AQA biology AS topic


Nucleic acids

Structure of RNA and DNA

  • Deoxyribonucleic acid and ribonucleic acid
  • double helix structure of DNA makes it immediately recognisable 
  • DNA carries genetic information

Nucleotide Structure

  • Made up of three components:
  • Pentose sugar (has five carbons)
  • Phosphate group
  • Nitrogen-containing organic bases (cytosine, thymine, uracil, adenine and guanine)
  • They are all joined by a condensation reaction to form a single nucleotide (mononucleotide). 
  • Two mononucleotides may be joined by condensation reaction between the deoxyribose sugar of one mononucleotide and the phosphate group of another forming a phosphodiester bond 
  • Creates a dinucleotide. 
  • This continues to form long chains (polynucleotides)

RNA structure

  • Ribonucleic acid (RNA) is a polymer made up of nucleotides.
  • Single, short, polynucleotide chain which the pentose sugar is ribose and organic bases are A,T,C,U 
  • One type of RNA transfers genetic information from DNA to the ribosomes. 
  • Ribosomes are made up of proteins and another type of RNA. 
  • A third type of RNA is involved in protein synthesis

DNA structure

  • 1953 = Watson and Crick worked on structure of DNA, following pioneering work by Rosalind Franklin on X-ray diffraction patterns of DNA
  • Opened door for many major developments in biology over the next half-century
  • In DNA pentose sugar is deoxyribose and the organic bases are A,T,C,G and is made up of two strands of nucleotides (polynucleotides). 
  • Each strand joined by hydrogen bonds between bases.
  • The phosphate and deoxyribose wind around each other to form a double helix and form backbone of the DNA molecule
  • DNA thought of to be a ladder, where phosphate and deoxyribose molecules alternate to form uprights and organic bases pair together to form rungs 

Base Pairing

  • Bases on the two strands of DNA attach to each other by hydrogen bonds.
  • Adenine joins with Thymine and Guanine joins with Cytosine, as a result of complementary pairing
  • The concentration of A and T, G and C are always the same in DNA. However, the ratio of A and T to G and C varies from species to species

Stability of DNA

  • Phosphodiester backbone protects the chemically reactive organic bases inside double helix
  • Hydrogen bonds link organic base pairs forming bridges (rungs) between the phosphodiester uprights. 3 hydrogen bonds between C and G, higher proportion of C-G, more stable DNA molecule 
  • there are other forces between base pairs that hold the molecules together (base stacking)

Function of DNA

  • DNA = hereditary material responsible for passing genetic information from cell to cell and generation to generation 
  • There are 3.2 billion base pairs in DNA of mammal cells. There is infinite variety of sequences and will provide genetic diversity. 
  • Function depends on sequence of base pairs
  • Very stable structure from generation to generation with no changes as rarely mutates
  • 2 separate strands joined by hydrogen bonds allows them to separate during replication and protein synthesis
  • large so can carry lots of genetic information. 
  • By having base pairs with helical cylinder of deoxyribose-phosphate backbone the genetic information is protected from being corrupted by outside chemicals and physical forces
  • Base pairs allow to replicate


No comments have yet been made