What you need to know about genetics

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What you need to know about genetics All
organisms have DNA and RNA as genetic material A
ll organisms use the same nucleotides All
organisms replicate, transcribe and translate DNA
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The central dogma of molecular biology
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What is different in prokaryotes from
eukaryotes? The prokaryotic genome is circular,
so it is repli- cated differently than linear
eukaryotic DNA What is gene expression? transcri
ption synthesis of RNA from a DNA template tra
nslation formation of a protein (amino acid
sequence) from RNA sequence How does this
process differ in prokaryotes?
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In prokaryotes, an mRNA molecule can
carry information for several genes e.g., all
of the genes in a biosynthetic pathway (this
doesnt happen in eukaryotes) The RNA polymerase
in constructed differently in
prokaryotes Process consists of initiation,
elongation, termination Same steps are seen in
translation
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What are the differences? In eukaryotes, RNA is
modified and transported from the nucleus The
ribosomes are different (70S in prokaryotes, 80S
in eukaryotes The proteins and enzymes are
different that make transcription and
translation occur
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The entire genomes (DNA sequences) of
many bacteria are known gt1000 times smaller than
human genome
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Gene expression is carefully controlled. Bacteria
must be able to adapt to environment and divide
quickly Some genes are constitutive (always
expressed) Some are inducible (only expressed
when needed) example enzymes that metabolize
lactose Some are repressible ( expressed most of
the time, but can be turned off) example
enzymes involved in biosynthesis These genes are
organized into operons
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Operons a group of genes expressed together
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Repressor binds to operator (a region of
DNA adjacent to genes) If something is bound to
operator, trasncription cannot occur Inducer
binds to repressor so it cant bind to
operator Therefore transcription can take
place Example lactose operon. genes are
expressed only if lactose (inducer) is present
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Biosynthetic genes are also arranged in
operons In this case, the repressor is the end
product of the pathway
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Some fundamentals of bacterial genetics Bacterial
DNA (like any DNA) can be altered
by mutations Mutations can result in changes in
proteins diversity acquisition of
resistance new traits can be transmitted to
other microbes
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What happens when DNA is mutated? Bases
(nucleotides) are changed (base substitution) Ba
ses are inserted or deleted Sequences of DNA
move around the genome (transposons)
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Point mutation a single amino acid is changed
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How many amino acids are changed?
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Mutations can occur spontaneously, or can be
induced By chemicals By radiation
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DNA damage occurs fairly frequently Cells have
several repair pathways available to
them Repair of errors in base incorporation Repa
ir of thymine dimers (due to UV
irradiation) Repair of damaged DNA Bypass of
damaged DNA (SOS repair)
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In bacteria, mutants can be identified by
plating on special minimal medium Auxotrophs
have lost the ability to synthesize certain
substances (Prototrophs retain the ability to
do so) A variety of selective strategies have
been developed
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The Ames test uses bacteria to test
chemicals for capacity to cause mutations, as
well as carcinogens (cancer-causing
chemicals) Much more rapid and inexpensive than
testing animals Results have been reliable and
reproducible Several tester strains are used,
since chemicals can cause point or frameshift
mutations (but not both)
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How does gene transfer occur in bacteria? Only
part of the bacterial DNA is transferred The new
DNA replaces existing DNA (homologous
recombination) Transformation- uptake of naked
DNA Transduction- by bacteriophages Conjugation-
bacterial cells come in direct contact with each
other. Plasmid is often transferred
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Transformant identified by selection
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Recipient bacteria must be competent to
take up and incorporate DNA Few strains of
bacteria are naturally competent (generally Gram
positive) Bacteria can be made artificially
competent calcium solutions electric current
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Transduction bacteriophage transfer DNA from
one bacterium to another
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Conjugation Direct contact between donor and
recipient must occur
Sex pilus is encoded by fertility (F) plasmid
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Note role of plasmids in conjugation What is a
plasmid? Small, circular double-stranded
DNA Common in prokaryotes May vary widely in
size Can encode a variety of genes usually
not essential bacterial genes but may
give bacterium new properties
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R plasmids- resistance to antibiotics,
metals Virulence factors (that make bacteria
pathogenic, or disease causing) Transposons can
insert themselves into genome (and out of
it) Thus bacteria have many ways of obtaining
new genes to enhance survival
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Natural methods of gene transfer have
been modified in biotechnology Deliberate
introduction of genes into recipient bacteria or
other cells Genes could not otherwise be
exchanged recombinant DNA
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Plasmid may be able to replicate frequently, so
many copies of the DNA are made The gene may be
expressed, so the protein product in made in
large quantities e.g., recombinant human
insulin, as opposed to extracts from pig
pancreases Genetic analysis determining DNA
sequences analyzing gene products expressing
DNA in a variety of cells What do the proteins
do?
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Summary DNA replication, transcription and
translation occur in all cells Prokaryotes have
a single, circular DNA genome They have different
enzymes and ribosomes but use the same
nucleotides They acquire new traits through
incorporating mutations. These can be analyzed
through selection techniques
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Bacteria do not reproduce sexually but can
acquire new DNA through transformation, transdu
ction or conjugation These natural processes
have been modified so that DNA can be
deliberately incorpor- ated into host microbes-
even genes that would normally never be
transferred this way