Nucleic acids and ATP

• Name the pentose sugars in DNA and RNA

  ~DNA: deoxyribose

  ~RNA: Ribose
• State the role of DNA in living cells

  ~Base sequence of genes codes for functional RNA and amino acid sequence of polypeptides

  ~Genetic information determines inherited characteristics = influences structure and function of organisms
• State the role of RNA in living cells

  mRNA: complementary sequence to one gene from DNA with introns spliced out. Codons can be translated into a polypeptide by ribosomes

  rRNA: component of ribosomes

  tRNA: supplies complementary amino acids to mRNA codons during translation
• How do polynucleotides form?

  Condensation reactions between nucleotides form strong phosphodiester bonds
• Describe the structure of DNA

  ~Double helix of two polynucleotide strands

  ~H-bonds between complementary purine and pyrimidine base pairs on opposite strands
• Relate the structure of DNA to its functions

  ~Sugar phosphate backbone and many H-bonds provide stability

  ~Long molecules stores lots of information

  ~Helix is compact for storage in nucleus

  ~Base sequence of triplets codes for amino acids

  ~Double stranded for semi conservative replication

  ~Complementary base pairing for accurate replication

  ~Weak H-bonds break so strands separate for replication
• Describe the structure of messenger RNA (mRNA)

  ~Long ribose polynucleotide but shorter than DNA

  ~Contains uracil instead of thymine

  ~Single stranded and linear

  ~Codon sequence is complementary to exons of one gene from one DNA strand
• Relate the structure of messenger RNA (mRNA) to its functions


  ~Breaks down quickly so no excess polypeptide forms

  ~Ribosome can move along strand and tRNA can binds to exposed bases

  ~Can be translated into a specific polypeptide by ribosomes
• Describe the structure of transfer RNA (tRNA)


  ~Single strand of about 80 nucleotides

  ~Folded into clover shape

  ~Anticodon on one end, amino acid binding site on the other
• Why did scientists initially doubt that DNA carried the genetic code

  Chemically simple molecule with few components
• Why is DNA replication described as 'semiconservative'

  ~Strands from original DNA molecule act as a template

  ~New DNA molecule contains one old strands and one new strand
• Outline the process of semiconservative DNA replication

  ~DNA helicase breaks H-bonds between base pairs

  ~Each strand acts as a template

  ~Free nucleotides from nuclear sap attach to exposed bases by complementary base pairing

  ~DNA polymerase catalyses condensation reactions that join adjacent nucleotides on new strand

  ~H-bonds reform
• Describe the Meselson-Stahl experiment

  ~Bacteria were grown in a medium containing heavy isotope 15N for many generations

  ~Some bacteria were moved to a medium containing light isotope 14N. Samples were extracted after one and two cycles of DNA replication

  ~Centrifugation formed a pellet. Heavier DNA settled closer to bottom of tube
• Describe the structure of adenosine triphosphate (ATP)

  Nucleotide derivative of adenine with three phosphate groups
• Explain the role of ATP in cells

  ~ATP hydrolase catalyses ATP into ADP and Pi

  ~Energy releases is coupled to metabolic reactions

  ~Phosphate group phosphorylates compounds to make them more reactive
• How is ATP resynthesised in cells

  ~ATP synthase catalyses condensation reaction between ADP and pi

  ~During photosynthesis and respiration
• Explain why ATP is suitable as the energy currency of cells

  ~High energy bonds between phosphate groups

  ~Small amounts of energy released at a time = less energy wasted as heat

  ~Single step hydrolysis = energy available quickly

  ~Readily resynthesised