MCAT Biology Flashcards

24 sample cards from the Biology deck

Sampled from the end of the deck, so these are different cards from the ones shown on the syllabus page.

1. What is the role of DNA polymerase I in prokaryotic replication?

   DNA polymerase I removes the RNA primers using its $5'\to 3'$ exonuclease activity and replaces them with DNA. It also has $3'\to 5'$ proofreading exonuclease activity.

2. Define the terms template strand, coding strand, and identify which RNA polymerase reads.

   RNA polymerase reads the template (antisense) strand $3'\to 5'$ to synthesize mRNA $5'\to 3'$. The coding (sense) strand has the same sequence as the mRNA (with T instead of U).

3. What three modifications convert pre-mRNA to mature mRNA in eukaryotes?

   (1) Addition of a 5' 7-methylguanosine cap; (2) addition of a 3' poly-A tail; (3) splicing out of introns and joining of exons by the spliceosome.

4. Distinguish introns from exons.

   Exons are the coding (expressed) sequences retained in mature mRNA and translated. Introns are the intervening non-coding sequences that are transcribed but spliced out before translation.

5. Explain the key features of the genetic code (degeneracy, wobble, start/stop).

   The code is read in triplet codons, is degenerate (multiple codons per amino acid), nearly universal, and non-overlapping. The wobble hypothesis allows flexible third-base pairing. AUG (Met) is the start codon; UAA, UAG, UGA are stop codons.

6. Name the three stages of translation and the ribosomal sites involved.

   Initiation (small subunit + initiator tRNA-fMet/Met binds start codon), elongation (aminoacyl-tRNA enters A site, peptide bond forms, translocation), and termination (release factor at stop codon). Sites: A (aminoacyl), P (peptidyl), E (exit).

7. State Mendel's Law of Segregation.

   Each organism carries two alleles for each gene, and these alleles segregate (separate) randomly during gamete formation so that each gamete receives only one allele. This reflects homolog separation in meiosis I.

8. State Mendel's Law of Independent Assortment.

   Alleles of different genes assort independently of one another during gamete formation, provided the genes are on different chromosomes (or far apart on the same chromosome).

9. For a monohybrid cross $Aa \times Aa$, give the genotypic and phenotypic ratios.

   Genotypic ratio is $1\,AA : 2\,Aa : 1\,aa$ ($1:2:1$). Phenotypic ratio (complete dominance) is $3$ dominant $: 1$ recessive ($3:1$).

10. For a dihybrid cross $AaBb \times AaBb$, state the classic phenotypic ratio.

    $9:3:3:1$ — 9 both dominant, 3 dominant A/recessive b, 3 recessive a/dominant B, 1 both recessive, assuming independent assortment and complete dominance.

11. Differentiate complete dominance, incomplete dominance, and codominance.

    Complete dominance: heterozygote shows the dominant phenotype. Incomplete dominance: heterozygote shows a blended intermediate phenotype (e.g., red $\times$ white $\to$ pink). Codominance: both alleles fully and simultaneously expressed (e.g., AB blood type).

12. Explain how ABO blood type illustrates multiple alleles and codominance.

    Three alleles ($I^A$, $I^B$, $i$) exist. $I^A$ and $I^B$ are codominant to each other and both dominant to $i$. Genotypes: $I^A I^A/I^A i$ = type A, $I^B I^B/I^B i$ = type B, $I^A I^B$ = type AB, $ii$ = type O.

13. What is the inheritance pattern of X-linked recessive traits?

    The gene is on the X chromosome. Males (XY) need only one recessive allele to be affected (hemizygous); females need two. Affected fathers cannot pass it to sons but pass carrier status to all daughters. Examples: hemophilia, color blindness.

14. Define epistasis and pleiotropy.

    Epistasis: one gene's alleles mask or modify the phenotypic expression of a different gene (e.g., coat color). Pleiotropy: a single gene influences multiple, seemingly unrelated phenotypic traits (e.g., sickle-cell).

15. How is recombination frequency used to construct genetic maps?

    Recombination frequency (percent recombinant offspring) estimates distance between two linked genes, where 1% recombination = 1 map unit (centimorgan, cM). Closer genes recombine less; frequencies above 50% indicate unlinked (independently assorting) genes.

16. Why does a recombination frequency of 50% indicate genes are unlinked?

    A maximum of 50% recombinants is what independently assorting genes produce, indistinguishable from genes on separate chromosomes. Genes physically far apart on the same chromosome undergo enough crossovers to appear unlinked.

17. State the Hardy-Weinberg allele and genotype frequency equations.

    For two alleles: $p + q = 1$ and $p^{2} + 2pq + q^{2} = 1$, where $p^{2}$ is homozygous dominant frequency, $2pq$ heterozygous frequency, and $q^{2}$ homozygous recessive frequency.

18. List the five conditions required for Hardy-Weinberg equilibrium.

    (1) No mutation, (2) no natural selection, (3) no gene flow (migration), (4) random mating, and (5) a very large (effectively infinite) population size (no genetic drift).

19. In a population, 16% of individuals show a recessive phenotype. Find $q$, $p$, and the heterozygote frequency.

    $q^{2} = 0.16 \Rightarrow q = 0.4$; $p = 1 - q = 0.6$; heterozygote frequency $2pq = 2(0.6)(0.4) = 0.48$ (48%).

20. Define genetic drift and the bottleneck and founder effects.

    Genetic drift is random change in allele frequencies, pronounced in small populations. Bottleneck effect: a drastic population size reduction randomly alters allele frequencies. Founder effect: a small group founds a new population with non-representative allele frequencies.

21. Distinguish the modes of natural selection: directional, stabilizing, and disruptive.

    Directional selection favors one extreme phenotype, shifting the mean. Stabilizing selection favors intermediate phenotypes, reducing variance. Disruptive (diversifying) selection favors both extremes over intermediates, potentially leading to speciation.

22. Differentiate allopatric and sympatric speciation.

    Allopatric speciation occurs when a physical/geographic barrier isolates populations, preventing gene flow. Sympatric speciation occurs without geographic isolation, via mechanisms like polyploidy, niche differentiation, or sexual selection within the same area.

23. Contrast the shapes and composition of Gram-positive and Gram-negative bacterial cell walls.

    Gram-positive bacteria have a thick peptidoglycan layer and no outer membrane, staining purple. Gram-negative bacteria have a thin peptidoglycan layer plus an outer membrane containing lipopolysaccharide (LPS, an endotoxin), staining pink/red.

24. Describe the three basic bacterial morphologies and the function of bacterial flagella, pili, and plasmids.

    Morphologies: cocci (spherical), bacilli (rod), and spirilla/spirochetes (spiral). Flagella enable motility; pili (fimbriae) aid adhesion and conjugation (sex pilus); plasmids are small circular extrachromosomal DNA carrying genes such as antibiotic resistance.

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