DNA Structure and Molecular Genetics

UNIT 4: MOLECULAR GENETICS

PART 2: DNA STRUCTURE

  • Definition of DNA

    • DNA = deoxyribonucleic acid

    • Type of macromolecule = nucleic acid

    • Polymer made of nucleotide monomers

    • Structure: double helix, resembling a twisted ladder

    • Sides of the ladder known as the sugar-phosphate backbone

    • Rungs of the ladder made up of nitrogenous bases

NUCLEOTIDE

  • A nucleotide consists of three components:

    • Phosphate group

    • Deoxyribose sugar (5 Carbon sugar)

    • Nitrogenous base

Structure of Nucleotide

  • Represents as:

    • HO-P-O-CH₂

    • 5' - 3' notation for the sugar

NITROGENOUS BASES

  • Types of nitrogenous bases categorized into Purines and Pyrimidines:

    • Purines (double ring structure):

    • (A) Adenine

    • (G) Guanine

    • Pyrimidines (single ring structure):

    • (T) Thymine

    • (C) Cytosine

Structures of Bases

  • Chemical Structures of the Bases:

    • Purines:

    • Adenine: NH₂-

    • Guanine: NH₂-

    • Pyrimidines:

    • Thymine: H₂N-

    • Cytosine: H₂N-

    • Uracil: H₂N-

ERWIN CHARGAFF'S CONTRIBUTION

  • Erwin Chargaff, an American biochemist, studied the percentage composition of nitrogenous bases (adenine, guanine, thymine, and cytosine) across different organisms.

Summary of Chargaff’s Data

  • Composition of nitrogenous bases (%):

    • Human:

    • Adenine: 31.0%

    • Thymine: 31.5%

    • Guanine: 19.1%

    • Cytosine: 18.4%

    • Fruit Fly:

    • Adenine: 27.3%

    • Thymine: 27.6%

    • Guanine: 22.5%

    • Cytosine: 22.5%

    • Corn:

    • Adenine: 25.6%

    • Thymine: 25.3%

    • Guanine: 24.5%

    • Cytosine: 24.6%

    • Yeast:

    • Adenine: 23.0%

    • Thymine: 23.3%

    • Guanine: 27.1%

    • Cytosine: 26.6%

    • Bacteria:

    • Adenine: 24.6%

    • Thymine: 24.3%

    • Guanine: 25.5%

    • Cytosine: 25.6%

Implications of Chargaff’s Findings

  • Observations indicate:

    • Guanine approximately equals Cytosine (G ≈ C)

    • Adenine approximately equals Thymine (A ≈ T)

  • Suggests base pairing rules:

    • G always pairs with C

    • A always pairs with T

    • In DNA, a purine always pairs with a pyrimidine

    • These pairs are known as complementary base pairs

COMPLEMENTARY BASE PAIRING

  • Pairing specifics:

    • Purines pair with Pyrimidines

  • Resulting structure characteristics:

    • Forms a double helical structure with a constant diameter

Illustration of Base Pairing

  • Example Pairing:

    • Adenine (A) with Thymine (T) and Guanine (G) with Cytosine (C)

    • Example shown with hydrogen bonding interactions between bases:

SIGNIFICANCE OF COMPLEMENTARY BASE PAIRS

  • The sequence of nucleotides on one strand of DNA determines the sequence of nucleotides on the complementary strand, resulting in specificity of genetic information

Homework Exercise

  • For the DNA strand given, write the complementary strand based on base pairing rules:

    • Strand 1: A T G C A C T A G C A A

    • Resulting Strand 2: T A C G T G A T C G T T

PHOSPHODIESTER BONDS

  • Nucleotide monomers within DNA are connected via strong electromagnetic attractions known as phosphodiester bonds.

  • Structure of deoxyribose sugar:

    • Carbons are numbered systematically from 1' to 5'

  • During nucleotide linkage:

    • The 5' carbon of one nucleotide joins with the 3' carbon of the successive nucleotide.

    • This establishes a 5'-3' configuration of the DNA strand.

DNA BACKBONE AND ANTIPARALLEL STRANDS

  • The backbone of DNA consists of strongly bonded phosphates and sugars between nucleotides.

  • In a double helix, the 5' end of one DNA strand's backbone is parallel with the 3' end of the other strand’s backbone.

  • This arrangement is known as antiparallel.

Antiparallel Analogy

  • Suggestion to consider an everyday analogy about the term antiparallel and relate it to the structure of DNA.

HYDROGEN BONDS BETWEEN DNA STRANDS

  • Two DNA strands are held together by weak hydrogen bonds between the base pairs.

    • Number of hydrogen bonds:

    • 2 hydrogen bonds hold A & T together

    • 3 hydrogen bonds hold G & C together

  • Despite individual weakness, the cumulative effect of many hydrogen bonds ensures strong DNA strand coherence.

FUNCTION OF DNA

  • DNA encodes all instructions necessary for protein synthesis.

  • Proteins are composed of amino acid polymers, being versatile macromolecules involved in diverse biological functions.

  • Interactions between proteins and their environments ultimately determine all the traits of an organism.