Download lecture 03b

Molecular Genetics of Bacteria
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• DNA contains info for making entire cell.
– info contained in segments called genes
– each gene codes for a protein (many are enzymes)
– different enzymes carry out different reactions.
– lots of different enzymes, lots of genes needed,
long DNA molecule
– Types of bacteria differ from one another because
their DNA is different.
– DNA is information in molecular form.
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Information is coded into the molecule
Other 3 bases are
Guanine, cytosine,
and thymine.
These 2 phosphate groups
are lost (to provide energy)
when nucleotides are
joined together, sugar of
one to phosphate of next.
The coded information is in
the order of the 4 bases.
DNA
structure
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Double-stranded molecule.
A always pairs with T
C always pairs with G
H-bonds between the bases
help hold the two strands
together. (rungs)
“Backbone”: alternating sugar
and phosphate groups.
D = deoxyribose (sugar)
P = phosphate
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Double helix
Helix: spiral. Two strands that bind to each
other and twist.
Because of “base pairing”, if you know the
sequence of one strand, you automatically
know the sequence of the other.
Long thin molecule: if as thick as spaghetti, a
bacterial DNA molecule would stretch from
here to Bono
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DNA replication
Complementary base-pairing allows
for a mechanism for 1 DNA molecule
to be copied to produce two identical
molecules.
Because each new double-stranded
molecule is made of one old strand
and one new strand, this is called
“semi-conservative” replication.
The same genetic information thus
appears in both new daughter cells.
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How long and thin? How many genes?
• Bacteria are all different. E. coli:
– An operon is a group of genes used together at the
same time. An average size operon is 4000 bp
– Total genome size of E.coli is about 4,000,000 bp
– E. coli has over 4,000 genes
One gene
http://www.sciencebuddies.org/mentoring/plugin_bac_d
iversity_bacteria_and_dna.jpg
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Not just for storage, but for use
• A cell doesn’t just store its genetic
information, it uses it.
– Some genes are needed all the time.
– Other genes are not; they are turned on and off as
needed to save energy
• Genes code for proteins
– Proteins are made at the ribosomes
– How to get a copy of the information to the
ribosomes? (DNA is too big and bulky)
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Transcription
• A copy of 1 or more genes is made
– The copy is in the form of RNA (mRNA), a much
smaller nucleic acid in which is coded the information
from one or more genes.
– The mRNA is complementary to one of the 2 strands of
the DNA (if the DNA strand has T, the RNA has A.
– The process of making the mRNA copy of a portion of
the DNA is known as transcription.
– At the beginning of each gene (or group of genes) there
is a sequence of DNA recognized by enzymes that make
mRNA. This sequence is called the promoter.
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RNA has differences
1) DNA is double stranded, but RNA is
single stranded.
2) RNA contains ribose instead
of deoxyribose
3) RNA contains uracil instead
of thymine.
www.layevangelism.com/.../ deoxyribose.htm
http://www.rothamsted.bbsrc.ac.uk/notebook/courses/guide/images/ur
acil.gif
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Translation
• Translation is protein synthesis, the actual making
of proteins by the ribosomes using the information
in a mRNA molecule.
– The actual decoding is done by small RNAs called
transfer RNAs (tRNA) that read the code in the mRNA
and bring the correct amino acid to be used in the protein.
– Review the definition of “primary structure” of a protein
– The genetic code is read as 3 bases in a row. Each
combination of 3 bases indicates a particular amino acid
– Because the DNA of bacteria is in the cytoplasm, not
within a nucleus, mRNA is used even before it’s done.
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Bacterial ribosomes
• Ribosomes have 2 subunits, a large and a small
• Large subunit: 50 S
– 33 polypeptides, 5S RNA, 23 S RNA
• Small subunit: 30 S
– 21 polypeptides, 16S RNA
• Ribosome structure and differences between prokaryotes
and eukaryotes are important.
– Prokaryotic ribosomes are 70S; eukaryotic are 80S
– Differences are the basis for success of many antibiotics
– S is Svedberg unit, how fast a particle travels during
centrifugation. Affected by both mass and shape.
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The Genetic Code-2
http://www.biology.arizona.edu/molecular_bio/problem_sets/nucleic_acids/graphics/gencode.gif
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tRNA: the decoder
a.a. attaches here
anticodon
http://www.designeduniverse.com/articles/Nobel_Prize/trna.jpg
Simultaneous transcription and translation
•No processing, no nucleus;
mRNA already where the
ribosomes are, so they get
started quickly.
http://opbs.okstate.edu/~petracek/Chapter%2027%20Figures/Fig%2027-30.GIF
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Plasmids
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• Plasmids: small, circular,
independently replicating pieces of
DNA with useful, not essential info
• Types of plasmids
– Fertility,
– resistance,
– catabolic,
– bacteriocin,
– virulence,
– tumor-inducing, and
– cryptic
http://www.estrellamountain.edu/faculty/farabee/biobk/14_1.jpg
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About plasmids-1
Fertility plasmid: genes to make
a sex pilus; replicates, and a copy
is passed to another cell.
Resistance plasmid: genes that
make the cell resistant to
antibiotics, heavy metals.
Catabolic plasmid: example, tol
plasmid with genes for breaking
down and using toluene, an
organic solvent.
www.science.siu.edu/.../ micr302/transfer.html
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About plasmids-2
• Bacteriocin plasmid: codes for bacteriocins, proteins that
kill related bacteria.
• Virulence plasmid: has genes needed for the bacterium to
infect the host.
• Tumor-inducing plasmid: The Ti plasmid found in
Agrobacterium tumefaciens. Codes for plant growth
hormones. When the bacterium infects the plant cell, the
plasmid is passed to the plant cell and the genes are
expressed, causing local overgrowth of plant tissue = gall.
Very useful plasmid for cloning genes into plants.
• Cryptic: who knows?
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Gene transfer
• Ways that bacteria can acquire new genetic info
– Transformation
• Taking up of “naked DNA” from solution
– Transduction
• Transfer of DNA one to cell to another by a virus
– Conjugation
• “Mating”: transfer of DNA from one bacterium to
another by direct contact.
Conjugation: bacterial sex
• If sex is the exchange of genetic
material, this is as close as bacteria
get. Conjugation is widespread and
does NOT require bacteria to be
closely related.
• Bacteria attach by means of a sex
pilus, hold each other close, and
DNA is transferred.
• Plasmids other than F plasmids, such
as resistance plasmids, can also be
exchanged, leading to antibioticresistant bacteria.
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DNA of a cell can be altered
• Mutations
– Any change in the DNA that can be passed on
– Copying mistakes, radiation, chemicals can be causes
– Most mutations harmful, but sometimes can help
• New DNA becoming permanent
– DNA from transformation, transduction, or conjugation
becomes permanent part of cell’s DNA.
– Some viruses can insert their DNA into the cell’s DNA
and hang out indefinitely
– These provide new genetic information.