An investigator is comparing DNA replication in prokaryotes and eukaryotes. He finds that the entire genome of E. coli (4 × 106 base pairs) is replicated in approximately 30 minutes. A mammalian genome (3 × 109 base pairs) is usually replicated within 3 hours. Which of the following characteristics of eukaryotic DNA replication is the most accurate explanation for this finding?

Simultaneous replication at multiple origins

Replication inhibition at checkpoint

Absence of telomerase enzyme activity

More efficient DNA polymerase activity

An investigator is studying the biology of human sperm cells. She isolates spermatogonia obtained on a testicular biopsy from a group of healthy male volunteers. She finds that the DNA of spermatogonia obtained from these men show a large number of TTAGGG sequence repeats. This finding can best be explained by increased activity of an enzyme with which of the following functions?

Proofreading of synthesized daughter strands

Production of short RNA sequences

A researcher is tracing the fate of C-peptide, a product of preproinsulin cleavage. Which of the following is a true statement regarding the fate of C-peptide?

C-peptide is packaged with insulin in secretory vesicles

C-peptide exits the cells via a protein channel

C-peptide is further cleaved into insulin

C-peptide is immediately degraded by the proteasome

C-peptide activates an intracellular signaling cascade

A 67-year-old man comes to the physician for a follow-up examination after he was diagnosed with mantle cell lymphoma. The physician recommends a chemotherapeutic regimen containing bortezomib. Which of the following best describes the effect of this drug?

Accumulation of ubiquitinated proteins

Preventing the relaxation of DNA supercoils

Inhibition of tyrosine kinase receptors

Stabilization of tubulin polymers

An investigator is studying DNA repair processes in an experimental animal. The investigator inactivates a gene encoding a protein that physiologically excises nucleotides from damaged, bulky, helix-distorting DNA strands. A patient with a similar defect in this gene is most likely to present with which of the following findings?

Dry skin and increased photosensitivity

Ataxic gait and facial telangiectasias

Malignant breast and ovarian growths

Leukocoria and a painful bone mass

Colorectal and endometrial cancers

A 52-year-old female was found upon mammography to have branching calcifications in the right lower breast. Physical exam revealed a palpable nodularity in the same location. A tissue biopsy was taken from the lesion, and the pathology report diagnosed the lesion as comedocarcinoma. Which of the following histological findings is most likely present in the lesion?

Pleomorphic cells surrounding areas of comedonecrosis

Disordered glandular cells invading the ductal basement membrane

Extensive lymphocytic infiltrate

Orderly rows of cells surrounding lobules

Cellular aging mechanisms for USMLE Step 2 CK: High-Yield Pathology Lessons

Cellular Senescence - The Ticking Clock

Telomere Shortening and Cell Division

Telomere Attrition: Finite cell divisions cause progressive shortening of chromosome ends (telomeres), acting as a mitotic clock.
Hayflick Limit: Normal somatic cells arrest after a fixed number of divisions (~50-70), entering senescence.
Mechanism: Critically short telomeres are sensed as DNA damage, activating tumor suppressor pathways (p53, Rb) to halt the cell cycle.

⭐ Senescent cells are not inert; they adopt a Senescence-Associated Secretory Phenotype (SASP), releasing pro-inflammatory cytokines (IL-6, IL-8) that can promote chronic inflammation and aging-related diseases.

Telomere Shortening - The End-Replication Problem

Telomeres: Repetitive DNA sequences (TTAGGG) capping chromosome ends to prevent degradation and fusion.
End-Replication Problem: DNA polymerase cannot fully replicate the 3' end of the lagging strand, causing progressive telomere shortening with each cell division.

Telomere shortening and telomerase action

Hayflick Limit: The finite number of divisions a normal cell can undergo before shortening triggers senescence.
Telomerase: A reverse transcriptase that adds telomeric DNA.
- Active in germ cells & stem cells, maintaining their length.
- Inactive in most somatic cells.

⭐ Reactivation of telomerase is a hallmark of ~90% of cancers, granting cells replicative immortality by overcoming the Hayflick limit.

Pathways & Proteins - The Cellular Housekeeping

Proteostasis Collapse: Age-related decline in maintaining a healthy cellular proteome, leading to the accumulation of misfolded or damaged proteins.
Key Pathways & Their Decline:
- Chaperone-mediated folding: ↓ Heat Shock Proteins (HSPs) impair correct protein folding.
- Ubiquitin-Proteasome System (UPS): Reduced efficiency in tagging (ubiquitination) and degrading cytosolic proteins.
- Autophagy-Lysosome Pathway: Impaired "self-eating" mechanism for clearing damaged organelles and long-lived proteins.

Protein Degradation Pathways in Cellular Aging

⭐ Lipofuscin: The yellow-brown "wear-and-tear" pigment seen in aging cells (heart, liver, neurons) is composed of oxidized lipid and protein aggregates within lysosomes, representing evidence of cumulative oxidative damage and declining autophagic clearance.

Telomere attrition is a key driver of replicative senescence, limiting cell divisions.

Accumulation of DNA damage and mutations, often from reactive oxygen species (ROS), contributes significantly.

Failed proteostasis leads to the buildup of misfolded proteins, impairing cellular function.

Deregulated nutrient sensing, particularly involving the mTOR pathway, influences aging.

Mitochondrial dysfunction results in decreased ATP production and increased ROS, creating a vicious cycle.

Stem cell exhaustion depletes the regenerative potential of tissues over time.

Continue reading on Oncourse

CONTINUE READING — FREE

or get the app

App Store|Google Play