Understanding the central dogma of molecular biology—how genetic information flows from DNA to RNA to protein—is fundamental for excelling in medical licensing exams like the USMLE. This module breaks down the key concepts of DNA structure, RNA types, transcription, translation, and the critical role of proteins in cellular function and disease.

Deoxyribonucleic acid (DNA) is a double-stranded helix that carries the genetic instructions for the development, functioning, growth, and reproduction of all known organisms and many viruses. It's composed of two polynucleotide chains that coil around each other to form a double helix. The sequence of nucleotides—adenine (A), guanine (G), cytosine (C), and thymine (T)—determines the genetic code.

Ribonucleic acid (RNA) is a single-stranded nucleic acid that plays a crucial role in protein synthesis and gene regulation. Unlike DNA, RNA typically uses uracil (U) instead of thymine (T) and has a ribose sugar instead of deoxyribose.

There are several key types of RNA, each with a specific role:

Messenger RNA (mRNA): Carries the genetic code transcribed from DNA to the ribosomes for protein synthesis. Transfer RNA (tRNA): Acts as an adapter molecule, bringing specific amino acids to the ribosome during translation, matching them to the mRNA codons. Ribosomal RNA (rRNA): A structural and catalytic component of ribosomes, the cellular machinery responsible for protein synthesis.

Protein synthesis is the process by which cells build proteins. It involves two main stages: transcription and translation. This is often referred to as the 'central dogma' of molecular biology.

Transcription is the process where a segment of DNA is copied into a complementary strand of mRNA. This occurs in the nucleus (in eukaryotes) and is catalyzed by the enzyme RNA polymerase. The DNA sequence serves as a template.

Translation is the process where the genetic code carried by mRNA is decoded to produce a specific sequence of amino acids, forming a polypeptide chain that folds into a functional protein. This occurs at the ribosomes in the cytoplasm.

The mRNA sequence is read in codons, which are sets of three nucleotides. Each codon specifies a particular amino acid or a stop signal. tRNA molecules, each carrying a specific amino acid and possessing an anticodon complementary to an mRNA codon, deliver the amino acids to the ribosome. The ribosome catalyzes the formation of peptide bonds between adjacent amino acids, building the polypeptide chain.

Mutations are changes in the DNA sequence. These can arise spontaneously or be induced by mutagens. Point mutations (single nucleotide changes), insertions, and deletions can alter the mRNA sequence, potentially leading to changes in the amino acid sequence of a protein. Depending on the type and location of the mutation, the resulting protein may be non-functional, have altered function, or even gain a new function. Understanding how mutations affect protein function is critical for comprehending genetic diseases.

For the USMLE, focus on:

• The structure of DNA and RNA, including base pairing rules.
• The processes of transcription and translation, including the key enzymes and molecules involved (RNA polymerase, ribosomes, mRNA, tRNA, rRNA).
• The genetic code and the concept of codons.
• Different types of mutations and their potential effects on protein function and disease.
• Examples of genetic disorders caused by molecular defects.