Genetic Information: DNA & RNA (College Board AP® Biology): Study Guide

Passing on heritable information

• The nucleic acid DNA is the primary source of heritable information

  • Genetic information can also be transmitted using another nucleic acid, RNA, e.g., in some viruses

• Genetic material is packaged differently in different types of organisms

DNA in eukaryotic cells

• Eukaryotic cells have multiple linear chromosomes comprised of DNA

• Chromosomes are condensed by winding them round histones and associated proteins

DNA in prokaryotic cells

• Prokaryotic cells carry most of their DNA within a single, circular chromosome

• They also contain plasmids; small, circular molecules of DNA that are separate to the chromosome

Nucleic acids as hereditary material

• The structure of DNA and RNA allows these molecules to store genetic information

• DNA and RNA are polymers of nucleotides; nucleotides contain:

  • a pentose sugar

    • DNA = deoxyribose

    • RNA = ribose

  • a phosphate group

  • a nitrogenous base

• The nitrogenous bases within nucleotides are:

• Purines:

  • adenine (A)

  • guanine (G)

• Pyrimidines:

  • cytosine (C)

  • thymine (T) in DNA

  • uracil (U) in RNA

• DNA and RNA nucleotides join to form polynucleotides with a sugar-phosphate backbone

• Single-stranded polynucleotides can join to form double-stranded molecules

  • DNA forms a double-stranded helix, while RNA is single-stranded, but can fold to form double-stranded regions

• Within double-stranded regions of DNA and RNA, the bases join together via hydrogen bonds in specific nucleotide base pairing:

  • Adenine pairs with thymine or uracil (A-T or A-U)

  • Cytosine pairs with guanine (C-G)

• This specific base pairing occurs because of the differences in structure between the nitrogenous bases:

  • G and A have a double-ring structure and are known as purine bases

  • C, T, and U have a single-ring structure and are pyrimidine bases

• A purine always pairs with a pyrimidine base to create base pairs of equal length; a single ring base matched to a double ring base

• Specific nucleotide base pairing has been conserved through evolution, meaning that it has been the same across evolutionary time, and is the same between different groups of organisms

DNA as the primary hereditary material

• DNA is well suited for storing genetic information because:

  • the deoxyribose sugar within DNA provides more stability than ribose in RNA, meaning that DNA is a stable molecule

  • the complementary base pairing that occurs in double-stranded DNA molecules means that one strand can always be determined by looking at the other strand; this means that DNA can be easily replicated from a single-strand

  • many combinations can be created from the four nitrogenous bases, meaning that a large volume of information can be stored