Forms of Nucleic Acids (College Board AP® Biology)

Structure & Function of DNA & RNA

• Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are both types of nucleic acid

• They are polynucleotides made up of many nucleotides linked together in a chain

• Both are found in all living cells and are both needed to build proteins, which are essential for the proper functioning of cells

• DNA and RNA are both important information-carrying molecules

• Although their main functions are slightly different, DNA and sometimes RNA are the primary source of heritable information

  • This means that offspring inherit characteristics as a result of the genetic code found in the nucleic acids which are passed from one generation to the next

An Overview of DNA Structure

• The individual units of DNA are called nucleotides

A DNA nucleotide base

• All DNA nucleotides contain the same phosphate and deoxyribose sugar, but differ from each other in the base attached

• There are four different bases, Adenine (A), Cytosine (C), Thymine (T) and Guanine (G)

• The bases on each strand pair up with each other with hydrogen bonds between complementary bases

• The bases always pair up in the same way:

  • Adenine always pairs with Thymine (A-T)

  • Cytosine always pairs with Guanine (C-G)

Complementary Base Pairing Diagram

DNA base pairs 

• DNA forms a double helix shape where

  • The phosphate and sugar section of the nucleotides form the ‘backbone’ of the DNA strand (like the sides of a ladder) through the formation of strong phosphodiester bonds

  • The base pairs of each strand project out and connect by complementary base pairing to form the rungs of the ladder

The DNA helix is made from two strands of DNA held together by hydrogen bonds

• It is this sequence of bases that holds the code for the formation of proteins

An Overview Of RNA Structure

• RNA nucleotides contain the nitrogenous bases adenine (A), guanine (G) and cytosine (C) but not thymine (T) – in place of this they contain the nitrogenous base uracil (U)

• The pentose sugar in RNA is ribose (instead of deoxyribose)

Comparing RNA to DNA Nucleotides Diagram

An RNA nucleotide compared with a DNA nucleotide

• RNA molecules are only made up of one polynucleotide strand (they are single-stranded)

• These chains are relatively short compared to DNA

• As with DNA, the phosphodiester bonds (between different nucleotides in the same strand) are strong covalent bonds with the nitrogenous bases sticking out sideways from the sugar/phosphate backbone

RNA Structure

The structure of RNA

Chromosomes in Eukaryotes & Prokaryotes

• Eukaryotes have cells with membrane-bound organelles

  • This includes the nucleus where DNA is stored

  • DNA in eukaryotic cells is typically linear and held in multiple chromosomes

Eukaryotic DNA Diagram

• Prokaryotes, on the other hand, do not have membrane-bound organelles

  • DNA is circular and free in the cytoplasm

Prokaryotic DNA Diagram

• Eukaryotic, and more commonly prokaryotic cells contain plasmids, small circular loops of DNA

Purines & Pyrimidines

• The nitrogenous base molecules that are found in the nucleotides of DNA (A, T, C, G) and RNA (A, U, C, G) occur in two structural forms: purines and pyrimidines

  • The bases adenine and guanine are purines – they have a double-ring structure

  • The bases cytosine, thymine and uracil are pyrimidines – they have a single-ring structure

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

• This gives stability to the structure of DNA, strengthened by the hydrogen bonds between pairs

Purine & Pyrimidine Diagram

The molecular structures of purines and pyrimidines are slightly different