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Nucleotides and nucleic acids
RNA and DNA
These are two nucleic acids made from nucleotides
A nucleotide has three components
 Phosphate group
 Pentose sugar
 Organic base
Feature
Pentose sugar
Purines
Pyrimidines

RNA
Ribose
Adenine and Guanine
Cytosine and uracil

DNA
Deoxyribose
Adenine and guanine
Cytosine and thymine

Structure of RNA
RNA is a Polynucleotide
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The sugar
involved is ribose
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There are 3 types of RNA
 Messenger RNA (mRNA) which carries the code held in the genes to the ribosomes
where the code is used to manufacture proteins
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 Ribosomal RNA (rRNA) which males up the ribosome
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the organic bases can pair up – one purine with one pyrimidine
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Adenine always pairs to thymine
(or uracil in RNA) using two hydrogen bonds
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The polynucleotide strands lie in opposite directions, which is known as
antiparallel
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It then twists to form a helix, called a double helix
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The bonds are
formed by condensation reactions and are called phosphodiester bonds
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These bonds form between the sugar of one nucleotide and the
phosphate of another, making a sugar phosphate backbone
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ADP and ATP
These are phosphorylated nucleotides
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The replication of DNA
DNA replication occurs in all living things in order to copy their DNA for biological
inheritance
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This process is known as semi-conservative replication
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This is catalysed by the enzyme helicase
 The two polynucleotide chains separate, exposing the bases
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Occasionally an incorrect base may be bonded into place,
this is known as a mutation
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The genetic code
The genetic code is universal – all organisms use the same code
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Three consecutive bases codes for one amino acid
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A sequence of amino acids forms one polypeptide
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The synthesis of polypeptides
This involves two stages – transcription and translation
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The double stranded DNA molecule is unwound and split by RNA
polymerase, which breaks the hydrogen bonds holding them together
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The coding strand carries the genetic code and the
complementary strand is a non-coding strand, which is used to build a copy of the coding
strand called (mRNA)
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Each triplet of bases on the (mRNA) is called a codon

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