Chromosome Structure and Cell Cycle

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Questions and Answers

What is a chromosome?

A chromosome is a single, long DNA molecule, which contains many genes, and is associated with proteins to help organize its structure. Chromosomes are found in the nucleus of eukaryotic cells and are visible under a microscope during cell division.

What is a chromatid?

A chromatid is one half of a duplicated chromosome. After DNA replication, a chromosome consists of two identical sister chromatids joined together at the centromere.

How many DNA double helices are present in an unreplicated chromosome?

Three
Four
Two
One (correct)

How many DNA double helices are present in a replicated chromosome?

Two (B) Signup and view all the answers

What are sister chromatids?

Two identical copies of a single chromosome (D) Signup and view all the answers

What are homologous chromosomes?

Pairs of chromosomes that are similar in shape, size, and genetic content (D) Signup and view all the answers

Which phase of the cell cycle does chromosome duplication occur?

S Phase (C) Signup and view all the answers

If a diploid cell has 8 chromosomes during G1, how many DNA double helices are present in G1?

8 (B) Signup and view all the answers

If a diploid cell has 8 chromosomes during G1, how many DNA double helices are present in G2?

16 (C) Signup and view all the answers

If a diploid cell has 8 chromosomes during G1, how many DNA double helices are present in Metaphase?

16 (D) Signup and view all the answers

If a diploid cell has 8 chromosomes during G1, how many DNA double helices are present in each daughter cell after cell division is complete?

8 (D) Signup and view all the answers

What is the role of the cytoskeleton in cytokinesis?

The cytoskeleton plays a critical role in cytokinesis by providing the structural framework and force necessary for cell division. Actin filaments form the contractile ring in animal cells, which helps pinch the cell membrane inward to create the cleavage furrow. In plant cells, microtubules guide vesicles to the center of the cell during cell plate formation, ensuring proper distribution of cell wall materials. Signup and view all the answers

What is MPF?

MPF, or M-phase Promoting Factor, is a protein complex that acts as a key regulator, triggering the start of M-phase (mitosis) in the cell cycle. Signup and view all the answers

What are the two main components of MPF?

MPF is composed of two main components: cyclin B and cyclin-dependent kinase 1 (Cdk1). Signup and view all the answers

How does MPF concentration change across the cell cycle?

Cyclin levels fluctuate during the cell cycle, gradually increasing throughout interphase (G1, S, and G2 phases) and peaking at the beginning of M-phase. Cdk levels remain constant, but are only active when bound to cyclin. Signup and view all the answers

How does MPF trigger M-phase?

When cyclin levels are high enough in G2, cyclin binds to Cdk1 to form active MPF. This activates a cascade of phosphorylation events that ultimately lead to the breakdown of the nuclear envelope, chromosome condensation, and the assembly of the mitotic spindle, ultimately pushing the cell into M-phase. Signup and view all the answers

What are the regulatory steps required for MPF activation?

MPF activation requires cyclin binding to Cdk1. Cdk1 is initially phosphorylated at two sites, an activating site and an inhibitory site. Before M-phase begins, a phosphatase enzyme removes the inhibitory phosphate, fully activating MPF. Active MPF can further activate phosphatases, creating a positive feedback loop that ensures a strong transition into M-phase. Signup and view all the answers

What causes the MPF concentration to decline sharply during M-phase?

During late M-phase, cyclin B is targeted for degradation by the anaphase-promoting complex (APC). The APC tags cyclin with ubiquitin, marking it for destruction by the proteasome. This leads to a sharp decline in MPF levels, as cyclin B is degraded, allowing the cell to exit mitosis and re-enter interphase. Signup and view all the answers

What category of proteins transfer a phosphate group to substrates?

The category of proteins that transfer a phosphate group to substrates is called kinases. Signup and view all the answers

Give an example of one such protein that plays a role in cell cycle regulation.

An example of a kinase that plays a crucial role in cell cycle regulation is the Cyclin-dependent kinase (Cdk). Signup and view all the answers

What are cell cycle checkpoints?

Cell cycle checkpoints are control points in the cell cycle that ensure each phase is completed correctly before moving to the next phase. Signup and view all the answers

Where in the cell cycle are the checkpoints found?

The cell cycle checkpoints are found at specific points within the G1, G2, and M phases. Signup and view all the answers

How do they differ?

Each checkpoint focuses on specific aspects of the cell cycle. The G1 checkpoint ensures that the cell has enough resources, has not suffered DNA damage, and is ready for DNA replication. The G2 checkpoint ensures that DNA replication is complete and accurate and that the cell is ready for mitosis. The M checkpoint ensures that all chromosomes are correctly attached to spindle fibers before the cell enters anaphase, preventing the incorrect separation of chromosomes. Signup and view all the answers

How are Rb, p53, and MPF involved in cell cycle checkpoints?

Rb, p53, and MPF are key players in regulating cell cycle checkpoints. Rb acts as a gatekeeper at the G1 checkpoint, ensuring that the cell enters S phase only when it is ready. p53 acts as a tumor suppressor that halts the cell cycle if DNA damage is detected. MPF is a protein complex that triggers the transition into M-phase. Signup and view all the answers

What events could lead to cell cycle arrest?

Several events can lead to cell cycle arrest. These include DNA damage, insufficient resources or signals, incomplete DNA replication, improper chromosome attachment, or the presence of abnormal chromosome numbers. These events can trigger checkpoint activation, halting the cell cycle to prevent the development of cancer. Signup and view all the answers

What is the cellular basis of cancer?

Cancer results from uncontrolled cell division due to a breakdown in the regulatory mechanisms that govern the cell cycle. In normal cells, strict controls ensure that division only occurs when it is beneficial for the organism. Cancerous cells ignore these signals, leading to abnormal growth and division. Signup and view all the answers

What defects are commonly found in cancer cells?

All of the above (F) Signup and view all the answers

Do all cancer cells have mutations in the same genes?

False (B) Signup and view all the answers

What are growth factors?

Growth factors are signaling molecules, usually proteins or polypeptides, released by certain cells to stimulate growth and division in nearby cells. Signup and view all the answers

What role do growth factors play in the control of the cell cycle?

Growth factors serve as a form of social control over the cell cycle, particularly at the G1 checkpoint. They activate intracellular signaling pathways that lead to the production of regulatory proteins, such as cyclins and E2F, which ultimately influence the decision of whether the cell will enter S-phase or remain in G1. Signup and view all the answers

What is the relationship of cancer to the G1 checkpoint?

Cancer is closely related to a loss of control over the G1 checkpoint, often due to defects in the pathways that normally respond to growth factors. Signup and view all the answers

How can overproduction of growth factors or oncogene activation contribute to cancer?

Cancer cells may overproduce growth factors themselves or mutate genes (proto-oncogenes) involved in growth factor signaling pathways, turning them into oncogenes. These mutations can lead to continuous signaling for the cell to divide, even in the absence of external growth factors. Signup and view all the answers

How can disruption of Rb function contribute to cancer?

In cancer, Rb protein is often defective or excessively phosphorylated due to high levels of cyclin-Cdk activity. This prevents Rb from binding to E2F, leading to uncontrolled cell cycle progression through G1, even without proper growth signals. Signup and view all the answers

How can mutations in tumor suppressors like p53 contribute to cancer?

p53 is another checkpoint regulator that can stop the cell cycle if DNA damage is detected. However, mutations in p53 can prevent this halt, allowing cells with damaged DNA to continue dividing. When p53 is nonfunctional, cells skip this crucial safety check, contributing to tumor formation and growth. Signup and view all the answers

How can loss of social control contribute to cancer?

Cancer cells often produce their own growth signals or ignore the need for external growth factors, allowing them to grow without the social control signals that normally restrict cell division to the needs of the organism. Signup and view all the answers

How are mitosis and meiosis similar?

Mitosis and meiosis are both processes of cell division that involve the duplication of chromosomes. Both processes are essential for maintaining the genetic material during cell division. Both processes also involve similar stages, such as prophase, metaphase, anaphase, and telophase. Signup and view all the answers

How do mitosis and meiosis differ?

Mitosis and meiosis differ in the number of cell divisions, the number of daughter cells produced, the ploidy of the daughter cells, and the types of cells in which they occur. Mitosis produces two diploid daughter cells, while meiosis produces four haploid daughter cells. Mitosis occurs in somatic cells, while meiosis occurs in germ cells. Signup and view all the answers

What is the difference between diploid and haploid cells?

Diploid cells contain two sets of chromosomes, one set inherited from each parent. Haploid cells contain only one set of chromosomes. Humans have 46 chromosomes in diploid cells and 23 chromosomes in haploid cells. Signup and view all the answers

What cells in the body are diploid?

Most cells in the body are diploid, including somatic cells such as skin, muscle, and liver cells. These cells undergo mitosis to produce more diploid cells for growth, repair, and maintenance. Signup and view all the answers

What cells in the body are haploid?

Only gametes (sperm and egg cells) are haploid cells. These cells are produced through meiosis, which reduces the chromosome number by half to prepare for sexual reproduction. Signup and view all the answers

What kind of diploid cells can become haploid cells?

Germ line cells are the only diploid cells in the body that can become haploid cells. Germ line cells are precursor cells located in the ovaries and testes, which go through meiosis to produce haploid gametes. Signup and view all the answers

What is ploidy?

Ploidy refers to the number of sets of chromosomes in a cell. Diploid cells contain two sets of chromosomes, while haploid cells contain one set of chromosomes. Signup and view all the answers

What are homologous chromosomes?

Homologous chromosomes are pairs of chromosomes that are similar in size, shape, and genetic content. They are not identical copies and may have different alleles for the same genes. These pairs are inherited from each parent, one homolog from the mother and one from the father. Signup and view all the answers

What is crossing over?

Crossing over is a process during meiosis in which homologous chromosomes exchange segments of genetic material. During this process, non-sister chromatids from homologous chromosomes align closely at points called chiasmata, where they break and exchange corresponding segments of DNA. This exchange leads to new combinations of alleles on each chromatid. Signup and view all the answers

When does crossing over occur?

Crossing over occurs during prophase I of meiosis, specifically when homologous chromosomes pair up in a process called synapsis before being separated in anaphase I. Signup and view all the answers

How does crossing over influence genetic diversity?

Crossing over creates new combinations of alleles on each chromatid, which leads to genetic recombination. As a result, each gamete receives a unique set of genetic information. This increased variability means that offspring inherit a mixture of traits, not just direct copies of either parent's chromosome arrangements. Signup and view all the answers

What is independent assortment?

Independent assortment is a process during meiosis in which homologous chromosome pairs are distributed randomly into daughter cells. This randomness leads to different combinations of maternal and paternal chromosomes in each gamete, contributing to genetic diversity. Signup and view all the answers

Which steps in meiosis lead to independent assortment?

Independent assortment occurs during metaphase I of meiosis when homologous chromosome pairs align at the metaphase plate. Each pair can align with either maternal or paternal chromosomes on either side of the plate, and this alignment is independent for each pair when they are separated during anaphase I, the chromosomes are distributed randomly to the daughter cells. Signup and view all the answers

Is self-fertilization likely to result in offspring that are genetically identical to the parent?

False (B) Signup and view all the answers

Is self-fertilization likely to result in offspring that are genetically identical to each other?

False (B) Signup and view all the answers

Flashcards

Chromosome

A single long DNA molecule containing many genes and associated with proteins to organize its structure.

Chromatid

One half of a duplicated chromosome, identical to its sister chromatid, joined at the centromere.