DNA Structure & Organization in the Cell

DNA Overview

• DNA carries hereditary information.

• gene = DNA region coding for a polypeptide or RNA.

• genome = complete hereditary information.

Key Scientists

• Friedrich Miescher (1869): Isolated nuclein (DNA).

• Pheobus Levene (1919): Isolated DNA (deoxyribose) and RNA (ribose).

• Nucleotides: DNA/RNA polymers.

  • Each nucleotide contains:

  • 5C (deoxy)ribose sugar

  • Phosphate group

  • Nitrogen-containing base:

    • Adenine

    • Guanine

    • Cytosine

    • Thymine (in DNA only)

    • Uracil (in RNA only)

• Erwin Chargaff (1940s): Chargaff’s Rule - $A=T$, $G=C$

• Rosalind Franklin (1952): DNA has a helical structure with bases inside and sugar-phosphate backbone outside.

• Watson & Crick (1953): DNA model.

DNA Structure

• Double helix with sugar-phosphate backbone.

• Complementary bases (purine + pyrimidine) joined by H-bonds: $A=T$ (2 H bonds), $G≡C$ (3 H bonds).

• Antiparallel strands run in opposite directions (3’ to 5’).

DNA in Prokaryotic Cells

• Circular, double-stranded DNA.

• Proteins compact DNA into circular structures.

• Mostly genes/regulatory sequences.

• DNA Supercoiling- compacts DNA

• chromosomal DNA and additional plasmid DNA

Plasmids

• Genes for beneficial traits:- digest new foods

  • resist antibiotics

• Replicated separately; transmitted via conjugation (e.g., antibiotic resistance).

DNA in Eukaryotic Cells

• DNA in nucleus; also in mitochondria/chloroplasts.

• DNA wrapped around histones to compact DNA.

• chromosome = condensed DNA around histones.

Chromatin vs Chromosome

• chromatin = loosely coiled DNA.

• chromosome = condensed for cell division.

Genome size

• Genome size not proportional to complexity or gene number.