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http://www.nsf.gov/news/overviews/biology/interact02.jsp
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From DNA to RNA to Protein
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DNA = Deoxyribonucleic acid
RNA = Ribonucleic acid
o Three forms: Messenger RNA, Ribosomal RNA, Transfer RNA
Type of nucleic acid—a long chain, each link of which is a main building block
called a NUCLEOTIDE
Each nucleotide is made up of 3 repeating sub-units: a) a PHOSPHATE group;
b) a PENTOSE sugar (ribose in RNA, or deoxyribose in DNA); and c) one of 5
possible nitrogenous BASES (adenine, guanine, cytosine, or thymine in DNA;
adenine, guanine, cytosine, or uracil in RNA).
1 DNA nucleotide looks like:
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1 RNA nucleotide looks like:
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Phosphate
Group
Ribose Sugar
4 types:
Nitrogenous
Base
Adenine (A) = Purine
Guanine
(G) = Purine
Cytosine (C) = Pyrimidine
Uracil (U) = Pyrimidine
replaces T in RNA
Practice
Match the bases to the correct group (complementary pairs)
Pyrimidines
Purines
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Practice
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Strand 1
Strand 2
P
S
P
A
T
P
S
S
P
C
G
S
Chargaff’s Rule
 In DNA, A = T; C = G
DNA is complementary
 bases on one strand match up
with the bases on the other
strand
(A=T and G=C)
 Example: Strand 1 - ATG GGC
Strand 2 - TAC CCG
Codon: Group of 3 bases
Phosphates + sugars
on the outside
Bases on the inside (Bases fit
like puzzle pieces)
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Shape is a double helix
o Double helix: 2 spirals wound around each other
Genes:
o
o
o
o
stretch of DNA that codes for a trait
Genetic code is a triplet code—consisting of 3 bases
The code is the order of the bases (letters)
Genes are hundreds or thousands of bases long
The genetic code is a sequence of DNA nucleotides in the
nucleus of a cell.
o Genes specify enzymes
o Genes specify polypeptide bonds
Eye color gene
Dimples gene
Hair color gene
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Replication
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Process by which DNA copies itself
The primary purpose of replication is to provide identical sets of instructions
(sets of identical DNA) to the two new cells when one cell divides.
In a cell, new nucleotides are added at a rate of about 50 per second, involving
more than a dozen enzymes.
Replication begins when the DNA molecule "unzips".
Happens in s phase of interphase
Semiconservative replication: Each new piece of DNA is made up of 1 old
strand and 1 new strand
Original DNA
DNA Unzips
Each original Strand
grows a new strand
**A mistake in DNA Replication is called a Mutation.
DNA never ever leaves the nucleus
 DNA is the master copy of the directions a cell needs to live so it
needs to be protected
DNA in
the
nucleus is
safe
DNA in the
cytoplasm can
be destroyed
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DNA Replication Kit
1. Unzip DNA Molecule
2. Move the DNA-nucleotides from the nucleotide pool (already cut out) into positions so that
their base ends fit with the exposed base-ends of each of the original, unzipped
3. DNA strands. In a cell, this typically starts at one end of a strand and works toward the
other. The other strand builds in the opposite direction.
4. First bring an "A" nucleotide which fits the upper left hand "T" nucleotide, then move another
"A" nucleotide to fit the lower right hand "T" nucleotide.
5. Continue adding the nucleotides which fit as you go (moving down the left hand strand, and up
the right hand strand) until both halves of the ladder have been matched with new nucleotides.
(Not all of the nucleotides will be used up. They remain as part of the nucleotide "pool" for the
next replication episode).
Notice the pattern? What always matches (fits) with T (thymine)?
What always matches with C (cytosine)?
What always matches with A (adenine)?
What always matches with G (guanine)?
How many DNA molecules did we start with?
How many DNA molecules do we have now?
In terms of their respective sequences of base pairs, they are ____________ (identical,
similar, or different).
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RNA
RNA is a copy of DNA that goes out into the cytoplasm to tell the cell what to do in order to
stay alive
The big differences between DNA and RNA:
RNA is a single strand, not double sided
RNAs have the nitrogen base uracil in place of thymine
The five carbon sugar in the nucleotides is ribose, not deoxyribose.
DNA
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How many
strands?
Nucleotide
subunit
Bases
Phosphate
Group
Deoxyribose
Sugar
Deoxyribose sugar
Thymine (T)
T–A
Adenine (A)
Guanine (G)
G–C
Cytosine (C)
RNA
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Nitrogen
Base
Phosphate
Group
Ribose
Sugar
Nitrogen
Base
ribose sugar
Uracil (U)
U– A
Adenine (A)
Guanine (G)
G–C
Cytosine (C)
Protein Synthesis
Think of protein synthesis as a construction process, in which the finished product is a
particular protein (perhaps an enzyme) that was assembled according to the directions from the
"Master Plan" (DNA) in the nucleus.
Steps in Protein Synthesis Process
Transcription
Translation
Initiation
Elongation
Termination
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Transcription
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Protein synthesis actually occurs in ribosomes found in the cytoplasm and on rough
endoplasmic reticulum.
The type of RNA called mRNA (messenger RNA) sends a message from DNA to the
cytoplasm
DNA safe in the nucleus
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Uses mRNA
To send a message to
the cytoplasm
Transcription process
o The DNA unzips, allowing the mRNA to move in and transcribe (copy) the genetic
information.
o mRNA matches up bases to one side of gene in DNA
 If the code of DNA looks like this : G-G-C-A-T-T, then the mRNA would look
like this C-C-G-U-A-A (remember that uracil replaces thymine)
o mRNA detaches from the DNA
o mRNA moves out of the nucleus and into the cytoplasm towards the ribosomes
Practice:
1. Divide the correct (top or bottom) DNA strand into groups of 3.
2. Find the start codon (in DNA= TAC, in RNA= AUG)
3. Find the stop codon (1 of 3: UAA (ATT), UGA (ACT), UAG (ATC))
4. Change the entire gene from DNA to RNA.
5. Decode the groups of 3 RNA nucleotides using your mRNA codon chart.
Use these top strands of DNA and convert them into protein sequences.
1.
GCTTCCTACGCTGGAACCGCGCGATTCATCGCT
DNA base sequence:________________________________________________
mRNA base sequence:_______________________________________________
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DNA
mRN
A
mRNA
Cytoplasm
of cell
Nucleus
Transcription
happens in the
nucleus. An
RNA copy of a
gene is made.
Then the mRNA
that has been
made moves out
of the nucleus
into the
cytoplasm
Once in the
cytoplasm, the
mRNA is used to
make a protein
How does mRNA tell the cell what to do?
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mRNA is a message that codes for a protein
Proteins are made in the cytoplasm and then work to keep the cell alive
Translation (protein synthesis): process of making a protein
Proteins are made up of amino acids (small building blocks)
There are 20 different types of amino acids
TRANSLATION
 Takes place inside the cytoplasm
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Process of Translation
mRNA moves out of the nucleus
and into the cytoplasm
Nucleus
Cytoplasm
Initiation Step
A small ribosomal subunit attaches to the mRNA
near the start codon (AUG). Then a large ribosomal
subunit joins the smaller unit.
Ribosomes
Transfer RNA (tRNA) decodes the mRNA and
brings the amino acids to build up the protein.
tRNA
Amino Acids
Amino
acid
Elongation Step
Anticodon (3 bases on tRNA)
matches up to codons on mRNA
at the P site.
Polypeptide synthesis occurs.
Once next tRNA is in place, the
peptide is transferred to the new
tRNA and translocation occurs:
The tRNA moves forward and the
peptide-bearing tRNA moves into
the P site. Process repeats to
create a chain.
Termination Step
The ribosome comes to a stop codon on the
mRNA. Protein (chain of amino acids)
detaches from ribosome and goes off to
work in the cell.
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Genetic Code
 Code that matches codons in mRNA to amino acids on tRNAs
Transcription
DNA
Directions to make
proteins are safely
stored in the nucleus
Translation
RNA
Carries the
directions to
the cytoplasm
Protein
Works to keep
the cell alive
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THE PROCESS OF PROTEIN SYNTHESIS: Practice
Construction metaphor
Think of protein synthesis as a construction process, in which the finished product is a
particular protein (perhaps an enzyme), and it was assembled according to the directions from
the "Master Plans" (DNA) in the nucleus.
1. FIRST, blueprint copies of the building plans must be made from the Master Plans (DNA) in
the nucleus, and sent out to the construction site (ribosomes in the cytoplasm):
a. Unzip the DNA, separating the two DNA strands. Use only the right strand for the
next step.
b. Move the blue mRNA nucleotides, one at a time, to positions where their base-ends fit
the exposed DNA base-ends, starting at one end of the DNA and working toward the
other end: A to T, U to A, etc. There will be some unused nucleotides left over in the
"nucleotide pool"; that's ok.
c. The chain of mRNA nucleotides (blue) would now be attached to each other, in a
sequence which matches (in a complementary way) the original DNA sequence. Move
the chain away from the DNA, "through" the "nuclear membrane", and over onto the
ribosome surface, with the base ends exposed upward.
d. The mRNA serves as a "blueprint" copy of the DNA message (gene), and carries that
message out of the nucleus and into the cytoplasm, where ribosomes help to assemble
a chain of amino acids into a sequence dictated by that message.
2. NEXT, the Construction Supervisor (ribosome) reads the blueprints (mRNA) for the building
(protein), and directs the assembly of all the building parts (amino acids) into their proper
places to make the finished building (protein). (In this simplified version, think of the
ribosome as the Supervisors' blueprint table, making it easier to read the blueprints).
Yellow "specialty" trucks (tRNA) pick up their appropriate loads of concrete, bricks, lumber,
glass, plumbing, etc.(green amino acids), and bring them only to specific locations at the
unloading dock, according the supervisors' directions (sequence of nucleotide shapes in the
mRNA "unloading dock"), so the 3-nucleotide sequence in the "bumper" of each tRNA truck
must fit a 3-nucleotide sequence in the mRNA "unloading dock".
a. Fit each amino acid (green) into its matching tRNA (yellow)
b. Move the tRNA (with its amino acid load) which fits the first 3-nucleotide sequence in
the mRNA ("UUU" at the left end), and position it so its nucleotide shapes are
touching the mRNA nucleotide shapes.
c. Move the next tRNA (with its load, too) which fits the NEXT 3-nucleotide sequence,
and position it so that their matching nucleotide base ends touch, too.
d. Finally, move the third tRNA and its amino acid load, and fit it into the last 3nucleotide sequence of the mRNA.
e. The three amino acids should be touching "head-to-tail" in such a way that they could
be glued together.
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f. Move the three amino acids away as a "polypeptide unit", representing a much
reduced version of the final protein product. (The yellow tRNA molecules would move
away and pick up new loads of amino acids, ready for the next assembly).
f. If you have done this properly, the first letter of the name for each amino acid
assembled here should spell out a simple 3-letter word.
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Mutation
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a change in the DNA sequence
It’s a mistake that’s made during replication or transcription
Important because provide new genetic variations required for evolution.
Involve single nucleotide in the DNA or large scale changes in chromosome strucutre
harmful: diseases or deformities
helpful: organism is better able to survive
neutral: organism is unaffected
if a mutation occurs in a sperm or egg cell, that mutation is passed onto offspring
if a mutation occurs in a body cell, that mutation affects only the organism and is not
passed onto offspring
Types of mutations
1. Point mutations: Bases are mismatched (change in single nucleotide)
 Deletions: a nucleotide is left out when the DNA is duplicated
 Substitutions: involve an addition of a nucleotide between two others
 Insertions: one nucleotide is mistakenly replaced by another, T for A for example
 Deletions & Insertions often called frame-shift mutations
 Harmful when: a mistake in DNA is carried into mRNA and results in the wrong
amino acid
Correct DNA
Correct mRNA
GAG
CTC
CUC
Point mutation in DNA
GCG
CTC
Correct amino acid
Mutated mRNA
CGC
Leucine
Wrong amino acid
Arginine
A should pair with T, but instead C is mismatched to T
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Not harmful when a mistake in DNA is carried into mRNA, but still results in the
correct amino acid
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2. Frameshift mutations: bases are inserted or deleted
 Are usually harmful because a mistake in DNA is carried into mRNA and results in
many wrong amino acids
Correct DNA:
ATA
TAT
CCG
GGC
TGA
ACT
Correct mRNA:
UAU
GGC
ACU
Glycine
Threonine
Correct amino acids: Tyrosine
Extra inserted base shifts how we read the codons (3 bases),
which changes the amino acids
Frameshift mutation
in DNA:
ATG
TAC
ACC
TGG
GTG
CAC
A
T
Mutated mRNA:
UAC
UGG
CAC
U
Wrong amino acids: Tyrosine Tryptophan Histadine
3. Chromosomal mutations
 chromosomes break or are lost during mitosis or meiosis
 broken chromosomes may rejoin incorrectly
 almost always lethal when it occurs in a zygote
4. Causes of mutations
 mutagens: anything that causes a change in DNA
 examples: X rays, UV light, nuclear radiation, asbestos,
cigarette smoke
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5. Examples
 Sickle Cell= Although several hundred HBB gene variants are known, sickle cell anemia
is most commonly caused by the hemoglobin variant Hb S. In this variant, the
hydrophobic amino acid valine takes the place of hydrophilic glutamic acid at the sixth
amino acid position of the HBB polypeptide chain. This substitution creates a
hydrophobic spot on the outside of the protein structure that sticks to the hydrophobic
region of an adjacent hemoglobin molecule's beta chain. This clumping together
(polymerization) of Hb S molecules into rigid fibers causes the "sickling" of red blood
cells.
 Huntington’s Disease=The HD gene (or IT15 gene), located on chromosome
number four, interferes with the manufacture of a particular protein known as
huntingtin. The protein huntingtin is comprised of amino acids strung together. One part
of the chain consists of the amino acid glutamine. In people without HD, this section
ranges between 12 and 34 glutamines in length, while a person with HD has a section of
more than 35 glutamines. (Abnormally long chains of glutamine are also associated with a
number of other neurological conditions.) NOT located on sex chromosomes.
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Cystic Fibrosis—malfunctioning CI channel protein in plasma membrane. About 70%
of mutations observed in CF patients result from deletion of three base pairs in CFTR's
nucleotide sequence. This deletion causes loss of the amino acid phenylalanine located at
position 508 in the protein. With normal CFTR, once the protein is synthesized, it is
transported to the endoplasmic reticulum (ER) and Golgi apparatus for additional
processing before being integrated into the cell membrane. When deltaF508 CFTR
reaches the ER, the quality-control mechanism of this cellular component recognizes that
the protein is folded incorrectly and marks the defective protein for degradation. As a
result, deltaF508 never leaves the ER. People who are homozygous for deltaF508
deletion tend to have the most severe symptoms of cystic fibrosis due to critical loss of
chloride ion transport [4]. This upsets the sodium and chloride ion balance needed to
maintain the normal, thin mucus layer that is easily removed by cilia lining the lungs and
other organs. The sodium and chloride ion imbalance creates a thick, sticky mucus layer
that cannot be removed by cilia and traps bacteria, resulting in chronic infections.