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Molecular Genetics
DNA: The Genetic Material
MAIN IDEA: The discovery
that DNA is the genetic code
involved many experiments.
Rosalind Franklin
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Used X-ray crystallography to study
DNA (pattern made when x-rays
bombard them)
Franklin concluded: DNA is a double
helix
Watson and Crick
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Heard of Franklin’s work before
she published it
Used her X-ray diffraction pictures
and other mathematical data
Designed the 3-D model of DNA
DNA Structure
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Double helix (twisted ladder)
Outside: sugar (deoxyribose) and
phosphate
Rungs: nitrogenous bases
Adenine, guanine, cytosine,
and thymine
 A-T, G-C
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Do Now
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One strand of DNA has the following
sequence of nitrogen bases:
ATTCGTAGCTAGCTAAC
What is the sequence of nitrogenous
bases on the complementary strand of
DNA?
How did scientists depend on each
other to discover DNA?
Answer
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ATTCGTAGCTAGCTAAC
The complementary strand is:
TAAGCATCGATCGATTG
12.2: Replication of DNA
MAIN IDEA: DNA replicates
by making a strand that is
complementary to each
original strand.
DNA Replication
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Occurs during S phase of
mitosis/meiosis
DNA must make an exact duplicate
of itself
“mistakes” (changes in the genetic
code) = mutations
Semiconservative
Replication
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Process proposed by Watson and
Crick
Original strands of DNA separate,
serve as templates (patterns), and
produce new DNA with one old
strand and one new strand
Step 1: Unwinding
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DNA helix must first untwist and “unzip”
(H-bonds break between nitrogen bases)
Step 2: Base Pairing
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Add new, complementary nucleotides
to either side
Step 3: Joining
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DNA replication may start at many
different places on one
chromosome
These sections must then be
joined together when complete
Result of Replication
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2 identical DNA molecules
Each molecule: one old strand
and one complementary new
strand
Semi-conservative
12.3: DNA, RNA, and Protein
MAIN IDEA: DNA codes
for RNA, which guides
protein synthesis.
Protein Synthesis
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DNA codes for RNA, which codes
for building proteins
One gene directs the synthesis of
one protein
DNA and RNA
DNA
 Has two strands
 Has thymine, not
uracil
 Cannot leave nucleus
 Bigger than RNA
 Only 1 type
 Has sugar
deoxyribose
RNA
 Has one strand
 Has uracil, not
thymine
 Can leave nucleus
 Smaller than DNA
 3 types
 Has sugar ribose
BOTH DNA and RNA
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Both…
Are made of nucleotides
Have adenine, cytosine, and
guanine
Carry genetic code
Are nucleic acids
Types of RNA
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3 types:
 Messenger RNA (mRNA) – takes
DNA’s message from nucleus to
ribosome
 Ribosomal RNA (rRNA) – makes up
the ribosome and helps make protein
 Transfer RNA (tRNA) – brings amino
acids (protein parts) to ribosome
Transcription
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DNA cannot leave nucleus, but RNA
can
DNA is first transcribed (copied) into
mRNA in the nucleus
DNA unzips, and complementary
mRNA strand is made
RNA nucleotides attached according
to base-pairs
Transcription
Practice transcription
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DNA code: TTTAGGCATCCG
What’s the complementary RNA
code?
Translation
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mRNA leaves nucleus through nuclear
pores goes to cytoplasm
mRNA joins ribosomes, and is
translated into a protein
tRNA brings over the appropriate
amino acid
Each amino acid joins to make a chain
 protein!
Reading the Code
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Codon – 3-base code of DNA
Each codon = 1 amino acid of
protein chain
64 codons, but only 20 amino acids
Some codons are repetitive and
code for the same amino acids
Some start and end the protein
Practice
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mRNA code: AUGCGGAUUUGA
1.
Separate into codons
Use the chart to translate
Write the amino acids in the chain
2.
3.
Summary
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One gene codes for one protein
Transcription (in nucleus)– DNA
copied to mRNA
Translation (in cytoplasm)–
ribosomes translate mRNA, and
tRNA attaches amino acids to make
proteins
Checkpoint
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What are the 3 types of RNA and
what do they do?
How many proteins does one gene
make?
What is transcription?
What is translation?
Draw a diagram, concept map, etc to
explain how DNA and RNA work
together to make proteins.
Gene Regulation and
Mutations
Chapter 12.4
Gene Regulation
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Gene expression is regulated by the
cell; mutations can affect this
expression
Mutations
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Change in DNA sequence, either a
single base pair (point mutation) or a
large segment of DNA
Can cause alternate phenotypes,
diseases/disorders, non-functioning
proteins
Caused by mutagens
Point Mutations
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Substitution – one base pair
exchanged for a different one
Can cause missense – wrong
amino acid is used
Can cause nonsense – codes for
a stop codon and ends protein
synthesis early
Examples
NORMAL:
THE BIG FAT CAT ATE THE WET RAT
MISSENSE SUBSTITUTION:
THE BIZ FAT CAT ATE THE WET RAT
NONSENSE SUBSTITUTION:
THE BIG RAT
Point Mutations cont’d
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Deletion – one base pair
eliminated
Insertion – one base pair added
Both result in frameshift, which is
a change in the groups of 3 bases
making codons
Examples
NORMAL:
THE BIG FAT CAT ATE THE WET RAT
DELETION (FRAMESHIFT):
THB IGF ATC ATA TET HEW ETR AT
INSERTION (FRAMESHIFT):
THE BIG ZFA TCA TAT ETH EWE TRA
Large Section Mutations
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Larger sections of DNA can be:
 Deleted
 Inserted
 Inverted (made backwards)
 Translocated (moved to a different
section or chromosome)
 Duplicated (copied again)
 Tandem repeats (copied many
times)
Think…
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What would be the result of a
missense substitution mutation to
an intron?
What would most likely be the
result of a mutation in the last
base position of a codon?