gene

Meaning: A gene is a segment of DNA that carries the information for a functional product. Transcription copies it into RNA. For protein-coding genes, the mRNA is translated into a polypeptide, while other genes give functional RNAs such as tRNA and rRNA. A gene includes its coding sequence and nearby regulatory regions, such as the promoter. In eukaryotes, exons are kept and introns are spliced out of the pre-mRNA. Alleles are different versions of a gene at the same locus.

On the MCAT: Passages ask you to predict the effect of a mutation: a silent, missense or nonsense point mutation, or a frameshift from an insertion or deletion not divisible by three. Expect Mendelian crosses using alleles, regulation of expression (the lac operon, where one promoter controls a cluster of genes in prokaryotes), and linkage, where genes farther apart on a chromosome recombine more often.

Common confusion: A gene is not the same thing as a protein, an allele or a chromosome. Much DNA is noncoding, and one gene can give several proteins through alternative splicing, so "one gene, one protein" is only a rough rule.