genome

Meaning: The genome is the complete set of genetic material of a cell, organism or virus. It includes genes plus regulatory sequences, introns, repeats and other noncoding DNA; some viruses have RNA genomes. In eukaryotes the nuclear genome is on linear chromosomes, and mitochondria (and chloroplasts) carry their own small circular genomes. Prokaryotes usually have one circular chromosome, sometimes with plasmids. Most somatic cells of one individual share the same genome, so different cell types arise largely from differential gene expression rather than different DNA.

On the MCAT: Passages on sequencing, differentiation, gene regulation or comparative genomics. Expect questions such as why a neuron and a muscle cell differ despite having the same genome (different genes are transcribed), or why genome size does not predict organism complexity, since only a small fraction of the human genome codes for protein.

Common confusion: The genome is not the same as gene expression, the transcriptome or the proteome. The genome stays essentially constant in an individual apart from mutation, whereas the transcriptome and proteome change with cell type and conditions. Genome is also not the same as genotype, which describes the alleles present at particular loci.