Lecture Notes: Structure and Condensation of DNA

Importance of DNA

• DNA is a unifying feature of living organisms.
• It contains the genetic code passed from parent to offspring.
• Unique DNA sequences make individuals distinct (except identical twins).
• DNA codes for proteins, which perform bodily functions and determine physical appearance.

Structure of DNA

• DNA is a macromolecule made up of nucleotides.
• Components of Nucleotide:
  • Phosphate Group: Consists of phosphorus bonded to four oxygens.
  • Sugar: Deoxyribose in DNA (different sugar in RNA).
  • Nitrogenous Base: Forms the genetic code.
• DNA forms a double helix (twisted ladder structure).
  • Sides of the Ladder: Made up of phosphate and sugar.
  • Rungs of the Ladder: Composed of pairs of nitrogenous bases.

Nitrogenous Bases

• Four types of nitrogenous bases in DNA:
  • Adenine (A)
  • Thymine (T)
  • Guanine (G)
  • Cytosine (C)
• Complementary Base Pairing:
  • A pairs with T
  • G pairs with C
• Hydrogen bonds connect the complementary bases (weak bonds that allow separation during replication and protein synthesis).
• The sequence of bases (e.g., ATGCCG) holds the genetic information.

Condensation of DNA

• DNA does not remain free-floating; it becomes condensed within the nucleus.
• DNA wraps around proteins called histones to form nucleosomes (bead-like structures).
• Chromatin:
  • Nucleosomes packed together form chromatin.
  • Chromatin is the usual form of DNA when the cell is not dividing.
• Chromosomes:
  • The most compact form of DNA, formed when chromatin is further condensed.
  • DNA is in chromosome form during cell division.

Future Topics

• Next discussion will address the number of chromosomes in organisms, as DNA is packed into multiple chromosomes based on its quantity.

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This summary captures the key points from the discussion on DNA structure and its condensation process in preparation for cell division.