BMEN 240 Exam III Review

What are the membrane-bound organelles in the cell and what are their functions?

Nucleus: stores DNA; ER: makes proteins and lipids; Golgi: modifies and sorts proteins and lipids; mitochondria: produce ATP; lysosomes: digest and recycle materials; peroxisomes: oxidize fatty acids and break down hydrogen peroxide; endosomes: sort internalized materials; vesicles: transport materials. In plants, chloroplasts perform photosynthesis and vacuoles store materials and maintain pressure.

What is the endosymbiotic theory of eukaryotic cell evolution?

Mitochondria and chloroplasts evolved from bacteria engulfed by an ancestral cell, establishing a mutually beneficial relationship.

What is required for a protein to be moved to its correct destination?

A sorting signal on the protein and cellular machinery that recognizes it; proteins without targeting signals generally remain in the cytosol.

What is required for a protein to be moved into the nucleus? Does it require other proteins?

A nuclear localization signal, importin transport receptors, and the Ran GTPase system.

How does a protein get out of the nucleus?

A nuclear export signal binds an exportin, which carries the protein through a nuclear pore with help from Ran-GTP.

Describe how proteins move through the nuclear pore complex.

Small proteins can diffuse through; larger proteins use transport receptors that interact with pore proteins. Transported proteins can remain folded.

How do proteins move into mitochondria and do they remain folded?

Targeting signals direct proteins through mitochondrial translocators; most enter unfolded, with help from chaperones, ATP, and the inner-membrane potential.

Which membrane-bound organelle has the most extensive network in the cell?

The endoplasmic reticulum.

T or F? Cytosolic proteins are translated by completely different sets of ribosomal subunits than ER proteins.

False. The same types of ribosomes make both; an ER signal sequence directs a translating ribosome to the ER.

If a protein is completely translated in the cytoplasm, where could it end up?

The cytosol, nucleus, mitochondria, or peroxisomes; also chloroplasts in plants. Some proteins enter the ER after translation.

If a protein is translated in the rough endoplasmic reticulum, where could it end up?

The ER, Golgi, endosomes, lysosomes, plasma membrane, or outside the cell through secretion.

Where does glycosylation take place?

Mainly in the ER and Golgi apparatus for proteins in the secretory pathway.

What occurs in the Golgi apparatus?

Proteins and lipids are modified, sorted, and packaged into vesicles for delivery.

In what organelle do proteins that are continuously secreted diverge from proteins whose secretion is regulated?

The trans-Golgi network.

Where and how are transmembrane proteins inserted into the membrane?

Most enter the ER membrane through translocators; hydrophobic signal and stop-transfer sequences anchor them in the lipid bilayer.

What is the function of chaperone proteins?

They help proteins fold correctly, prevent aggregation, and help maintain unfolded proteins during transport.

Which proteins are responsible for docking a vesicle to the cell membrane during exocytosis and are the target of botulinum toxin?

SNARE proteins promote vesicle docking and membrane fusion; botulinum toxin cleaves specific SNAREs.

What molecules generate vesicles during endocytosis?

Clathrin and adaptor proteins help form coated buds; dynamin pinches off many endocytic vesicles.

What cellular process allows engulfing large particles, including other eukaryotic cells?

Phagocytosis.

What process do some viruses trick the cell into using to gain entry?

Receptor-mediated endocytosis.

What is the role of the lysosome and how does it function?

It digests and recycles macromolecules using acid hydrolases; proton pumps maintain its acidic interior.

Compare and contrast phagocytosis and autophagy.

Both deliver material to lysosomes for digestion. Phagocytosis takes in large external particles; autophagy breaks down the cell’s own components.

How can autophagy be a survival mechanism for cancer cells?

It recycles cellular components to provide nutrients and energy during stress or nutrient shortages.

What are four ways that animal cells use extracellular molecules to communicate with one another?

Endocrine signaling through blood, paracrine signaling to nearby cells, synaptic signaling through neurotransmitters, and contact-dependent signaling between touching cells.

What kind of signaling molecules have intracellular receptors?

Small molecules that cross the plasma membrane, including steroid hormones, thyroid hormones, and nitric oxide.

How can one hormone have different effects in different cells?

Different cells have different receptor types, signaling proteins, and target genes.

What is a fast way for a cell to signal another cell and what requires more time?

Responses that change existing proteins or ion channels are fast; responses requiring gene expression and new protein synthesis take longer.

What are the three kinds of cell-surface receptors?

Ion-channel-coupled receptors, G-protein-coupled receptors, and enzyme-coupled receptors.

What are the components of the G-protein signaling system?

A signal molecule, GPCR, heterotrimeric G protein with α, β, and γ subunits, and an effector enzyme or ion channel.

What enzyme generates cAMP and what activates it?

Adenylyl cyclase makes cAMP from ATP; activated Gαs bound to GTP stimulates it.

How can a rise in cAMP activate gene transcription?

cAMP activates protein kinase A, which phosphorylates transcription factors such as CREB.

How can phospholipase C generate two signaling systems?

PLC splits PIP2 into IP3 and DAG; IP3 promotes calcium release, while DAG helps activate protein kinase C.

How does PLC cause a rise in intracellular Ca2+?

PLC produces IP3, which opens IP3-gated calcium channels in the ER membrane.

What is a kinase?

An enzyme that adds a phosphate group to a target molecule, usually using ATP.

What is a phosphatase?

An enzyme that removes a phosphate group from a target molecule.

What stimulates the phosphorylation of receptor tyrosine kinases (RTKs)?

Ligand binding brings receptor partners together or rearranges them, allowing their kinase domains to phosphorylate one another.

What happens after RTKs are phosphorylated?

Their phosphotyrosines recruit intracellular signaling proteins that activate pathways controlling growth, division, survival, or differentiation.

What is a survival factor for cells?

An extracellular signal that suppresses apoptosis and helps a cell stay alive.

Why are RTKs usually slower than GPCRs?

RTK responses involving gene expression and protein synthesis take longer than rapid GPCR effects on existing proteins; receptor type alone does not determine speed.

When two cells bind via special membrane-bound receptors, what kind of signaling can occur?

Contact-dependent, or juxtacrine, signaling.

How do small nonpolar hormones cross the cell membrane?

They dissolve in the lipid bilayer and cross by simple diffusion.

If a drug opens a sodium channel on a neuron, will it make it more or less likely to fire an action potential? What if it opens a potassium channel?

Under usual resting conditions, sodium channels depolarize the neuron and make firing more likely; potassium channels generally make firing less likely.

What component of the cytoskeleton provides the cell with the most mechanical strength?

Intermediate filaments, which resist stretching and mechanical stress.

Which component of the cytoskeleton serves as the main transportation route for synaptic vesicles in neurons?

Microtubules carry synaptic vesicle precursors along axons.

What motor protein moves mitochondria toward the synapse?

Kinesin generally moves mitochondria toward microtubule plus ends at the axon terminal.

What do microtubules need to grow?

Available α/β-tubulin dimers carrying GTP; new microtubules are typically nucleated by γ-tubulin complexes.

What would happen to the ER and Golgi if they were not connected to the microtubular network?

The ER would lose much of its extended distribution, and the Golgi would fragment and disperse.

What cellular structure is capable of independent movement and what cytoskeletal elements are involved?

Cilia and flagella beat using microtubules and dynein motors.

If colchicine is added to the medium of growing cells, would there be more, fewer, or the same number of cells?

Fewer than in an untreated culture because colchicine disrupts microtubules and blocks mitosis.

What is the most abundant cytoskeletal element in the cell?

Actin filaments, or microfilaments, are generally the most abundant.

What motor proteins move along actin filaments?

Myosins.

How would adding phalloidin to cultured cells affect their movement?

If it enters the cells, phalloidin stabilizes actin filaments and reduces movement by preventing normal actin turnover; it poorly crosses intact membranes.

What is the contractile unit of the skeletal muscle cell and what are its principal components?

The sarcomere, consisting mainly of thin actin filaments and thick myosin filaments between two Z discs.

What stimulates contraction of a skeletal muscle cell?

A muscle action potential triggers calcium release from the sarcoplasmic reticulum; calcium binds troponin, exposing myosin-binding sites on actin.

What is required for the myosin head to release actin?

Binding of a new ATP molecule to myosin.

What molecule prevents myosin from binding actin in a skeletal muscle cell?

Tropomyosin blocks the myosin-binding sites on actin when calcium is low.

What symptoms are expected when ciliary and flagellar dynein is inactive in Kartagener’s syndrome?

Poor mucus clearance causes recurrent respiratory infections and bronchiectasis; sperm motility is impaired, fertility may be reduced, and organ positions can be reversed.

When a somatic cell replicates its DNA it goes from 2N to 4N. What does that mean?

Its DNA amount doubles, producing two sister chromatids per chromosome. Strictly, DNA content changes from 2C to 4C while ploidy remains 2n.

During what stage of the cell cycle do cells spend most of their time and what is their DNA status?

Most time is spent in interphase, often mainly G1. DNA content is 2C in G1, increases during S, and is 4C in G2; the questions call these 2N and 4N.

After which stage do cells become 4N?

After S phase, DNA replication is complete and DNA content is 4C, called 4N in these questions.

What do CDKs have to bind to become active?

Cyclins; full activation also depends on appropriate regulatory phosphorylation.

Are the regulators of the cell cycle conserved?

Yes. Core regulators such as cyclins and CDKs are evolutionarily conserved among eukaryotes.

How are cyclins degraded so quickly?

Ubiquitin ligases tag them with ubiquitin, targeting them for destruction by proteasomes.

What is the trigger that starts a round of cell division?

In typical animal cells, mitogens activate G1 cyclin–CDK activity and promote commitment to the cell cycle; M-CDK activation later triggers mitosis.

What are tumor suppressor genes?

Genes whose products restrain cell division, maintain genome integrity, or promote cell death; loss of function can contribute to cancer.

What tumor suppressor is mutated in most cancers?

TP53, which encodes p53, is the most frequently mutated tumor suppressor; it is mutated in roughly half of cancers, rather than all or necessarily most.

What happens when S-CDK is activated?

It triggers DNA replication at licensed origins and helps prevent DNA from being replicated again during the same cycle.

When is the centrosome replicated?

Centrosome duplication begins around the G1/S transition and proceeds during S phase.

When can microtubules start to interact with the chromosomes?

During prometaphase, after the nuclear envelope breaks down.

What is the kinetochore and what happens there?

A protein complex assembled at each chromosome’s centromere; spindle microtubules attach there to align and separate chromosomes.

What proteins are involved with cytokinesis?

Actin and myosin II form the contractile ring that pinches an animal cell in two.

What proteins are involved with apoptosis?

Caspases execute apoptosis by cleaving cellular proteins; Bcl-2-family proteins regulate the mitochondrial pathway.

T or F: All cell death is bad for the organism.

False. Programmed cell death supports development, tissue maintenance, and removal of damaged or dangerous cells.

What is required to keep most cells alive?

Most animal cells require extracellular survival signals that suppress apoptosis, along with nutrients and suitable conditions.

On a DNA-content flow-cytometry graph, where are G1, G2/M, and S cells, and what is the longest phase?

G1 cells form the lower-DNA peak at 2C; G2/M cells form the higher-DNA peak at 4C; S cells lie between. In a steadily dividing population, the largest fraction generally indicates the longest phase; the missing graph is needed to identify it.

What is an advantage of sexual reproduction?

It generates genetic variation, which can help populations adapt to changing environments.

What is an advantage of asexual reproduction?

It allows rapid reproduction without finding a mate and preserves successful genetic combinations.

What cells produce haploid cells?

Diploid germ-line cells undergo meiosis to produce haploid cells.

How is meiosis similar to mitosis?

Both use spindle microtubules to separate chromosomes and usually involve chromosome condensation and cytokinesis.

How is meiosis different from mitosis?

Meiosis uses two divisions after one DNA replication to produce haploid products, with homolog pairing and crossing-over; mitosis usually preserves ploidy in two daughter cells.

What are two differences between male and female gametogenesis?

In humans, one primary spermatocyte produces four functional sperm, while one primary oocyte produces one functional egg plus polar bodies. Sperm production begins at puberty; oocyte meiosis begins before birth and includes long arrests.

What process creates the bivalent?

Synapsis pairs homologous chromosomes during prophase I, forming a bivalent containing four chromatids.

When do the maternal and paternal chromosomes segregate during meiosis?

Homologous maternal and paternal chromosomes separate during anaphase I.

On what cellular structure does crossing-over take place?

Between nonsister chromatids of paired homologous chromosomes in a bivalent during prophase I; crossover connections later appear as chiasmata.

What protein must be degraded for sister chromatids to segregate during meiosis II?

Centromeric cohesin must be cleaved by separase, allowing sister chromatids to separate.

What are the two mechanisms that generate genetic diversity during meiosis?

Crossing-over and independent assortment of homologous chromosomes.

What is nondisjunction and how does it affect cells?

Failure of homologous chromosomes or sister chromatids to separate properly; it produces cells with extra or missing chromosomes.

When do genes not follow Mendel’s law of segregation?

Nondisjunction can prevent normal allele segregation. Linked genes violate independent assortment, not the law of segregation.

Are all mutations bad?

No. Mutations can be harmful, neutral, or beneficial depending on their effects and the environment.

How many combinations of gametes can independent segregation of maternal and paternal chromosomes create in humans?

2^23 = 8,388,608 possible chromosome combinations per person, ignoring crossing-over.

Why is there even more diversity among gametes?

Crossing-over creates new allele combinations within chromosomes, and mutations can introduce new alleles.