Molecular Biology and Genetics - Lecture Eleven : DNA structure

4 important questions on Molecular Biology and Genetics - Lecture Eleven : DNA structure

What are the components of DNA ?

DNA are made up of nucleotides that are linked together by phosphodiester bonds.
A nucleotide is composed of :
  • Pentose sugar : Either deoxyribose (DNA) or ribose (RNA)
  • Phosphate group
  • Nitrogenous nucleobase
- Pyrimidine bases (Cytosine, thymine and uracil) : 1 ring
- Purine bases (Adenine and Guanine : 2 rings 

What are phosphodiester bonds in the DNA ?

  • These are formed via a condensation reaction
  • As H2O is a byproduct that forms alongside this bond between two nucleotides
  • 3’ carbon of pentose sugar on nucleotide 1 attaches to the phosphate on nucleotide 2
  • Many individual nucleotides eventually form a single polynucleotide (nucleic acid)
  • The phosphates forms the backbone of nucleic acids

What is the Chargaff's Rules ?

1st Rule :
  • The number of adenine (A) nucleotides = thymine (T) in DNA nucleotides
  • The number of adenine (A) nucleotides = uracil (U) in RNA molecules
  • The number of guanine (G) nucleotides = cytosine (C) in both RNA and DNA nucleotides
→ Therefore, the A pairs with either T (DNA) or U (RNA), and G pairs with C
2nd Rule :
  • The composition of DNA varies between species
→ Thus, DNA is likely the genetic material accounting for differences between species
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What is the Watson-Crick Structural Model of DNA ?

  • Antiparallel DNA Strands :
- One runs from 5’ → 3’ direction , the other from 3’ → 5’ direction
- Parallel, but in the opposite direction
  • Double Helix
  • Complementary Base Pairing
- Each purine pairs with an associated pyrimidine (A and T pair with 2 hydrogen bonds whereas G and C pair with 3 hydrogen bonds) 
  • Sugar-phosphate ‘backbone’ is on the outside
  • Bases face inward



This model leads deciphering how the genetic code works and deciphering the mechanisms behind DNA replication.

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