7.1 DNA Structure

Question 1

Describe the structure of DNA including the antiparallel strands, 3’ - 5’ linkages and hydrogen bonding between purines and pyrimidines

Answer 1

• The carbon atoms in deoxyribose are numbered, with the nitrogenous bases attached to C1and the phosphate group attached to C5
  
  • Nucleotides are joined by a covalent phosphodiester bonds between the C5phosphate group and the C3hydroxyl group
  • The chain is constructed 5’ - 3’, as new nucleotides bond to the exposed C3 hydroxyl group
  • The nitrogenous bases interact via hydrogen bonding (complementary base pairing)
  • In order for the bases to associate one strand must run antiparallel to the other (3’ - 5’)

Question 2

State the number of hydrogen bonds between A and T

Answer 2

2

Question 3

State the number of hydrogen bonds between G and C

Answer 3

3

Question 4

Outline the structure of nucleosomes

Answer 4

• The DNA of eukaryotes associates with proteins called histones
  
  • DNA is wound around an octamer of histones (8 histones)
  • The octamer and DNA combination is secured to a H1 histone, forming a nucleosome

Question 5

Outline the function of nucleosomes

Answer 5

Nucleosomes serve two main functions
• They protect DNA from damage
• They allow long lengths of DNA to be packaged (supercoiled) for mobility during mitosis/meiosis
• When supercoiled, DNA is not accessible for transcription

Question 6

Define exon

Answer 6

The part of the gene which codes for a protein (expressed sequence)

Question 7

Define intron

Answer 7

A non coding sequence of DNA within a gene which is cut out by enzymes when RNA is made into mature mRNA (interrupting sequence)

Question 8

Outline the features of exons (unique or single copy genes)

Answer 8

• Smaller proportion of genome
  
  • Occur once in the genome
  • Long base sequences
  • Usually genes
  • May be translated
  • Low rate of mutation
  • Very similar between individuals
  • Not used for DNA profiling

Question 9

Outline the features of introns (repetitive sequences)

Answer 9

• Larger proportion of genome
  
  • Occur many times in the genome
  • Short base sequences
  • Not genes
  • Never translated
  • Higher rate of mutation
  • Varies greatly between individuals
  • Used for DNA profiling

Question 10

Distinguish between eukaryotic and prokaryotic DNA with regards to introns

Answer 10

Eukaryotic DNA contains introns but prokaryotic DNA does not