Genetics - DNA Structure and Synthesis Notes

DNA Structure

• DNA is double stranded with nitrogenous bases facing inward.
• Nucleotide subunits link to form a single strand.
• Two DNA strands run anti-parallel and are held by hydrogen bonds between nitrogenous bases.
• Double-stranded DNA forms a helix.
• DNA replication is semi-conservative.

Components of DNA

• Nucleotide units: pentose sugar (deoxyribose), phosphate group, and a nitrogenous base.
• Nitrogenous bases: Adenine (A), Guanine (G), Thymine (T), Cytosine (C).
• Nucleotide units link via phosphodiester linkages.
• Each strand has a free 5’ phosphate group at one end and a free 3’ hydroxyl group at the other end.

Double Helix

• Two nucleic acid strands form a double helix.
• Strands run anti-parallel.
• Sugar-phosphate backbone is on the outside.
• Complementary base pairing: Adenine (A) pairs with Thymine (T); Guanine (G) pairs with Cytosine (C).
• Bases held together by hydrogen bonds.

Semi-Conservative Replication

• Strands separate, and each acts as a template.
• Complementary base pairing ensures new halves are built correctly.

Replication Process

• Unwinding: DNA helix unwinds, and strands separate by breaking hydrogen bonds.
• Complementary base pairing: New strand is built.
• Joining: DNA polymerase links nucleotides to form the new chain.
• DNA polymerase adds nucleotides in a 5’ to 3’ direction (only to the 3’ end).

Enzymes Involved in DNA Replication

• Helicase: Untwists the DNA double helix.
• Single-strand binding protein: Keeps separated DNA strands apart.
• DNA polymerase: Adds nucleotides to the new DNA strand.
• Ligase: Joins DNA fragments together.