Aim to answer every question before checking. Missed questions point you to the sections you need most.
1. (2.1) G1 phase of the cell cycle is best described as:
B. G1 (gap 1) is the growth phase after mitosis where the cell doubles in size, synthesizes proteins/organelles, and decides whether to enter S phase. S phase (not G1) replicates DNA.
2. (2.1) DNA replication in the cell cycle occurs during:
D. S = synthesis. Each chromosome is duplicated to form two sister chromatids joined at the centromere. After S phase, a human cell has 46 chromosomes with 92 chromatids total.
3. (2.2) During metaphase of mitosis:
C. Metaphase = alignment. Prophase: condensation + envelope breakdown. Anaphase: sister chromatid separation. Telophase: envelope reforms.
4. (2.2) The primary purpose of mitosis is:
A. Mitosis yields two identical 2n daughter cells for growth and repair. Meiosis (B/D) halves chromosome number, and crossing over (C) is specific to meiosis I.
5. (2.3) The G1/S checkpoint primarily asks:
D. The restriction point (G1/S) checks size, nutrients, and DNA integrity before committing to replication. p53 halts the cycle here if DNA is damaged.
6. (2.3) The retinoblastoma protein (Rb) controls the cell cycle by:
B. Hypo-phosphorylated Rb binds E2F, preventing transcription of S-phase genes. Growth signals activate Cyclin D/CDK4, which phosphorylates Rb, releasing E2F and unlocking S phase. Loss of Rb causes unchecked proliferation.
7. (2.4) Meiosis I differs fundamentally from mitosis because:
A. Meiosis I is the reductional division: homologs pair as bivalents and separate, producing two haploid cells (each chromosome still has two sister chromatids). Meiosis II then separates the sisters, like mitosis.
8. (2.4) A human primary spermatocyte entering meiosis I has a DNA content of:
C. S phase precedes meiosis I, so the primary spermatocyte is 2n = 46 chromosomes but 4C of DNA (each chromosome has two chromatids). After meiosis I: 2 secondary spermatocytes (n, 2C). After meiosis II: 4 spermatids (n, 1C).
9. (2.5) Crossing over occurs during:
B. Homologous chromosomes synapse and exchange segments at chiasmata during pachytene of prophase I. This is the only phase where homologs pair and recombine.
10. (2.5) Crossing over contributes to:
D. Recombination between homologs (plus independent assortment) is the chief source of genetic variation in sexual reproduction. The further apart two loci sit, the more often they are separated by crossover.
11. (2.6) A single primary oocyte ultimately gives rise to:
A. Oogenesis is asymmetric: each meiotic division sheds a small polar body, concentrating cytoplasm and nutrients into the single functional ovum.
12. (2.6) Spermatogenesis differs from oogenesis in that:
C. Sperm production is continuous after puberty, symmetric, and yields 4 viable sperm per primary spermatocyte. Oogenesis starts in fetal life, pauses in prophase I, resumes one oocyte at a time, and yields 1 ovum.
13. (2.7) Spermatogenesis takes place in the:
D. Seminiferous tubules house spermatogonia, Sertoli cells (nurse cells), and developing sperm. The epididymis stores and matures sperm, the vas deferens transports them, and the prostate contributes seminal fluid.
14. (2.7) Sertoli cells function primarily to:
B. Sertoli cells surround developing sperm, respond to FSH, and form tight junctions that isolate spermatogenesis from the immune system. Leydig cells (interstitial) produce testosterone.
15. (2.8) Ovulation releases the oocyte arrested at:
C. The secondary oocyte resumes meiosis at ovulation but halts again at metaphase II. Meiosis II completes only if fertilization occurs.
16. (2.8) The corpus luteum secretes primarily:
A. The corpus luteum is the post-ovulation remnant of the follicle. Its progesterone dominates the luteal phase and maintains the endometrium. FSH/LH come from the pituitary; hCG comes from the embryo.
17. (2.9) The mid-cycle LH surge triggers:
D. A sudden spike in LH (driven by rising estrogen's positive feedback) ruptures the dominant follicle and releases the secondary oocyte ~24-36 hours later.
18. (2.9) During the luteal phase, progesterone acts to:
B. Progesterone ("pro-gestation") thickens and stabilizes the uterine lining. If no implantation occurs, the corpus luteum regresses, progesterone falls, and menstruation follows.
19. (2.10) Fertilization in humans normally occurs in the:
A. Sperm meet the oocyte in the wide ampullary section of the oviduct. Implantation happens days later in the uterine wall.
20. (2.10) The cortical reaction prevents polyspermy by:
C. Cortical granules fuse with the plasma membrane and release enzymes that modify ZP glycoproteins, converting the zona to an impenetrable fertilization membrane (the "slow block" to polyspermy).
21. (2.1) Which cyclin/CDK complex drives entry into mitosis (M-phase)?
B. Cyclin B partners with CDK1 to form M-phase Promoting Factor, which phosphorylates nuclear lamins and other targets to initiate mitosis.
22. (2.3) When p53 detects DNA damage it:
D. p53 is the "guardian of the genome." It upregulates p21 (a CDK inhibitor) to halt the cycle and activates pro-apoptotic genes if DNA damage cannot be repaired. p53 mutations are found in ~50% of human cancers.
23. (2.4) Non-disjunction during meiosis II produces gametes with:
C. Failure of sister chromatids to separate in meiosis II gives aneuploid gametes. Fertilization then yields trisomies (e.g., Down syndrome, trisomy 21) or monosomies.
24. (2.10) hCG from the early embryo maintains the corpus luteum by:
A. hCG (secreted by the trophoblast) binds LH receptors on the corpus luteum, keeping it alive and producing progesterone through the first trimester until the placenta takes over hormone production.
