exam questions

• Describe how nucleotides join to form DNA (2)

  -       Phosphodiester bonds form between phosphate group of one nucleotide and pentose sugar of another

  -       Via a condensation reaction 
• describe the differences between the structure of DNA and structure of RNA (3)

  • DNA is double stranded whereas RNA is single stranded 
  • DNA is a helix structure and RNA is not 
  • DNA has a thymine base whereas RNA has a uracil base 
  • DNA contains deoxyribose whereas RNA contains ribose
• Explain 2 features if DNA that help stabilise its structure (2)

  -       Hydrogen bonds between the base pairs, many hydrogen bonds provide strength 

  -       Phosphodiester bonds between the DNA nucleotides
• Compare and contrast the structures and functions of DNA and RNA (5)

  -       DNA is double stranded whereas RNA is single stranded 

  -       DNA contains deoxyribose whereas RNA contains ribose sugar 

  -       Both contain adenine, guanine, and cytosine 

  -       RNA contains uracil whereas DNA contains thymine

  -       Both have a sugar phosphate backbone 

  -       DNA is read by polymerases whereas RNA is read by ribosomes 

  -       DNA is structures into chromosomes whereas RNA is structured into tRNA, mRNA or rRNA

   

   
• Describe the structure of DNA (5) 

  -       A polymer of nucleotides

  -       Each nucleotide made up of deoxyribose pentose sugar, a phosphate group and a nitrogenous base

  -       2 anti parallel strands made from sugar phosphate backbone 

  -       Form a double helix structure 

  -       Deoxyribose sugar and pentose group of adjacent DNA nucleotides joined by phosphodiester bonds

  -       Bases A and T joined by 2 hydrogen bonds whereas C and G are joined by 3 hydrogen bonds
• Describe 2 advantages to DNA having a double helix (2)

  -       Gives stability to the molecule 

  -       Protects the bases from digestive enzymes or mutagens 

  -       Allows for semi conservative replication
• Describe the difference in the structure of purines and pyrimidines (2)

  -       Purines are A and G – have a double ring structure

  -       Pyrimidines are T and C- have a single ring structure
• Explain how the structure of DNA relates to its function (6)

  -       Polymer of nucleotides= means it is a large molecule which helps restrict DNA to the nucleus which protects it/preserves genetic code 

  -       Phosphodiester bonds between the nucleotides- strong covalent bonds of the backbone help preserve the base sequence

  -       Double helix is 2 strands held together by hydrogen bonds= hydrogen bonds help increase stability of DNA however weakness of hydrogen bonds means they can be easily broken during replication and transcription.

   
• Explain the meaning of each of these terms (2) 

  Triplet code= every 3 bases(codon) attract one specific amino acid.

  Non overlapping=no bases are shared between the codons, all have 3 in each 

  Degenerate= different triplet bases(codons) can code for the same amino acid