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12–3 RNA and Protein Synthesis
12-3 RNA and Protein Synthesis
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12–3 RNA and Protein Synthesis
Comparing
The transcription process is similar to
replication.
•Transcription and replication both involve complex
enzymes and complementary base pairing.
•The two processes have different end results.
– Replication copies
all the DNA;
transcription copies
a gene.
– Replication makes
one copy;
transcription can
make many copies.
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12–3 RNA and Protein Synthesis
12–3 RNA and Protein
Synthesis
Genes are coded DNA instructions that control the
production of proteins.
Genetic messages can be decoded by copying part
of the nucleotide sequence from DNA into RNA.
RNA contains coded information for making proteins.
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12–3 RNA and Protein Synthesis
12–3 RNA and Protein
Synthesis
•RNA carries DNA’s
instructions.
•The central dogma states
that information flows in one
direction from DNA to RNA to
proteins.
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12–3 RNA and Protein Synthesis
12–3 RNA and Protein
Synthesis
•The central dogma includes
three processes.
– Replication
– Transcription
– Translation
•RNA is a link between DNA
and proteins.
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12–3 RNA and Protein Synthesis
The Structure of RNA
The Structure of RNA
There are three main differences between RNA and DNA:
•The sugar in RNA is ribose instead of deoxyribose.
•RNA is generally single-stranded instead of double
stranded.
•RNA contains uracil in place of thymine.
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12–3 RNA and Protein Synthesis
Types of RNA
Types of RNA
There are three main types of RNA:
• messenger RNA
• ribosomal RNA
• transfer RNA
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12–3 RNA and Protein Synthesis
Types of RNA
Messenger RNA (mRNA) carries copies of
instructions for assembling amino acids into proteins.
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12–3 RNA and Protein Synthesis
Types of RNA
Ribosome
Ribosomal RNA
Ribosomal RNA (rRNA) forms part of ribosomes
where proteins are made.
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12–3 RNA and Protein Synthesis
Types of RNA
Amino acid
Transfer RNA
During protein construction, transfer RNA (tRNA)
brings amino acids from the cytoplasm to a ribosome.
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12–3 RNA and Protein Synthesis
Protein Synthesis
DNA
molecule
DNA strand
(template)
5
3
TRANSCRIPTION
mRNA
5
3
Codon
TRANSLATION
Protein
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Amino acid
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12–3 RNA and Protein Synthesis
Transcription
Transcription
1. A transcription complex consisting of RNA and other
proteins recognizes the start site of a gene and begins to
unwind the DNA.
2. RNA polymerase reads one side of the DNA template
and strings together a complementary strand of RNA
nucleotides.
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12–3 RNA and Protein Synthesis
Transcription
Transcription
3. The growing RNA strand
hangs freely as it is transcribed
and detaches completely once
the entire gene is transcribed.
4. DNA Molecule
5. Nucleotides
6. Newly synthesized RNA
strand
7. RNA polymerase
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12–3 RNA and Protein Synthesis
Transcription
RNA
RNA polymerase
DNA
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12–3 RNA and Protein Synthesis
The Genetic Code
The Genetic Code
The genetic code is the “common language” of
mRNA instructions and is shared by almost all
organisms.
The code is written using four “letters” (the bases:
A, U, C, and G).
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12–3 RNA and Protein Synthesis
The Genetic Code
A codon consists of three consecutive nucleotides
on mRNA that specify a particular amino acid.
•The start codon signals the start of translations and the amino acid
methionine.
•The stop codon is three codons that signal the end of a chain of amino
acids.
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The Genetic Code
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The Genetic Code
• The genetic code matches each codon to its amino
acid or function.
–three stop codons
–one start codon, codes for methionine
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Translation
Translation
Translation is the decoding of an mRNA message into a
polypeptide chain (protein).
–Ribosomes are the site of protein synthesis; made of
rRNA and proteins
–Catalyzes the formation of peptide bonds between
amion acids
During translation, the cell uses information from
messenger RNA to produce proteins.
Nucleus
mRNA
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12–3 RNA and Protein Synthesis
Reading frame
Codon
• A reading frame is the series of 3 nonoverlapping nucleotides read, in
order, by a cell.
• 3 different reading frames are possible for each mRNA molecule.
• Codons must be read in the correct reading frame for the correct protein
to be made.
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Anticodon
Amino acids are linked to become a protein.
• An anticodon is a set of three nucleotides that is complementary to an
mRNA codon.
• An anticodon is carried by a tRNA.
• Transfer RNA (tRNA) is a type of RNA that carries amino acids from
the cytoplasm to the ribosome. One end has a specific anticodon, the
other end attaches to a specific amino acid.
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12–3 RNA and Protein Synthesis
Ribosomes
Ribosomes consist of two subunits
• The large subunit has three binding sites for tRNA.
• The small subunit binds to mRNA.
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Translation Process
1. Ribosome assembles at the start codon;
complementary tRNA molecule pair with the exposed
codon.
Lysine
Phenylalanine
tRNA
Methionine
Ribosome
mRNA
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Start codon
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Translation
2. Protein Synthesis-ribosome helps bond the new
amino acid to the start codon and breaks the bond
between the amino acid and the first tRNA.
Lysine
tRNA
Translation direction
mRNA
Ribosome
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Translation
3. Ribosome pulls the RNA strand the length of one codon; first
tRNA returns to the cytoplasm; another codon is exposed for
tRNA binding. The process continues until the ribosome reaches
Polypeptide
a stop codon.
Ribosome
tRNA
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mRNA
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12–3 RNA and Protein Synthesis
Codon
Genes and Proteins
Codon Codon
DNA
Single strand of DNA
mRNA
Codon Codon Codon
mRNA
Protein
Alanine Arginine Leucine
Amino acids within
a polypeptide
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12–3 RNA and Protein Synthesis
1. Amino acid (Start codon methionine)
2. Polypeptide bond
3. Large subunit
4. tRNA
5. codons
6. Small subunit
7. mRNA
8. anticodon
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Continue to:
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The role of a master plan in a building is similar
to the role of which molecule?
a. messenger RNA
b. DNA
c. transfer RNA
d. ribosomal RNA
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A base that is present in RNA but NOT in DNA is
a. thymine.
b. uracil.
c. cytosine.
d. adenine.
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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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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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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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