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SBI3U
DNA STRUCTURE
DNA nucleotide  monomer of DNA
 Composed of a phosphate, deoxyribose sugar, nitrogenous base (A,T,C,G)
o Adenine always pairs with Thymine
o Cytosine always pairs with Guanine
 called complementary base pairs
Note: Two types of bases:
1. Purines – double ring structure (e.g. adenine, guanine)
2. Pyrimidines – single ring structure (e.g. cytosine, thymine)
o A forms 2 hydrogen bonds with T
o C forms 3 hydrogen bonds with G
 Double stranded structure forms a “Double Helix”
 The code of the DNA is in the sequence of bases
o A sequence of 3 bases (called a codon) codes for an amino acid
e.g. AAA codes for phenylalanine
AGT codes for serine
 A gene is composed of a specific sequence of nucleotides (bases) that
codes for a specific amino acid sequence  specific protein
 A chromosome is composed of hundreds to thousands of genes
Importance of the Double Helix Structure:
 Ensures no errors are made during replication
o Having a complementary strand acts like a “spell check”
o If an incorrect or damaged base is added, it will be replaced with a
proper base
DNA REPLICATION
 Process by which other copies of DNA is made
 Occurs in the nucleus, before cell division
STEPS:
1. The original DNA molecule unzips with the help of an enzyme that breaks
the bonds between the complementary base pairs
2. Free nucleotides bond to each of the unzipped parent chains (other
enzymes help do this)
 each parent strand is used to build a complementary strand
Recall: Adenine bonds to Thymine, Cytosine bonds to Guanine
3. Sugar and phosphate of the adjacent nucleotides bond to form the
backbone of the new DNA strand
4. Two new copies of DNA are produced from the original DNA molecule.
 this is referred to as “Semi-Conservative” method