DNA Structure & Replication

Chromosomes

• Composed of 60% protein and 40% nucleotide.

  • Long tangled molecules of DNA wound around protein.

• Hard to distinguish individual chromosomes → Collectively called chromatin.

• Each chromosome (46 per human cell) contains 100s to 1000s of genes.

• Gene: a segment of DNA that encodes the sequence of a particular protein.

DNA Structure

• Has a shape called a double helix, which resembles a twisted ladder.

• Monomer is the nucleotide.

• Polymer is the nucleic acid.

• DNA is formed by two polymers attached in the middle by H-bonds.

• There are 4 possible bases in DNA, and therefore, there are 4 different nucleotides.

  • Pyrimidines (1 ring): thymine & cytosine.

  • Purines (2 rings): adenine & guanine

• Only pyrimidine pairs up with purine → A ratio of 1 pyrimidine: 1 purine.
  → DNA is always “3 rings” across.

• Interactions between the different nucleotides cause the twisting of the ladder.

Comparison between DNA and RNA

• Name

  • DNA: deoxyribonucleic acid

  • RNA: ribonucleic acid

• Sugar

  • DNA: deoxyribose

  • RNA: ribose

• Location in cell

  • DNA: nucleus and mitochondria

  • RNA: nucleus, nucleolus, cytoplasm, and ER (ribosomes)

• Shape

  • DNA: double helix

  • RNA: single strand

• Bases

  • DNA: A, T, C, G

  • RNA: A, U, C, G ( U = uracil is also a pyrimidine)

DNA replication

• Mitosis: DNA must be precisely copied so that daughter cells are identical to parent cells.

• Due to the complementary base pair, each DNA strand can be used as a template to build the opposite complementary strand.

• Semi-conservative replication occurs because half of the new DNA molecule is new and half is conserved (saved) from the original molecule.

• DNA replication has 3 major steps:

  • Unzipping

  • Complementary base pairing

  • Joining adjacent nucleotides

Unzipping

• An enzyme called DNA helicase “unwinds” the DNA by disrupting the H-bonds between the paired nucleotides.

• Proteins at the “knot” prevent the “rezipping.”

Complementary base pairing

“Free” nucleotides in the cytoplasm form H-bonds to the newly exposed bases.

Joining adjacent (side by side) nucleotides

An enzyme called DNA polymerase attaches the sugar-phosphate backbone of the NEW strand of DNA.