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Lecture 6
Microbial Genetics:
DNA Replication
Gene Expression
Genetics
• Genome=
• Cells genome organized into
chromosomes
• Chromosome=
• Gene= segment of the DNA that codes for
one protein
Bacterial Chromosome
• Single circular chromosome composed of
DNA
• Looped and folded and attached at one or
more points to the plasma membrane
• Supercoiled
Bacterial Plasmids
• Many prokaryotic cells also contain
plasmids
• They replicate independently from the
chromosome
Nucleic Acids
• 2 types of nucleic acids:
– Deoxyribonucleic acid (DNA)
– Ribonucleic acid (RNA)
• Subunit: Nucleotides
Nucleotide
Nitrogen containing bases
• 5 Different:
• Purines: Adenine (A)
Guanine (G)
• Pyrimadines: Thyamine (T)
Cystosine (C)
Uracil (U)
Synthesis of DNA
• Dehydration synthesis- forming of covalent
bonds between nucleotides
• Forms between phosphate group of one
nucleotide and sugar of another nucleotide
• Phosphate joins #3 carbon of one sugar
with #5 carbon of the other
• Results in backbone of alternating sugar
and phosphate molecules
Double Helix of DNA
• Strand are held together by hydrogen
bonds
– A pairs with T
– G pairs with C
•
•
•
•
# of A= # of T
# of G=# of C
DNA sequence: read from 5’ to 3’
Sequence example: ATTAGCA etc.
DNA Replication
DNA Replication
• Purpose is to create new DNA strand, so
that upon binary fission, each of the 2 cells
receives a complete copy of DNA
• Bidirectional- from distinct starting pointproceeds in both directions
• Semi- conservative- each of the 2 DNA
helix’s generated contains 1 new strand
and 1 old strand
First Stage: Initiation
• DNA unwinds and strands separate
• As the DNA unzips, two replication forks
form and move in opposite directions away
from the origin
Second Stage: Elongation
• Enzymes synthesize a new stand to pair
with each original strand
• Nucleotides can only be added in 3’ to 5’
direction
• This creates leading and lagging strands
• The lagging strand is synthesized in
Okazaki fragments, which are joined by
DNA ligase
Figure 8.4
Third Stage: Termination
• Two DNA helices separate from each
other
• Each chromosome now contains one old
and one new strand
Figure 8.5
Figure 8.6b
Gene Expression:
Transcription
Translation
Central Dogma of Molecular
Biology
• DNA  RNA  Protein
• Gene Expression: The production of a
protein product from a gene
– Involves two steps: transcription and
translation
Gene Expression
•
Series of two processes that link genes
to proteins
1. Transcription: synthesis of RNA from
DNA
2. Translation: synthesis of protein from
RNA
Transcription
• DNA used as template
• Use one strand of DNA to make mRNA
molecule
• mRNA is complementary to one strand of
DNA
Initiation of Transcription
• Transcription begins when RNA
polymerase recognizes and binds to
sequence of nucleotides in the DNA called
the promoter
• The promoter orients the RNA polymerase
in one of two possible directions, telling it
which DNA strand to use
Transcription- Elongation
• RNA polymerase moves along template
strand of DNA, synthesizing the
complementary single-stranded RNA
molecule
• RNA synthesized in 5’ to 3’ direction,
nucleotides added to 3’ end
• Very fast: 30 nucleotides per second
Transcription- Termination
• When RNA polymerase encounters
terminator it falls off DNA
• Once terminated RNA is called mRNA
Figure 8.7 (Overview) (1 of 7)
mRNA
• Messenger RNA
• Temporary copy of genetic information
• 3 nucleotides of DNA  3 nucleotides of
RNA
• 3 nucleotides of RNA is a codon
• One codon codes for one amino acid
• String of amino acids with proper 3-D shape 
protein
Translation
• Process by which information on mRNA is
decoded to synthesize the specified
protein
• Proteins synthesized by adding amino
acids sequentially
• Remember: one codon  one amino acid
• How many amino acids would one protein
contain if it was translated from an mRNA
that is 690 nucleotides long?
• AUGCGGCAGACCAAACGAUUGGUUGC
GUAA
• How many codons?
10
• List the codons:
AUG CGG CAG ACC AAA CGA UUG GUU
GCG UAA
The Genetic Code: Universal for all
living things
Translation
• Process of translation requires three major
components
– mRNA
– Ribosomes
– tRNA
Ribosomes
• Serve as sites of translation, or sites of
protein synthesis
• Prokaryotic ribosomes are 70S
– Large subunit- 50S
– Small subunit- 30S
tRNA
• Transfer RNA
• Carries amino acids to the ribosome
• Recognize and base-pair with a specific
codon and deliver appropriate amino acid
to site
• Recognition occurs because each tRNA
has an anti-codon, which is
complementary to codon on mRNA
Initiation of Translation
• Translation begins as the mRNA is still
being synthesized
• 30S subunit binds to ribosome-binding site
• tRNA and 50S subunit soon join
• AUG- start codon- codes for methionine
Elongation
• Ribosome moves along mRNA
• As the next codon is exposed, a new tRNA
with correct anti-codon moves in
• As each tRNA brings in the correct amino
acid it forms a covalent bond to it’s
neighboring amino acid
• Elongation continues until stop codon is
reached
Regulation of Gene Expression
• Protein synthesis requires a huge amount
of energy
• Regulation of protein synthesis conserves
energy for the cell
• Repression and Induction
• Operon model of gene expression
Repression and Induction
• Repression: inhibits gene expression and
decreases the synthesis of enzymes
– Mediated by regulatory proteins called
repressors
• Induction: process that turns on the
transcription of a gene
– Mediated by regulatory proteins called
inducers
Operon model of gene
expression
• Read over Operon Model of Gene
Expression before class (page 229-231)
• Work in groups to understand the concept