DNA Structure and Replication

DNA Structure and Replication

Nucleic Acids

  • Definition: The macromolecule that holds genetic material (DNA).

  • Genes: Sections of DNA that serve as the blueprint/instructions for making proteins.

    • Located at specific points on a chromosome.

    • Function of Proteins: Carry out all cellular activity.

  • Types of Nucleic Acids:

    • DNA: Deoxyribonucleic acid

    • RNA: Ribonucleic acid

Nucleotides

  • Definition: The monomer of nucleic acids that make up the larger macromolecule.

  • Components:

    • Sugar:

    • Deoxyribose in DNA

    • Ribose in RNA

    • Phosphate

    • Nitrogen Base:

    • Adenine (A)

    • Guanine (G)

    • Cytosine (C)

    • Thymine (T) (found only in DNA)

    • Uracil (U) (found only in RNA)

DNA Structure

  • Discovery: The structure of DNA was discovered by James Watson and Francis Crick in 1953.

  • Structure Description:

    • Double Helix: Resembles a twisted ladder.

    • Sugar-Phosphate Backbone: Formed by sugar and phosphate.

    • Bonding: Nitrogen bases bond in the middle with weak hydrogen bonds, while all other bonds are strong covalent bonds.

Complementary Base Pairing Rules

  • Definition: Nitrogen bases bond only to their specific complementary base pair through hydrogen bonds.

  • Pairs:

    • A’s bond with T’s

    • C’s bond with G’s

Types of Nitrogen Bases

  • Classification:

    • Purines (small word, big base):

    • Adenine (A)

    • Guanine (G)

    • Pyrimidines (big word, small base):

    • Cytosine (C)

    • Thymine (T)

  • Chargaff’s Rules:

    • A = T and C = G, indicating the pairing fidelity of bases.

  • Bonding Strength:

    • A forms double hydrogen bonds with T.

    • C forms triple hydrogen bonds with G.

DNA Orientation

  • Antiparallel Structure:

    • Definition: The strands run in opposite directions.

    • Strand Directionality:

    • The first strand runs in a 5’ to 3’ direction.

    • The second strand runs in a 3’ to 5’ direction.

    • Phosphate End: Always the 5’ end.

    • Deoxyribose Sugar End: Always the 3’ end.

RNA Structure

  • Structure: Single strand of nucleotides with exposed bases.

  • Base Pairing with DNA:

    • A’s bind with U’s

    • C’s bind with G’s

Comparison: DNA vs. RNA

Characteristic

DNA

RNA

Types of Nitrogen Bases

A, T, C, G

A, U, C, G

Type of Sugar

Deoxyribose

Ribose

Shape

Double helix

Single strand

Basics of Heredity

  • Chromosomes: Tightly coiled strands of DNA.

    • Different organisms have different numbers of chromosomes.

    • Examples:

      • Humans have 23 pairs (46 total: 23 from mom and 23 from dad).

      • Dogs have 37 pairs (74 total: 37 from mom and 37 from dad).

  • Gene: A section of DNA that encodes the instructions to produce a protein.

    • One chromosome can contain thousands of genes linked together.

DNA Replication Background

  • Definition: The process by which a cell makes an identical copy of its DNA when it is ready to divide, termed DNA Replication.

    • Occurs in the nucleus during the S Phase (Synthesis) of Interphase.

    • Ensures that each newly formed cell has the exact same DNA as the original cell.

Steps of DNA Replication

  1. Unzipping the DNA:

    • Enzyme Helicase unzips the DNA into two strands at several locations called origins of replication.

    • Diagram:

    3' ext{ } 3'
    5' ext{ } 5'

  2. Adding Complementary Nucleotides:

    • Enzyme DNA Polymerase adds complementary nucleotides to the template strands.

    • A’s bond to T’s and C’s bond to G’s.

    • DNA Polymerase only adds nucleotides to the free 3’ end of the template strand, forming new strands in the 5’ to 3’ direction ONLY.

    • Diagram:

    3' ext{ } 5'
    5' ext{ } 3'

Role of Enzymes in DNA Replication

  • Primase:

    • Required for DNA synthesis; acts like a “key” for a car ignition.

    • Synthesizes short RNA primers that help initiate DNA polymerase activity.

  • DNA Polymerase:

    • Adds nucleotides to RNA primer (

    • FUNCTION #1: Creates polynucleotides).

    • After nucleotides are added, RNA primer is removed and replaced with DNA (FUNCTION #2).

    • Proofreading Function: Checks the strand for errors before completing the backbone (FUNCTION #3).

  • DNA Ligase:

    • Seals the gaps in DNA and connects DNA pieces by making phosphodiester bonds.

Leading and Lagging Strands

  • Leading Strand:

    • Synthesized toward the replication fork (5’ to 3’ direction).

    • Requires only ONE RNA primer produced by Primase.

    • Generated continuously.

  • Lagging Strand:

    • Synthesized away from the replication fork, leading to discontinuous replication.

    • Forms Okazaki Fragments, which are short pieces of DNA.

    • Requires many RNA primers created by Primase.

    • Joining of Okazaki Fragments: Completed by DNA ligase which “stitches” fragments together.

Semi-Conservative Replication

  • Definition: Each newly formed DNA molecule consists of one “old” strand (template) and one “new” strand.

  • Mechanism:

    • Each parent strand serves as a template that determines the sequence of new bases, leading to a complementary strand production.

    • Newly synthesized double helix comprises one “old” and one “new” DNA strand.

Summary of DNA Replication Steps

  1. Unzip the DNA using helicase.

  2. Enzymes assist in finding complementary bases and binding them according to base-pairing rules (A-T and C-G).

  3. Two identical DNA molecules are formed, with each containing an “old” strand and a “new” strand.

Considerations in DNA Replication Process

  • Possible Mistakes:

    • What kinds of mistakes could occur during this process?

    • What would the implications be if these mistakes occurred?

      • Differences between mutations in body cells (e.g., brain cells) vs. germ cells (e.g., sperm cells).

      • Differences between mutations in a single gene vs. an entire chromosome.