Difficulty: Introductory | Prerequisites: None, though familiarity with basic cell biology (organelles, cell membrane) is helpful.
This topic covers the molecular structure of DNA and RNA, the landmark experiments that proved DNA is the hereditary molecule, and the mechanism cells use to copy their DNA before dividing. It sits at the heart of introductory biology: everything from genetics to evolution to biotechnology builds on it. You should already be comfortable with the idea that cells contain a nucleus and that traits are inherited, but no molecular detail is assumed. If you can follow this material, the protein synthesis and gene control notes that come next will make far more sense.
DNA is a double-stranded helix that stores genetic information; RNA is its single-stranded working copy. Several classic experiments (Griffith, Hershey-Chase, Chargaff, Watson and Crick) established that DNA, not protein, carries hereditary information and revealed its structure. During replication, enzymes unwind the double helix and build two identical copies using semiconservative replication, so each daughter molecule keeps one original strand.
DNA (deoxyribonucleic acid)
A double-stranded nucleic acid that carries the complete set of genetic instructions for an organism. In simple terms, it is the master copy of every instruction your cells need.
RNA (ribonucleic acid)
A single-stranded nucleic acid involved in decoding and expressing the instructions stored in DNA. Think of it as the working blueprint copied from the master plan.
Nucleotide
The monomer (building block) of nucleic acids, consisting of a sugar, a phosphate group, and a nitrogenous base. Think of it as one rung plus its side-rails on a ladder.
Nitrogenous base
The variable part of a nucleotide. DNA uses adenine (A), thymine (T), guanine (G), and cytosine (C). RNA swaps thymine for uracil (U).
Complementary base pairing
The rule that A always pairs with T (or U in RNA), and G always pairs with C, held together by hydrogen bonds. This is the reason DNA can be copied faithfully.
Helicase
An enzyme that unwinds the DNA double helix at the replication fork. Think of it as the zipper-puller that separates the two strands.
DNA polymerase
The enzyme that builds a new DNA strand by adding nucleotides to the 3' end of a growing chain. It also proofreads for errors.
RNA primer / primase
A short RNA sequence laid down by the enzyme primase that gives DNA polymerase a starting point. DNA polymerase cannot begin from scratch; it needs this primer.
Topoisomerase
An enzyme that relieves the tension ahead of the replication fork by preventing the DNA from supercoiling. It works alongside helicase.
Single-stranded binding proteins (SSBPs)
Proteins that stabilise the unwound DNA strands and prevent them from snapping back together or folding on themselves.
Leading strand
The new DNA strand synthesised continuously in the direction of the replication fork. It begins with the old 3' strand as its template.
Lagging strand
The new DNA strand synthesised in short fragments (Okazaki fragments) away from the replication fork. It begins with the old 5' strand as its template.
Okazaki fragments
Short segments of DNA produced on the lagging strand, later joined together by DNA ligase.
DNA ligase
An enzyme that seals the gaps between Okazaki fragments on the lagging strand, creating a continuous strand.
Semiconservative replication
The model of DNA replication in which each daughter molecule contains one original (parental) strand and one newly synthesised strand.
Chromatin
The loosely packed form of DNA and histone proteins found in the nucleus during interphase. Think of it as thread before it is wound into yarn.
Nucleosome
A unit of DNA wrapped tightly around a core of histone proteins, the first level of DNA packaging.
Chromosome
A fully condensed, supercoiled structure of DNA and histones, visible during cell division. Humans have 46 chromosomes (23 pairs).
Sister chromatids
Two identical copies of a chromosome joined at the centromere after DNA replication. The familiar X shape is a pair of sister chromatids.
Centromere
The densely packed protein region that holds sister chromatids together until they separate during cell division.
Autosome
Any chromosome that is not a sex chromosome. In humans, these are pairs 1 through 22.
Sex chromosome
The chromosome pair that determines biological sex: XX in females, XY in males (in mammals and many other animals).
DNA is double-stranded (two antiparallel strands running 5' to 3' and 3' to 5'), uses deoxyribose as its sugar, and contains thymine (T) as one of its four nitrogenous bases.
The two strands are held together by hydrogen bonds between complementary bases.
The backbone is made of alternating sugar and phosphate groups.
RNA is single-stranded, uses ribose as its sugar, and substitutes uracil (U) for thymine.
A helpful analogy: DNA is the master plan locked in the vault (the nucleus). RNA is the working blueprint carried out to the construction site (the ribosome).
Griffith injected mice with two strains of Streptococcus pneumoniae: a virulent smooth (S) strain and a harmless rough (R) strain.
Live S strain killed the mouse.
Live R strain did not kill the mouse.
Heat-killed S strain did not kill the mouse.
Heat-killed S strain mixed with live R strain killed the mouse.
The harmless R bacteria had picked up something from the dead S bacteria and transformed into a deadly form. This demonstrated that cells contain heritable information (a "transforming principle") that can be transferred between organisms. The experiment proved transformation occurs but did not identify DNA specifically.
Hershey and Chase used T2 bacteriophages (viruses made of only DNA and protein) that infect E. coli.
They labelled one batch with a radioactive isotope that tags protein and another batch with a radioactive isotope that tags DNA.
After the phages infected the bacteria, the cells were agitated to remove the outer protein coats.
The protein isotope was found in the surrounding medium (outside the cell).
The DNA isotope was found inside the bacterial cells.
This proved that DNA, not protein, is injected into the host and carries the hereditary information.
Erwin Chargaff analysed the base composition of DNA from multiple species and found a consistent pattern:
The amount of adenine (A) always equals the amount of thymine (T).
The amount of guanine (G) always equals the amount of cytosine (C).
These ratios pointed toward specific base pairing and were a critical clue for working out DNA's structure.
James Watson and Francis Crick synthesised data from Chargaff, Rosalind Franklin's X-ray crystallography, and others to propose that DNA is a double helix with nitrogenous bases on the inside and a sugar-phosphate backbone on the outside. They are credited with discovering the structure of DNA.
Hydrogen bonds hold the complementary bases together across the two strands (A-T and G-C).
G-C pairs are held by 3 hydrogen bonds; A-T pairs by 2. This difference in bond number is why G never pairs with T.
Peptide bonds are found in polypeptide chains (proteins), not in DNA itself. Know the distinction.
DNA strands have directionality. One end is called 5' (carries a phosphate group) and the other is 3' (carries a hydroxyl, OH, group).
The template strand is read 3' to 5' by DNA polymerase.
New nucleotides are added only to the 3' end of the growing strand.
This directionality is the reason the leading and lagging strands behave differently during replication.
DNA in its loosest form is called chromatin, a long thread of DNA wrapped around histone proteins.
DNA wraps tightly around histones to form nucleosomes (the "beads on a string" structure).
Nucleosomes coil and supercoil further to form a fully condensed chromosome, visible under a microscope during cell division.
Humans have 46 chromosomes arranged in 23 pairs.
An unreplicated chromosome is a single linear DNA molecule wrapped around histones.
After replication, it becomes a pair of sister chromatids joined at the centromere. The X shape you see in diagrams is this replicated form.
Pairs 1 to 22 are autosomes (not involved in sex determination).
Pair 23 consists of the sex chromosomes: XX in females, XY in males. This pattern holds across mammals, fruit flies, and many other animals.
DNA replication is semiconservative: each new double helix contains one old strand and one newly built strand. It occurs during interphase (before mitosis).
Helicase unwinds the double helix at the replication fork.
RNA primase lays down a short RNA primer. DNA polymerase binds to the primer and begins adding complementary nucleotides.
The leading strand is synthesised continuously; the lagging strand is synthesised in Okazaki fragments.
DNA ligase joins the fragments on the lagging strand.
Step 1: Unwinding
Helicase separates the two strands, creating a replication "bubble" with replication forks at each end. Topoisomerase works ahead of helicase to relieve the tension caused by unwinding. Single-stranded binding proteins (SSBPs) coat the exposed strands to stop them re-pairing or folding.
Step 2: Priming
RNA primase synthesises a short RNA primer complementary to the template strand. DNA polymerase cannot start a new chain on its own; the primer provides the free 3'-OH group it needs.
Step 3: Elongation
DNA polymerase reads the template strand 3' to 5' and builds the new strand 5' to 3', adding nucleotides only to the 3' end.
Leading strand: The template runs 3' to 5' into the fork, so DNA polymerase can follow the fork continuously, producing one unbroken strand. It needs only a single RNA primer.
Lagging strand: The template runs 5' to 3' into the fork, so DNA polymerase must work away from the fork. Each time the fork opens further, a new RNA primer is laid down and a short Okazaki fragment is synthesised. DNA ligase then glues the fragments together into a continuous strand.
Step 4: Proofreading
DNA polymerase has a built-in proofreading function: it checks each newly added nucleotide and removes mismatches. Despite this, some errors slip through, which is one source of mutations.
Helicase: unwinds DNA at the replication fork.
Topoisomerase: prevents supercoiling ahead of the fork.
SSBPs: stabilise single-stranded DNA.
Primase: synthesises RNA primers.
DNA polymerase: adds nucleotides (3' end only) and proofreads.
DNA ligase: seals Okazaki fragments on the lagging strand.
DNA replication is the basis of PCR (polymerase chain reaction), the technique used in forensic DNA profiling, paternity testing, and COVID-19 diagnostic tests. Understanding base pairing and replication enzymes is also essential to gene therapy and CRISPR gene editing, both of which rely on the cell's own replication and repair machinery.
Students often think the two DNA strands run in the same direction. They do not. The strands are antiparallel: one runs 5' to 3' while the other runs 3' to 5'.
Students often confuse "chromosome" with "chromatid." A chromosome is one DNA molecule packaged with histones. Sister chromatids are the two identical copies of that chromosome joined at the centromere after replication.
Students often think DNA polymerase can start building a new strand from nothing. It cannot. It always requires an RNA primer to provide a free 3'-OH group first.
Students often think Griffith's experiment identified DNA as the transforming substance. It did not. Griffith showed transformation happens; Hershey and Chase later proved the transforming substance is DNA.
Know every replication enzyme and its specific role. This is called out explicitly in the source notes as a likely exam topic.
Be able to distinguish the leading strand from the lagging strand, including which parental strand each one uses as a template (leading begins with the old 3' strand; lagging begins with the old 5' strand).
Understand semiconservative replication and be able to explain why each daughter molecule is half old, half new.
Be ready to compare Griffith's experiment with Hershey-Chase: what each proved and what each left unanswered.
Know Chargaff's rules (A = T, G = C) and how they supported the Watson-Crick model.
Understand the 5' and 3' convention: which end carries the phosphate, which carries the hydroxyl, and why nucleotides are added only to the 3' end.
True or False: DNA polymerase adds nucleotides to the 5' end of the growing strand. (False, it adds to the 3' end.)
True or False: In Hershey and Chase's experiment, the radioactive protein was found inside the bacterial cells. (False, the protein label stayed in the surrounding medium.)
Fill in the blank: According to Chargaff's rules, if a DNA sample is 30% adenine, it must also be 30% ________. (Thymine.)
True or False: The lagging strand is synthesised as one continuous piece. (False, it is synthesised in short Okazaki fragments.)
Fill in the blank: The enzyme that seals the gaps between Okazaki fragments is called ________. (DNA ligase.)
Q: What did Griffith's experiment demonstrate, and what question did it leave unanswered?
A: It demonstrated that a "transforming principle" could transfer heritable information from dead virulent bacteria to live harmless bacteria, changing them into a virulent form. It did not identify what that transforming substance was.
Q: Describe how Hershey and Chase proved that DNA, not protein, is the hereditary material.
A: They labelled bacteriophage DNA and protein with different radioactive isotopes, then tracked which isotope entered the bacterial cell during infection. Only the DNA label was found inside the cells, proving DNA carries the genetic instructions.
Q: Why does the lagging strand require multiple RNA primers while the leading strand needs only one?
A: DNA polymerase can only add nucleotides in the 5' to 3' direction. The leading strand's template runs 3' to 5' into the fork, so synthesis is continuous and needs just one primer. The lagging strand's template runs 5' to 3' into the fork, so polymerase must work away from the fork in short segments, each requiring its own primer.
Q: Explain what "semiconservative replication" means and why it matters.
A: Each daughter DNA molecule retains one original parental strand and one newly synthesised strand. This ensures that the genetic information is faithfully copied, because each new strand is built using the old strand as a direct template.
Q: A DNA sample is found to be 22% guanine. What are the percentages of the other three bases?
A: Cytosine = 22% (G always equals C). The remaining 56% is split equally between adenine and thymine, so each is 28%.
DNA replication is the prerequisite for both mitosis and meiosis; without faithful copying, cell division would produce defective daughter cells. The enzymes covered here (especially RNA polymerase and DNA polymerase) reappear in protein synthesis, where DNA is transcribed into mRNA and then translated into protein. Mutations, which arise from replication errors, are the raw material for evolution by natural selection.
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