5: DNA & Chromosomes

• DNA


  Deoxyribonucleic acid, the molecule that carries the genetic information in living organisms
• DNA molecule


  • Consists of two long polynucleotide chains
  • Each chain is composed of four types of nucleotide subunits
  • The two strands are held together by hydrogen bonds between the base portions of the nucleotides
• Nucleotide


  Composed of a nitrogen-containing base, a five-carbon sugar (deoxyribose), and a phosphate group
• Bases in DNA


  • Adenine (A)
  • Cytosine (C)
  • Guanine (G)
  • Thymine (T)
• Polynucleotide chain


  Nucleotides covalently linked together through the sugars and phosphates, forming a backbone of alternating sugar-phosphate-sugar-phosphate
• Phosphodiester bond


  Bonds that link the 5' end of one sugar to the 3' end of the next, giving the DNA strand a chemical polarity
• Polarity of DNA strand

  One end is the 3' end (hydroxyl group), the other end is the 5' end (phosphate group)
• Base pairing in DNA

  Adenine (A) always pairs with Thymine (T), Guanine (G) always pairs with Cytosine (C)
• Base pair


  A purine (two-ring base) paired with a pyrimidine (single-ring base)
• DNA double helix


  • Two polynucleotide chains wound around each other
  • The two strands run antiparallel to each other
  • Contains 10 base pairs per helical turn
• The complementarity of the base pairs allows each strand to serve as a template for the other strand during DNA replication
• Heredity


  The mechanism by which genetic information is accurately copied and transmitted from one generation to the next
• The structure of DNA provides a mechanism for heredity by enabling the accurate replication and transmission of genetic information
• Complementarity


  A always matches T, and C always matches G on the opposite DNA strand
• Complementarity is of crucial importance when it comes to both copying and maintaining the DNA structure
• Genes encode information that must be copied and transmitted accurately when a cell divides
• DNA strand polarity


  The two ends of a DNA strand are chemically different
• 1. C base pairs are more stable than A-T base pairs
• DNA double helix


  • The two strands are held together by hydrogen bonds between complementary base pairs
  • The strands are antiparallel (oriented in opposite directions)
• The nucleotides are linked together by phosphodiester bonds that connect the 3' hydroxyl group of one sugar and the 5' phosphate of the next
• This linkage gives each polynucleotide strand a chemical polarity, with a 3' end and a 5' end
• DNA encodes information in the order, or sequence, of the nucleotides along each DNA strand

  
• Organisms differ from one another because their respective DNA molecules have different nucleotide sequences and carry different biological messages
• DNA messages must somehow be able to encode proteins
• The linear sequence of nucleotides in a gene must spell out the linear sequence of amino acids in a protein
• The correspondence between the 4-letter nucleotide alphabet of DNA and the 20-letter amino acid alphabet of proteins is the genetic code
• The amount of information in an organism's DNA is staggering
• Eukaryotic cells package their very long, double-stranded DNA molecules into chromosomes
• Chromosomes fit inside the cell nucleus and can be accurately apportioned between daughter cells during cell division
• Specialized proteins bind to and fold the DNA, generating a series of coils and loops to package the DNA
• Bacterial DNA is also associated with proteins that condense it, but the structure is different from eukaryotic chromosomes
• In eukaryotes, nuclear DNA is distributed among a set of different chromosomes
• Each human cell contains two copies of every chromosome, one from each parent
• The maternal and paternal versions of each chromosome are called homologous chromosomes
• The human genome contains approximately 3.2 x 10^9 nucleotide pairs of DNA

  
• Chromosomes can be distinguished from one another using chromosome-specific DNA probes coupled to fluorescent dyes (chromosome painting)
• Chromosomes can also be distinguished by staining patterns that reflect differences in A-T and G-C content
• An ordered display of the full set of human chromosomes is called a karyotype
• Cytogeneticists analyze karyotypes to detect chromosomal abnormalities associated with inherited disorders and cancer
• The most important function of chromosomes is to carry genes, the functional units of heredity