Source: Chapters 4–6, University of Central Florida
Tags: DNA structure, double helix, Watson and Crick, base pairing, nucleotides, purines, pyrimidines, Chargaff's rules, supercoiling, topoisomerase, B-form DNA, Z DNA, melting temperature, Tm
Difficulty: Intermediate | Prerequisites: Basic chemistry (hydrogen bonding, covalent bonds, acid-base), introductory cell biology.
This chapter covers the physical and chemical structure of DNA, from the discovery of the double helix through to supercoiling and alternative DNA conformations. It sits at the foundation of everything else in molecular biology: you cannot understand replication, transcription, or repair without knowing how the molecule is built and how it behaves. You should already be comfortable with basic organic chemistry concepts (functional groups, bond types) and the idea that DNA carries genetic information. If Chapters 1–3 covered the evidence that DNA is the genetic material, this chapter tells you what that material looks like and why its shape matters.
DNA is a double-stranded, antiparallel helix held together by hydrogen bonds between complementary bases (A with T, G with C) and stabilised by base stacking and hydrophobic exclusion of water. The molecule can adopt several conformations (B, A, Z), can be supercoiled (positively or negatively), and its stability depends on GC content. Topoisomerases manage supercoiling by cutting and re-joining strands.
Purine
A two-ring nitrogenous base. In DNA, the purines are adenine (A) and guanine (G). Think of it as the "bigger" bases, with two fused rings.
Pyrimidine
A single-ring nitrogenous base. In DNA, the pyrimidines are thymine (T) and cytosine (C). In simple terms, these are the smaller, one-ring bases.
Nucleoside
A base attached to a sugar (ribose or deoxyribose) by a glycosidic bond, with no phosphate group. The sugar has a 3' OH (in deoxyribonucleosides, the 2' position carries an H rather than an OH). Think of it as a nucleotide minus the phosphate.
Nucleotide
A nucleoside with one or more phosphate groups attached at the 5' carbon of the sugar via an ester bond. In simple terms, this is the monomer unit of DNA and RNA, the building block that gets polymerised.
Phosphodiester bond
The covalent linkage between the 3' OH of one nucleotide and the 5' phosphate of the next. This bond forms the sugar-phosphate backbone of DNA and RNA. Think of it as the "chain link" connecting one nucleotide to the next in a strand.
Glycosidic bond
The bond between the base and the 1' carbon of the sugar. Formed by a condensation (dehydration) reaction.
Antiparallel
The two strands of DNA run in opposite directions: one 5' to 3', the other 3' to 5'. This orientation is critical for base pairing geometry.
Chargaff's rules
The observation that in double-stranded DNA, the amount of A equals T, and the amount of G equals C. Erwin Chargaff disproved the earlier tetranucleotide hypothesis (which assumed all four bases were present in equal amounts).
Tautomer
A structural isomer of a base produced by a proton shift (e.g. keto to enol, amino to imino). Tautomers are rare but can cause mispairing during replication. In simple terms, it is an alternative chemical form of a base that can trick the polymerase into inserting the wrong partner.
Melting temperature (Tm)
The temperature at which 50% of double-stranded DNA in a sample has denatured into single strands. Tm increases with higher GC content because G:C pairs have three hydrogen bonds compared to two for A:T.
B-form DNA
The most common conformation of DNA under physiological conditions. Right-handed helix, ~10.5 base pairs per turn, 3.4 nm pitch, 2 nm diameter.
A-form DNA
A right-handed helix that is shorter and wider than B-form. Typically seen in dehydrated conditions or in RNA-DNA hybrids.
Z DNA
A left-handed helix found in sequences with alternating purines and pyrimidines (often GC repeats). It is found in the human body and is believed to play a role during transcription.
Supercoiling
The overwinding (positive) or underwinding (negative) of the DNA helix. Supercoiled DNA is more compact. Negative supercoiling stores energy that aids strand separation and is the predominant form in most cells.
Topoisomerase
An enzyme that changes the linking number of DNA by cutting one (Type I) or both (Type II) strands, passing DNA through, and re-sealing. Topoisomerase uses a tyrosine residue to form a transient covalent bond with the phosphate backbone.
Gyrase
A Type II topoisomerase found in bacteria that introduces negative supercoils.
Linking number
The total number of times one strand of DNA winds around the other in a closed circular molecule. It equals twist plus writhe.
Writhe
The coiling of the DNA helix axis in three-dimensional space (the "shape" of supercoiling, as distinct from the twist of the helix itself).
Major groove / minor groove
The two grooves that run along the outside of the double helix, created because the glycosidic bonds of a base pair are not directly opposite each other. Proteins read more information from the major groove, where the pattern of hydrogen bond donors, acceptors, and methyl groups differs for each base pair.
Zinc finger domain
A small protein structural motif that can recognise 3 bases in the major groove of DNA. Zinc fingers can form dimers and use different amino acid codes to recognise specific DNA sequences.
Quadruple helix DNA (G-quadruplex)
A four-stranded DNA structure that is guanine-rich and commonly found at telomeres.
I-motif DNA
A four-stranded structure formed by cytosine-rich sequences under acidic conditions, often found at telomeres alongside G-quadruplexes.
Francis Crick recognised the connection between B-form DNA diffraction patterns and the structure of hemoglobin, which helped him interpret the helical parameters.
Watson and Crick's first (incorrect) model placed the bases on the outside of the structure. Their successful model placed bases inside, paired by hydrogen bonds.
Erwin Chargaff disproved the tetranucleotide hypothesis by showing that base ratios vary between species but that A = T and G = C within a species. He did not show equal concentrations of all four bases.
Linus Pauling proposed a three-chain model of DNA with phosphates on the inside, which worried Watson and Crick. He had earlier discovered the alpha-helix by folding paper.
Rosalind Franklin's Photo 51 was an X-ray diffraction image of B-form DNA. It revealed the helical nature, periodicity, and water content of DNA.
A nucleoside is a base + sugar (no phosphate). A nucleotide is a base + sugar + at least one phosphate.
The base attaches to the 1' carbon of the sugar via a glycosidic bond.
The phosphate attaches to the 5' carbon via an ester bond.
All three bond types (ester, glycosidic, phosphodiester) are formed by condensation reactions (removal of water).
The 3' OH of one nucleotide attacks the alpha-phosphate of the incoming nucleotide to form a phosphodiester bond.
DNA polymerase synthesises DNA in the 5' to 3' direction and reads the template in the 3' to 5' direction.
A pairs with T via two hydrogen bonds. G pairs with C via three hydrogen bonds.
Base pairing uses hydrogen bonds between complementary bases on opposite strands, not van der Waals forces.
Purines (A, G) typically adopt the amino and keto conformations. Pyrimidines (C, T) also adopt amino and keto forms. The enol and imino forms are rare tautomers.
If adenine = 30% of a DNA sample, then T = 30%, leaving 40% for G + C, so G = 20% and C = 20%.
A keto-to-enol proton shift on thymine produces enol thymine. A similar shift on guanine produces enol guanine, which can mispair with thymine.
Tautomeric (rare) forms exist at much lower concentrations than the common (keto/amino) forms.
Tautomers are a source of replication errors because DNA polymerase cannot always detect mispairing caused by rare tautomeric forms.
B-form DNA: ~10.5 base pairs per turn, right-handed, 3.4 nm pitch (rise per full turn), 0.34 nm per base pair, 2 nm diameter.
The mica experiment (binding DNA to mica, digesting with DNase) determined the periodicity of the helix.
The two strands are antiparallel and held together by hydrogen bonds between bases plus covalent phosphodiester bonds along each backbone.
The grooves arise because the two glycosidic bonds of a base pair are offset, not at 180 degrees.
The major groove provides more chemical information to proteins (pattern of donors, acceptors, methyl groups), making it the primary reading surface.
In the major groove, a protein can distinguish all four base pair orientations (A:T, T:A, G:C, C:G).
In the minor groove, a protein can distinguish A:T from G:C using hydrogen bonding and the methyl group of thymine, but the information is more limited.
The functional groups on DNA bases that interact with DNA-binding proteins are some of the same groups involved in hydrogen bonding between the bases.
Three factors stabilise dsDNA: base pairing (hydrogen bonds), base stacking (van der Waals and hydrophobic interactions between adjacent bases), and exclusion of water from the hydrophobic interior.
Base stacking is a weak, attractive force. Phosphodiester linkages and disulfide bridges are covalent, not weak forces, and are not classified among the weak stabilising interactions.
Tm increases with GC content because G:C base pairs have three hydrogen bonds (vs. two for A:T), requiring more energy to separate.
DNA nucleotide precursors are built up from amino acid building blocks, not taken directly from food.
Z DNA is left-handed, found in alternating purine-pyrimidine sequences, present in the human body, and believed to play a role during transcription.
Quadruple helix DNA (G-quadruplex) is guanine-rich and commonly found at telomeres.
A left-handed helix can also represent positively supercoiled DNA when drawn as a plectonemic coil.
Positive supercoiling (overwinding) keeps DNA tightly wound and helps organisms that live at high temperatures.
Negative supercoiling (underwinding) stores energy that aids in strand separation during replication and transcription. Most cellular DNA is negatively supercoiled.
Writhe is the crossing of the helix axis over itself (figure-of-eight shapes). Twist is the winding of strands around each other.
In gel electrophoresis, supercoiled DNA migrates furthest (most compact), followed by linear DNA, then nicked circular DNA (least compact, migrates slowest).
Type I topoisomerases change the linking number by 1 (cut one strand). Type II topoisomerases change it by 2 (cut both strands).
Gyrase is a bacterial Type II topoisomerase that introduces negative supercoils.
Topoisomerase forms a transient covalent bond between its active-site tyrosine and the DNA phosphate backbone. (Note: thymine at the active site attacking the backbone is a false statement often used as a distractor.)
Topotecan and resveratrol are known topoisomerase inhibitors.
Students often confuse nucleosides with nucleotides. A nucleoside has no phosphate group. As soon as a phosphate is added, it becomes a nucleotide (or nucleotide monophosphate).
"Chargaff showed all four bases are in equal concentration" is wrong. Chargaff showed A = T and G = C, which varies by species. The tetranucleotide hypothesis (equal amounts) is what he disproved.
Students sometimes think tautomers are common forms of the bases. They are rare, and that rarity is exactly why they cause occasional mispairing rather than constant errors.
Saying DNA polymerase "synthesises 3' to 5'" is backwards. Synthesis is always 5' to 3'; the template is read 3' to 5'.
⚠️ Know the difference between nucleoside and nucleotide, and which bonds connect each component (glycosidic for base-sugar, ester for sugar-phosphate).
⚠️ Be able to calculate base composition from Chargaff's rules (if A = 30%, then T = 30%, G = 20%, C = 20%).
⚠️ Understand that Tm correlates with GC content (three H-bonds vs. two), not with hydrophobicity.
⚠️ Be ready to identify supercoiling direction, writhe vs. twist, and the function of Type I vs. Type II topoisomerases.
⚠️ The major groove is where proteins read specific base pair identity. The minor groove provides less discriminating information.
⚠️ Topoisomerase uses a tyrosine (covalent bond to phosphate backbone), not thymine.
True or False: Purines have two rings, and adenine and guanine are both purines.
Fill in the blank: DNA polymerase synthesises in the _______ to _______ direction.
True or False: The melting temperature (Tm) is the point at which all DNA in a sample is denatured.
Fill in the blank: Type I topoisomerases change the linking number by _______.
True or False: Z DNA is a right-handed helix.
True.
5' to 3'.
False. Tm is the point at which 50% of the DNA is denatured.
False. Z DNA is left-handed.
Q: What did Erwin Chargaff's work demonstrate, and how did it contribute to understanding DNA structure?
A: Chargaff showed that in any given species' DNA, the amount of adenine equals thymine, and the amount of guanine equals cytosine. This disproved the tetranucleotide hypothesis (which assumed all four bases were present in equal amounts) and provided key evidence for complementary base pairing.
Q: If a DNA sample contains 22% guanine, what are the percentages of the other three bases?
A: Cytosine = 22% (G always equals C). That accounts for 44%, leaving 56% for A + T combined, so adenine = 28% and thymine = 28%.
Q: Explain why DNA with a higher GC content has a higher melting temperature.
A: G:C base pairs are held together by three hydrogen bonds, compared to two for A:T pairs. More energy (higher temperature) is therefore required to break apart DNA that is rich in GC pairs.
Q: How does topoisomerase interact with the DNA backbone, and what is the role of its active-site tyrosine?
A: The tyrosine residue in topoisomerase's active site forms a transient covalent bond with the phosphate backbone of DNA. This allows the enzyme to cut the strand(s), pass DNA through (or rotate), and re-seal the break, thereby changing the linking number and relieving supercoiling tension.
Q: What is the functional difference between positive and negative supercoiling?
A: Positive supercoiling (overwinding) compacts DNA and helps maintain helix integrity at high temperatures, as seen in thermophilic organisms. Negative supercoiling (underwinding) stores energy that facilitates strand separation, aiding processes like replication and transcription. Most cellular DNA carries negative supercoils.
Q: A student claims the alpha-helix fits perfectly into the minor groove of DNA. Is this correct?
A: No. The alpha-helix fits into the major groove, not the minor groove. This is a commonly tested point. The major groove is wider and provides more chemical information for sequence-specific recognition.
Base pairing rules here underpin DNA replication fidelity (Chapter 7+) and the mechanisms of mismatch repair.
Supercoiling and topoisomerase function connect directly to replication fork dynamics and transcription regulation.
The concept of tautomeric mispairing returns in discussions of spontaneous mutation (mutagenesis chapters).
DNA double helix, Watson and Crick model, Rosalind Franklin Photo 51, X-ray crystallography, Chargaff's rules, base composition, complementary base pairing, purine, pyrimidine, adenine, thymine, guanine, cytosine, nucleoside vs nucleotide, phosphodiester bond, glycosidic bond, antiparallel strands, major groove, minor groove, zinc finger, B-form DNA, A-form DNA, Z DNA, G-quadruplex, quadruple helix, telomeres, supercoiling, positive supercoiling, negative supercoiling, linking number, twist, writhe, topoisomerase, gyrase, Type I topoisomerase, Type II topoisomerase, melting temperature, Tm, GC content, tautomer, enol, imino, keto, amino, base stacking, mica experiment, gel electrophoresis DNA migration