MCAT Genetics 2 — Questions and Answers
Question 1: Trisomy 21 (Down syndrome) most commonly results from which of the following mechanisms?
- A deletion on chromosome 21
- Nondisjunction during meiosis (Correct answer)
- Robertsonian translocation involving chromosome 21
- A point mutation in the DYRK1A gene
Correct answer: Nondisjunction during meiosis
Approximately 95% of Down syndrome cases result from nondisjunction (failure of chromosomes to separate) during meiosis I, most often in the mother, producing a gamete with two copies of chromosome 21.
Question 2: DNA methylation of CpG islands in a gene's promoter region typically results in:
- Increased gene expression due to enhanced polymerase binding
- Gene silencing and decreased transcription (Correct answer)
- Increased mutation rate at that locus
- Histone acetylation and chromatin relaxation
Correct answer: Gene silencing and decreased transcription
Methylation of CpG islands in promoter regions recruits methyl-CpG-binding repressor proteins and blocks transcription factor binding, leading to gene silencing.
Question 3: During eukaryotic RNA processing, which modification is added to the 5' end of pre-mRNA?
- A poly-A tail (200–250 adenine nucleotides)
- A 7-methylguanosine (m7G) cap (Correct answer)
- A Kozak consensus sequence
- An AAUAAA polyadenylation signal
Correct answer: A 7-methylguanosine (m7G) cap
A 7-methylguanosine cap is co-transcriptionally added to the 5' end of pre-mRNA, protecting it from exonucleolytic degradation and facilitating ribosome recognition for translation initiation.
Question 4: During translation, which RNA molecule carries specific amino acids to the ribosome?
- mRNA (messenger RNA)
- rRNA (ribosomal RNA)
- tRNA (transfer RNA) (Correct answer)
- snRNA (small nuclear RNA)
Correct answer: tRNA (transfer RNA)
Transfer RNA (tRNA) molecules serve as adaptors, each carrying a specific amino acid and containing an anticodon that base-pairs with the complementary mRNA codon at the ribosome.
Question 5: The lac repressor protein is inactivated (prevented from binding the operator) when it binds to:
- The operator DNA sequence directly
- Allolactose (a metabolite of lactose) (Correct answer)
- Cyclic AMP (cAMP)
- RNA polymerase sigma factor
Correct answer: Allolactose (a metabolite of lactose)
Allolactose, produced from lactose by beta-galactosidase, acts as the true inducer by binding to the lac repressor and causing a conformational change that prevents operator binding.
Question 6: Which of the following best defines genetic drift?
- Directed change in allele frequencies due to differential reproductive success
- Movement of alleles between populations through migration
- Random fluctuations in allele frequencies due to chance events, especially in small populations (Correct answer)
- Changes in allele frequencies caused by new mutations
Correct answer: Random fluctuations in allele frequencies due to chance events, especially in small populations
Genetic drift refers to stochastic, non-directional changes in allele frequencies caused by random sampling events, with its effects most pronounced in small populations.
Question 7: In recombinant DNA technology, restriction endonucleases are primarily used to:
- Synthesize new DNA strands from an RNA template
- Join two DNA fragments with compatible ends
- Cut DNA at specific palindromic recognition sequences (Correct answer)
- Amplify a specific DNA sequence exponentially
Correct answer: Cut DNA at specific palindromic recognition sequences
Restriction endonucleases recognize specific short DNA sequences (typically 4–8 bp palindromes) and cleave the DNA at or near those sites, generating defined fragments for cloning or analysis.
MCAT Genetics 2
Trisomy 21 (Down syndrome) most commonly results from which of the following mechanisms?