Genetics and Molecular Biology

Genetics and molecular biology tie together the central dogma of information flow and the rules of inheritance — two of the most heavily tested areas of the MCAT Bio/Biochem section. The exam rewards conceptual mastery: knowing why replication is semiconservative, how mutations change proteins, and how to predict offspring ratios, rather than rote memorization of enzyme names.

Core Idea

• The central dogma runs DNA → RNA → protein. DNA is replicated, transcribed into RNA, and translated into polypeptides; information flows in this direction (with retroviruses using reverse transcriptase as the exception).
• Structure dictates function. Antiparallel, complementary strands make semiconservative replication and templated transcription possible; the double helix is held by hydrogen bonds (A-T with two, G-C with three).
• Inheritance is probability applied to alleles. Punnett squares and the laws of segregation and independent assortment let you predict genotype and phenotype ratios.

DNA Replication

Replication is semiconservative: each daughter molecule keeps one parental strand and one new strand (proven by Meselson-Stahl). Synthesis always runs 5' to 3', reading the template 3' to 5'.

• Helicase unwinds the double helix at the origin; topoisomerase relieves supercoiling ahead of the fork.
• Primase lays down an RNA primer; DNA polymerase III extends it in prokaryotes.
• The leading strand is synthesized continuously toward the fork; the lagging strand is made in Okazaki fragments away from the fork.
• DNA polymerase I removes primers, and ligase seals the nicks. Polymerases proofread with 3' to 5' exonuclease activity.

Transcription and RNA Processing

RNA polymerase synthesizes mRNA 5' to 3' from a DNA template, needing no primer. In eukaryotes the primary transcript (pre-mRNA) is processed in the nucleus:

• A 5' cap (7-methylguanosine) protects the transcript and aids ribosome binding.
• A poly-A tail is added at the 3' end for stability and export.
• Splicing removes introns and joins exons via the spliceosome. Alternative splicing lets one gene encode multiple proteins.

Translation and the Genetic Code

Translation occurs on ribosomes (rRNA + protein) at the A, P, and E sites. mRNA codons are read in triplets; tRNA carries amino acids and pairs its anticodon to the codon.

• Start codon AUG (methionine) sets the reading frame; stop codons UAA, UAG, UGA end translation.
• The code is degenerate (multiple codons per amino acid), near-universal, and read without overlap.
• Wobble in the third codon position allows one tRNA to read several codons.

Mutations and Inheritance

Point mutations change one base: silent (no amino-acid change), missense (one amino acid swapped), or nonsense (premature stop). Frameshift mutations (insertions/deletions not in multiples of three) shift the reading frame and usually devastate the protein.

Mendelian inheritance: the law of segregation says allele pairs separate into gametes; independent assortment says genes on different chromosomes sort independently. A monohybrid cross of heterozygotes gives a 3:1 phenotypic (1:2:1 genotypic) ratio; a dihybrid cross gives 9:3:3:1.

Non-Mendelian patterns: incomplete dominance blends phenotypes (red x white to pink); codominance expresses both alleles fully (AB blood type); sex-linked (X-linked) recessive traits appear more often in males. Gene regulation in prokaryotes uses operons — the lac operon is inducible (on with lactose), the trp operon is repressible (off with tryptophan).

High-Yield Exam Patterns

• Leading vs. lagging strand and Okazaki fragments are classic figure questions — track directionality (always 5' to 3' synthesis).
• Expect a codon chart to translate mRNA or identify a mutation's effect; know silent vs. missense vs. nonsense outcomes.
• Pedigree analysis: unaffected parents with an affected child signals recessive; a trait skipping generations and hitting mostly males signals X-linked recessive.
• Punnett square math: memorize 3:1, 1:2:1, and 9:3:3:1; use the product rule for independent events.
• Blood-type crosses (ABO) test codominance plus multiple alleles simultaneously.
• The lac and trp operons test inducible vs. repressible regulation logic.

Common Traps to Avoid

• Confusing transcription (DNA to RNA, in the nucleus) with translation (RNA to protein, at the ribosome).
• Assuming synthesis can run 3' to 5' — polymerases only add to a 3' hydroxyl, so synthesis is always 5' to 3'.
• Mixing up incomplete dominance (blended intermediate) with codominance (both alleles fully shown).
• Forgetting that a silent mutation can still be a base change — "no mutation" and "no phenotype change" are not the same.
• Treating the genetic code as fully universal; it is near-universal, with rare exceptions (e.g., mitochondria).