Biology - Chapter 2: Nucleic Acids

Compare and contrast the structure of DNA and RNA. (6)

- Similarities

• Phosphodiester bonds between nucleotides

• Polymers of nucleotides

• Cytosine, Guanine, Adenine

• Phosphate, pentose, nitrogenous base

- Differences

• DNA→Deoxyribose / RNA→ Ribose

• DNA→Thymine / RNA→Uracil

• DNA→Double stranded helix / RNA→Single Stranded

• DNA→Longer / RNA→Shorter

Describe the structure of the double helix of DNA. (4)

• Two/2 polynucleotide strands made of nucleotides joined by phosphodiester bonds

• Two strands held together by hydrogen bonds between complementary base pairs A=T and C=G

• Each nucleotide made up of a phosphate group, a deoxyribose and a nitrogenous base (A, T, C or G)

• Two polynucleotide strands running antiparallel OR sugarphosphate backbone with nitrogenous bases on the inside

Describe and explain how the structure of DNA is related to its function. (6)

• Base sequence codes for amino acids in polypeptide

• Double stranded so DNA replication is semi-conservative

• Coiled helix so compact / stores a lot of information is small volume

• Sugar-phosphate backbone to give stability / protects bases

• Long/large molecule so lots of information stored

• (Relatively) weak hydrogen bonds so strands can be separated for semi-conservative replication

• Complementary base pairing so accurate replication

Describe how DNA is replicated. (6)

• Hydrogen bonds broken by DNA helicase

• Both strands act as a template

• Free DNA nucleotides attach

• Based on complementary base pairs: Adenine to Thymine and Cytosine to Guanine

• DNA polymerase joins adjacent nucleotides by phosphodiester bond in condensation reaction

• Semi-conservative replication: each molecule of DNA is made of one new and one original/old strand

Describe how ATP is synthesised. (3)

• Joins one adenine, one ribose and three phosphate /Pi

• In condensation reactions

• By ATP synthase ADP + Pi → ATP

Describe how ATP is broken down. (2)

• By ATP hydrolase

• ATP --> ADP + Pi OR to form adenosine diphosphate (ADP) and (inorganic) phosphate (Pi)

Explain the advantages of using ATP as a energy-source molecule in cells. (4)

• Hydrolysed in one step / quickly so it is an immediate/instantaneous source of energy

• Releases small and manageable amounts of energy so little energy lost as heat

• Cannot diffuse / be lost from cell

• Reformed/re-synthesised from one reaction / rapidly

Describe and explain the features that make water important for living organisms. (6)

• Metabolite in photosynthesis / hydrolysis / respiration

• Solvent to transport substances

• High specific heat capacity so buffers (sudden) changes in temperature

• Cohesion so supports large columns of water/transpiration stream

• Large latent heat of vapourisation so provides cooling effect when evaporates

• Cohesion so large surface tension to support small organisms