DNA Replication, Transcription, and Translation: MCAT Guide to the Central Dogma

The central dogma of molecular biology describes the flow of genetic information: DNA is replicated to make more DNA, DNA is transcribed into RNA, and RNA is translated into protein. This framework underlies almost all of molecular biology and appears throughout MCAT passages — in genetics, cell biology, pharmacology, and virology questions.

For the MCAT, you need to understand all three processes mechanistically: which enzymes are involved, what direction the synthesis goes, and the key differences between how these processes work in prokaryotes versus eukaryotes.

DNA replication is the process of copying double-stranded DNA before cell division. It is semiconservative — each new DNA molecule consists of one original strand and one newly synthesized strand.

The process begins at origins of replication. In prokaryotes, there is a single origin; in eukaryotes, there are multiple origins per chromosome, allowing replication to proceed simultaneously at many locations. Helicase unwinds and separates the double helix at the replication fork, breaking hydrogen bonds between base pairs.

Primase synthesizes a short RNA primer (typically 5–10 nucleotides) to provide a free 3'-OH group. DNA polymerase can only extend an existing strand — it cannot start a new chain from scratch. This is why a primer is required.

DNA polymerase III (in prokaryotes) or DNA polymerase delta/epsilon (in eukaryotes) extends the new strand in the 5'→3' direction by adding nucleotides complementary to the template strand (which it reads 3'→5'). The leading strand is synthesized continuously; the lagging strand is synthesized discontinuously as a series of Okazaki fragments, each requiring its own primer.

DNA polymerase I (prokaryotes) removes RNA primers and replaces them with DNA. DNA ligase seals the nicks between adjacent fragments by forming phosphodiester bonds. Single-strand binding proteins (SSBPs) stabilize the unwound single-stranded DNA. Topoisomerase relieves the torsional stress ahead of the replication fork.

Key MCAT fact: telomerase extends the ends (telomeres) of linear chromosomes in eukaryotes by adding repetitive sequences using an RNA template it carries. Without telomerase, chromosomes shorten with each replication cycle.

Transcription produces an RNA copy of a segment of DNA. In prokaryotes, the same RNA polymerase transcribes all RNA types. In eukaryotes, there are three RNA polymerases: RNA Pol I (rRNA), RNA Pol II (mRNA and most snRNA), and RNA Pol III (tRNA, 5S rRNA). RNA Pol II is the most MCAT-relevant.

RNA polymerase reads the template strand of DNA in the 3'→5' direction and synthesizes RNA in the 5'→3' direction. No primer is needed — RNA polymerase can initiate a new chain directly.

Transcription starts when RNA polymerase binds a promoter sequence on the DNA. In eukaryotes, the TATA box (located ~25 bp upstream of the transcription start site) is a core promoter element. Transcription factors help RNA Pol II recognize and bind the promoter.

Transcription ends at a terminator sequence. In prokaryotes, terminators include intrinsic (hairpin-based) and Rho-dependent mechanisms. In eukaryotes, the process is more complex — cleavage and polyadenylation are part of termination.

In eukaryotes, the initial transcript (pre-mRNA) undergoes three major modifications before it leaves the nucleus as mature mRNA.

5' capping: A 7-methylguanosine cap is added to the 5' end of the pre-mRNA shortly after transcription begins. The cap protects the mRNA from degradation and is recognized by ribosomes during translation initiation.

3' polyadenylation: A poly-A tail (a string of ~100–250 adenine nucleotides) is added to the 3' end after the pre-mRNA is cleaved at a polyadenylation signal. The poly-A tail increases mRNA stability and aids in export from the nucleus.

Splicing: Introns (non-coding sequences) are removed from the pre-mRNA and exons (coding sequences) are joined together by the spliceosome, a complex of small nuclear RNAs (snRNAs) and proteins. Alternative splicing allows one gene to produce multiple different proteins depending on which exons are included.

Prokaryotes do not perform mRNA processing — their transcription and translation are coupled (occurring simultaneously in the cytoplasm). This distinction is a classic MCAT question target.

Translation occurs at ribosomes and converts the mRNA sequence into a protein sequence. The ribosome reads mRNA codons (three-nucleotide sequences) in the 5'→3' direction. Each codon corresponds to a specific amino acid according to the genetic code, which is nearly universal across all life.

The start codon is AUG, which codes for methionine (formyl-methionine in prokaryotes). The three stop codons are UAA, UAG, and UGA — none of them code for amino acids. They signal the ribosome to release the polypeptide.

Transfer RNAs (tRNAs) bring amino acids to the ribosome. Each tRNA has an anticodon that is complementary to a specific codon, and it carries the corresponding amino acid at its 3' end. The ribosome has three sites: the A site (aminoacyl — incoming tRNA), the P site (peptidyl — growing chain), and the E site (exit — outgoing tRNA).

Translation is divided into initiation (ribosome assembles at AUG), elongation (amino acids added one at a time as the ribosome moves 5'→3' along the mRNA), and termination (stop codon reached, release factors trigger polypeptide release).

In prokaryotes, ribosomes are 70S (made of 30S and 50S subunits). In eukaryotes, ribosomes are 80S (made of 40S and 60S subunits). This difference is clinically important — many antibiotics target the prokaryotic 70S ribosome specifically, sparing eukaryotic cells.

Knowing the prokaryote/eukaryote differences for each process is a high-yield MCAT topic because these differences appear in antibiotic and antiviral mechanism questions.

Replication: Multiple origins (eukaryotes) vs. single origin (prokaryotes). Telomerase (eukaryotes only). Histone-associated DNA (eukaryotes) vs. naked circular DNA (prokaryotes).

Transcription: Three RNA polymerases (eukaryotes) vs. one (prokaryotes). Nuclear processing (eukaryotes only). Coupled transcription-translation (prokaryotes only).

Translation: 80S ribosomes (eukaryotes) vs. 70S (prokaryotes). Poly-A tail and 5' cap required for eukaryotic translation initiation (prokaryotes have Shine-Dalgarno sequences instead). Organelle ribosomes in mitochondria and chloroplasts are 70S — this is why mitochondria are susceptible to aminoglycoside antibiotics.