DNA Structure - U4

Composition

• Deoxiribose sugar (pentose sugar)

• Phosphate group

• Nitrogenous base

  • Purines (2 C rings)

    • Adenine

    • Guanine

  • Pyramidines (1 C ring)

    • cytosine

    • Thymine

Base Pairing

• A wth T (2 hydrogen bonds)

• C with G (3 hydrogen bonds)

Strong Covalent bonds

• Phosphodiester bond (bonds sugar and phosphate group

• Glycosidic bond: binds sugar and nitrogenous base

Strand are antiparallel

Chargaff’s Rule

%A +G = %T + C

Packaging

DNA must be packaged to fit within the nucleus

1. DNA double helix wraps around histone protein

• Known as nucleosome

• basic unit of chromatin

2. Nucleosomes continue to coil around themselves

• Solenoids are 30nm thick of chromatin fiber (nucleosomes wrapped together)

3. Chromatin Fiber continues to coil around itself

• Loosely packed chromatin is known as euchromatin (700nm)

  • Makes most of the chromosome

• Genes can be expressed

4. Chromatin fiber is compacted and compressed

• Known as heterochromatin

  • Found in centromeres and telomeres

• Cannot express genes

5. Chromosome is composed of both heterochromatin and euchromatin

Gene

A segment of DNA which codes for a specific protein

• Located in the gene locus of a chromosome

• All of the genes in a species is called it’s genome

Prokaryotic Genome

• One single chromosome

• Loop of DNA

• Limited genome

  • Not many genes

  • Prokaryores reproduce asexually —> offspring are genetic clones

    • Low genetic variation

To solve low genetic variation:

Prokaryotes absorb as much foreing DNA as possible

• Plasmids are small circular pieces of DNA that provide new traits to the prokaryotes

  • Come from various sources

  • Conjugation - A plasmid is tranferred directly from one baterium to another

Eukaryotic Genome

• Significantly larger than prokaryotes

  • 3 billion pase pairs

    • 20-25000 genes

• All fits on 23 chromosome pairs (46 chromosomes)