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Microbial Genetics Part 1
Genetics can be a challenge to understand. Use
the McGraw Hill website to supplement this
lecture.
www.mhhe.com/cowan1
Please do not spend any time studying the Lactose
Operon.
Over view of Microbial Genetics
• 1. Replication of DNA: occurs before each cell
division is complete.
• 2. Transcripton: DNA is converted to RNA and
occurs to carry on life processes.
• 3. Translation: RNA is converted to protein
(enzymes).
• 4. Genetic Transfer and Recombination: How do
we get genetic diversity (antibiotic resistance for
example) within the microbial community?
Replication
• Each organism has its own genome. A genome is all of the
cell’s genetic information. Included in the genome are
chromosomes and plasmids, as well as other DNA that is
sometimes found within microbes.
– Chromosomes are structures made up of DNA that carry hereditary
information. (Remember that they are circular in bacteria.)
– Genes are segments of DNA within chromosomes, that code for
functional products. For example, the insulin gene codes for the
final insulin product.
• Each organism has a genotype and a phenotype.
– The Genotype is the genetic make up of the organism. In other
words, all the genes that it has.
– The Phenotype is the manifest characteristics due to the genes it
has. In other words, do you have blue eyes or green eyes?
•
DNA is composed of 4 nucleotides: Adenine
(A), Thymine (T), Cytosine (C), and Guanine
(G).
–
•
•
•
AT are complementary base pairs and CG are
complementary base pairs.
To give you an idea of the size of DNA in
bacteria, E-coli has 4 million bps. = 4,000 Kb in
its one chromosome.
Each chromosome consists of 2 strands of DNA
bases bound together. They are not identical to
each other but are complementary to each other.
In order to know which end is which, they are
labeled 3’ and 5’. (See Fig. 9.4 in your
textbook)
–
Note that the 3’side of the nucleotide has a different
chemical group from that of the 5’ side.
Steps of Replication
• 1. DNA is partially unwound with the help of an enzyme
called a helicase. The point where the helicase pauses the
unwinding is called the replication fork.
• 2. A molecule, called an RNA primer, is place on the DNA
to help the nucleotides begin to bind. The complementary
bases are then added to the template (parent) strand using
an enzyme called polymerase.
– DNA can only replicate in the 5’to 3’ direction. The reason is
because the chemical group on 3’ side of the nucleotide acts like a
hand that can grab onto the next nucleotide on its 5’side.
– Since the DNA strands are complementary, (also called
antiparallel) only one strand can replicate quickly and easily in the
5’ to 3’ direction. This is called the “leading strand”.
– .
– A little help is needed for the opposite strand so that it too can be
replicated.
– The first step consists of multiple RNA primers placed along the
template strand. These primers provide the necessary hand for the
nucleotides to grab onto. Then they can replicate the strand 5’ to 3’
for a short distance. These fragments of DNA are called Okazaki
Fragments.
– Once the strand has been replicated, the RNA primers are cut out
and replaced by the missing nucleotides. This strand is called the
“lagging strand” because it takes longer for it to be replicated.
• 3. Once the strands are replicated up to the replication
fork, the helicase unwinds the DNA some more and the
replication fork moves down strand to a new location.
• 4. The newly replicated DNA rewinds. One new strand
winds together with one old strand.
– This process of replication is called Semi-conservative Replication
because one half of the template DNA is kept with one half of the
new DNA.
• In Prokaryotes, replication begins at a specific site
in the chromosome called the origin of replication.
• Because prokaryotes have a circular chromosome
replication can proceed bi-directionally or rolling
circle.
– Bi-directional means that replication starts at the origin
of replication and proceeds right and left on both
strands. See Fig. 9.6
– Rolling Circle means that replication only occurs right
or left from the origin of replication but as it proceeds,
the DNA comes off of the chromosome in a motion
similar to a tape dispenser. When replication is
complete, the new chromosome is stitched into a circle
using an enzyme called ligase.
• The replication speed for E.coli is estimated to be
1000 nucleotides/sec.
Transcription
• Formation of RNA from DNA
– The nucleotides are essentially the same as the DNA nucleotides.
The main difference is that Uracil (U) replaces Thymine (T) in
RNA. In other words, anytime T would have been placed in the
new strand, U is put in that spot instead.
• RNA is single stranded, not double stranded.
• 3 types of RNA are formed:
– mRNA, messenger RNA is the template for protein synthesis.
– tRNA, transfer RNAs are taxis for amino acids during protein
synthesis.
– rRNA, ribosomal RNAs are the site of protein synthesis. They put
the protein together.
• 1. RNA synthesis begins at a place on DNA
called the promoter.
• 2. DNA is unwound.
• 3. A primer is put in place and
complementary bases are added replacing T
with U.
• 4. As the RNA strand is synthesized it
comes off of the template and the DNA
strands rewind.
• Once the RNA strand is finished, it folds
into a shape that gives it its final function.
Translation
• mRNA codes for functional proteins
• 1. Ribosomes bind to the mRNA.
– Each ribosome has 2 assembly sites amino acids.
– Each protein is made up of a string of amino acids
bound together.
– 3 nucleotides of mRNA is called a codon
– A codon codes for 1 amino acid.
– There is also a start codon, which signals to the
ribosomes and tRNA that translation starts here, and a
stop codon. The stop codon doesn’t code for any
amino acids. It is like a bump in the road that bumps the
ribosomes off when translation is done.
• 2. tRNA binds to an amino acid and takes it to the
ribosome.
– Each tRNA has one that binds to a specific amino acid.
It can never bind to any other kind of amino acid.
– The other end of the tRNA has an anti-codon. The anticodon is complimentary to a specific codon on the
mRNA.
– So, the tRNA binds to its specific amino acid, then goes
to the ribosome. It then enters the open assembly site
and if the mRNA codon and the tRNA anti-codon
match, then the amino acid is bound to its neighboring
amino acid in the adjacent assembly site. (See figure
9.13)
• 3. Amino acid elongation
– As the amino acids are synthesized, the ribosome
moves down the mRNA one codon at a time. This frees
up one assembly spot in the ribosome for a new tRNA
to bring a new amino acid.
– Amino acid elongation occurs until the ribosomes have
traveled down the length of the mRNA. Then the new
amino acid chain is release and the ribosomes fall off of
the mRNA.
• 4. Protein folding
– The newly formed chain of amino acids has many
different charges on it due to the variety of chemical
structures of the amino acids. Once the chain is
released from the ribosome, it then folds into a
functional shape based upon the charges and shapes of
the amino acids. (Remember that negative repels
negative, positive repels positive, and negative and
positive attract.)