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Nucleic Acids & Protein
Synthesis
DNA (Deoxyribonucleic acid)
 The primary function of DNA in organisms is to store
& transmit genetic information for the synthesis of
proteins
 Group of macromolecules called nucleic acids
 Organic compound
 Made up of repeating subunits called nucleotides
 Consists of 2 long chains of nucleotides
Nucleotides
 Made up of three parts
1. Sugar molecule called deoxyribose
2. Phosphate group
3. Nitrogen-containing base
Nitrogen-containing Bases
 There are 4 different bases which are classified by their
size:
o Purines (larger in size having 2 carbon rings): Adenine
(A) and Guanine (G)
o Pyrimidines (smaller in size having 1 carbon ring):
Thymine (T) and Cytosine (C)
History of DNA
 Johann Friedrich Miescher (1869), a Swiss biochemist,
isolated DNA
 Because the molecules were acidic in nature and found
in the nucleus Miescher called them nucleic acids
 Rosalind Franklin and Maurice Wilkins developed X-
ray photographs of DNA
 The photographs indicated that DNA was a helix with a
sugar-phosphate backbone
 In 1953, James Watson (an American biologist) and
Francis Crick (a British physicist) suggested a model of
DNA
 They proposed that DNA was composed of 2 long chains
of nucleotides coiled around each other to form a spiral
staircase called a Double Helix
Watson & Crick’s DNA Model
 It can be compared to a twisted ladder
 The sides of the ladder are made of alternating
deoxyribose sugar & phosphate groups connected by
covalent bonds
 The rungs of the ladder are made of paired nitrogen-
containing bases connected by weak hydrogen bonds
 Erwin Chargoff, an American chemist, determined
how the nitrogen-containing bases paired up
 Pairs of bases are called Complementary Base Pairs
 Base pairing rules
1.
2.
Cytosine bonds with Guanine
Thymine bonds with Adenine
 Cytosine and Guanine form three hydrogen bonds
 Thymine and Adenine form two hydrogen bonds
 If one strand of DNA has a nitrogen base
sequence of T-A-G-C-A-T, what would be
the sequence of bases for the
complementary strand?
 The sequence of nitrogen-containing bases in DNA
make up the codes for specific amino acids and amino
acids make up proteins
 In order for a new cell to function properly, it must
contain a complete set of genetic instructions. Every
new cell must receive an exact copy of the DNA.
Replication of DNA
 Replication is the process of copying DNA in a cell
 During replication, the two nucleotide chains separate
by unwinding and each chain serves as a template for a
new nucleotide chain
Steps of Replication
1. The separation of the two nucleotide chains

The point at which the chains separate is called
the Replication Fork

Enzymes called Helicases move along the
DNA molecule and break the weak hydrogen
bonds between the complementary nitrogencontaining bases
2. New chains of DNA are assembled
 Enzymes called DNA polymerases bind
to the separated chains of DNA
 As they move along the chain,
complementary nucleotides are joined
together to assemble new DNA
3. The DNA strands twist back together
creating two identical DNA molecules
(double helixes)
 Each DNA molecule is made of one
chain from the original DNA and one
newly replicated chain
 The replication process occurs at many points along
the DNA molecule
 This allows the copying of DNA to be completed faster
 The cell is now ready to undergo cell division (Mitosis or
Meiosis)
Accuracy and Repair
 DNA replication if highly accurate
 1 error in every 10,000 paired nucleotides
 A change in the nucleotide sequence called
a Mutation can occur at any spot along the
DNA molecule
 Cells contain proofreading enzymes and
repair processes that can detect and fix
errors in the complementary base-pairing of
DNA
 Not all errors are detected or can be fixed by
enzymes in the cell, therefore mutations can
occur
 Mutations can either be genetic or
environmental in nature