Questions: Nucleic Acid Structures

Question 1

What structural feature distinguishes B-DNA from A-DNA, and why do RNA duplexes adopt the A-form rather than B-form?

• A. B-DNA and A-DNA are identical except for their sequence
• B. B-DNA has a wide, shallow major groove and narrow, deep minor groove with 10 bp per turn, while A-DNA has a narrow, deep major groove and shallow, wide minor groove with 11 bp per turn. RNA duplexes adopt A-form because the 2'-OH group on ribose creates a steric clash in the B-form geometry — the C3'-endo sugar pucker required to accommodate the 2'-OH forces the A-form helix parameters
• C. A-DNA is always left-handed while B-DNA is right-handed
• D. The only difference is that A-DNA uses adenine while B-DNA uses all four bases

Answer: (B)

The distinction between B-form and A-form DNA is fundamentally a matter of sugar pucker and the geometric consequences that follow. In B-DNA, the deoxyribose sugar adopts a C2'-endo pucker, producing a helix with 10 bp per turn, 3.4 A rise per bp, and characteristic groove dimensions that proteins like transcription factors read for sequence recognition. In A-DNA (and all RNA duplexes), the sugar adopts a C3'-endo pucker, producing a wider, shorter helix with 11 bp per turn and 2.6 A rise. The 2'-OH of ribose sterically favors the C3'-endo conformation, which is why RNA duplexes are exclusively A-form. This structural difference has functional consequences: the A-form geometry of RNA duplexes creates a distinct pattern of groove accessibility that RNA-binding proteins recognize, and the wider major groove of A-form RNA is less accessible for sequence-specific reading.

Question 2

RNA is structurally limited to forming simple Watson-Crick duplexes, similar to DNA.

• T. True
• F. False

Answer: False

RNA forms vastly more complex three-dimensional structures than DNA. While RNA does form Watson-Crick duplexes (in A-form), single-stranded RNA folds back on itself through a hierarchy of structural organization: secondary structure (stems, internal loops, bulges, hairpins, junctions), tertiary interactions (pseudoknots where a loop base-pairs with a region outside its own stem, long-range kissing loops, A-minor motifs where adenines dock into the minor groove of helices, ribose zippers), and quaternary assembly (subunit interfaces in the ribosome). This structural complexity enables RNA to perform catalysis (ribozymes — the ribosome's peptidyl transferase center is an RNA enzyme), gene regulation (riboswitches that change conformation upon binding metabolites), and structural scaffolding (the ribosomal RNA that organizes ribosomal protein assembly). The structural repertoire of RNA rivals that of proteins.

Question 3

How do riboswitches use structural transitions to regulate gene expression, and what structural methods have revealed their mechanism?

Model answer: Riboswitches are structured RNA elements in the 5' untranslated regions of mRNAs that directly bind small-molecule metabolites (adenine, guanine, SAM, thiamine pyrophosphate, glycine, etc.) and undergo conformational changes that regulate transcription or translation of the downstream gene. The riboswitch has two functional domains: an aptamer domain that binds the ligand with high specificity and affinity, and an expression platform that transduces binding into a regulatory output (typically by forming or disrupting a transcription terminator hairpin or a Shine-Dalgarno sequestering structure). X-ray crystallography of riboswitch aptamer domains bound to their ligands has revealed the atomic details of recognition — showing how RNA pockets achieve molecular recognition rivaling antibody-antigen interactions, using base stacking, hydrogen bonding to Watson-Crick edges, sugar edges, and Hoogsteen edges, and metal-ion-mediated contacts. Structures of both ligand-bound and ligand-free states (when obtainable) reveal the conformational switch.

The adenine riboswitch structure (Serganov et al., 2004) showed that a single nucleotide change in the binding pocket switches specificity from adenine to guanine — a remarkable example of how RNA structure achieves molecular selectivity. CRISPR-Cas structures have similarly revolutionized understanding of RNA-guided DNA recognition, showing how guide RNA geometry determines target specificity.

Question 4

What structural insights about protein-nucleic acid recognition have emerged from structures of CRISPR-Cas complexes?

Model answer: Structures of CRISPR-Cas complexes (Cas9, Cas12, Cas13 bound to guide RNA and target DNA/RNA) revealed several key principles: (1) the guide RNA forms a scaffold that positions the spacer sequence for base-pairing with the target, with the protein providing a binding channel that accommodates the RNA-DNA heteroduplex; (2) Cas9 recognizes the PAM (protospacer adjacent motif) through direct protein-DNA contacts in the major groove, explaining PAM specificity; (3) target strand unwinding proceeds directionally from the PAM-proximal seed region, creating an R-loop structure; (4) the HNH and RuvC nuclease domains cleave opposite strands at defined positions relative to the PAM. These structures enabled rational engineering of Cas9 variants with altered PAM specificity, reduced off-target activity (high-fidelity variants that destabilize mismatched R-loops), and new functionalities (base editors, prime editors) — a direct example of how structural biology drives biotechnology.

The Cas9 structures also revealed why off-target cleavage occurs: mismatches in the PAM-distal region of the guide-target duplex are tolerated because the protein does not contact every base pair with equal stringency. This structural insight guided the engineering of high-fidelity Cas9 variants (eSpCas9, HiFi Cas9) that introduce additional contacts or energetic penalties for mismatches.