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12–3 RNA and Protein Synthesis
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What is RNA?
Ribonucleic acid.. It too like DNA is a
nucleic acid!
 RNA contains codes for making
proteins (body tissues such as
muscle, hair, enzymes, etc..)
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Let’s compare RNA to
DNA
DNA
RNA
Deoxyribose
Sugar
Ribose
A, T, G, C
A, U, G, C
Double
Nitrogenous
Bases
Strands
Nucleus
Location
Anywhere in
cell
Single
3 Types of RNA
 Messenger mRNA
 Ribosomal rRNA
 Transfer tRNA
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Messenger RNA (mRNA) Makes a copy of DNA. Every 3
letters on an mRNA is a codon (we will get there, don’t worry
)
ribosomal RNA (rRNA) are what ribosomes are made of.
Types of RNA
transfer RNA (tRNA) Carry or “transfer” amino acids to the
ribosome.
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The Genetic Code
 The genetic code is the “language” of
mRNA instructions.
 The code is written using four
“letters” (the bases: A, U, C, and G).
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The Genetic Code
A codon consists of three consecutive nucleotides on
mRNA that specify a particular amino acid.
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For Example…
 Consider the following RNA sequence:
UCGCACGGU
This sequence would be read 3 bases at a time as:
UCG-CAC-GGU
The codons represent the different amino acids:
Serine-Histidine-Glycine
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Because there are 4
different bases….
 There are 64 possible 3-base codons (4 x 4 x 4 = 64).
 Look at Figure 12-17 in your book on pg. 303.
 This shows all 64 possible codons of the genetic code.
 See mRNA Codon/Amino Acid Chart Handout!!!
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Looking at Figure 1217….
 As you can see, some amino acids can be specified by
more than one codon.
 For example, 6 different codons specify the amino acid
Leucine and 6 others specify Arginine.
 There is also one codon, AUG, that can either specify
methionine or serve as the initiation, or “start” codon for
protein synthesis.
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See if you can figure this
out…
 What amino acid is specified by CAU?
 What is the codon for tryptophan?
 What are two possible codons for glutamine?
 What amino acid is usually the first amino acid
of a protein? How do you know?
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Video Time
So… What the
heck is protein
synthesis?
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Step 1 of Protein
Synthesis
1. Transcription: mRNA is transcribed
from DNA in the nucleus and
released into the cytoplasm
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Step 2 of Protein
Synthesis
2.Translation: mRNA attaches to a
ribosome and an amino acid is
brought by a tRNA to the mRNA.
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It works like an assembly line
Lysine
Translation direction
mRNA
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tRNA
Ribosome
-The process continues until the ribosome reaches a stop
codon.
-It then releases the newly formed polypeptide (which
creates a protein) and the mRNA molecule completes the
process of translation.
Polypeptide
Ribosome
tRNA
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mRNA
Why do we care about what protein
synthesis does?
Proteins determine eye
color, hair color, etc...
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RNA Editing
 Some DNA within a gene is not needed to produce
a protein. These areas are called introns.
 The DNA sequences that code for proteins are
called exons.
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12–3
Click to Launch:
Continue to:
- or -
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12–3
A base that is present in RNA but NOT in DNA is
a. thymine.
b. uracil.
c. cytosine.
d. adenine.
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12–3
The nucleic acid responsible for bringing
individual amino acids to the ribosome is
a. transfer RNA.
b. DNA.
c. messenger RNA.
d. ribosomal RNA.
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12–3
A region of a DNA molecule that indicates to an
enzyme where to bind to make RNA is the
a. intron.
b. exon.
c. promoter.
d. codon.
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12–3
A codon typically carries sufficient information to
specify a(an)
a. single base pair in RNA.
b. single amino acid.
c. entire protein.
d. single base pair in DNA.
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