DNA and RNA

• both types of nucleic acid
• found in all living cells and both carry information
• DNA (deoxyribonucleic acid) is used to store genetic information
• RNA (ribonucleic acid) is similar in structure to DNA

  • main function is to transfer genetic information from the DNA to the ribosomes
• ribosomes are the body's protein factories - read RNA to make polypeptides (proteins) in the process of translation
• ribosomes made from RNA and proteins
• Molecules of DNA and RNA are polymers of nucleotides
• a nucleotide is a type of biological molecule which is made from three different components:

  • pentose sugar
  • nitrogen-containing organic base
  • phosphate group
• many nucleotides join together to form polynucleotide strands or chains. the nucleotides join up via condensation reactions between the phosphate of one nucleotide and the sugar of another

  • forms a phosphodiester bond (consisting of the phosphate group and 2 ester bonds)
  • chain of sugars and phosphates is known as the sugar-phosphate backbone
• DNA has a double helix structure:

  • this means that a DNA molecule is formed from 2 separate strands which wind around each other to form a spiral
  • the strands are polynucleotides
  • made up of lots of nucleotides joined together in a long chain
  • DNA molecules are very long and coiled very tightly - lot of genetic information can fit into a small space in the cell nucleus
• DNA nucleotide structure:

  • made from a phosphate group, the pentose sugar deoxyribose and a nitrogen-containing base
  • each DNA nucleotide has the same sugar and phosphate
  • base on each nucleotide can vary - adenine, cytosine, guanine, thymine
• Complementary base pairing:

  • two DNA polynucleotide strands join together by hydrogen bonds between the bases
  • each base can only pair with one other
  • A-T
  • C-G
  • equal amount of A/T and C/G
  • 2 hydrogen bonds between A and T
  • 3 hydrogen bonds between C and G
• adenine and guanine are purines - 2 ring structures with a nitrogen atom in the middle/ 2 fused carbon-nitrogen rings
• cytosine and thymine are pyrimidines - single carbon-nitrogen ring
• two polynucleotide strands run are antiparallel - run in opposite directions. two antiparallel strands twist to form a DNA double helix
• RNA structure:

  • made of nucleotides containing a sugar, a phosphate group and one of 4 different bases
  • nucleotides also form a polynucleotide strand with a sugar-phosphate backbone
  • sugar in RNA nucleotides is a ribose sugar - still pentose
  • uracil (pyrimidine) replaces thymine as a base - uracil always pairs with adenine
  • nucleotides form a single polynucleotide strand - not double
  • RNA strands are much shorter than most DNA polynucleotides
• DNA:

  • double stranded - twisted into a double helix and held together by hydrogen bonds
  • deoxyribose sugar
  • A/T/C/G
  • Long
• RNA:

  • single-stranded
  • ribose sugar
  • A/U/G/C
  • Relatively short