Grade 11-12 (age 16-18)

high school DNA Replication & Gene Expression

After its two strands separate, DNA is copied by using each strand as a template for a new one, so that every copy contains one original strand and one new strand: semiconservative replication. DNA polymerase can extend a new strand only in the 5' → 3' direction, so one strand (the leading strand) is made continuously behind the fork, while the other (the lagging strand) is made in short Okazaki fragments that are joined afterward. Meselson and Stahl proved semiconservative replication by centrifuging DNA labeled with heavy nitrogen (¹⁵N) and light nitrogen (¹⁴N), and the copied genetic information is expressed as protein through transcription and translation. In the diagram you can see the fork climb as replication proceeds while the leading strand and Okazaki fragments lengthen, and watch the bands in the centrifuge tubes shift from generation to generation.

Why Accurate Copying Matters

💡 Copying a Book of 3 Billion Letters Without a Single Mistake

①Human DNA has about 3 billion base pairs, and every one of them has to be copied each time a cell divides

②DNA polymerase even corrects wrongly added bases, so errors are very rare, about one in a billion bases

③Once the two strands separate, each serves as a template, and a new strand is built by complementary base pairing, A with T and G with C

④So each of the two copied DNA molecules is made of one original strand and one new strand (semiconservative replication)

The Replication Fork: Leading and Lagging Strands

Replication progress5

Enzymes of Replication

helicase (unwinds the strands) → primase (RNA primer) → DNA polymerase (builds the new strand) → DNA ligase (joins fragments)

DNA polymerase cannot start a new strand on its own; it adds nucleotides to the end of a primer

🔍 Leading and Lagging Strands

①DNA polymerase extends a new strand only in the 5' → 3' direction

②On the leading strand the new strand grows in the same direction the fork moves, so it is made continuously, right behind the fork

③On the lagging strand it grows away from the fork, so a short Okazaki fragment is made separately each time more template is exposed

④Each fragment starts from an RNA primer; later the primer is replaced with DNA and DNA ligase joins the fragments into one strand

The Meselson–Stahl Experiment: Proof of Semiconservative Replication

GenerationF₁

💡 Reading the Results

①After E. coli grows for many generations on ¹⁵N medium, both DNA strands contain ¹⁵N, so centrifugation gives a single heavy band (P)

②Moved to ¹⁴N medium and allowed to replicate once, every DNA molecule has one ¹⁵N strand and one ¹⁴N strand, giving a single intermediate band (F₁)

③After one more replication, intermediate (¹⁵N-¹⁴N) and light (¹⁴N-¹⁴N) bands appear in a 1 : 1 ratio (F₂)

④Conservative replication would have given heavy and light bands in F₁, and dispersive replication would have given a single band in F₂ as well, so only semiconservative replication explains these results

Gene Expression: The Central Dogma

The Central Dogma

DNA → (transcription) → mRNA → (translation) → protein

genetic information flows from DNA through RNA to protein

📐 Transcription and Translation

①Transcription: RNA polymerase reads the template strand of DNA and makes mRNA; in eukaryotic cells this happens in the nucleus

②Translation: a ribosome reads the mRNA and links amino acids into a protein; this happens in the cytoplasm

③Each set of three consecutive mRNA bases (a codon) specifies one amino acid

④The start codon is AUG (methionine), and the stop codons UAA, UAG and UGA specify no amino acid

Worked Examples and Exam Practice

Example 1

In a double-stranded DNA, adenine (A) makes up 30% of all bases. What percentage is guanine (G)?

In double-stranded DNA, A pairs with T and G with C, so A = T and G = C.

If A is 30%, T is also 30%, making 60% together; G and C share the remaining 40% equally, so G = 20%.

▸ 20%

Complementary base pairing makes A = T and G = C (Chargaff's rule). Together with the four bases summing to 100%, the share of one base gives all the others.

Example 2

E. coli grown only on ¹⁵N is moved to ¹⁴N medium and its DNA replicates just once (F₁). What bands appear when this DNA is centrifuged?

In semiconservative replication, each copied DNA molecule has one ¹⁵N strand and one newly made ¹⁴N strand.

So all the DNA is ¹⁵N-¹⁴N (intermediate density), and a single intermediate band appears.

▸ a single intermediate (¹⁵N-¹⁴N) band

A single intermediate band in F₁ is the key evidence that rules out conservative replication. Move the diagram on to F₂ and it splits into an intermediate band and a light band.

Mock-exam style

Which statement about DNA replication and gene expression is correct?

①DNA is replicated conservatively.

②The lagging strand is made discontinuously, forming Okazaki fragments.

③Transcription takes place on ribosomes.

④Stop codons in translation specify methionine.

⑤The Meselson–Stahl experiment proved conservative replication.

▸ ② The lagging strand is made discontinuously, forming Okazaki fragments.

The lagging strand grows away from the fork, so it is made as short Okazaki fragments that DNA ligase then joins.

Replication is semiconservative (①, ⑤), transcription is carried out by RNA polymerase while translation takes place on ribosomes (③), and stop codons specify no amino acid (④).