ECA
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Study Notes

Unit 4: Nucleic Acids

Nucleotides

Nucleic acids (DNA and RNA) are polymers of nucleotides. Each nucleotide has three parts: a 5-carbon sugar, a phosphate group, and a nitrogenous base.

  • 5-carbon sugar (the pentagon in diagrams)
  • Phosphate group
  • Nitrogenous base (this is where the amino group –NH₂ appears in nucleic acids)
  • Nucleic acids contain C, H, O, N, and P
  • Nucleic acids carry the genetic code

Bonds in DNA

Two different types of bonds hold DNA together.

  • The nitrogenous bases of the two strands are held together by hydrogen bonds
  • The backbone (sugar–phosphate) of DNA and RNA is held together by covalent bonds

The Genetic Code: Codons

DNA sequences are often written in groups of three bases (for example 3' ATA TAT GCT ACT GCA AAG 5') because DNA is read in codons — three bases at a time. Each codon specifies an amino acid, which is how DNA directly codes for the primary structure of proteins.

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