DNA: Structure and Replication

DNA Structure Basics

  • Involved in inheritance; stores, retrieves, copies, and transfers genetic information.

  • Macromolecule (polymer) composed of nucleotide subunits.

  • A nucleotide consists of a phosphate group, a deoxyribose sugar, and a nitrogenous base.

  • Four nitrogenous bases: Adenine (A), Thymine (T), Guanine (G), Cytosine (C).

  • Base pairing rules: A pairs with T (forming 2 hydrogen bonds), G pairs with C (forming 3 hydrogen bonds).

  • DNA forms a double helix with two sugar-phosphate backbones; strands are held together by hydrogen bonds between base pairs.

  • Strands are antiparallel: one runs 5'-to-3', the other 3'-to-5'.

  • DNA is universal: A, T, C, G code information in all life forms.

  • RNA differs from DNA: it uses Uracil (U) instead of Thymine (T); A pairs with U in RNA.

DNA in Eukaryotes

  • Organisms (animals, plants, fungi, protists) have linear DNA strands located in the nucleus.

  • DNA is packaged by winding around histone proteins to form nucleosomes.

  • Nucleosomes coil to form chromatin, which further condenses into visible chromosomes during cell division.

  • Human cells contain 23 pairs of chromosomes.

  • Mitochondria and chloroplasts (organelles) possess their own unbound, circular DNA.

DNA in Prokaryotes

  • Single-celled organisms (e.g., bacteria) have a single, circular DNA molecule.

  • DNA floats freely in the cytosol within a region called the nucleoid, as there is no membrane-bound nucleus.

  • Referred to as "naked" DNA because it is not associated with histone proteins.

  • Most prokaryotic DNA lacks introns (non-coding DNA).

DNA Replication

  • Purpose: To precisely copy DNA for cell division (growth, repair) and genetic inheritance.

  • Semi-conservative process: Each new DNA molecule consists of one original (template) strand and one newly synthesized strand.

  • Key steps:

    1. The DNA double helix unwinds.

    2. The two original strands separate.

    3. New complementary strands are synthesized against each template strand using free nucleotides.

  • Enzymes involved:

    • Helicase: Unwinds the DNA helix, separating the two strands.

    • DNA Polymerase: Adds free nucleotides to the 3' end of the growing strand, catalyzing new strand synthesis in the 5'-to-3' direction.