Transcript Microbial Genetics

Genetics is the study of heredity which is concerned
with how:
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information in nucleic acids is expressed
nucleic acids are duplicated and transmitted to
progeny
these processes account for the characteristics of
progeny
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The heredity material
found in cells
Large molecules that are
acidic in nature
Associated with the
nuclear material of cells
Two types
Deoxyribonucleic Acid
Ribonucleic Acid
Is responsible for all cellular
activity.
Directs the production of proteins.
Is double stranded and helical.
Is maintained by hydrogen bonds
(weak bonds)
Is very stable and can survive
 Temperatures as high as 70 C
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High salt concentrations
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Acid environments
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Ribonucleic acid (RNA) is a
biologically important type of
molecule that consists of a long
chain of nucleotide units.
Each nucleotide consists of a
nitrogenous base, a ribose sugar,
and a phosphate. RNA is very
similar to DNA, but differs in a few
important structural details: in the
cell, RNA is usually single-stranded.
Three types of RNA
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mRNA messenger
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tRNA
transfer
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rRNA
ribosomal
mRNA messenger
Is complementary to one strand
of DNA and functions to carry
the genetic material from the
chromosome to the ribosome.
Transcription
tRNA transfer
Is responsible to transfer
information from mRNA to
rRNA.
Translation
rRNA ribosomal
Is associated with the ribosome
and accepts information from
tRNA and correlates the
information to synthesize
proteins.
Protein Synthesis
Are constructed from a string of
small molecules called
Nucleotides.
Nucleotides consist of a
5-carbon sugar (pentose), one or
more phosphate groups, and a
base containing nitrogenous
rings.
Purines
Contain 2 nitrogenous rings
Adenine and Guanine
Pyrimidines
Contain 1 nitrogenous ring
Cytosine and Thymine in DNA
Uracil replaces Thymine in RNA
Adenine always pairs with
Thymine in DNA (A-T)
Uracil replaces Thymine in RNA
Guanine always pairs with
Cytosine (G-C) and are stronger
bonds.
The central dogma theory of
molecular biology is represented
by a simple pathway: DNA—
>RNA-->protein, which
demonstrates the flow of genetic
information in a living cell.
The major processes involved in
this pathway are replication,
transcription, and translation.
In DNA replication, the DNA
polymerase enzyme replicates all
the DNA in the nuclear genome
in a semi-conservative manner,
meaning that the double
stranded DNA is separated into
two and a template is made by
DNA polymerase.
This allows genomic material to
be duplicated so it can be evenly
partitioned between two somatic
cells (daughter cells) upon
division.
The process in which DNA is
copied into RNA by RNA
Polymerase is called
transcription.
Three forms of RNA are produced
here: messenger RNA (mRNA),
ribosomal RNA (rRNA), and t
ransfer RNA (tRNA).
1. DNA guides the synthesis of
mRNA which in turn directs the
order in which amino acids are
assembled into proteins.
2. DNA directs its own
replication by giving rise to two
complete, identical DNA
molecules.
This replication is necessary
because each cell must inherit a
complete set of all genes in
order to carry out the cell’s life
processes.
Another process in this
pathway is reverse
transcription, which involves
copying RNA information into
DNA using reverse transcriptase.
Recently, this processes has
been defined and may expand
the central dogma.
For example, retroviruses use
the enzyme "reverse
transcriptase" to transcribe DNA
from a RNA template.
The viral DNA then integrates into the
nucleus of the host cell. Then it is
transcribed, and further translated into
proteins.
This biological process effectively adds
another pathway to the central dogma
of molecular biology.
Bacteria contain 1 chromosome
Many contain plasmids
When bacterial chromosomes
replicate both strands are
duplicated. Each strand functions
as a template.
During replication, enzymes known
as polymerases transport
nucleotides from the cytoplasm
that are complimentary to the
template and fit them into place,
resulting in two strands, one
parental and one new one
During replication, enzymes known
as polymerases transport
nucleotides from the cytoplasm that
are complimentary to the template
and fit them into place, resulting in
two strands, one parental and one
new one.
The replication is said to be semiconservative because the parental
strand is conserved (remains the
same ).
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DNA unwound with enzyme (replication fork)
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Complementary bases added to template (parent strand)
using enzyme
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Replication fork moves down strand
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Newly replicated DNA rewinds
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Process called Semiconservative Replication
Copied in 5’ to 3’ direction
Polymerase can only add nucleotides to 3’ end
In Prokaryotes, replication begins at specific site in
chromosome called the origin of replication
Replication of DNA begins a specific site on the DNA template
termed the origin and proceeds in both directions from the
origin until nuclear division and cytokinesis take place.
Replication speed = 1000 nucleotides/sec
Transcription is the synthesis
of RNA and involves the
assembly of nucleotides by an
enzyme, RNA polymerase.
1. RNA polymerase binds to DNA
at a promoter site near the
gene to be transcribed.
2. RNA polymerase travels the length
of the DNA using it as a template to
duplicate.
3. The RNA polymerase continues
until it reaches a termination site at
which time the transcription is
complete.
Protein synthesis is carried out in
the cytoplasm.
It begins with DNA duplication by
mRNA (Transcription)
mRNA then migrates to the
ribosome where tRNA transfers
information from mRNA to rRNA
(Translation).
Protein synthesis is
continuous and takes place in
three stages:
1. Initiation
2. Elongation
3. Termination
Initiation
The beginning of protein
synthesis starts methionine
which is the start codon.
Start codon is know as
formylmethoinine (f-met).
It is coded as AUG.
Elongation
By a complex that begins with
f-met, amino acids attach to
form a chain (amino acids
joined repeatedly to form
proteins)
Termination
Ends when the synthesis
comes to a termination
codon.
Termination codons are codes
as UAA, UAG, and UGA).
A codon is a group of three
nucleotides in DNA which acts
as a code in the placing of
an amino acid in a protein
molecule.
A codon is a group of three
nucleotides in DNA which acts
as a code in the placing of
an amino acid in a protein
molecule.
A codon is a group of three
nucleotides in DNA which acts
as a code in the placing of
an amino acid in a protein
molecule.
AUG begins protein synthesis
UAA, UAG, UGA are
termination codons.
One base pair is exchanged for another in the DNA molecule
One or more base pairs are inserted in the DNA molecule.
One or more base pairs are deleted in the DNA molecule
There is a rearrangement of sections in the DNA molecule.
There is an exchange of DNA region with another DNA molecule
(Recombination).
Some mutations harmful, some beneficial, some neutral
Some plasmids encode for
genes that enhance
pathogenicity of a bacterium
E-coli, plasmid encodes
toxins produced and
bacterial attachment to
intestinal cells
Transposons: are small pieces of DNA (2,000 – 20,000 base
pairs) fond in chromosomes and plasmids.
They are able to direct synthesis of copies of themselves and
become incorporated into the chromosome.
Called “jumping genes” because of their ability to insert
themselves into a chromosome or change their locations.
Chemical and physical agents that cause mutations.
UV light is absorbed by pyrimidines (cytosine and thymine). UV
causes adjacent thymines in the same strand to react and bond
with each other.
Thymine dimers are replication errors in transcription; if not
correct , can lead to cellular death.
◦ Agents in environment that directly or indirectly
cause mutation
Nitrous acid alters the chemical structures of adenine,
cytosine, and guanine so that they change the base pairing
which introduces mutation during DNA replication.
There are many agents in environment that directly or
indirectly cause mutation
Most mutations are harmful but some are beneficial because
they introduce variability into the progeny which promotes
survival.
Recombination: exchange of homologous genes on a
chromosome
Transformation: genes transferred from one bacterium to
Another. After cell death, some bacteria are lysed and release
cellular contents into surrounding environment. The recipient
cell is in a physiological state that will allow it to take up DNA.
Transformation occurs naturally among a few organisms..
Another form of transfer of DNA from
one cell to another.
Requires cell to cell contact and
transfer of plasmids through
f-factor pili.
◦ Donor cells carry plasmid
◦ Recipient cells usually do not
have plasmid
Streptococcus pneumoniae: S
form encapsulated, R form
non-encapsulated
◦ Experiment which placed heat
killed S form with live R form.
◦ Resulted in live S form.
Transfer of genes from a
donor to a recipient by a
bacteriophage