2.6 (A1.2, D1.1) The Structure and Replication of DNA

Bell Work

  • List one thing you know about DNA.

  • Objective: Students will be able to explain the structure of DNA and how it replicates.

DNA Structure and Replication (2.6 A1.2, D1.1)

Structure of DNA

  • DNA stands for deoxyribonucleic acid.

  • DNA is a nucleic acid, which is a long polymer molecule composed of monomers called nucleotides.

Parts of a Nucleotide

  • Three components of a nucleotide:

    • Nitrogenous Base: Variable component (adenine, thymine, cytosine, guanine)

    • Phosphate Group: Links nucleotides together.

    • Sugar (Deoxyribose): Provides structure and connectivity.

Double Helix Structure

  • DNA is structured as a "double helix"—two spirals or two strands made of many nucleotides; these strands are wound around each other.

DNA Backbone

  • The backbone of DNA consists of alternating sugar (deoxyribose) and phosphate groups, forming a sugar-phosphate backbone.

Complementary Base Pairing

  • Rungs of the DNA ladder are made up of base pairs:

    • Adenine (A) bonds with Thymine (T) (A = T) [2 hydrogen bonds]

    • Guanine (G) bonds with Cytosine (C) (G ≡ C) [3 hydrogen bonds]

  • Chargaff's rules state that the bases are complementary, meaning A always pairs with T and G always pairs with C.

Discovery of DNA Structure

  • Model proposed by Watson and Crick in 1953.

DNA Replication

  • Objective: Explain and model how DNA is replicated.

  • DNA replication is the process where a cell copies all of its DNA.

    • This is crucial to ensure that when a cell undergoes division, the resulting cells have identical DNA sets necessary for proper function.

Steps of DNA Replication

  1. Separation of Strands: The DNA molecule unwinds into two strands by the action of helicase, which breaks hydrogen bonds between the base pairs.

  2. Template Formation: The separated unpaired nucleotides serve as templates to create new strands.

  3. Nucleotide Addition: DNA polymerase joins free-floating complementary nucleotides to form new strands.

  4. Result: Two new strands are formed that are complementary to the original strands.

Semiconservative Replication

  • Each resulting DNA molecule consists of one original strand and one new strand, yielding two identical DNA molecules.

DNA Technology

PCR (Polymerase Chain Reaction)

  • A technique to amplify (increase the amount of) DNA from a small sample.

  • Useful for detecting genetic material (e.g., SARS-CoV-2).

  • Involves primers, free DNA nucleotides, Taq DNA polymerase, and thermal cycling for DNA replication.

Gel Electrophoresis

  • Method for separating DNA or proteins to identify sources.

  • Enzymes are used to cut DNA into fragments of varying lengths.

  • Fragments are separated by an electric current in a gel; smaller fragments move further.

  • Banding patterns can be used for DNA profiling and establishing genetic relationships.