Nucleic Acids: Sugars & Bases - USMLE Step 1 Highlights
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Nucleic Acids: Sugars & Bases - USMLE Step 1 Focus
DNA:
deoxyribose, phosphate, adenine, guanine, cytosine, thymine.
RNA:
ribose, phosphate, adenine, guanine, cytosine, uracil.
Differences Between DNA and RNA:
Ribose vs. deoxyribose (presence or absence of 2' hydroxyl).
Thymine in DNA vs. Uracil in RNA.
Purines vs. Pyrimidines:
Purines: adenine and guanine. Mnemonic: “Pure As Gold” for purines.
Pyrimidines: cytosine, thymine (DNA), uracil (RNA). Mnemonic: “CUT the Py” for pyrimidines.
Chargaff’s Rule:
Base-pairing proportions in double-stranded DNA.
Hydrogen Bonding and DNA Stability:
G-C pairs form 3 H-bonds (stronger), A-T/U form 2 H-bonds (weaker).
Important for understanding DNA melting temperatures and replication fidelity.
Chemical Structure of Nucleotides:
Base + sugar + phosphate.
Nucleotide vs. nucleoside distinctions.
Numbering and Directionality of DNA/RNA:
Prime notation (1', 2', 3', etc.) for sugar carbons.
5’ to 3’ strand orientation.
Application on Step 1:
1. Recognition of molecular structures.
2. Effects of mutations (e.g., GC-rich regions).
3. Biochemical pathways (linked to purine/pyrimidine synthesis).
USMLE Step 1 Questions