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Chapter 12
DNA and RNA
What is DNA again?
Deoxyribonucleic acid
 Long double-stranded molecule of nucleotides
 Stores genetic code that is transferred to future
generations and cells
 Codes for synthesis of proteins
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What is a nucleotide
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A nucleotide has 3 parts:
◦ A sugar (deoxyribose for DNA)
◦ A phosphate group
◦ A nitrogen base
Nitrogen base pairing
The order of nitrogen bases are what
determines your genetic code
 The order of the bases determines the proteins
that are made
 4 different bases exist:
◦ Purines: Guanine, Adenine
Grade A Pure
◦ Pyrimidines: Cytosine, Thymine
 Pairing is also called Chargaff’s Rule
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Chargaff’s Rule
Result: A=T, G=C
But what is the structure?
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In the early 1950s Rosalind Franklin used X-ray
diffraction took a “picture” of DNA
◦ Difficult to tell what the structure is from the
image
◦ Strands twisted around one another
But what is the structure?
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James Watson and Francis Crick used Franklin’s
images and their models to determine the
correct structure: double helix
◦ Two strands wound around one another
◦ “Twisted ladder”
◦ Later discovered that hydrogen bonds hold
the two sides of the ladder together
◦ Can only from between adenine and thymine,
or guanine and cytosine (A=T, G=C)
◦ The base pairing explained Chargaff’s Rule
DNA Structure
DNA Structure
DNA is very long; E. coli’s contains 4,639,221
base pairs (length =1.6 mm)
◦ Must fit in an organism 1/1000 its length
◦ Must be packed very tightly to fit
 Human cell contains almost 100 times the base
pairs
◦ Chromatin consists of DNA packed around
proteins called histones
◦ These compact together during mitosis to
create our visible chromosomes
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DNA Structure
DNA replication
Remember that before a cell divides it has to
copy its DNA
◦ If we separate each strand, we can use it to
make a copy of the other
◦ If we have an A, we match it with a T; if a G,
we match it with a C (called complementary)
 In prokaryotes, replication begins at one point
and proceeds typically in both directions
 In contrast, eukaryotes begin replication in
hundreds of locations (due to its longer length)
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DNA Replication
DNA replication
Two replication forks formed, each strand has a
new complementary strand added
◦ TACGTT  ATGCAA
◦ Results in two identical DNA molecules
◦ One strand is new, the other old
 DNA was unzipped (breaking of hydrogen
bonds)
◦ DNA polymerase (enzyme) joins individual
nucleotides to produce a DNA molecule
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What is RNA?
Ribonucleic acid
 Intermediate molecule between DNA and
proteins
 Differs from DNA in 3 ways:
◦ Single stranded (vs. double)
◦ Ribose instead of deoxyribose
◦ Utilizes uracil instead of thymine (A-U, G-C)
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Type of RNA

There are 3 types of RNA:
◦ messenger RNA (mRNA)- carries copies of
the protein instructions
◦ ribosomal RNA (rRNA)- makes up the
ribosome, along with several proteins
◦ transfer RNA (tRNA)- take the appropriate
(base pairing anti-codon to codon) amino acid
to the ribosome
Types of RNA
Protein synthesis: how we make
proteins?
Two processes or steps:
◦ Transcription- mRNA is made from the DNA
and travels to the cytoplasm to find a
ribosome
◦ Translation- tRNA brings to the ribosome the
amino acids to build the primary structure of
a protein and the result is a free polypeptide
that will then fold up into the shape of the
protein
 Remember: amino acids are held together by
peptide bonds
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Transcription
Transcription
RNA polymerase binds to DNA and separates
the strands
 RNA polymerase uses one strand to serve as a
template to create a strand of RNA
 RNA polymerase only binds to regions of DNA
called promoters (landing pad)
 The RNA is next edited, removing the introns,
while leaving the exons (exons=expressed)

RNA Splicing
Translation

Proteins are made by joining amino acids (1 of
20) into long chains (polypeptides)
◦ How do 4 letters lead to so many different
amino acids?
◦ Read 3 letters at a time, called a codon
◦ UCGCACGGU
◦ UCG-CAC-GCU
◦ Serine-Histidine-Glycine
◦ Some codons code for the same amino acid
◦ AUG is the start codon
Translation
Codon – 3 nucleotides that code for an amino
acid. Found on the mRNA.
 Anticodon - Fit the codon. Found on the
tRNA

DNA: master plan; never brought to work site
 mRNA: blue prints made from master plan
 Ribosomes: building site
 tRNA: trucks unloading materials

Codons
Codons
Translation
Transcription and Translation
Mutations
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Mutations – changes made in the genetic
material; can occur during copying
Point mutation – effect only one nucleotide.
Frameshift mutation – an insertion or
deletion that effects the remaining string of
nucleotides.
Chromosomal mutations – changes in the
number or structure of chromosomes.
Polyploidy – when an organism has extra sets
of chromosomes.
Mutations
THE FAT CAT ATE THE RAT
◦ Delete H (Frameshift / deletion)
 TEF ATC ATA TET HER AT
◦ Change H to L (Point mutation)
 TLE FAT CAT ATE THE RAT
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Mutations
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DNA: TAC GCA TGG AAT
RNA: AUG CGU ACC UUA
AA:
Met Arg Thr Leu
◦ Substitution
DNA: TAC GTA TGG AAT
RNA: AUG CAU ACC UUA
AA:
Met His Thr Leu
◦ Insertion
DNA: TAT CGC ATG GAA T
RNA: AUA GCG UAC CUU A
AA:
Ile
Ala Tyr
Leu