DNA Replication & Gene Expression

DNA replicates semiconservatively so each molecule keeps one original strand and one new strand. Polymerase builds only 5' to 3', so the leading strand is continuous and the lagging strand is made as Okazaki fragments. The Meselson–Stahl experiment proved semiconservative replication, and genetic information flows from DNA to mRNA to protein. Slide replication progress and generation to explore the fork and centrifuge bands.

Intro — Why accurate replication matters

💡 Analogy: copying a 3-billion-letter book without a single typo

①Human DNA ≈ 3 billion base pairs

②Every cell division replicates all of it

③Error rate: ~1 per billion — astonishing accuracy

④Semi-conservative: 1 original strand + 1 new strand → 2 DNAs

⑤Original strand serves as "template" → complementary base pairing (A-T, G-C)

Replication Fork — Leading and Lagging Strands

Replication progress5

Core replication enzymes

helicase → primer (primase) → DNA polymerase → ligase

unwind → mark start → synthesize → join fragments

🔍 Leading vs Lagging Strand

①DNA polymerase synthesizes only in 5'→3' direction

②Leading strand: same direction as fork → continuous synthesis

③Lagging strand: opposite direction → discontinuous (Okazaki fragments)

④Okazaki fragments joined by DNA ligase

⑤Primer (RNA) marks start → later replaced by DNA

Meselson-Stahl Experiment — Proof of Semi-conservative Replication

Generation (0=P, 1=F₁, 2=F₂)1

💡 Experiment Interpretation

①P: only ¹⁵N (heavy) DNA → heavy band only

②F₁: all DNA is ¹⁵N-¹⁴N → intermediate band only

③F₂: ¹⁵N-¹⁴N (intermediate) + ¹⁴N-¹⁴N (light) → two bands

④Only semi-conservative replication explains this result

⑤Conservative or dispersive would show different patterns

Gene Expression — Central Dogma

Central Dogma

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

flow of genetic information: DNA → RNA → protein

📐 Key differences between transcription and translation

①Transcription: DNA → mRNA | RNA polymerase | in nucleus

②Translation: mRNA → protein | ribosome | in cytoplasm

③1 codon (3 bases) = 1 amino acid

④Start codon: AUG (methionine)

⑤Stop codons: UAA, UAG, UGA (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 pairs with C one-to-one (A=T, G=C).

If A is 30%, then T is 30% (60% together); the remaining 40% is split evenly between G and C, so G = 20%.

▸ 20%

Complementary pairing (A-T, G-C) makes A=T and G=C (Chargaff rule). The four bases sum to 100%.

Example 2

E. coli grown only on ¹⁵N (heavy nitrogen) is moved to ¹⁴N medium and divides once (F₁). What band(s) appear on centrifugation?

In semi-conservative replication, each daughter DNA has one ¹⁵N strand and one newly made ¹⁴N strand.

So every DNA becomes ¹⁵N-¹⁴N (intermediate weight), giving a single intermediate band.

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

A single intermediate band at F₁ is the key proof of semi-conservative replication. At F₂ it splits into an intermediate band and a light band.

Mock-test style

Which statement about DNA replication and gene expression is correct?

①DNA replication is conservative

②The lagging strand is synthesized discontinuously as Okazaki fragments

③Transcription occurs on the ribosome

④A stop codon specifies methionine

⑤The Meselson-Stahl experiment proved conservative replication

▸ ② The lagging strand is synthesized discontinuously as Okazaki fragments

The lagging strand runs opposite to the fork, so it is made as short Okazaki fragments that ligase then joins.

Replication is semi-conservative (①, ⑤); transcription is done by RNA polymerase in the nucleus while translation is on the ribosome (③); and stop codons specify no amino acid (④).

Summary

DNA replication

semi-conservative: original strand + new strand

helicase → primase → DNA polymerase (5'→3') → ligase

🎯 Exam Points

①Semi-conservative replication: proven by Meselson-Stahl

②DNA polymerase: synthesizes only 5'→3'

③Leading (continuous) vs Lagging (Okazaki fragments, discontinuous)

④Transcription: DNA → mRNA (template 3'→5' read, mRNA 5'→3')

⑤Translation: codon-anticodon pairing → amino acid chain | AUG starts, UAA/UAG/UGA stop

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