Unit 3 Edit 6.09
Microbial Genetics
DNA – Deoxyribonucleic acid – the chemical code for protein
synthesis
 subunits are nucleotides
 Phosphate, sugar and nitrogen base


Variation of bases = protein structure
Adenine, Thymine, Cytosine, Guanine
o Bases pair A-T, C-G
Structure – double stranded, helical
 Long chains of nucleotides
 Backbone alternating sugar, phosphate
 Strands are joined by hydrogen bonds between
nitrogen bases
 Antiparallel arrangement
DNA Replication
 Required before all cell division
 Result is two identical molecules
 Process is semi-conservative
o Each new molecule has one strand of the
original DNA and newly formed strand
 30 enzyme process
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Unit 3 Edit 6.09

Why is it important to replicate the DNA exactly?

Because of antiparallel arrangement of the DNA, the
two halves of the molecule are replicated differently
o
o
Leading strand is replicated continuously
Lagging strand is replicated in fragments called
Okazaki fragments
Enzymes
 Helicase – unwinds DNA, separates nucleotides
 DNA polymerase – adds nucleotides to growing chain,
proofreads new DNA strand
 Ligase – in the lagging strand, it joins the Okazaki
fragments
Terminology
 Triplet –

Gene –

Genome –
RNA – Ribonucleic acid – similar to DNA, assists in protein
synthesis
 Differences between DNA and RNA
o RNA is single stranded
o Thymine base is replaced with Uracil
o Sugar is ribose (not deoxyribose)
o 3 types of RNA
 mRNA
 tRNA
 rRNA
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
Ribosomal RNA – together with proteins, makes up the
structure of the ribosome - where protein synthesis
takes place. Translation takes place in the ribosome.

Messenger RNA – carries the coded information for
making a protein from DNA to ribosome for translation,
this process is called transcription.

o
RNA polymerase – enzyme that is responsible
for assembling nucleotides to make mRNa
o
Codon – each 3 nucleotides code for one amino
acid and is called a codon
o
See mRNA amino acid on page 225
o
Degeneracy of the code – most amino acids are
coded for by more than one codon, why is this a
good idea?
Transfer RNA – carries the amino acid to the ribosome
where they are joined to form the protein.
o Called translation
o
tRNA reads mRNA to determine correct amino
acid
o
Small molecule, contains 1 anticodon
o
Brings correct amino acid to growing protein
chain
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Protein synthesis is regulated to conserve energy

Repression – proteins bind to mRNA to prevent
transcription

Induction – process that starts the transcription
process
Terminology

Genotype

Phenotype

3 types of genes – structural, regulatory,
RNA
Protein Structure

Primary structure

Secondary – pleating and helical

Tertiary – more folding

Quarternary – several molecules make up a
protein
Examples:
Mutation
 Many mutations are silent or neutral, especially if they
are in the third position of the codon

Two types of structural changes that lead to mutation
o Base substitution – a single base in DNA is
replaced with another
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o

It is rare that a mutation is beneficial, most are fatal or
neutral
o Missense mutation – the wrong amino acid is
inserted, creating a completely new primary
structure
o

Frameshift – one or many nucleotides are
deleted or inserted
Nonsense mutation – a substitution that results
in a codon that does not code for an amino
acid. Often stops transcription, a stop code
Mutagens
o Chemical
o
Radiation
o
Random
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