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• Nucleic acids are made of nucleotides
similar to how proteins are made of amino
acids
• each nucleotide consists of 3 parts
– a 5 carbon sugar (deoxyribose or ribose)
– a phosphate group
– a nitrogenous base (adenine, thymine, cytosine,
guanine, and uracil)
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• Nucleotides are derived from Nucleosides
• each nucleoside consists of 3 parts
– a 5 carbon sugar (deoxyribose or ribose)
– a nitrogenous base (adenine, thymine, cytosine,
guanine, and uracil)
– three phosphate group
– The energy from releasing the two phosphate
groups to form the nucleotide is used for the
process of DNA/RNA synthesis
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Nucleotide AMP
Nitrogen
Base
(Adenine)
5’ end
Phosphate
Group
Sugar
3’ end
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Nucleoside NTP
Nitrogen
Base
5’ end (Adenine)
Sugar
Phosphate
Group
3’ end
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DNA structure
• 2 strands twisted into a helix
• sugar -phosphate backbone
• nitrogenous bases form steps in ladder
– constancy of base pairing
– A binds to T with 2 hydrogen bonds
– G binds to C with 3 hydrogen bonds
• antiparallel strands 3’to 5’ and 5’to 3’
• each strand provides a template for the exact
copying of a new strand
• order of bases constitutes the DNA code 5
Significance of DNA structure
1. Maintenance of code during reproduction.
Constancy of base pairing guarantees that
the code will be retained.
2. Providing variety. Order of bases
responsible for unique qualities of each
organism.
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Genetics – the study of heredity
1. transmission of biological traits from
parent to offspring
2. expression & variation of those traits
3. structure & function of genetic material
4. how this material changes
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Levels of genetic study
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Levels of structure & function of the
genome
• genome – sum total of genetic material of an organism
(chromosomes + mitochondria/chloroplasts and/or
plasmids)
– genome of cells – DNA
– genome of viruses – DNA or RNA
• chromosome – length of DNA containing genes
• gene-fundamental unit of heredity responsible for
a given trait
– site on the chromosome that provides information for
a certain cell function
– segment of DNA that contains the necessary code to
make a protein or RNA molecule
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Genomes vary in size
• smallest virus – 4-5 genes
• E. coli – single chromosome containing
4,288 genes; 1 mm; 1,000X longer than cell
• Human cell – 46 chromosomes containing
31,000 genes; 6 feet; 180,000X longer than
cell
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DNA replication is
semiconservative because each
chromosome ends up with one
new strand of DNA and one old
strand.
Semi-conservative replication of DNA
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DNA replication
• Begins at an origin of replication
• Helicase unwinds and unzips the DNA double
helix
• An RNA primer is synthesized
• DNA polymerase III adds nucleotides in a 5’ to 3’
direction
• Leading strand – synthesized continuously in 5’
to 3’ direction
• Lagging strand – synthesized 5’ to 3’ in short
segments; overall direction is 3’ to 5’
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Bacterial replicon
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Flow of genetic information
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• What are the products that genes encode?
– RNAs and proteins
• How are genes expressed?
– transcription and translation
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Gene expression
• Transcription – DNA is used to synthesize
RNA
– RNA polymerase is the enzyme responsible
• Translation –making a protein using the
information provided by messenger RNA
– occurs on ribosomes
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• Genotype - genes encoding all the potential
characteristics of an individual
• Phenotype -actual expressed genes of an
individual (its collection of proteins)
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DNA-protein relationship
1. Each triplet of nucleotides (codon) specifies
a particular amino acid.
2. A protein’s primary structure determines its
shape & function.
3. Proteins determine phenotype. Living things
are what their proteins make them.
4. DNA is mainly a blueprint that tells the cell
which kinds of proteins to make and how to
make them.
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DNA-protein relationship
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3 types of RNA
• messenger RNA (mRNA)
• transfer RNA (tRNA)
• ribosomal RNA (rRNA)
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DNA
Transcription
RNA polymerase
RNA
Translation
ribosomes
PROTEINS
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Transcription
1. RNA polymerase binds to promoter region
upstream of the gene
2. RNA polymerase adds nucleotides
complementary to the template strand of a
segment of DNA in the 5’ to 3’ direction
3. Uracil is placed as adenine’s complement
4. At termination, RNA polymerase recognizes
signals and releases the transcript
• 100-1,200 bases long
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Transcription
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Translation
• Ribosomes assemble on the 5’ end of a
mRNA transcript
• Ribosome scans the mRNA until it reaches
the start codon, usually AUG
• A tRNA molecule with the complementary
anticodon and methionine amino acid enters
the P site of the ribosome & binds to the
mRNA
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Translation
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Interpreting the DNA code
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Translation elongation
• A second tRNA with the complementary
anticodon fills the A site
• A peptide bond is formed
• The first tRNA is released and the ribosome slides
down to the next codon.
• Another tRNA fills the A site & a peptide bond is
formed.
• This process continues until a stop codon is
encountered.
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Translation termination
• Termination codons – UAA, UAG, and
UGA – are codons for which there is no
corresponding tRNA.
• When this codon is reached, the ribosome
falls off and the last tRNA is removed from
the polypeptide.
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Polyribosomal complex
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Eucaryotic transcription &
translation differs from procaryotic
1. Do not occur simultaneously. Transcription
occurs in the nucleus and translation occurs in
the cytoplasm.
2. Eucaryotic start codon is AUG, but it does not
use formyl-methionine.
3. Eucaryotic mRNA encodes a single protein,
unlike bacterial mRNA which encodes many.
4. Eucaryotic DNA contains introns – intervening
sequences of noncoding DNA- which have to be
spliced out of the final mRNA transcript.
5. A 5’ cap and polyA tail are added to eucaryotic
RNA. 36
Split gene of eucaryotes
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Regulation of protein synthesis &
metabolism
Operons
• a coordinated set of genes, all of which are
regulated as a single unit.
• 2 types
– inducible – operon is turned ON by substrate:
catabolic operons- enzymes needed to metabolize
a nutrient are produced when needed
– repressible – genes in a series are turned OFF by
the product synthesized; anabolic operon –
enzymes used to synthesize an amino acid stop
being produced when they are not needed
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Lactose operon: inducible operon
Made of 2 segments:
1. Control locus- composed of promoter and operator
2. Structural locus- made of 3 genes each coding for an
enzyme needed to catabolize lactose –
b-galactosidase – hydolyzes lactose
permease - brings lactose across cell membrane
b-galactosidase transacetylase – uncertain function
Regulator- gene that codes for repressor
Not part of the operon
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Lac operon
• Normally off
– In the absence of lactose the repressor binds
with the operator locus and blocks transcription
of downstream structural genes
• Lactose turns the operon on
– Binding of allolactose to the repressor protein
changes its shape and causes it to fall off the
operator. RNA polymerase can bind to the
promoter. Structural genes are transcribed.
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Lactose operon
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Tryptophan operon: repressible
• Normally on and will be turned off when
nutrient is no longer needed.
• When excess tryptophan is present, it binds
to the repressor and changes it. Then the
repressor binds to the operator and blocks
tryptophan synthesis.
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Repressible operon
(a) Operon On. A repressible operon
remains on when its nutrient products
(here tryptophan) are in great demand
by the cell because the repressor is
unable to bind to the operator at low
nutrient levels.
Enzymes
synthesize
tryptophan
Tryptophan
immediately used in
metabolism
(a) Operon Off. The operon is
Tryptophan
repressed when (1) tryptophan builds
accumulates
up and, serving as a corepressor,
activates the repressor. (2) The
repressor complex binds to the
operator and blocks RNA polymerase
from transcribing the genes for
Tryptophan
tryptophan biosynthesis
synthesis 44
inhibited
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www.ndsu.nodak.edu/.../ prokaryo/prokaryo3.htm
Antibiotics that affect gene
expression
• Rifamycin – binds to RNA polymerase
• Actinomycin D - binds to DNA & halts mRNA
chain elongation
• Erythromycin & spectinomycin – interfere with
attachment of mRNA to ribosomes
• Chloramphenicol, linomycin & tetracycline-bind
to ribosome and block elongation
• Streptomycin – inhibits peptide initiation &
elongation
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Mutations – changes in the DNA
• Point mutation – addition, deletion or
substitution of a few bases
• Missense mutation – causes change in a
single amino acid
• Nonsense mutation – changes a normal
codon into a stop codon
• Silent mutation – alters a base but does not
change the amino acid
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Excision repair
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Ames Test
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Types of intermicrobial exchange
conjugation requires the attachment of two
related species & formation of a
bridge that can transport DNA
transformation transfer of naked DNA
transduction DNA transfer mediated by bacterial
virus
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conjugation
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transformation
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Generalized transduction
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Specialized transduction
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Transposons –DNA segments that shift
from one part of the genome to another
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Applications of mechanisms of
genetic variation
• Whether it is conjugation, transduction or
transformation, scientists have used these or
modification of these methods for research.
• Scientists have also used transposons and chemicals
and radiation for inducing mutations in many different
organisms (incuding bacteria) to study the effect of
those mutations on those organisms.
• The field of molecular biology/genetic engineering and
biotechnology have been made possible by using these
mechanisms as tools.
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