MCAT Genetics 2

Question 1

Trisomy 21 (Down syndrome) most commonly results from which of the following mechanisms?

Accepted Answer

Nondisjunction during meiosis

Answer

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:

Accepted Answer

Gene silencing and decreased transcription

Answer

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?

Accepted Answer

A 7-methylguanosine (m7G) cap

Answer

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?

Accepted Answer

tRNA (transfer RNA)

Answer

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:

Accepted Answer

Allolactose (a metabolite of lactose)

Answer

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.

MCAT - Medical College Admission Test Practice Test

MCAT - Medical College Admission Test Practice Test

MCAT Genetics 2