Biology, Genetics

The Genetic Code



Deoxyribonucleic acid (DNA) = the molecule of heredity

  • Inherited traits are determined by genes
    • Genes are transmitted from parents to offspring in reproduction
  • Genes are composed of DNA
  • An individual gene contains information that a living entity uses to perform one or several functions

Structure of DNA = a double helix composed of two intertwined strands

  • Watson and Crick = proposed the 3D structure of DNA in the 1950s
    • Had the help of Rosalin Franklin
  • Primary features of the model:
    • Typically, double-stranded
      • Each strand is a separate molecule of DNA
      • Each strand is comprised of a linear sequence of paried subunits called nucleotides
        • Each nucleotide contains any one of the four bases:
          1. Adenine
          2. Thymine
          3. Guanine
          4. Cytosine
  • Nucleotides = derivatives of purine and pyrimidine DNA molecules
    • Aside: nucleotides vs. nucleosides
      • Nucleotides are molecule derivatives either purine or pyrimidine that bind to ribose/deoxyribose phosphate
      • Nucleosides are molecule derivatives of either purine or pyrimidine that bind to ribose/deoxyribose
  • Nucleic acids = chains of nucleotides
    • Connected via phosphodiester bonds (phosphate bridges at 3′ and 5′ positions)
    • Chargaff’s Rule = no rules dictating nucleotide compostition in DNA EXCEPT there are equal numbers of adenosine & thymine (A = T) and cytosine & guanine (C = G)
  • DNA strands are joined together by nucleotide base pairing
    • Adenine and thymine are connected via two hydrogen bonds (dotted lines): adenine and thymine
    • Cytosine and guanine are connected via three hydrogen bonds (dotted lines): guanine and cytosine
  • Importantly, each DNA strand has polarity (i.e., directionality) and are always oriented antiparallel to one another (i.e., in opposite directions):
    • 3′ end
    • 5′ end

Genes are ultimately recipes for proteins

  • The genetic information contained in the nucleotide sequence of DNA specifies a particular type of protein

Central Dogma = an ever-present staple of molecular genetics

  • DNA → RNA → protien
    • Importantly, DNA doesn’t directly encode proteins
    • RNA is an intermediate step between the DNA code and protein synthesis
      • RNA contains the base uracil in the place of thymine and the has a sugar ribose backbone instead of deoxyribose
      • Three types of RNA
        • messenger RNA (mRNA) = codes for a protein to be translated
        • ribosomal RNA (rRNA) = codes for an RNA structure that makes up ribosomes
        • transfer RNA (tRNA) = codes for an RNA structure that aids in the translation of mRNAs into amino acid sequences
  • Transcription = the process in which DNA is converted into RNA
    • Note: the definition of transcribe is to make a full copy or to record
  • Translation = the process in which RNA is converted into protein
    • The sequences of bases in mRNA codes for a protein sequence of amino acids
    • The mRNA is translated into codons which specify a particular amino acid sequence
      • Codon = a non-overlapping, adjacent group of three bases
      • Each codon codes for a single amino acid
      • This step in the translation process occurs in the ribosome (which is comprised of rRNA)


  • Exam 1 study material from my genetics class in summer 2014
  • Class notes from my biochemistry class in spring 2015

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