DNA Structure and Replication — Quick Notes

Genetic Material: Core Characteristics

Griffith transformation: Type IIIS (virulent) DNA can transform Type IIR (nonvirulent) bacteria.
Transformation requires DNA; DNase (DNA destruction) stops transformation; RNase and protease do not.
Viruses transfer genetic material; Hershey–Chase experiments show DNA (not protein) is the genetic material when labeled with 32P vs 35S.

DNA is composed of nucleotides: sugar (deoxyribose), phosphate, and a nitrogenous base.
Four bases: Adenine (A), Guanine (G) – purines; Cytosine (C), Thymine (T) – pyrimidines.
RNA uses Uracil (U) instead of Thymine (T).

A pairs with T via 2 hydrogen bonds; G pairs with C via 3 hydrogen bonds.
Strands are antiparallel.
Phosphodiester backbone links 5'-phosphate to 3'-OH of adjacent nucleotides.
Double helix dimensions:
- 0.34 nm per base pair; ~2 nm diameter; ~3.4 nm per turn (10 bp per turn).
Bases stack in the backbone; rungs formed by hydrogen bonds between complementary bases.

Replication must be highly accurate: ~1 error per 10^6 bp; rapid replication (e.g., E. coli ~1000 nucleotides per second).
Requirements: template strand, building blocks (nucleotides), enzymes and proteins.
Semiconservative model: each new DNA molecule contains one old strand and one new strand.

New DNA synthesized from deoxyribonucleoside triphosphates (dNTPs).
DNA polymerase adds nucleotides to the 3' end of the growing strand; synthesis proceeds 5' → 3'.

Leading strand: continuous synthesis; Primer required only at the origin end.
Lagging strand: discontinuous synthesis via Okazaki fragments; Primase lays RNA primers to provide 3'-OH for DNA polymerase.
DNA polymerase I replaces RNA primers with DNA nucleotides.
DNA ligase seals nicks in the sugar-phosphate backbone.

Replication occurs at a replication fork with unwinding of the DNA.
Template strands: read 3'→5' while new strands are synthesized 5'→3'.
Diagram concepts: leading vs lagging strands defined by direction relative to fork movement.

Linear chromosomes face end-replication issues; removal of RNA primers creates gaps.
Telomeres: protective ends that prevent shortening; telomerase extends telomeres to counteract end-replication problems.
Telomerase is a ribonucleoprotein that extends ends; most somatic cells have little telomerase; re-expression can lead to immortal cells.
Telomerase activity is detected in ~90% of cancers.

Werner syndrome linked to telomere maintenance defects.
Key summary points:
- DNA is the molecule of heredity with two antiparallel, complementary strands.
- Replication is semiconservative and 5'→3'.
- Telomerase solves the end-replication problem.
- Information flow: DNA → RNA → protein; some viruses reverse this path (RNA → DNA).

Transcription and Translation follow, with information flow summarized as: DNA to RNA to protein; in some viruses, RNA to DNA or RNA to RNA.

Which enzyme would stop transformation if DNA carried hereditary information? DNAse would prevent transformation.
Which bases are purines? Adenine (A) and Guanine (G).
Which base pair is stronger? Guanine–Cytosine (G–C) with 3 hydrogen bonds.
What does 5' and 3' refer to? The ends defined by the number of the carbon atoms involved in phosphodiester bonds (5' phosphate to 3' hydroxyl).
If a DNA molecule is 10% adenine, what is the cytosine content? 40% (Chargaff's rule: A=T and G=C).