How the MCAT tests molecular biology

Molecular biology is AAMC content category 1B — the transmission of genetic information from the gene to the protein. The MCAT tests the flow of information (replication, transcription, translation), the enzymes involved, and how mutations and regulation change the output. It connects genetics to biochemistry.

workspace_premium The master diagram

The central dogma organizes the whole topic: DNA is copied (replication) and read into RNA (transcription), which is decoded into protein (translation). Locate any process on this map.

DNA structure

DNA stores information in the sequence of four bases along a double helix of two antiparallel strands.

5′3′ 3′5′ AT2 H-bondsTA2 H-bondsGC3 H-bondsCG3 H-bonds Base pairing A – T (2 bonds) G – C (3 bonds) Strands are antiparalleland complementary.
DNA base pairing. Adenine pairs with thymine (2 hydrogen bonds) and guanine with cytosine (3 hydrogen bonds); the two strands run antiparallel (5′→3′ opposite 3′→5′).
  • Each nucleotide is a sugar (deoxyribose), a phosphate, and one of four bases (A, T, G, C).
  • Complementary base pairing means one strand’s sequence dictates the other’s — the basis of copying and reading DNA.
  • The antiparallel arrangement (one strand 5′→3′, the other 3′→5′) is essential to how enzymes work on it.

DNA replication

Before a cell divides, it copies its DNA semiconservatively: the strands separate and each templates a new complementary strand, so every daughter molecule is half old, half new.

  • Helicase unwinds the double helix; DNA polymerase adds new nucleotides 5′→3′.
  • One strand (leading) is synthesized continuously; the other (lagging) is made in Okazaki fragments joined by ligase.
  • Proofreading by DNA polymerase keeps replication remarkably accurate.

The central dogma: transcription and translation

Genes are expressed in two steps — transcription copies a gene into messenger RNA, and translation decodes that RNA into a protein.

DNA RNA Protein transcription translation replication
The central dogma. DNA is replicated to pass on, transcribed into RNA, and translated into protein — the one-way flow of genetic information.
  • Transcription (nucleus): RNA polymerase reads a DNA template and builds an mRNA copy (using U instead of T).
  • RNA processing (eukaryotes): a 5′ cap, poly-A tail, and splicing out of introns produce mature mRNA.
  • Translation (ribosome): the mRNA is decoded into a chain of amino acids.

The genetic code and translation

The ribosome reads mRNA three bases at a time. Each three-base codon specifies one amino acid, delivered by a matching tRNA.

AUGCCUUACGAA 5′3′ codon 1codon 2 ribosome tRNAUAC aa polypeptide The ribosome reads mRNA 3 bases (a codon) at a time; each tRNA anticodon delivers one amino acid.
Translation. The ribosome reads mRNA codon by codon; each tRNA’s anticodon pairs with a codon and adds its amino acid to the growing polypeptide.
  • The code is degenerate (multiple codons per amino acid) but unambiguous (each codon means one thing).
  • AUG is the start codon (methionine); three stop codons end translation.
  • The reading frame is set by the start codon, so insertions or deletions can cause frameshifts.

Mutations and gene regulation

Changes to DNA and control of when genes are expressed shape the proteins a cell makes.

  • Point mutations: silent (no change), missense (different amino acid), or nonsense (premature stop).
  • Frameshift mutations: insertions or deletions shift the reading frame, usually more disruptive.
  • Gene regulation: cells control transcription (e.g., the lac operon in bacteria; transcription factors in eukaryotes) so the right proteins are made at the right time.

How to study it — and the traps to avoid

Molecular biology rewards knowing the flow and the enzymes. Map every process onto the central dogma.

  1. Memorize base pairing (A-T, G-C) and that replication is semiconservative.
  2. Separate transcription (nucleus, makes RNA) from translation (ribosome, makes protein).
  3. Read codons in the correct frame from the start codon.
  4. Classify mutations by their effect on the protein.
error Highest-yield trap

Transcription and translation are different steps. Transcription makes RNA from DNA in the nucleus; translation makes protein from RNA at the ribosome. Mixing up the two — or where they occur — is a frequent error.