• The nucleic acids DNA and RNA are polymers of nucleotides
• Both DNA and RNA are?polymers?that are made up of?many repeating units?called?nucleotides
• Each nucleotide is formed from:
  • A?pentose sugar?(a sugar with 5 carbon atoms)
  • A nitrogen-containing?organic base?(with either 1 or 2 rings of atoms)
  • A?phosphate group?(this is acidic and negatively charged)
• The base and phosphate group are both?covalently bonded?to the sugar

The basic structure of a nucleotide

• Nucleotides join together in chains to form DNA or RNA strands
• The?phosphate group of one nucleotide?forms a covalent bond to the?pentose sugar of the next one
  • This carries on to form a?large polymer
• This forms a 'sugar-phosphate backbone' with a base linked to each sugar
• The polymer of nucleotides is known as a?strand
• DNA is double-stranded, RNA is usually single-stranded
• There are just?4 separate bases?that can be joined in?any combination/sequence
  • Because the?sugar and phosphate are the same?in every nucleotide
• This sequence is the basis of the genetic code as a?store of genetic information

• Like DNA, the nucleic acid RNA (ribonucleic acid) is a?polynucleotide?– it is made up of?many nucleotides?linked together in a chain
• Like DNA, RNA nucleotides contain the nitrogenous bases adenine (A), guanine (G) and cytosine (C)
• Unlike DNA, RNA nucleotides?never contain?the nitrogenous base?thymine?(T) – in place of this they contain the nitrogenous base?uracil?(U)
• Unlike DNA, RNA nucleotides contain the pentose sugar?ribose?(instead of deoxyribose)

An RNA nucleotide compared with a DNA nucleotide

• Unlike DNA, RNA molecules are only made up of?one polynucleotide strand?(they are?single-stranded)
• Unlike DNA,?RNA polynucleotide chains are relatively?short compared to DNA
• Like DNA,?the?sugar-phosphate bonds?(between different nucleotides in the same strand) are strong?covalent?bonds
• Like DNA,?the nitrogenous bases?stick out sideways?from the sugar-phosphate backbone

The structure of RNA

Nucleotide Structure Summary Table

• DNA is a double helix made of two antiparallel strands of nucleotides linked by hydrogen bonding between complementary base pairs
• The nucleic acid DNA is a?polynucleotide?– it is made up of?many nucleotides?bonded together in a?long chain

A DNA nucleotide

• DNA molecules are made up of?two polynucleotide strands?lying side by side, running in opposite directions – the strands are said to be?antiparallel
• Each DNA polynucleotide strand is made up of?alternating deoxyribose sugars and phosphate groups bonded together?to form the?sugar-phosphate backbone
• Each DNA polynucleotide strand is said to have a?3’ end and a 5’ end?(these numbers relate to which carbon atom on the pentose sugar could be bonded with another nucleotide)
• Because the strands run in opposite directions (they are?antiparallel), one is known as the?5’ to 3’ strand?and the other is known as the?3’ to 5’ strand
• The nitrogenous bases of each nucleotide project out from the backbone?towards the interior?of the double-stranded DNA molecule

A single DNA polynucleotide strand showing 3 nucleotides in a sequence

Hydrogen bonding

• The two antiparallel DNA polynucleotide strands that make up the DNA molecule are?held together by hydrogen bonds?between the nitrogenous bases
• These hydrogen bonds always occur between the?same pairs of bases:
  • The purine?adenine?(A) always pairs with the pyrimidine?thymine?(T) – two hydrogen bonds are formed between these bases
  • The purine?guanine?(G) always pairs with the pyrimidine?cytosine?(C) – three hydrogen bonds are formed between these bases
  • This is known as?complementary base pairing
  • These pairs are known as?DNA base pairs

A section of DNA – two antiparallel DNA polynucleotide strands held together by hydrogen bonds

Double helix

• DNA is not two-dimensional as shown in the diagram above
• DNA is described as a?double helix
• This refers to the?three-dimensional shape?that DNA molecules form

DNA molecules form a three-dimensional structure known as a DNA double helix

• Francis Crick and James Watson were two Cambridge scientists who worked together to establish the?double helix?structure of DNA in 1953
• They used data from?their previous experiments?on the composition of DNA
• Published findings from?other research labs?played a role in developing their model
  • Rosalind Franklin, Edwin Chargaff and Linus Pauling, all of whom were leading research efforts in?other universities, contributed important data to Crick & Watson's discovery
  • This suggests that there was a close-knit collaboration, but in fact, there was?a lot of competition between the groups?to make the breakthrough discovery
• Physical?model-making?played a large role in their success
• Early versions of the model were rejected?for various reasons
  • It was?not compact enough
  • It would have been too?unstable?(DNA is a highly stable molecule)
  • It did not allow?equivalent amounts?of A and T, and C and G bases to be present (Chargaff's findings)
  • It fitted together much better once the?second strand was flipped?to become antiparallel
• Their final model was constructed carefully with?clamps,?metal rods?for bonds and with the?correct bond angles
• Their model was?the basis of a lot of genetic research?globally in the years that followed
  • Notably, Crick and Watson's model sparked work to prove?the way in which DNA replicates?in cells

NOS: Using models as a representation of the real world; Crick and Watson used model making to discover the structure of DNA

• Models in science are built?to represent concepts?and ideas in a way that can be pictured by our brains
• Models can be?accepted or rejected?based on experimental data generated by further research
• Crick and Watson?built physical, scale?models of DNA to explain biological observations
  • Using?simple laboratory equipment?(clamps, stands, metal rods etc)
  • They?adapted their models?by making them more realistic eg. by building in the correct bond angles within molecules
• They built successive models, using?trial-and-error?to arrive at the finalised model
• Their?first model of DNA was rejected, based on the findings of Rosalind Franklin
  • Crick and Watson?received the Nobel prize?for their work
  • Franklin died aged 37 so never received the recognition she deserved for?her significant role?in defining DNA structure
• Today,?sophisticated computer modelling?is performed, that
  • Takes the place of physical model-making?and provides further explanation of the functions of various biomolecules
  • Shortens the 'trial-and-error' cycles?of model-making as experienced by Crick and Watson