Cell Cycle, Homeostasis, and Cell Signaling

Comprehensive practice flashcards covering cell cycle phases, checkpoints, mitosis, cancer, homeostatic feedback loops, and cellular communication and signal transduction pathways.

What is the cell cycle?

The life of a cell from its initial formation until it divides.

How is eukaryotic DNA packaged and organized from double helix to chromosome?

DNA wraps around proteins called histones to form nucleosomes. Strings of nucleosomes assemble into chromatin, which remains non-condensed when the cell is not dividing and condenses into densely packed chromosomes after DNA replication.

What is the functional difference between a centromere and a kinetochore?

A centromere is the constricted region on each sister chromatid where they are most closely attached, whereas a kinetochore is a protein complex on the centromere that links each sister chromatid to the mitotic spindle.

What structures and process are illustrated in this diagram?

The diagram shows a pair of homologous chromosomes (one maternal and one paternal with identical lengths and centromere positions) undergoing DNA replication to form two pairs of duplicated sister chromatids joined at their centromeres.

How does genome structure differ between prokaryotes and eukaryotes, and what are the specific chromosome numbers for humans, chimps, and elephants?

Prokaryotes contain a singular, circular DNA molecule, whereas eukaryotes possess one or more linear chromosomes. Humans have $46$ chromosomes, chimps have $48$, and elephants have $56$.

How do somatic cells differ from gametes regarding ploidy, division method, and human chromosome number?

Somatic cells are diploid ($2n = 46$ in humans: $23$ from maternal parent and $23$ from paternal parent) and divide by mitosis. Gametes are reproductive cells (eggs/sperm) that are haploid ($n = 23$ in humans) and divide by meiosis.

What major events occur during each of the three subphases of interphase ($\text{G}_1$, $\text{S}$, and $\text{G}_2$)?

During $\text{G}_1$ ("first gap"), the cell is metabolically active, performs normal functions, and duplicates organelles; during $\text{S}$ ("synthesis"), DNA replication produces sister chromatids; during $\text{G}_2$ ("second gap"), protein synthesis occurs, large amounts of ATP are made, and centrosomes complete replication.

What is the $\text{G}_0$ phase of the cell cycle?

A non-dividing, quiescent state entered from the $\text{G}_1$ checkpoint when cells receive a "stop" signal. Some specialized cells (like muscle and nerve cells) remain in $\text{G}_0$ permanently, while others can reenter the cell cycle upon receiving appropriate cues.

What key cellular events occur in early prophase compared to late prophase (prometaphase)?

In early prophase, chromosomes condense, nucleoli disappear, the mitotic spindle begins forming, and centrosomes move apart. In late prophase (prometaphase), chromosomes condense further, the nuclear envelope breaks down, and spindle microtubules invade the nuclear region to attach to kinetochores.

What key events occur during metaphase of mitosis?

Centrosomes reach opposite poles of the cell, spindle microtubules attach to each chromosome's kinetochores, and chromosomes line up along the metaphase plate.

What occurs during anaphase of mitosis?

Paired sister chromatids separate into individual chromosomes and move toward opposite poles of the cell as kinetochore microtubules shorten, while non-attached microtubules cause the cell to elongate.

How does cytokinesis differ between animal and plant cells?

In animal cells, cytokinesis occurs through the formation of a cleavage furrow driven by a contractile ring of actin filaments. In plant cells, vesicles produced by the Golgi apparatus migrate to the center of the cell to form a cell plate.

In a human cell, what are the numbers of chromosomes and chromatids during metaphase, anaphase, and at the end of mitosis?

Metaphase: $46$ chromosomes and $92$ chromatids. Anaphase: $92$ chromosomes and $92$ chromatids. End of mitosis (separated daughter cells): $46$ chromosomes and $46$ chromatids in each cell.

What does each of the three major cell cycle checkpoints ($\text{G}_1$, $\text{G}_2$, and $\text{M}$) check before issuing a "go" signal?

The $\text{G}_1$ checkpoint checks cell size, growth factors, and DNA damage; the $\text{G}_2$ checkpoint checks completion of DNA replication and DNA damage (triggering apoptosis if unrepairable); the $\text{M}$ (spindle) checkpoint checks proper attachment of microtubules to all kinetochores at metaphase.

What mechanism of CDK activation is illustrated in this diagram?

Cyclin binds to CDK, forming a cyclin-CDK complex that is subsequently phosphorylated by a Cyclin-Activating Kinase (CAK). Once fully activated, the complex phosphorylates a target protein that advances the cell cycle.

How do the cellular concentrations of cyclins and cyclin-dependent kinases (CDKs) behave across the cell cycle?

Cyclin concentrations fluctuate because cyclins are synthesized and degraded at specific stages of the cycle, whereas CDK concentrations remain constant throughout all phases and require their specific cyclin to become active.

How do contact inhibition and anchorage dependence regulate normal cell division?

Contact (or density) inhibition uses cell-surface receptors to detect physical contact with neighboring cells and halts the cycle in $\text{G}_1$. Anchorage dependence requires cells to be attached to adjacent cells or the extracellular matrix in order to divide.

What are three major biological differences between normal cells and cancer cells in culture?

Normal cells follow cell cycle checkpoints, divide $20\text{--}50$ times in culture before dying, and undergo apoptosis upon significant DNA damage. Cancer cells bypass checkpoints, divide infinitely (are "immortal"), evade apoptosis, and carry an average of $60$ or more mutations on growth-regulating genes.

What is the difference between a benign tumor, a malignant tumor, and metastasis?

A benign tumor consists of abnormal non-cancerous cells that remain localized at the primary site. A malignant tumor contains cancerous cells that lose anchorage dependence and invade surrounding tissue. Metastasis occurs when malignant cells break away and spread to distant body sites.

What is the definition of homeostasis and what are physiological set points?

Homeostasis is the state of relatively stable internal conditions. Set points are specific values that the body maintains within an acceptable fluctuation range (for example, the body temperature set point is $98.6^\circ\text{F}$, with a normal range of $97^\circ\text{F}$ to $99^\circ\text{F}$).

What are the four functional components of a biological feedback loop?

Stimulus (a variable that causes a change), Receptor/sensor (sensory organs that detect the stimulus and alert the brain/control center), Effector (a muscle or gland that carries out the response), and Response (the physiological reaction that modifies the effect of the stimulus).

How does negative feedback regulate human body temperature in response to heat and cold?

When body temperature rises (heat stimulus), skin temperature receptors signal sweat glands (effectors) to release sweat (response). When temperature drops (cold stimulus), skin temperature receptors signal muscles (effectors) to shiver (response), both counteracting the initial temperature change.

What type of feedback loop is demonstrated in this childbirth diagram, and what are its components?

A positive feedback loop: the baby pushing on the cervix is the stimulus; cervical nerve cells are receptors sending signals to the brain; the pituitary gland is the effector releasing oxytocin; and oxytocin stimulating stronger uterine contractions is the response, which further amplifies cervical pressure.

How is disease defined in the context of homeostasis, and what are two specific examples?

Disease is a condition where the body is unable to maintain homeostatic regulation. Examples include cancer (inability to regulate cell division and growth) and diabetes (inability to regulate blood glucose levels).

What direct intercellular communication structures are shown in this diagram?

Animal cells utilize gap junctions formed by connexon protein channels (left), while plant cells utilize plasmodesmata channels that cross cell walls (right) to allow direct exchange of substances between adjacent cytoplasms.

What form of cell communication is illustrated in this immune interaction diagram?

Direct contact through cell-cell recognition: a T-cell receptor (TCR) on an immature T cell recognizes an antigen bound to MHC on an antigen-presenting cell, driving maturation into either a $\text{CD4}^+$ helper T cell or a $\text{CD8}^+$ cytotoxic T cell.

What is paracrine signaling, and what is an example of it during an immune response?

Paracrine signaling occurs when a secretory cell releases local chemical ligands that diffuse across extracellular fluid to elicit a response in nearby target cells. An example is mast cells releasing histamine upon allergen exposure to induce a localized inflammatory response.

How does synaptic signaling function as a specialized form of paracrine signaling?

Neurons secrete neurotransmitter molecules from an axon terminus that diffuse across the narrow synaptic cleft to bind receptors on a target cell, inducing a chemical change within the receiving cell.

What is autocrine signaling, and what example occurs in immune regulation?

Autocrine signaling occurs when a cell releases a ligand that binds to receptors on its own surface, regulating its own activity via feedback. An example is a macrophage releasing cytokines that bind to its own receptors to adjust its activity.

How does long-distance hormone signaling travel in plants versus animals?

In plants, hormones travel through vascular tissue (xylem and phloem) or diffuse through the air to reach target tissues. In animals, endocrine signaling involves specialized cells releasing hormones into the bloodstream/circulatory system to reach distant target cells (e.g., insulin released by the pancreas).

What are the three structural domains of a cell-surface receptor, and what types of ligands do they bind?

They consist of an extracellular ligand-binding domain, a hydrophobic transmembrane domain spanning the plasma membrane, and an intracellular domain that transmits the signal. They bind polar, water-soluble, or large molecules that cannot cross the phospholipid bilayer.

How does the intracellular receptor pathway shown in this diagram work?

A small, hydrophobic signaling molecule (such as a steroid hormone) diffuses through the plasma membrane into the cytoplasm and binds an intracellular receptor, altering its conformation and allowing the ligand-receptor complex to enter the nucleus and bind target gene DNA to regulate transcription.

What are the three universal stages of cell signaling?

1. Reception: The ligand binds specifically to a receptor. 2. Transduction: The extracellular signal is converted into an intracellular cascade through conformational changes, relay enzymes, and second messengers. 3. Response: A cellular process is altered, such as gene expression, secretion, or cell growth.

What opposing roles do protein kinases and protein phosphatases play in signal transduction cascades?

Protein kinases relay and amplify signals by transferring phosphate groups from ATP to specific target proteins, altering protein shape and activity (acting like an "on" switch). Protein phosphatases remove those phosphate groups, returning proteins to their inactive baseline state and shutting off the pathway.

What is the function of second messengers, and what are two prominent examples?

Second messengers are small, non-protein, water-soluble molecules or ions that diffuse rapidly through the cytosol to relay and amplify extracellular signals to downstream proteins. Prominent examples include cyclic AMP ($\text{cAMP}$) and calcium ions ($\text{Ca}^{2+}$).

What sequence of events leads to G protein activation and signaling at a GPCR?

Ligand binding causes a conformational change in the GPCR, prompting the associated inactive G protein to release GDP and bind GTP. The active G protein separates and binds a downstream effector enzyme, which produces second messengers that activate protein kinases to elicit a response.

How do ligand-gated ion channels convert a chemical signal into an intracellular electrical or chemical event?

When a specific ligand binds to the extracellular receptor site of the channel protein, the channel "gate" opens (or closes), allowing the rapid diffusion of specific ions (such as $\text{Na}^+$, $\text{Ca}^{2+}$, or $\text{K}^+$) across the membrane along their concentration gradient.

What is signal transduction "cross talk"?

Interactions between different signaling pathways where components (such as an activated kinase) from one pathway phosphorylate or regulate elements within another distinct pathway, integrating multiple inputs before initiating a cellular response.