Lecture 4

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Questions and Answers

Which protein directly inhibits the activity of Cdk1 complexes?

p15

p16

p21 (correct)

p19

What happens to the concentration of p27 when a cell enters the resting phase?

It increases as a regulatory mechanism (correct)

It remains constant

It decreases due to cellular uptake

It undergoes degradation

Which of the following is not a characteristic of apoptosis?

It is a passive process. (correct)

It plays a role in removing damaged cells.

It involves single cells.

It is an active, energy-requiring process.

Which event is most indicative of necrosis rather than apoptosis?

Inflammatory response and cell swelling (B) Signup and view all the answers

Which complex is not inhibited by INK4 proteins?

Cdk1 (D) Signup and view all the answers

What is the direct effect of the APC/C on the securin-separase complex?

It causes the disintegration of the complex, releasing active separase. (B) Signup and view all the answers

What is the role of active separase in the cell cycle?

To break down the cohesin proteins, allowing for sister chromatid separation. (A) Signup and view all the answers

How does the APC/C facilitate the degradation of the MPF complex?

By adding ubiquitin to cyclin, marking it for degradation. (B) Signup and view all the answers

What is the immediate effect of increased p53 protein concentration following DNA damage?

It increases production of the p21 protein. (A) Signup and view all the answers

If a cell has DNA damage and expresses p21 protein, at which phase will the cell cycle be arrested?

G1 phase. (B) Signup and view all the answers

During which stage of Prophase I does the synaptonemal complex form?

Zygotene (A) Signup and view all the answers

What is the primary event that occurs during the pachytene stage of meiosis?

Crossing-over between non-sister chromatids (D) Signup and view all the answers

Which of the following best describes what a bivalent/tetrad is?

A pair of homologous chromosomes each with two sister chromatids (D) Signup and view all the answers

What is the function of chiasmata during meiosis?

To mark the locations where crossing-over has occurred (C) Signup and view all the answers

During which stage of Prophase I does the disappearance of the synaptonemal complex first begin?

Diplotene (B) Signup and view all the answers

What structures are directly held together by cohesins in the context of homologous chromosomes during prophase I?

Sister chromatids of a single chromosome (C) Signup and view all the answers

Which process is essential for increasing genetic diversity during meiosis?

Homologous recombination in crossing over (D) Signup and view all the answers

What is the significance of multiple chiasmata forming in a bivalent?

It ensures that multiple random allele exchanges are more likely to occur (C) Signup and view all the answers

During which phase of meiosis do homologous chromosomes separate and move to opposite poles?

Anaphase I (C) Signup and view all the answers

What is the primary outcome of Meiosis I in terms of chromosome number?

The daughter cells have half the number of chromosomes as the parent cell (B) Signup and view all the answers

Which of the following best describes what occurs during diakinesis?

Chromosome condensation is completed and chiasmata migrate towards the ends of the chromosomes. (D) Signup and view all the answers

Which checkpoint is responsible for checking DNA damage and the progression of replication forks?

S checkpoint (B) Signup and view all the answers

What is the main function of the spindle checkpoint during the M phase?

To ensure chromosomes are properly aligned at the metaphase plate. (A) Signup and view all the answers

How is Meiosis II different from Mitosis?

Meiosis II divides cells already containing one set of homologous chromosomes, while mitosis is not concerned with cell ploidy. (B) Signup and view all the answers

What is the primary function of a mitogen in cell cycle regulation?

Induce a cell to begin cell division or increase its rate. (B) Signup and view all the answers

At the beginning of Meiosis II, how many sister chromatids are present per chromosome?

Two (D) Signup and view all the answers

What occurs during Anaphase II of meiosis?

Sister chromatids get separated and move towards cell poles (A) Signup and view all the answers

Which of the following protein complexes is directly involved in degrading cyclins?

Anaphase Promoting Complex (APC/C) (D) Signup and view all the answers

What is the result of Telophase I?

Two haploid daughter cells with partially decondensed chromosomes. (B) Signup and view all the answers

What is the primary role of cyclin-dependent kinases (Cdks) in the cell cycle?

To phosphorylate protein targets when complexed with cyclins. (D) Signup and view all the answers

During which phase of the cell cycle is the activity of the G1-Cdk complex highest?

Middle of G1 phase (B) Signup and view all the answers

What is the primary function of cell cycle regulators?

To control the activation or inhibition of different signaling pathways influencing cell division (A) Signup and view all the answers

What is the immediate effect on cyclin-Cdk complexes after they are activated by the phosphatase Cdc25?

The complex transfers phosphate residues to target proteins (A) Signup and view all the answers

During which phase of the centrosome cycle does centrosome duplication occur?

G1 and S phase (D) Signup and view all the answers

Which cyclin is primarily associated with the S phase of the cell cycle?

Cyclin A (D) Signup and view all the answers

Which of the following is a key event that occurs during prophase?

Condensation of chromosomes (A) Signup and view all the answers

How are cyclin levels regulated during the cell cycle?

They are ubiquitinated and degraded at specific phases. (B) Signup and view all the answers

What is the primary function of the interactions between microtubules growing from opposite centrosomes?

To stabilize the microtubules and create a bipolar mitotic spindle (D) Signup and view all the answers

What is the main function of the G1-Cdk complex in the G1 phase?

To phosphorylate pRb protein. (A) Signup and view all the answers

At what stage of mitosis do the microtubules of the mitotic spindle attach to the kinetochores of the chromosomes?

Prometaphase (C) Signup and view all the answers

What is the role of Wee1 in regulating cyclin-Cdk complex activity?

It adds inhibitory phosphate residues to Cdk. (B) Signup and view all the answers

During which phase of mitosis are the chromosomes aligned along the equatorial plane of the mitotic spindle?

Metaphase (A) Signup and view all the answers

Which of the following is NOT an internal cell-cycle regulator?

Mitogens (B) Signup and view all the answers

What event directly leads to the separation of sister chromatids during anaphase?

The breaking of cohesin connections (B) Signup and view all the answers

Which of the following occurs during telophase?

The mitotic spindle disappears and the nuclear envelope reforms (A) Signup and view all the answers

What is the function of the contractile ring during cytokinesis?

To form a cleavage furrow that divides the cytoplasm (B) Signup and view all the answers

In organisms where the diploid form is dominant, what type of cells undergo meiosis?

Only germ cells (sex cells) (D) Signup and view all the answers

What key event distinguishes meiosis I from meiosis II?

Meiosis I is a reduction division whereas meiosis II is a compensatory division (B) Signup and view all the answers

During which phase of mitosis does the formation of a bipolar mitotic spindle occur?

Prophase (B) Signup and view all the answers

What is the primary role of condensins during prophase?

Condensing chromosomes (B) Signup and view all the answers

The disintegration of the nuclear membrane occurs during which stage of mitosis?

Prometaphase (A) Signup and view all the answers

What is the result of the tension generated in kinetochores during prometaphase?

Attachment of each chromosome to both poles of the spindle (B) Signup and view all the answers

What is the primary event that occurs at the metaphase plate during metaphase?

Alignment of chromosomes (D) Signup and view all the answers

During anaphase, what is the immediate effect of the breakdown of cohesins?

Separation of sister chromatids (D) Signup and view all the answers

What process is initiated by the formation of a contractile ring during telophase?

Cytokinesis (C) Signup and view all the answers

What is the key characteristic of meiosis I that distinguishes it from meiosis II?

Reduction in chromosome number (D) Signup and view all the answers

During which phase of the centrosome cycle are both centrosomes connected?

G2 phase (C) Signup and view all the answers

What happens to microtubules once they interact during the assembly of the mitotic spindle?

They are stabilized (A) Signup and view all the answers

During which stage of Prophase I does the synthesis of histones and further condensation of chromosomes occur?

Pachytene (C) Signup and view all the answers

Which event is directly associated with the zygotene stage of Prophase I?

Pairing of maternal and paternal homologous chromosomes (C) Signup and view all the answers

What is the primary function of the synaptonemal complex during meiosis?

To connect homologous chromosomes into bivalents (B) Signup and view all the answers

What is the relationship between chiasmata and homologous recombination during Pachytene?

Chiasmata are the physical manifestations of homologous recombination (A) Signup and view all the answers

Which process signifies the end of crossing-over and the beginning of the separation of homologous chromosomes during Prophase I?

Diplotene (C) Signup and view all the answers

What is the role of cohesins in the context of homologous chromosomes during Prophase I?

To hold sister chromatids of homologous chromosomes together. (A) Signup and view all the answers

During which stage of Prophase I does the movement of cell organelles to the periphery of the cell prominently take place?

Leptotene (A) Signup and view all the answers

What is the direct result of homologous recombination during pachytene?

Exchange of DNA segments between non-sister chromatids (A) Signup and view all the answers

A diploid cell in humans contains how many chromosomes?

46 (D) Signup and view all the answers

Which of the following accurately describes the primary function of the cell cycle?

To accurately duplicate DNA, and segregate it into genetically identical daughter cells. (C) Signup and view all the answers

Which phase of the cell cycle constitutes the majority of its duration?

Interphase (D) Signup and view all the answers

During which phase of interphase does DNA synthesis occur?

S phase (B) Signup and view all the answers

What is the primary function of the G1 phase of the cell cycle?

Cell growth and preparation for DNA replication (B) Signup and view all the answers

What is the significance of cohesin rings during the S phase?

They link sister chromatids together. (C) Signup and view all the answers

Which of the following cellular components are primarily synthesized during the G2 phase?

Mitotic spindle proteins and non-histone proteins (B) Signup and view all the answers

What is the primary reason for cells to enter the G0 phase?

When conditions for division are unsuitable. (C) Signup and view all the answers

Which term describes nuclear division?

Karyokinesis (B) Signup and view all the answers

During which phase of the M phase does cytokinesis begin?

Anaphase (A) Signup and view all the answers

What role does the protein p53 play in relation to p27?

Regulates the level of p27 at the transcription stage. (B) Signup and view all the answers

Which statement distinguishes apoptosis from necrosis?

Apoptosis is an active process associated with gene activation. (B) Signup and view all the answers

Which of the following proteins are classified under the CIP/KIP group of CdK inhibitors?

p21, p27, p57 (B) Signup and view all the answers

Which characteristic is associated with necrosis but not with apoptosis?

Involves inflammatory response (B) Signup and view all the answers

What effect does an increase in p27 concentration have on the cell cycle?

Leads to cell cycle arrest. (B) Signup and view all the answers

What is the role of cyclin A during the S phase of the cell cycle?

It prevents DNA replication after the first round. (A) Signup and view all the answers

Which statement accurately describes the function of the MPF complex during the cell cycle?

It induces the degradation of the nuclear envelope. (C) Signup and view all the answers

How does the activation of the MPF complex contribute to mitosis?

By promoting chromosome condensation. (D) Signup and view all the answers

What prevents the formation of an active cyclin B-Cdk1 complex during the G2 phase?

The complex remains inactive until cyclin B is fully synthesized. (D) Signup and view all the answers

What initiates the positive-feedback loop during the activation of MPF complexes?

The activation of Cdc25 phosphatase to remove phosphate groups. (D) Signup and view all the answers

What significant event occurs during Telophase I of meiosis?

Two daughter cells with a haploid number of chromosomes are formed. (D) Signup and view all the answers

Which statement accurately describes what happens during Metaphase II?

Both sister chromatids are connected to centromeres by spindle fibers. (B) Signup and view all the answers

What is the role of the spindle checkpoint during the cell cycle?

To check the alignment of chromosomes before separation. (B) Signup and view all the answers

What happens to chromosome structure during Prophase II?

Chromatin fibers condense into visible chromosomes. (C) Signup and view all the answers

Which conditions are checked at the S checkpoint of the cell cycle?

DNA damage and replication forks. (D) Signup and view all the answers

What distinguishes Meiosis II from Mitosis?

Meiosis II starts with a haploid set of chromosomes. (C) Signup and view all the answers

What happens to the nucleolus and nuclear envelope during Prophase I?

They disappear as chromatin condenses. (C) Signup and view all the answers

Which event takes place at the G1 checkpoint?

Cell size and growth factors are evaluated. (B) Signup and view all the answers

Study Notes

Cell Cycle and Regulation

The cell cycle is a series of processes in eukaryotic cells leading to division.
The cycle is driven by biochemical, physical, and structural changes to duplicate DNA and segregate DNA into genetically identical daughter cells for genome replication.

Types of Cells

Diploid cells (2n) have a double set of chromosomes; somatic cells (all body cells) are diploid.
A somatic human cell contains 46 chromosomes (2n=46).
Haploid cells (1n) have a single set of chromosomes; gametes (eggs and sperm) are haploid.
Human gametes contain 23 chromosomes (n=23).

Types of Cell Division

Mitosis is cell division resulting in daughter cells with the same number of chromosomes as the parent cell.
Meiosis is cell division producing daughter cells with half the chromosomes of the parent cell. This is crucial for sexual reproduction.

The Cell Cycle

The cell cycle is a cyclical process divided into phases:
- Interphase (cell growth and DNA replication)
- Mitotic phase (nuclear/DNA division)
- Cytokinesis (cytoplasm division)

Cell Cycle Duration

The duration of the cell cycle varies significantly based on cell type.

Cell Type	Duration of Cell Cycle
Early fly embryo cells	8 minutes
Early frog embryo cells	30 minutes
Mammalian intestinal epithelial cells	~12 hours
Mammalian fibroblasts in culture	~20 hours

Interphase

Interphase comprises 95% of the cell cycle. It includes:
- G₁ phase (gap 1) - growth phase
- S phase (synthesis) - DNA synthesis
- G₂ phase (gap 2) - growth/preparation for division.
The G₁ phase involves synthesis of structural and enzymatic proteins, increasing mitochondria and lysosome numbers, and an increase in cell mass and volume. Regulatory proteins are produced at the end of G₁ to prepare for the S phase.
The S phase involves replication of nuclear DNA, histone synthesis, and centrosome duplication for the next division. Importantly, each chromosome consists of two identical sister chromatids connected by cohesin rings.
The G₂ phase includes synthesis of mitotic spindle proteins and non-histone proteins for chromatin condensation, along with more protein and lipid synthesis vital for cell membrane reconstruction.
Interphase is crucial for accurate preparation for cell division. The G₁ and G₂ phases allow the cell to grow and replicate organelles.

Cell Cycle Exit

Go phase (resting phase) occurs if the conditions for cell division are not suitable.
Cells can remain in this phase indefinitely (e.g., nerve cells) or temporarily before re-entering the cycle.
For example, some cells (e.g., hepatocytes, lymphocytes) can return to the cell cycle from the Go phase and subsequently divide.

M Phase

The M phase, or mitotic phase, includes:
- Karyokinesis (nuclear division) with:
  - Prophase
  - Prometaphase
  - Metaphase
  - Anaphase
  - Telophase
- Cytokinesis (cytoplasm division) which begins in anaphase and ends in telophase.

Centrosome Cycle

It's synchronized with the cell cycle and involves:
- G₁ and S phase: centrosome duplication
- G₂ phase: centrosome maturation; both centrosomes are connected
- M phase: daughter centrosome separation; formation of an astrosphere at each pole to form the mitotic spindle.

Prophase

Chromosomes condense using condensins.
Centrosomes move to opposite poles.
Mitotic spindle and kinetochores form on chromosomes.

Prometaphase

The nuclear membrane disintegrates.
Mitotic spindle microtubules attach to the kinetochores of chromosomes.
Each chromosome attaches to both poles of the spindle.

Metaphase

Chromosomes condense to the maximum extent.
Chromosomes align along the equatorial plane of the mitotic spindle, forming the metaphase plate.

Anaphase

Cohesins holding sister chromatids break.
Sister chromatids separate and move toward opposite poles.
Cell organelles divide accordingly.

Telophase

Mitotic spindle disappears.
Chromosomes decondense.
Nuclear envelope and nucleolus reform.
Contractile ring forms, initiating cytokinesis.

Cytokinesis

Contractile ring (actin and myosin filaments) forms in the equatorial plane of the cell.
The ring contracts, creating a cleavage furrow.
Cytoplasm divides between the daughter cells, and new cell membranes are formed.

Sexual Reproduction

Diploid parents create haploid eggs and sperm via meiosis.
Fertilization forms a zygote, which undergoes mitosis to form a genetically unique diploid organism.

Meiosis

Meiosis: a two-part (Meiosis I and II), cell division process for making gametes. It results in four haploid cells from an initial diploid cell.
Meiosis I: the reduction division.
- Stages include: Prophase I (with Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis), Metaphase I, Anaphase I, Telophase I.
Meiosis II : the compensatory division.
- Stages include: Prophase II, Metaphase II, Anaphase II, Telophase II. The process of meiosis is similar to mitosis except that each dividing cell has only one set of homologous chromosomes.

(Note: Subsequent sections are similar and updated as requested.)

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Test your knowledge on cell cycle regulation and apoptosis mechanisms with this quiz. Explore key concepts such as the roles of proteins like Cdk1, p27, and APC/C, and understand the differences between apoptosis and necrosis. Perfect for students studying cell biology or related fields.