Nucleotides and nucleic acids

• Draw the structure of a nucleotide.

  A) 5 carbon sugar

  B) nitrogenous base

  C) phosphate group

  
• Name the pentose sugars in DNA and RNA.

  DNA: deoxyribose

  RNA: ribose
• Describe how polynucleotide strands are formed and broken down.

  Condensation reactions between nucleotides form strong phosphodiester bonds (sugar-phosphate backbone). Hydrolysis reactions use a molecule of water to break these bonds.

  Enzymes catalyse these reactions.
• Describe the structure of DNA.

  Molecule twists to form double helix of 2 deoxyribose polynucleotide strands (so there are 2 sugar-phosphate backbones).

  H-bonds form between complementary base pairs (AT and GC) on strands that run antiparallel.
• Name the purine bases and describe their structure.

  • adenine C5H5N5
  • guanine C5H5N5O

  
• Name the pyrimidine bases and describe their structure.

  • thymine C5H6N2O2
  • cytosine C4H5N3O
  • uracil C4H4N2O2

  
• Name the complementary base pairs in DNA and RNA.

  • DNA: 2 H-bonds between adenine (A) + thymine (T)
  • RNA: 2 H-bonds between adenine (A) + uracil (U)
  • Both have 3 H-bonds betwen guanine (G) + cytosine (C)
• Why is DNA replication described as semiconservative?

  Strands from original DNA molecule act as templates.

  New DNA molecule contains 1 old strand and 1 new strand (specific base pairing enables genetic material to be conserved accurately)
• Explain the role of DNA helicase in semiconservative replication.

  Breaks H-bonds between base pairs to form 2 single strands, each of which can act as a template.
• How is a new strand formed during semiconservative replication?

  1. Free nucleotides from nuclear sap attach to exposed bases by complementary base pairing
  2. DNA polymerase joins adjacent nucleotides on new strands in a 5'-3' direction via condensation reactions to form phosphodiester bonds
  3. H-bonds reform
• Identify features of the genetic code.

  • Non-overlapping: each triplet is only read once
  • Degenerate: more than one triplet codes for the same amino acid (64 possible triplets for 20 amino acids)
  • Universal: same bases and sequences used by all species
• How does a gene determine the sequence of amino acids in a protein?

  Consists of base triplets that code for a specific amino acid
• Describe how DNA can be purified by precipitation.

  • Add ethanol and a salt to aqueous solution
  • Nucleic acids precipitate out of solution
  • Centrifuge to obtain pellet of nucleic acid
  • Wash pellet with ethanol and centrifuge again
• What does transcription produce and where does it occur?

  • produces mRNA
  • occurs in nucleus
• Outline the process of transcription.

  1. RNA polymerase binds to promoter region on a gene
  2. Section of DNA uncoils into 2 strands with exposed bases. Antisense strand acts as template.
  3. Free nucleotides are attached to their complementary bases
  4. RNA polymerase joins adjacent nucleotides to form phosphodiester bonds
• What happens after a strand of mRNA is transcribed?

  • RNA polymerase detaches at terminator region
  • H-bonds reform and DNA rewinds
  • Splicing removes introns from pre-mRNA in eukaryotic cells
  • mRNA moves out of nucleus via nuclear pore and attaches to ribosome
• What does translation produce and where does it occur?

  • Produces proteins
  • Occurs in cytoplasm on ribosomes (which are made of protein + rRNA)
• Outline the process of translation.

  1. Ribosome moves along mRNA until 'start' codon
  2. tRNA anticodon attaches to complementary bases on mRNA
  3. Condensation reactions between amino acids on tRNA form peptide bonds. Requires energy from ATP hydrolysis
  4. Process continues to form polypeptide chain until 'stop' codon is reached
• Describe the structure of adenosine triphosphate (ATP) and adenosine diphosphate (ADP).

  • nucleotide derivative of adenine
  • ATP has 3 inorganic phosphate groups
  • ADP has 2

  
• What is a mutation?

  An alteration to the DNA base sequence. Mutations often arise spontaneously during DNA replication.
• DNA and RNA are nucleic acids: polymers that are made up of many repeating units (monomers) called nucleotides
• Each nucleotide is formed from:

  • A pentose sugar (a sugar with 5 carbon atoms)
  • A nitrogen-containing organic base
  • A phosphate group
• The basic structure of a nucleotide

  A) phosphate

  B) pentose

  C) nitrogenous

  
• The components of a DNA nucleotide are:

  • A deoxyribose sugar with hydrogen at the 2' position
  • A phosphate group
  • One of four nitrogenous bases - adenine (A), cytosine(C), guanine(G) or thymine(T)
• The components of an RNA nucleotide are:

  • A ribose sugar with a hydroxyl (OH) group at the 2' position
  • A phosphate group
  • One of four nitrogenous bases - adenine (A), cytosine(C), guanine(G) or uracil (U)
• The presence of the 2' hydroxyl group makes RNA more susceptible to hydrolysis

  • This is why DNA is the storage molecule and RNA is the transport molecule with a shorter molecular lifespan
• An RNA nucleotide

  A) phosphate

  B) ribose

  C) base

  
• A DNA nucleotide

  A) phosphate

  B) deoxyribose

  C) base

  
• The bases adenine and guanine are purines – they have a double ring structure
• The bases cytosine, thymine and uracil are pyrimidines – they have a single ring structure
• The molecular structures of the phosphate group and sugars in RNA and DNA

  A) nucleotide

  B) ribose

  C) deoxyribose

  
• The molecular structures of purines and pyrimidines are slightly different

  A) purine

  B) guanine

  C) pyrimidine

  D) uracil

  E) cytosine

  
• Nucleotide Structure Table

  A) deoxyribose

  B) ribose

  C) thymine

  D) uracil

  E) double

  F) single

  
• Separate nucleotides are joined together via condensation reactions

  • These condensation reactions occur between the phosphate group of one nucleotide and the pentose sugar of the next nucleotide
• A condensation reaction between two nucleotides forms a phosphodiester bond

  • It is called a phosphodiester bond because it consists of a phosphate group and two ester bonds
• The chain of alternating phosphate groups and pentose sugars produced as a result of many phosphodiester bonds is known as the sugar-phosphate backbone (of the DNA or RNA molecule)
• As the synthesis of polynucleotides requires the formation of phosphodiester bonds, the same is true for the reverse process: the breakdown of polynucleotides requires the breakage of phosphodiester bonds
• A section of a single polynucleotide strand showing a phosphodiester bond (and the positioning of the two ester bonds and the phosphate group that make up the phosphodiester bond)

  A) phosphate

  B) pentose

  C) base

  D) phosphodiester

  E) ester

  F) backbone

  
• In all organisms this energy is required for:

  • Anabolic reactions (building larger molecules from smaller molecules)
  • Moving substances across the cell membrane or moving substances within the cell
• In animals energy is also required for:

  • Muscle contraction – to coordinate movement at the whole-organism level
  • The conduction of nerve impulses