W - BIOL200 - 3.1-3.2 DNA STRUCTURE, MESELSON - STAHL EXPERIMENT

What is the structure of DNA?

Double helix with sugar-phosphate backbone and nucleotide bases.

 How are DNA strands oriented?

Anti-parallel (5'→3' and 3'→5').

Which base pairs with adenine in DNA?

 Thymine (T).

Which base pairs with guanine in DNA?

Cytosine (C).

What bonds connect base pairs in DNA?

 Hydrogen bonds.

How many hydrogen bonds between A–T?

2 hydrogen bonds.

How many hydrogen bonds between G–C?

 3 hydrogen bonds.

 Which bases are purines?

Adenine (A) and Guanine (G).

Which bases are pyrimidines?

Cytosine (C), Uracil (U), and Thymine (T).

 Mnemonic to remember pyrimidines?

CUT the Py = Cytosine, Uracil, Thymine.

Mnemonic to remember purines?

Pure As Gold = Adenine and Guanine.

 What replaces thymine in RNA?

Uracil (U).

Why are C–G bonds stronger?

 They have 3 H-bonds; need more energy to break.

 What are the 3 parts of a nucleotide?

Phosphate group, pentose sugar, nitrogenous base.

 What bond joins sugar to base in nucleotide?

 Glycosidic bond.

 What type of sugar is in DNA?

 Deoxyribose (missing OH at 2' carbon).

How is the DNA backbone elongated?

By forming phosphodiester bonds.

Where do phosphodiester bonds form?

 Where do phosphodiester bonds form?

What are the 3 levels of DNA structure?

• Primary: nucleotide sequence

• Secondary: double helix

• Tertiary: chromatin (DNA/protein complex)

What are the 3 DNA replication models?

Conservative, Semi-conservative, Dispersive.

What does semi-conservative mean?

Each new DNA has one old strand and one new strand.

What isotope was used to label "heavy" DNA?

Nitrogen-15 (¹⁵N).

What isotope was used to label "light" DNA?

 Nitrogen-14 (¹⁴N).

What did 1st round in ¹⁴N show?

All DNA was 50% heavy / 50% light → ruled out conservative model.

What did 2nd round in ¹⁴N show?

 DNA was either 50/50 or all light → ruled out dispersive model.

Conclusion of Meselson-Stahl experiment?

 DNA replicates by a semi-conservative mechanism.