Source: Molecular Biology 1, Chapters 5 & 6 (University of Central Florida)
Tags: DNA structure, SNPs, single nucleotide polymorphism, base pairing, purines, pyrimidines, tautomers, major groove, minor groove, B-DNA, A-DNA, Z-DNA, quadruplex DNA, melting temperature, Tm, supercoiling, topoisomerase, RNA world hypothesis
Difficulty: Intermediate | Prerequisites: Basic chemistry (hydrogen bonding, covalent bonds), introductory biology (cell structure, central dogma).
Big picture: These notes cover the physical and chemical structure of DNA, from the scale of individual nucleotides up to the higher-order coiling of chromosomes. You will also encounter the key differences between DNA and RNA, and why those differences matter for biological function. This material sits at the foundation of everything else in molecular biology: replication, transcription, and repair all depend on understanding how DNA is built, how it melts, how proteins read it, and how it is packaged. If you are coming in cold, make sure you are comfortable with hydrogen bonding and the general idea of complementary base pairing before diving in.
The human genome is vast (roughly 20 million kilometres of DNA across all cells), yet individuals differ by only about 0.1% due to single nucleotide polymorphisms (SNPs). DNA's double helix is stabilised by complementary base pairing, base stacking, and hydrophobic effects. Proteins read DNA through the major and minor grooves, and the helix itself can adopt several forms (B, A, Z, quadruplex) depending on conditions. Supercoiling compacts DNA further, managed by topoisomerases.
Single nucleotide polymorphism (SNP)
A variation at a single position in a DNA sequence among individuals. The rate of SNP occurrence is approximately 0.000001%, and about 0.1% of the differences between any two human genomes are attributable to SNPs.
In simple terms, an SNP is a one-letter spelling difference in your DNA compared to someone else's.
Purine
A two-ringed nitrogenous base. In DNA, the purines are adenine (A) and guanine (G). Purines favour the keto tautomeric form.
Think of purines as the larger bases, the ones with two fused rings.
Pyrimidine
A single-ringed nitrogenous base. In DNA, the pyrimidines are cytosine (C) and thymine (T). Pyrimidines favour the amino tautomeric form.
Think of pyrimidines as the smaller bases, one ring each.
Tautomeric form
An isomer of a molecule that differs in the position of a hydrogen atom and a double bond. For nucleotide bases, the keto and amino forms are the most stable and biologically relevant tautomers.
In simple terms, tautomers are the same molecule wearing a slightly different hat, and the wrong hat can cause mispairing during replication.
Base stacking
The arrangement of bases on top of one another within the DNA helix, stabilised by van der Waals interactions and the exclusion of water from the hydrophobic bases.
Think of it as the bases sitting like coins in a stack, sharing electrons between them and hiding from water.
Melting temperature (Tm)
The temperature at which half of a DNA sample is in the single-stranded form. Higher G/C content raises the Tm because G-C base pairs have three hydrogen bonds, compared to two for A-T pairs.
In simple terms, Tm tells you how much heat it takes to unzip half of your DNA. More G-C pairs means a tougher zip.
Major groove
The wider of the two grooves that spiral along the DNA double helix. It contains more chemical information and is the primary site where proteins read DNA sequences without opening the helix.
Minor groove
The narrower groove. It contains less distinguishing chemical information than the major groove, making it harder for proteins to identify specific sequences here alone.
B-DNA
The most common form of DNA under physiological conditions (high humidity). Right-handed helix with the classic Watson-Crick geometry.
A-DNA
A right-handed DNA form found under low-humidity conditions. Shorter and wider than B-DNA.
Z-DNA
A left-handed DNA helix. It accounts for a small portion of cellular DNA and is found on nucleosomes. The 5' strand crosses over the 3' strand in a left-handed diagonal.
Quadruplex DNA
A four-stranded DNA structure. Quite unstable and associated with certain cancers.
Supercoiling
The over- or under-winding of the DNA helix. Negative supercoiling (underwinding) loosens the helix; positive supercoiling (overwinding) tightens it.
Topoisomerase I
An enzyme that relieves supercoiling in single-stranded DNA without requiring ATP. It cuts one strand, allows rotation, and re-seals.
Topoisomerase II
An enzyme that cuts double-stranded DNA to relieve supercoiling. Requires ATP. Also acts on DNA dimers.
Reannealing
The process of complementary DNA strands re-forming a double helix after denaturation. More complex DNA takes longer to reanneal because more bonds must re-form in the correct orientation.
Photo 51
The famous X-ray diffraction image of DNA. Taken by Maurice Wilkins and Raymond Gosling under the supervision of Rosalind Franklin. It was critical evidence for the helical structure of DNA.
RNA world hypothesis
The proposal that early life was based entirely on RNA, before DNA and proteins evolved. The ribosome supports this: peptide bond formation within the ribosome is catalysed by RNA, suggesting RNA preceded protein.
Laid end to end, the DNA in a human body stretches roughly 20 million kilometres.
There are approximately six billion copies of the human genome across all human cells worldwide.
SNPs can be advantageous (e.g. increased protective pigmentation, greater red blood cell production) or disadvantageous (e.g. predisposition to disease).
Humans share a surprising amount of genetic material with mice, fruit flies, and even bananas.
Scientists in Switzerland and Germany demonstrated this by inserting human DNA into fruit flies; the flies accepted and used the DNA as though it were their own.
Purines (A, G) favour the keto tautomeric form; pyrimidines (C, T) favour the amino form.
Complementary base pairing (A with T, G with C) is fundamental to helix stability.
Base stacking adds further stability: electrons are shared between stacked bases, and the hydrophobic bases are shielded from water.
The major and minor grooves arise from the geometry of the sugar-phosphate backbone. Sugars protrude at 120 or 240 degrees around the helix.
240 degrees for the major groove, 120 degrees for the minor.
If the angle were 180 degrees, both grooves would be equal in width.
The major groove contains more chemical information, allowing proteins to distinguish all four base pairs through hydrogen bonding and nonpolar contacts without opening the helix.
In the major groove, all base-pair combinations (GC, CG, AT, TA, and mixed) can be distinguished.
In the minor groove, GC/CG and AT/TA pairs cannot be distinguished from one another, though mixed pairs (GC vs AT and CG vs AT) can be.
B-DNA: found at high humidity, most representative of DNA under physiological conditions.
A-DNA: found at low humidity.
Z-DNA: left-handed helix, small proportion of cellular DNA, found on nucleosomes.
Quadruplex DNA: four-stranded, unstable, linked to certain cancers.
Tm is the temperature at which half the DNA is single-stranded.
G/C-rich DNA has a higher Tm (three hydrogen bonds per pair vs two for A/T).
More complex DNA takes longer to reanneal because more bonds must form in the correct orientation.
Negative supercoiling: the DNA is underwound (fewer helical turns). This loosens the helix and is the predominant form in cells.
Positive supercoiling: the DNA is overwound (more helical turns).
Topoisomerase I relieves supercoiling in single-stranded DNA, does not require ATP.
Topoisomerase II cuts double-stranded DNA and requires ATP.
RNA contains a 2' hydroxyl group that DNA lacks, which allows proteins to distinguish between the two.
RNA uses uracil in place of thymine.
These differences allow RNA to form more complex secondary structures than DNA.
The RNA world hypothesis proposes that RNA preceded both DNA and proteins. Evidence: peptide bond formation in the ribosome is catalysed by an RNA molecule (the ribosomal RNA), not a protein.
Students often assume that the major and minor grooves are simply "big" and "small" versions of the same thing. They are not: the major groove carries far more chemical information for protein recognition.
A common error is thinking Z-DNA is the "default" left-handed form. Z-DNA is rare and context-specific; B-DNA is the standard form under physiological conditions.
Students sometimes conflate Tm with complete denaturation. Tm is defined as the point where half the DNA is single-stranded, not all of it.
Mixing up topoisomerase I and II is frequent. Remember: Topo I handles single-strand nicks and needs no ATP; Topo II cuts double-stranded DNA and requires ATP.
⚠️ Be able to explain why G/C-rich DNA has a higher Tm than A/T-rich DNA, with reference to hydrogen bond numbers.
⚠️ Know the structural and functional differences between the major and minor grooves.
⚠️ Distinguish between negative and positive supercoiling and the role of each topoisomerase.
⚠️ Understand the evidence for the RNA world hypothesis (ribosomal peptide bond formation by RNA).
⚠️ Photo 51 and the roles of Franklin, Wilkins, and Gosling come up as historical context questions.
True or false: A-T base pairs have three hydrogen bonds. (False, they have two.)
Fill in the blank: The ______ groove contains more chemical information for protein recognition. (major)
True or false: Topoisomerase I requires ATP. (False.)
True or false: Z-DNA is a right-handed helix. (False, it is left-handed.)
Fill in the blank: The RNA world hypothesis is supported by the fact that ______ in the ribosome is catalysed by RNA. (peptide bond formation)
Q: Why is the Tm of G/C-rich DNA higher than that of A/T-rich DNA?
A: G-C base pairs are held together by three hydrogen bonds, whereas A-T pairs have only two. More energy (higher temperature) is therefore needed to separate the strands of G/C-rich DNA.
Q: What is the difference between negative and positive supercoiling?
A: Negative supercoiling is underwinding of the helix (fewer turns), which loosens the DNA. Positive supercoiling is overwinding (more turns), which tightens it. Topoisomerase I relieves single-strand supercoils without ATP; topoisomerase II cuts double-stranded DNA and requires ATP.
Q: How do proteins recognise specific DNA sequences without opening the double helix?
A: Proteins read the pattern of hydrogen bond donors and acceptors, and nonpolar contacts, exposed in the major groove. The major groove provides enough chemical information to distinguish all four base-pair combinations.
Q: What structural feature allows proteins to distinguish RNA from DNA?
A: The 2' hydroxyl group on the ribose sugar of RNA, which is absent in DNA's deoxyribose.
Q: What evidence supports the RNA world hypothesis?
A: Peptide bond formation in the ribosome is catalysed by ribosomal RNA, not by protein. This suggests RNA had catalytic function before proteins evolved.
DNA supercoiling connects directly to chromosome packaging and the role of histones (typically covered in chromatin structure chapters).
The melting temperature concept reappears in PCR primer design and hybridisation techniques.
The RNA world hypothesis links forward to ribozymes and the catalytic properties of RNA covered in later chapters on gene expression.
DNA double helix, Watson-Crick base pairing, nucleotide structure, purine vs pyrimidine, keto tautomer, amino tautomer, base stacking interactions, hydrophobic effect in DNA, major groove recognition, minor groove, B-form DNA, A-form DNA, Z-form DNA, G-quadruplex, melting temperature, Tm, hyperchromicity, reannealing, DNA denaturation, negative supercoil, positive supercoil, topoisomerase I, topoisomerase II, gyrase, Photo 51, Rosalind Franklin, X-ray crystallography, RNA world, ribozyme, 2' hydroxyl group, uracil vs thymine, SNP, single nucleotide polymorphism, genetic variation, human genome