Home / Courses / Genetics / Unit 6

Genetics Unit 6: DNA Structure and Replication

This unit covers how scientists proved DNA is the hereditary material, how its double-helix structure works, and how cells copy it accurately before each division.

Covers: Evidence that DNA is the genetic material, Structure of the double helix, Chargaff's rules and base pairing, Semiconservative replication (Meselson-Stahl), Replication enzymes and the fork, Leading and lagging strands, Proofreading, telomeres and telomerase

Study guide

Three experiments identified DNA as the genetic material. Griffith found that heat-killed virulent pneumococci could transform harmless live bacteria into virulent ones. Avery, MacLeod and McCarty showed the transforming factor survived protein- and RNA-destroying enzymes but was destroyed by DNase, so it was DNA. Hershey and Chase labeled bacteriophage DNA with ³²P and protein with ³⁵S; after infection, ³²P entered the bacteria while ³⁵S stayed outside. Together these showed that DNA carries the instructions passed between generations.

DNA is a polymer of nucleotides, each with a deoxyribose sugar, a phosphate and one of four bases: the purines adenine and guanine and the pyrimidines thymine and cytosine. Phosphodiester bonds link the 3′ carbon of one sugar to the 5′ carbon of the next, giving each strand direction. Watson and Crick, using Franklin's X-ray data, proposed two antiparallel strands wound into a right-handed helix, with about 10.5 bp per turn and 0.34 nm between pairs. A pairs with T by two hydrogen bonds and G with C by three, so A = T and G = C in double-stranded DNA (Chargaff's rules).

Base pairing suggested that each strand can serve as a template, and Meselson and Stahl confirmed semiconservative replication. They grew E. coli in heavy ¹⁵N, moved it to ¹⁴N, and separated DNA by density. After one generation all DNA was hybrid, ruling out conservative replication. After two generations, there were equal hybrid and light bands, ruling out the dispersive model. Each new duplex keeps one original strand and one new strand.

Replication begins at origins and proceeds in both directions. Helicase unwinds the helix, single-strand binding proteins keep strands apart, and topoisomerase relieves supercoiling ahead of the fork. DNA polymerases can only add nucleotides to an existing 3′-OH, so primase first lays down a short RNA primer. Synthesis is always 5′→3′. The leading strand is made continuously toward the fork; the lagging strand is made away from the fork as Okazaki fragments. In bacteria, DNA polymerase III does most synthesis, DNA polymerase I replaces primers with DNA, and DNA ligase seals the nicks.

Accuracy comes in layers. Polymerase selects the correct base most of the time, its 3′→5′ exonuclease proofreads, and mismatch repair fixes most remaining errors, giving about one mistake per 10⁹ to 10¹⁰ bases. Eukaryotes replicate long linear chromosomes from many origins because their forks move slowly. Linear ends create the end-replication problem: the last primer on the lagging strand cannot be replaced, so telomeres would shorten each division. Telomerase, an enzyme with its own RNA template, adds repeats to the ends; it is active in germ cells, stem cells and most cancer cells.

Key ideas

• Hershey and Chase showed that phage DNA, not protein, enters infected cells.
• DNA strands are antiparallel, with A–T (2 hydrogen bonds) and G–C (3) pairing.
• Chargaff's rules A = T and G = C hold for double-stranded DNA only.
• Meselson and Stahl showed replication is semiconservative.
• Polymerases add only to a 3′-OH, so all synthesis is 5′→3′ and needs a primer.
• The lagging strand is made as Okazaki fragments joined by ligase.
• Telomerase uses an RNA template to extend chromosome ends.

Formulas and rules

Common mistakes

• Writing a complementary strand without reversing it into 5′→3′ order.
• Applying Chargaff's rules to single-stranded genomes.
• Thinking DNA polymerase can start a strand without a primer.
• Confusing the 3′→5′ proofreading exonuclease with the 5′→3′ primer-removing activity.
• Thinking one generation of Meselson-Stahl data rules out the dispersive model.

Before the test, make sure you can

• Describe the Griffith, Avery and Hershey-Chase experiments and what each showed.
• Describe DNA structure, including antiparallel strands and base pairing.
• Calculate base percentages with Chargaff's rules and write complementary strands correctly.
• Predict Meselson-Stahl band patterns under each replication model.
• Explain the roles of each enzyme at the replication fork.
• Explain why the lagging strand is discontinuous.
• Calculate replication time and number of origins.
• Explain the end-replication problem and telomerase.

Words to know

Nucleotide

A sugar, phosphate and base; the monomer of DNA and RNA.

Purine

A two-ring base: adenine or guanine.

Pyrimidine

A one-ring base: cytosine, thymine or uracil.

Phosphodiester bond

The covalent bond linking the 3′ carbon of one nucleotide to the 5′ phosphate of the next.

Antiparallel

The two DNA strands run in opposite 5′→3′ directions.

Chargaff's rules

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

Semiconservative replication

Each new DNA duplex contains one parental and one new strand.

Origin of replication

A site where DNA unwinding and replication begin.

Helicase

Enzyme that unwinds the double helix at the fork.

Topoisomerase

Enzyme that relieves supercoiling by cutting and rejoining DNA.

Primase

Enzyme that makes short RNA primers for DNA synthesis.

DNA polymerase III

The main bacterial replicative polymerase, with 3′→5′ proofreading.

DNA polymerase I

Bacterial polymerase that removes RNA primers and fills the gaps.

DNA ligase

Enzyme that seals nicks in the DNA backbone.

Leading strand

New strand made continuously toward the replication fork.

Lagging strand

New strand made discontinuously away from the fork.

Okazaki fragment

A short DNA piece made on the lagging strand.

Telomerase

A reverse transcriptase with an RNA template that extends telomeres.

Sample practice questions

30 more questions like these, from level 1 to 5, are in the practice set.