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RNA & Protein Synthesis
Chapter 12-3
What is DNA used for?

DNA molecules are the coded instructions used by
cells to build proteins (Protein Synthesis)
 Proteins are the keys to everything about a cell
(looks, functions, growth, passing of genetic
information)
 Proteins are integral pieces of many cell
structures (ex. Cell membrane) and perform
many cell functions (regulating reactions,
transport, motion, protection, support,
communication)
 It is the presence of specific proteins that
determines how an organism develops & what
characteristics an organism will have
What is DNA used for?
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Proteins are polymers made out of monomers called
amino acids
 There are 20 different amino acids
 Polypeptide – a chain of amino acids
Each protein contains a combination of any or all of
the 20 different amino acids
The properties of proteins are determined by the
order in which the different amino acids are joined
In order to build a protein, cells must “know” which
amino acids to use & what order to put them in
A DNA gene indicates which amino acids to use, and
what order to put them in, to build a specific protein
What is DNA used for?
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DNA molecules contain many individual genes –
each gene is the instructions to build one
specific protein
DNA only holds the instructions, it does not
actually do the building of protein
RNA molecules carry out the process of making
proteins
Analogy - A dictionary is to words
as DNA is to proteins
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In order to spell a word, you
must know which of the 26
letters to use and what order to
put them in
 A dictionary holds this
information
In order to build a protein, a cell
must know which of the 20
amino acids to use and what
order to put them in
 DNA holds this information
The Structure of RNA
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RNA (ribonucleic acid) is a long chain of
nucleotides
There are 3 main differences between DNA
and RNA:
1. The sugar in RNA nucleotides is ribose
2. RNA molecules are single-stranded
3. RNA contains the base uracil (U), but not
thymine (T)
Types of RNA

There are 3 main types of RNA:
1. Messenger (mRNA) – an RNA copy of a
single gene that “carries” instructions from
DNA to ribosome
2. Ribosomal (rRNA) – makes up ribosome
and assembles amino acids making a protein
chain
3. Transfer (tRNA) – transfers amino acids
from the cytoplasm to the ribosome for
assembly
Protein Synthesis – How do cells
make protein?

Protein synthesis is a 2-step process:
1. Transcription – an mRNA copy of a
specific gene is made
 mRNA is a copy of a single gene, NOT an
entire DNA molecule
2. Translation – information from mRNA is
used to build a protein
Building
a protein is like building a house…
 You need a master set of instructions for the
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house in the construction office – this is like the
DNA in the nucleus
You need a work site to build the house – this is
like the ribosome
You need a copy of the master plan (blueprint)
that can travel to the work site – this is like the
mRNA
You need workers to deliver the materials to
build the house to the job site – this is like the
tRNA
You need builders to put all the materials
together and make the house – this is like the
rRNA
Building a protein is like building a house…
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The blueprint is copied and the copy is brought
to the work site. Workers bring the correct
materials to the work site and set them up
according to the information in the blueprint.
Builders put everything together building the
house.
An mRNA copy is made of DNA and the mRNA
travels to the ribosome. The tRNA brings the
correct amino acids to the ribosome and lines
them up according to the information in the
mRNA. The rRNA puts the amino acids together
forming the protein.
The Genetic Code – How does an
mRNA molecule “tell” a cell what amino
acids to use and where to put them?
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The mRNA instructions are called the genetic
code
The genetic code is read 3 bases at a time
Each 3-base sequence is equivalent to 1 amino
acid
Codon – A 3-base sequence of mRNA that
specifies a single amino acid in the protein chain
A Triplet Code
The Genetic Code - Example
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mRNA sequence UCGCACGGU has 3 codons
and is read like this:
 UCG – CAC – GGU
Each codon stands for a specific amino acid
 UCG = serine
 CAC = histidine
 GGU = glycine
The polypeptide created from that mRNA
sequence would look like this:
 serine – histidine – glycine
Transcription – A closer look
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In eukaryotic cells, transcription takes place in the
nucleus, in two steps:
1. RNA polymerase “unzips” DNA at the correct
location
2. RNA polymerase uses one strand of DNA to
form a complementary mRNA molecule
 DNA:
AGCGTGCCA
 mRNA: U C G C A C G G U
mRNA can travel to a ribosome where the protein
can be assembled
Transcription is like finding the recipe you want in
a cookbook and copying it down onto a single
sheet of paper – the copied recipe can then be
brought to the kitchen
Transcription – Animated Movie
Transcription – 3D Animation
Translation – A closer look
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Translation takes place on ribosomes and involves
mRNA, rRNA, and tRNA molecules
tRNA is responsible for bringing the correct amino
acids to the ribosome
 tRNA molecules have amino acids bonded at one
end, and three unpaired bases (anticodons) at
the other
 tRNA anticodons match up with complementary
mRNA codons
 Different tRNA molecules carry different amino
acids
Translation – 5 Steps
1. mRNA attaches to ribosome – translation begins at
AUG, the “start” codon
2. Two complementary tRNA molecules carrying
amino acids bond with the first two mRNA codons
3. The ribosome bonds the amino acids together, and
breaks the bonds between the tRNA molecules an
amino acids
4. The first tRNA is released and the ribosome moves
down the mRNA to the 3rd codon, where another
complementary tRNA brings in a 3rd amino acid
which is bonded to the first two
5. The ribosome continues down the mRNA in this
way until a “stop” codon is reached and the
finished protein is released
Translation – Animated Movie
Translation – 3D Animation
Transcription – DNA “unzips”
Transcription – mRNA is made
Transcription – mRNA is made
Transcription – mRNA is made
Transcription – mRNA is made
Transcription – mRNA is made
Transcription – mRNA is made
Transcription – mRNA is made
Transcription – mRNA is made
Transcription – mRNA is made
Transcription – mRNA is made
Transcription – 3D Animation
Translation – ribosome attaches
to mRNA molecule
Translation – translation begins at
AUG, the “start” codon
Translation – tRNA anticodons bond
with complementary mRNA codons
Translation - 2 complementary tRNA molecules
carrying amino acids bond with the first 2 mRNA
codons
Translation – the ribosome bonds the 2 amino
acids together, breaks the bond between the first
tRNA and its amino acid, and brings in a 3rd tRNA
Translation – the first tRNA is released, ribosome
bonds the 2nd and 3rd amino acids together and
breaks bond between 2nd tRNA and its amino acid
Translation – ribosome moves
down the mRNA one codon
Translation – a 4th tRNA bonds with the 4th
mRNA codon, bringing in another amino acid to
add to the growing polypeptide chain
Translation – the 2nd tRNA is released, the
3rd and 4th amino acids are bonded together
Translation – ribosome moves
down the mRNA one codon
Translation – ribosome continues down the
mRNA in this way until a “stop” codon is reached
and the finished protein is released
Translation – 3D Animation
Why is it important to eat protein?
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Your body needs 20 different amino acids
to make all the necessary proteins
Your body is only able to produce 12 of
these amino acids on its own
The other 8 amino acids come from foods
you eat that contain protein (meat, nuts,
dairy products, beans, etc.) – These are
called essential amino acids because you
cannot survive without eating them
The bottom line about DNA…
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No DNA = No Protein = No Cells = No Life
Reproduction is all about passing DNA
from one cell to another and from one
generation of organisms to the next
How To
Transcribe
DNA:
mRNA:
A
T
C
G
U
A
G
C
How To Transcribe – Example 1
 DNA:
 mRNA:
TAC TCA CGC ATC
AUG AGU GCG UAG
How to Translate
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Every 3 mRNA bases
together stand for 1
amino acid in the
protein chain – these
pieces of mRNA are
called codons
Use the amino acid
chart to figure out
which amino acid
each codon stands for
How to Translate – Example 1
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mRNA: AUG AGU GCG UAG
AUG = Met (start codon)
AGU = Ser
GCG = Ala
UAG = STOP
Protein: Met-Ser-Ala-STOP
Transcribe and Translate –Example 2
DNA : TAC TGA ATA CCT CAA GGA GGC ACC TGG ACT
mRNA:
AUG ACU UAU GGA GUU CCU CCG UGG ACC UGA
Protein: Met-Thr-Tyr-Gly-Val-Pro-Pro-Trp-Thr-Stop
Transcribe and Translate –Example 3
DNA : TAC AAA GGA CGA GTA GTT TAA GCA AGA ATT
mRNA:
AUG UUU CCU GCU CAU CAA AUU CGU UCU UAA
Protein: Met-Phe-Pro-Ala-His-Glu-Ile-Arg-Ser-Stop