer 32

A structure that forms during prophase I of meiosis.

This structure consists of both sets of homologous chromosomes, each of which includes two sister chromatids, all in close proximity to each other.

Question 33

During which stage of meiosis do daughter cells become haploid?

Answer 33

After meiosis I.

Meiosis I is known as “reductional division,” since homologous chromosomes separate during this stage. This leaves two daughter cells, each with a single chromosome from each pair and its replicated copy. Meiosis II, or “equational division,” separates these copies.

Question 34

Mitosis is broadly similar, but not identical, to meiosis I. Name three differences between the two processes.

Answer 34

• Mitosis involves the separation of sister chromatids, while meiosis I involves the separation of homologous chromosomes.
• Mitosis produces diploid cells, while the products of meiosis I are haploid.
• Crossing over occurs in prophase of meiosis I, but does not occur at all during mitosis.

Question 35

Which process of genetic recombination occurs during prophase I of meiosis?

Answer 35

Crossing over, or the trading of DNA segments between homologous chromosomes, occurs when these chromosomes are paired together in prophase I.

Question 36

Which two processes ensure that none of the four daughter cells produced during meiosis are genetically identical?

Answer 36

1. crossing over
2. independent assortment

Crossing over is a method of genetic recombination that occurs during prophase I of meiosis. In this process, homologous chromosomes exchange genetic material, increasing total variation.

The principle of independent assortment, or Mendel’s second law, states that genes for different traits separate into gametes independently of each other (as long as those traits are not linked).

Question 37

An individual develops a disease in which certain cells undergo mitosis rapidly, with interphase dramatically shortened. What name is commonly used for this condition?

Answer 37

Cancer

Cancer can be promoted by many factors, including carcinogens, DNA damage, failure of tumor suppressor genes, and failure of apoptosis, or a combination of these events.

Question 38

In which situations might a cell undergo apoptosis?

Answer 38

Also known as programmed cell death, this generally occurs in damaged cells. However, it can also serve as a normal part of development.

Viral infection, UV-induced damage, and hypoxia can all induce apoptosis.