Nucleic acids 8a

• What does DNA stand for?

  Deoxyribonucleic acid
• What does RNA stand for?


  Ribonucleic acid
• What is DNA and RNA?

  They are information carrying molecule, examples of biological molecules called nucleic acid. DNA stores genetic information and RNA transfers genetic information from DNA to ribosomes. Ribosomes are formed from RNA as well as protein
• What is the monomer unit for DNA and RNA?


  Nucleotides
• Both DNA and RNA are polymers of monomer units called nucleotides
• What are the 3 components of nucleotides?

  • A phosphate group
  • A pentose sugar (5 carbon atoms)
  • organic base containing nitrogen
• What do nucleotides join together to form?

  Polynucleotide strands.
• The chain is held together because the phosphate group of each nucleotide is linked to the sugar of the next by strong covalent bonds called?

  Phosphodiester bonds
• What does the ‘sugar-phosphate backbone‘ mean?

  As the phosphate group and sugar are identical throughout the chain, this is the term used
• What type of reaction is used to join nucleotides together?

  Condensation reaction
• What is the only way in which one polynucleotide chain can differ from another?

  Sequence of bases in the polynucleotide
• Describe the 3 components of a DNA nucleotide

  • Deoxyribose is the pentose sugar
  • Nitrogen containing base of either Adenine, Guanine, Cytosine or Thymine
  • a phosphate group
• Which bases are the purines?

  Adenine and guanine
• Which bases are the pyrimidines?

  Cytosine and Thymine
• What is the difference between purines and pyrimidines?

  Purines have 2 rings whereas Pyrimidines have only 1
• Which are the base pairing?

  Adenine ( purine) and Thymine (pyrimidine)

  Guanine ( purine) and Cytosine ( pyrimidine)
• Why do purines and pyrimidines always pair together?

  to make sure the ladder is of a regular length
• James Watson and Francis Crick worked out the structure of DNA in 1953
• DNA consists of two polynucleotide chains, one running the opposite way to the other (anti-parallel) twisting to form a double helix. The bases in each strand are held together by hydrogen bonding
• How many hydrogen bonds are formed between each base pairing?


  Adenine forms 2 H bonds with thymine

  Guanine forms 3 H bonds with cytosine.
• What term is used to describe the pairing of bases?

  Complementary base pairing
• A sample of DNA was analysed and 15% of the nucleotides contained adenine. What % of the nucleotides would you expect to contain guanine?

  35% of the nucleotides will be guanine. Because base parings are always equal. If 15% are adenine , then 15% must be thymine. This leaves 70% to be the base pairing of cytosine and guanine so 70/2 = 35% must be guanine.
• Some viruses have single stranded DNA. How could an analysis of a piece of DNA tell you if it was single stranded?


  There wouldn't be equal proportions of A -T and C-G
• How does the structure of DNA relate to its function?

  • It's a stable molecule due to the double helix and there are many Hydrogen bonds which are strong collectively. it's also strong due to the covalent bonds in the sugar-phosphate backbone.
  • Complementary base pairing allows DNA to replicate itself exactly when cells divide as weak H bonds allow strands to separate in this process
• How does the structure of DNA relate to its function?


  • Compact, DNA molecules are long so contain large amounts of coded info. However, the double helix shape allows DNA to fit inside the nucleus of the cell
  • It has a precise genetic code, determined by the sequence of bases, which controls protein synthesis.
• The DNA of a eukaryotic organism is stored in the nucleus, but protein synthesis occurs on ribosomes found in the cytoplasm and the rough ER. So the genetic code on the DNA must be transferred from the nucleus to the cytoplasm by copying sections of DNA that code for polypeptides are copied onto a single stranded molecule called RNA.
• Describe the structure of RNA

  • A Pentose sugar Called ribose
  • an organic base- Adenine, Guanine, cytosine or Uracil
  • a phosphate group
• Is uracil purine or pyrimidine?

  Pyrimidine
• Outline semi-conservative replication

  • Two strands of the DNA molecule are seperated
  • Both strands act as a template for the formation of.a new complementary strand
  • following replication, each new DNA molecule consists of one original strand and one new strand
• What does semi-conservative replication mean?

  half of the original DNA molecule is conserved in each new molecule
• Describe the process of semi-conservative replication

  An enzyme called DNA helicase breaks the weak hydrogen bonds between complementary bases in the p olynucleotide strands. Each exposed strand acts as template for the formation of a new strand. New DNA nucleotides attach to exposed bases and attach by complementary base pairing so H bonds form between the bases. Finally, the enzyme DNA polymerase joins new nucleotides together forming phosphodiester bonds.
• Describe the experiment carried out by Meselson and Stahl in 1958

  They used the bacteria E.coli which have circular DNA and they were grown in a medium containing the heavy isotopes 15N, all the DNA was labelled with 15N. These cultures were then transferred to a medium containing the light isotope 14N and allowed to grow. After some time, the samples were taken and the DNA extracted and centrifuged.
• Explain what happened?

  In Generation 0, all the DNA was ‘heavy’

  In Generation 1, there was two hybrid molecules of DNA.

  In Generation 2, there was 50% hybrid and 50% light

  In Generation 3, there was 25% hybrid and 75% light DNA.. With each successive generation, the proportion of hybrid DNA halves and all the remaining DNA is ‘light’
• What is ATP?

  Adenosine triphosphate is the energy currency used by cells to drive their metabolic reactions.
• What group of molecules does ATP belong to?

  Nucleotides
• What does an ATP molecule Consist of?

  Adenine, ribose sugar, and three phosphate groups.
• Describe how ATP is synthesised?

  by the addition of ADP to an inorganic phosphate ( Pi) , this condensation reaction is catalysed by the enzyme ATP synthase. The addition of of phosphate is called phosphorylation. ATP synthesis requires energy input from respiration or photosynthesis..
• Describe the breakdown of ATP


  The useful energy is stored in the phosphate-phosphate bonds between the 2nd and 3rd phosphate. This is due to the covalent bond between the groups being unstable and so easily broken by the enzyme ATP hydrolase in a hydrolysis reaction. A Pi is removed to form ADP and energy is released.
• What Is the equation for the ATP cycle?

  ADP + Pi —> ATP

  enegry input.

  ATP —> ADP + Pi

  enegry released
• Why is ATP an immediate energy source?

  Not a good long term energy store due to instability of its phosphate bonds, cells can only maintain a few seconds supply. However, it’s not a problem as ATP is rapidly reformed from ADP and Pi.