DNA structure

DNA nucleotide

• pentose sugar (deoxyribose)

• nitrogenous base attached to C-1 of sugar

• phosphate group on C-5 of sugar

• the next nucleotide attaches at C-3

  • Adenine

  • Cytosine

  • Guanine

  • Thymine

RNA nucleotide

• Has ribose instead of deoxyribose

• ribose has hydroxyl group at C-2 instead of hydrogen

  • Adenine

  • Cytosine

  • Guanine

  • Uracil

Purines

• Composed of 2 linked rings that contain N atoms

  • Guanine

  • Adenine

Pyrimidines

• Composed of single rings containing N atoms

  • Cytosine

  • Uracil

  • Thymine

Sugar phosphate Backbone

• Phosphate at 5’ position of one nucleotide attaches to hydroxyl at 3’ position of the next nucleotide to form the backbone of the DNA strand

• Nucleotides are held together by phosphodiester bonds

• Glycosidic bonds hold sugars to bases

Complementary Base pairing

• A bonds with T with 2 H bonds

• G bonds with C with 3 H bonds

• Distance between each pair is 0.34 nm

• Each full twist of helix is 3.4 nm

• Width of 2 nm

Antiparallel strands

• 5’ end of one strand is matched with 3’ end of the other

• Therefore, they run in opposite directions

• 5’ has phosphate on C-5

• 3’ has hydroxyl on C-3

Double Helix structure

• strands twist in a clockwise direction

• Looks like a twisted ladder

• 10 base pairs per turn

• One side of the turn is called the major groove and is wider; the other is called the minor groove

DNA organizations in Eukaryotes

• To fit in the cell, it is wound around special proteins called histones

• Histones are positively charged so the negatively charged DNA strand are attached to them

• The DNA wraps around a cluster of eight histones, producing a nucleosome

• Further packing of DNA occurs by coiling strings of nucleosomes into cylindrical fibres called solenoids (6 nucleosomes)

• Most of the DNA in interphase is in the form of loosely packed nucleosomes or solenoids

• During prophase, the solenoids can be supercoiled to form the typical X-shaped chromosomes