05 November 2014

# 34 DNA structurer and replication

DNA molecule consists of nucleotides in which the sugar component is deoxyribose whereas the RNA molecule has nucleotides in which the sugar is a ribose.


  • are monomers for nucleic acid polymers, such as DNA and RNA. The genetic material (DNA) is a polymer of 4 different nucleotides. The genetic information is coded in the sequence of nucleotides in a DNA molecule. 
  • adenosine triphosphate (ATP) - the nucleotide molecule that doesn't include the phosphate group - is energy carrier in metabolic pathways. 
  • are components of some important coenzymes:  flavin adenine dinucleotide (FAD), nicotinamide adenine dinucleotide phosphate (NADP) and Coenzyme A. 
They consist of:
  1. A 5-carbon sugar (deoxyribose in DNA; ribose in RNA)
  2. a phosphate group. 
  3. a 1 or 2 ring nitrogen-containing base

The bases are usually referred to by their first letters:
  • A = adenin, 
  • G =guamine, 
  • C = cystosine, 
  • T = thymine (methyl +uracil)
  • U = uracil.
  • A and G = purine bases (2 carbon-nitrogen rings). 
  • C, T and U = pyrimidine bases (1 carbon-nitrogen ring).

Nucleotides bond together to make nucleic acids. They form covalent bonds between the phosphate group of one and the sugar of another. This takes place through a condensation reaction.

Structure of DNA and RNA

DNA and RNA are polynucleotides (long chains of nucleotides).
DNA = A, G, C, T
RNA = A, G, C, U

+ RNA molecule -  a single strand (may be folded up on itself). 

+ DNA molecule 
  • 2 strands run in opposite directions (anti-parallel) and twist round each other --> double helix. 
  • There are H bonds between the bases on the 2 strands. 
  • H bonding only occurs between A-T and C-G (complementary base pairing).  

The significance of complementary base pairing
  • Only certain pairings of bases are possible ( A-T and G-C) => 2 strands of the double helix are complementary, each the predictable counterpart of the other.
  • Since the 2 strands of DNA are complimentary, they can separate from one another and each can serve as a template for building a new partner (if you know the sequence of one DNA strand then you can easily figure out the sequence of the other strand).
  • Thus, DNA replication is semi-conservative, with each of the two daughter DNA molecules having one old strand derived from the parent and one newly made strand.
  • The complementary base pairing results in the two daughter DNA molecules being identical.

DNA Replication

New DNA molecules need to be made before a cell can divide. The 2 daughter cells must each receive a complete set of DNA. The base sequences on the new DNA molecules must be identical with those on the original set. DNA replication takes place in the nucleus, during interphase.

Semi-conservative DNA replication

• Hydrogen bonds between the bases along part of the two strands are broken. This 'unzips' part of the molecule, separating the two strands.

• Nucleotides that are present in solution in the nucleus are moving randomly around. By chance, a free nucleotide will bump into a newly exposed one with which it can form hydrogen bonds. Free nucleotides therefore pair up with the nucleotides on each of the DNA strands, always A with T and C with G. DNA polymerase links together the phosphate and deoxyribose groups of adjacent

  • As each strand retains half of the original DNA material, this method of replication is called semi-consservative. 

3 types of DNA replication 
  • Semi-conservative replication: each new DNA has 1 old strand and 1 new one. 
  • Conservative replication: each new DNA has 2 new strands containing all of the new DNA base pairs. The two original template DNA strands stay together in a double helix. 
  • Dispersive replication: each new DNA has distinct regions of DNA composed of either both original strands or both new strands.


 Syllabus 2015

 (a) describe the structure of RNA and DNA and explain the importance of base pairing and the different hydrogen bonding between bases (includes reference to adenine and guanine as purines and to cytosine, thymine and uracil as pyrimidines. Structural formulae for bases is not required but the  recognition that purines have a double ring structure and pyrimidines have a single ring structure should be included);

(b) explain how DNA replicates semi-conservatively during interphase;

Syllabus 2016 - 2018

6.1 Structure and replication of DNA

Understanding the structure of nucleic acids allows an understanding of their role in the storage of genetic information and how that information is used in the synthesis of proteins.

a) describe the structure of nucleotides, including the phosphorylated nucleotide ATP (structural formulae are not required)

b) describe the structure of RNA and DNA and explain the importance of base pairing and the different hydrogen bonding between bases (include reference to adenine and guanine as purines and to cytosine, thymine and uracil as pyrimidines. Structural formulae for bases are not required but the recognition that purines have a double ring structure and pyrimidines have a single ring structure should be included)

c) describe the semi-conservative replication of DNA during interphase

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