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Genetics
AP Biology
The Discovery of DNA Structure
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Rosalind Franklin: x-ray diffraction photographs
of DNA
Watson & Crick built model based on x-ray
diffraction photos
DNA Structure
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Deoxyribose sugar backbone, alternating with
phosphate groups
Nitrogenous bases held together by hydrogen
bonds: Adenine, Thymine, Cytosine and
Guanine
Arranged in a double helix
Chargaff’s base pairing rules: A-T; G-C
Anti-parallel nature of DNA
5’ 3’
3’ 5’
DNA Replication
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Makes an exact copy
Double helix separates at Origins of Replication
Each strand serves as the template for a new
strand
Semi-conservative replication
DNA polymerase builds new strand 5’ to 3’ (adds
nucleotides onto 3’ end (OH)
DNA replication occurs in a 5’ to 3’ direction: Leading strand
Other side is constructed in 5’ to 3’ fragments called Okasaki
fragments : Lagging strand
From Gene to Protein:
DNA contains information of amino acid sequence in
proteins
Transcription
Synthesis of RNA using the DNA as a template:
contains gene’s protein building instructions
RNA Structure:
Contains ribose sugar, Base thymine is replaced
with uracil, single stranded
Types of RNA:
Ribosomal RNA (rRNA) – manufactured in nucleolus
Transfer RNA (tRNA)
Messenger RNA (mRNA)
Single stranded RNA
molecules fold into
secondary structures
Uracil
Characteristic
secondary structure
of tRNA
RNA Editing
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RNA molecule made by transcription –
premRNA
Contains stretches of bases – “introns” that do
not code for proteins
Introns are removed and coding sequences
“exons” are spliced together
Role of RNA Splicing
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“One gene, One polypeptide” hypothesis: a gene
of DNA codes for one protein.
Several proteins can be manufactured from a
single gene due to alternative splicing
Translation
tRNAs bring amino acids to the ribosome to assemble
proteins
Translation – the specifics
Amino acids joined to tRNAs by a specific
aminoacyl-tRNA synthetase
 Ribosomal Structure:
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P site – holds tRNA carrying amino acid the growing
polypetide chain
 A site – holds the tRNA carrying the next amino
acid to be added to the chain
 E site – discharged tRNAs leave through this site
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Translation – the specifics continued
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Initiation: mRNA, initiator tRNA, ribosomal subunits
assemble. Initiator tRNA sits in P site and A site is
vacant
Elongation:
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Codon recognition: H bonding between tRNA and codon
in A site. Requires elongation factors & GTP
Peptide bond formation: an rRNA molecule catalyzes
formation of peptide bond growing chain from P site to the
new amino acid in the A site
Translocation: the tRNA in the A site (now has polypetide
chain) moves to P site. Blank tRNA in P site moved to E site.
Termination: stop codon – release factor binds to A site &
causes the addition of water – hydrolyzes tRNA from protein
chain.
DNA Mutations - Effects on
Translation
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Point mutations
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Insertions and Deletions
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Frameshift mutations
Base pair substitution
Missense – still codes for amino acid
 Nonsense – prematurely codes for a stop codon
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Control of Transcription in
Eukarytoic cells
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Transcription is most often the result of a chemical
signal
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Some genes are constitutively active – meaning they are always
turned on in a cell
Chemical signals often in the form of hormones –
either steroidal or peptide
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Peptide hormones cannot diffuse into the cell and bind
hormone receptors on the cell membrane
Steroid hormones are able to diffuse easily into the nucleus
where they bind steroid hormone receptors that function as
transcription factors (transcription factors “turn on/off” the
transcription of a gene
Peptide hormones
Binding of
peptide hormone
to cell membrane
receptor
Activation of cellular
signaling molecules
that carry signal to
the nucleus
Examples of peptide hormones: insulin,
adrenaline, vasopressin, etc.
Change in
transcription of a
gene
Steroid hormone signaling
Examples of steroid
hormones: estrogen,
testosterone,
glucocorticoids
Intronic RNA
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Once thought to be completely nonfunctional
Within the last 5-7 years: microRNAs
Now found that microRNAs can bind to newly
transcribed mRNA and target them for
degradation
Another way that the cell controls what proteins
are made
Can be synthesized in
the laboratory and can
be used to shut down
production of a
protein – possible
therapeutic uses