Nucleic Acids, DNA and RNA


Differences between DNA and RNA

DNA- Stores genetic information to allow for growth and development 

Contains phosphate group, DEOXYRIBOSE SUGAR and a nitrogenous base (A/T/C/G)

Double polynucleotide strand 

Longer than RNA 

RNA- Transfers genetic information from DNA to the ribosomes (site of protein synthesis) = translation of RNA into polypeptides 

Contains phosphate group, RIBOSE SUGAR and a nitrogenous base (A/U/C/G) 

Single polypeptide strand 

Shorter than DNA 

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Purines and Pyrimidines


The bases are A & G 

They have 2 carbon-nitrogen rings 

Tip to remember: purine is shorter word = more carbon-nitrogen rings 


The bases are C, T & U 

They have 1 carbon-nitrogen ring

Tip to remember: there is a y in thymine and cytosine like in pyrimidine (think of U being like thymine so its just the same) 

Purines always pair with pyrimidines = twist to form a double helix 

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DNA Replication

Semi-conservative replication = contains 1 new strand and 1 old strand 

1. DNA helicase "unzips" the DNA strands by breaking the hydrogen bonds between the complementary base pairs on the polynucleotide strands with each strand being kept apart by single-strand binding proteins

2. Each strand now acts as a template to form the new strands. Free nucleotides bases in the nucleus pair up to the exposed nitrogenous bases via complementary base pairing 

3. DNA polymerase catalyses the formation of the new strand through condensation reactions 

4. Hydrogen bonds form between bases on the new and old strands

5. Identical copy of the DNA is made 

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Structure and Function of DNA

  • Sugar phosphate backbone provides strength and stability 
  • Helical structure so it is compact 
  • Large molecule to store lots of information 
  • Double stranded so replication can occur semi-conservatively 
  • Weak hydrgoen bonds for replication
  • Base sequence codes for proteins
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