DNA Structure and Function

Overview of DNA
  • Definition: DNA stands for deoxyribonucleic acid.
  • Function: Acts as a blueprint for an organism, encoding the genetic information necessary for forming and functioning specific proteins.
  • Gene: A segment of DNA that encodes for a specific protein.
Structure of DNA
  • Components:
    • Nitrogenous Bases: Adenine (A), Thymine (T), Guanine (G), Cytosine (C).
    • Sugar: Deoxyribose (missing an OH group at the 2' carbon).
    • Phosphate group: Connects the sugar molecules in the DNA backbone.
  • Orientation:
    • 3' and 5' Ends: Nomenclature indicating the directionality of the sugar molecules. The 3' end has a hydroxyl (-OH) group while the 5' end has a phosphate group. This directionality is crucial for DNA replication.
  • Base Pairing:
    • Thymine (T) pairs with Adenine (A) via 2 hydrogen bonds.
    • Guanine (G) pairs with Cytosine (C) via 3 hydrogen bonds.
  • Strands Orientation: DNA strands are antiparallel; one runs from 3' to 5' and the other from 5' to 3'.
Location of DNA
  • Eukaryotic Cells: DNA is found within the nucleus.
  • Prokaryotic Cells: DNA is located in a region called the nucleoid, not a true nucleus.
DNA Replication
  • Purpose: To create exact copies of DNA, essential for cell division.
  • Process: Semi-conservative replication where each original strand serves as a template for a new strand.
    • Helicase: Unzips the double helix by separating the two strands of DNA.
    • DNA Polymerase: Enzyme that synthesizes the new DNA strand by adding complementary nucleotides.
    • Leading Strand: Synthesized continuously in the 3' to 5' direction toward the replication fork.
    • Lagging Strand: Synthesized in short fragments (Okazaki fragments) away from the replication fork due to opposite orientation (5' to 3').
Transcription and Translation
  • Transcription: Process of synthesizing RNA from a DNA template.
    • Key Differences Between DNA and RNA:
    • Thymine (T) in DNA is replaced by Uracil (U) in RNA.
    • Sugar in RNA contains an OH group at the 2' position (ribonucleic acid).
  • Translation: Process of synthesizing proteins based on the RNA sequence.
    • Ribosomes: Organelles that facilitate translation by reading the mRNA and constructing polypeptide chains from amino acids brought in by tRNA.
    • Each codon (set of three nucleotides) on mRNA corresponds to one amino acid.
Central Dogma of Molecular Biology
  • Concept: The flow of genetic information from DNA to RNA to protein.
    • DNA Replication: Creation of new DNA strands.
    • RNA Transcription: Conversion of DNA sequences to RNA.
    • Protein Translation: The process by which RNA sequences are used to build proteins.