Free MCAT study guide — Biological and Biochemical Foundations of Living Systems
This guide covers the central dogma of molecular biology: how genetic information flows from DNA to RNA to protein. The MCAT tests your understanding of DNA replication machinery (helicase, primase, DNA polymerase III, ligase), the differences between leading and lagging strand synthesis, and the significance of Okazaki fragments. You must also know the mechanics of transcription in both prokaryotes and eukaryotes, including promoter recognition, RNA polymerase function, and post-transcriptional modifications such as 5' capping, polyadenylation, and splicing. Translation requires knowledge of ribosome structure (40S and 60S subunits in eukaryotes), tRNA charging by aminoacyl-tRNA synthetases, codon-anticodon base pairing, and the three stages: initiation, elongation, and termination. The MCAT frequently presents experiments involving mutations, antibiotics that target specific steps (such as rifampicin blocking transcription or chloramphenicol blocking the peptidyl transferase center), and gel electrophoresis data. Understanding the wobble hypothesis, reading frame shifts, and the regulation of gene expression through operons (lac, trp) and eukaryotic enhancers/silencers is essential for scoring well on this topic.
DNA replication is semiconservative, meaning each daughter double helix contains one original and one newly synthesized strand, as demonstrated by the Meselson-Stahl experiment. Replication begins at origins of replication (oriC in E. coli) where helicase unwinds the double helix, creating a replication fork. Single-stranded binding proteins (SSBs) stabilize the unwound strands, while topoisomerase (gyrase) relieves supercoiling ahead of the fork. Primase synthesizes short RNA primers to provide 3'-OH groups for DNA polymerase III, which synthesizes DNA in the 5' to 3' direction. The leading strand is synthesized continuously, while the lagging strand is synthesized in short Okazaki fragments that are later joined by DNA ligase after RNA primers are removed by DNA polymerase I. Proofreading occurs via the 3' to 5' exonuclease activity of DNA polymerase III, resulting in an error rate of approximately one in a billion base pairs after mismatch repair.
Transcription produces a single-stranded RNA molecule complementary to the template (antisense) strand of DNA. In prokaryotes, RNA polymerase recognizes promoter sequences (such as the -10 and -35 boxes) with the help of sigma factor. In eukaryotes, transcription factors (TFIID binds the TATA box) recruit RNA polymerase II to the promoter. Elongation proceeds 5' to 3' as RNA polymerase reads the template 3' to 5'. Termination in prokaryotes occurs via rho-dependent or rho-independent (hairpin loop) mechanisms. Eukaryotic pre-mRNA undergoes three key modifications: addition of a 7-methylguanosine 5' cap (protects from degradation, aids ribosome binding), polyadenylation of the 3' end (poly-A tail of 100-250 adenines for stability and export), and splicing to remove introns via the spliceosome (snRNPs). Alternative splicing allows one gene to encode multiple protein variants, greatly increasing proteomic diversity.
Translation occurs on ribosomes, which consist of a small subunit (30S in prokaryotes, 40S in eukaryotes) and a large subunit (50S/60S). The process has three phases. During initiation, the small subunit binds the mRNA (at the Shine-Dalgarno sequence in prokaryotes or the 5' cap in eukaryotes), and the initiator tRNA carrying methionine (fMet in prokaryotes) occupies the P site. During elongation, aminoacyl-tRNAs enter the A site, a peptide bond forms via the peptidyl transferase activity of the large subunit (a ribozyme), and translocation moves the ribosome one codon along the mRNA. Termination occurs when a stop codon (UAA, UAG, UGA) enters the A site and release factors trigger hydrolysis of the polypeptide from the tRNA. Post-translational modifications include proteolytic cleavage, glycosylation, phosphorylation, and ubiquitination. Polyribosomes (polysomes) allow multiple ribosomes to translate a single mRNA simultaneously.
The genetic code is degenerate (multiple codons can code for the same amino acid), universal (nearly all organisms use the same code), unambiguous (each codon specifies only one amino acid), and non-overlapping. There are 64 codons: 61 sense codons and 3 stop codons. The wobble hypothesis explains how the third position of a codon can form non-standard base pairs with the anticodon, allowing fewer tRNAs to recognize all codons. Point mutations can be silent (no amino acid change), missense (different amino acid), or nonsense (premature stop codon). Frameshift mutations from insertions or deletions alter the entire downstream reading frame and are typically the most deleterious. Understanding sickle cell disease as a classic missense mutation (Glu to Val in beta-globin) is high-yield for the MCAT.
Prokaryotic gene expression is primarily regulated at the level of transcription through operons. The lac operon is an inducible system: in the absence of lactose, the repressor binds the operator and blocks transcription. Allolactose (an isomer of lactose) binds the repressor, causing it to release from the operator. The trp operon is a repressible system: tryptophan acts as a corepressor, binding the repressor and enabling it to block transcription when tryptophan levels are high. Eukaryotic regulation is more complex and occurs at multiple levels: chromatin remodeling (histone acetylation activates, methylation can activate or repress), transcriptional control (enhancers, silencers, transcription factors), post-transcriptional control (alternative splicing, miRNA), translational control, and post-translational modifications. Epigenetic modifications like DNA methylation (at CpG islands) can silence genes without changing the DNA sequence.
Cells employ multiple DNA repair pathways to maintain genomic integrity. Mismatch repair corrects errors missed by proofreading during replication. Base excision repair (BER) removes damaged bases via glycosylases, while nucleotide excision repair (NER) removes bulky lesions such as thymine dimers caused by UV radiation. Defects in NER cause xeroderma pigmentosum (XP), a condition with extreme sensitivity to sunlight and high skin cancer risk. Double-strand breaks are repaired by either homologous recombination (high fidelity, uses sister chromatid as template) or non-homologous end joining (NHEJ, error-prone but available throughout the cell cycle). The tumor suppressor p53 plays a central role in the DNA damage response, activating cell cycle arrest and apoptosis when damage is irreparable. Understanding BRCA1/2 gene function in homologous recombination is clinically relevant and may appear in passage-based questions.
DNA polymerase synthesizes in the 5' to 3' direction only; it requires a primer with a free 3'-OH group.
Okazaki fragments are 1,000-2,000 nucleotides in prokaryotes and 100-200 in eukaryotes.
Telomerase (a reverse transcriptase) extends telomeres to prevent chromosome shortening in germ cells and stem cells.
The Shine-Dalgarno sequence in prokaryotes is analogous to the Kozak sequence in eukaryotes for ribosome positioning.
Antibiotics targeting translation: chloramphenicol (50S, blocks peptidyl transferase), tetracycline (30S, blocks A site), erythromycin (50S, blocks translocation).
Rifampicin inhibits bacterial RNA polymerase; alpha-amanitin inhibits eukaryotic RNA polymerase II.
AUG is the universal start codon and codes for methionine (formyl-methionine in prokaryotes).
Introns are removed, exons are expressed. Remember: 'ex-ons are ex-pressed.'
The spliceosome consists of snRNPs (small nuclear ribonucleoproteins) that recognize splice sites at intron-exon boundaries.
Prokaryotic transcription and translation are coupled (occur simultaneously); eukaryotic processes are separated by the nuclear envelope.
Deamination of cytosine produces uracil, which is recognized and repaired by base excision repair.
The codon for sickle cell disease changes GAG (Glu) to GUG (Val) at position 6 of beta-globin.
Confusing the template strand (3' to 5', read by polymerase) with the coding strand (5' to 3', same sequence as mRNA except T instead of U).
Forgetting that RNA polymerase does NOT require a primer, unlike DNA polymerase.
Mixing up the ribosomal sites: A site (aminoacyl/arrival), P site (peptidyl/polypeptide), E site (exit).
Believing the genetic code is overlapping -- it is non-overlapping and read in triplets.
Confusing inducible (lac) and repressible (trp) operons -- inducible systems are OFF by default, repressible systems are ON by default.
Forgetting that eukaryotic mRNA is monocistronic (one gene per transcript) while prokaryotic mRNA can be polycistronic.
Assuming all mutations are harmful -- silent mutations and some missense mutations have no phenotypic effect.
Practice tracing information flow through the central dogma: given a DNA sequence, write the mRNA transcript, then determine the amino acid sequence using a codon table. Do this repeatedly until it becomes automatic. Pay attention to reading frame, start codons, and stop codons. The MCAT frequently asks you to identify how a specific mutation would affect the protein product.
For passage-based practice, focus on interpreting experimental data involving gel electrophoresis, PCR, and Western/Northern/Southern blots. Many MCAT questions present an unfamiliar experimental setup and ask you to predict results or explain observations. Practice identifying which antibiotic or inhibitor would block a specific step, and be comfortable with operon diagrams showing different combinations of mutations in the promoter, operator, and structural genes.
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