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TRANSCRIPT
Protein Synthesis
Unit 6
Goal: Students will be able to describe the
processes of transcription and translation.
Types of RNA
• Messenger RNA (mRNA) makes a copy of
DNA, carries instructions for making
proteins,
• Transfer RNA (tRNA) decodes mRNA and
transfers amino acids to the ribosome
• Ribosomal RNA (rRNA) build the protein
by bonding amino acids together.
Ribosomes are made of rRNA.
Central Dogma
• DNA RNA Protein• DNA is the instructions for making the
protein
• RNA copies the instructions & builds the
protein
• The protein is built by the ribosome and
used either by that cell or transported to
other cells.
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DNA RNA Protein
Nuclear
membrane
Transcription
RNA Processing
Translation
DNA
Pre-mRNA
mRNA
Ribosome
Protein
Eukaryotic Cell
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Codons & Amino Acids
• Codon = 3 base (letter) sequence
on mRNA
• Amino Acid = monomer of protein
•1 codon CODES FOR 1 amino acid
• Some amino acids have multiple codons.
• 64 possible codons; 20 Amino Acids in nature
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Codon Chart•Chart tells which Amino Acid that codon means•Find the correct letter in the 1st
position 2nd
position 3rd
position (where do those meet?)•Example: AUGcodes for Met (Methionine)
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Name the Amino Acids
• GGG?
• UCA?
• CAU?
• GCA?
• AAA?
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Start/Stop Codons
• There will always be a “Start” codon
(AUG) at the beginning. It is like
capitalizing the 1st letter of a sentence.
• There will always be a “Stop” codon
(UGA, UAA, UAG) at the end. It is like a
period at the end of a sentence.
Protein Synthesis: Protein synthesis
uses the information in genes to make
proteins. 2 Steps
1.Transcription
–Copying the DNA into mRNA
2.Translation
–tRNA decodes mRNA & delivers the
amino acids
–rRNA bonds the amino acids
together to build the protein
Transcription: Where?
• mRNA is made in the NUCLEUS
• mRNA will leave the nucleus through the
pores in the nuclear membrane
• (DNA does NOT leave the nucleus)
Transcription: Involved Molecules
• DNA (instructions for making the protein)
• mRNA (copy of the instructions)
• RNA polymerase (enzyme that builds
mRNA)
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RNA Polymerase = The Enzyme responsible for Transcription
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Transcription: Produces?• mRNA (messenger
RNA)
• Long Straight chain of Nucleotides
• Made in the Nucleus
• Copies DNA & leaves through nuclear pores
• Contains the Nitrogen Bases A, G, C, U ( no T )
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Remember the Complementary Bases
On DNA:A=TC=G
On RNA:A=UC=G
DNA RNA
G C C G
T A A U
Transcription: What Happens?Big Idea: DNA is copied into a
complementary sequence of mRNA.
1.RNA polymerase is the enzyme that is responsible for
transcription
2.RNA polymerase reads the gene and adds the correct
complementary RNA base to make a new strand of
RNA.
– DNA = GCCATT
– mRNA = CGGUAA
3. The mRNA gets processed (edited and packaged)
4. mRNA leaves the nucleus and goes to the
cytoplasm.
RNA
DNA
RNApolymerase
Adenine (DNA and RNA)
Cystosine (DNA and RNA)
Guanine(DNA and RNA)
Thymine (DNA only)
Uracil (RNA only)
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Transcription: Goes to?•mRNA leaves the nucleus through the pores in the nuclear membrane, travels through the cytoplasm, and goes to the ribosomes
Translation
Translation: Where? • Ribosome
–Free floating ribosome in cytoplasm
(makes proteins for that cell)
OR
–attached ribosome on the Rough
ER (makes proteins to be exported
to other cells)
Translation: Involved Molecules
• mRNA (copy of the instructions)
• tRNA (translates the instructions)
• rRNA (builds the protein)
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Transfer RNAamino acid
attachment site
U A C
anticodoncopyright cmassengale
• Decodes (translates) the mRNA
• Delivers (transfers) amino acids to the ribosome
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Codons and Anticodons• The 3 bases of an anticodon are
complementary to the 3 bases of a codon
• Ex: Anticodon UGA
• Codon ACU
UGA
ACU
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Ribosomal RNA (rRNA)
•Ribosomes are made of rRNA
•Site of Protein Synthesis
•Forms peptide bonds between amino acids
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Translation: Produces?• Chain of
amino acids
• Polypeptide
(protein)
Translation: What Happens?
• Big Idea: The mRNA is then
TRANSLATED by tRNA at
the RIBOSOME (made of
rRNA). The RIBOSOME then
BUILDS the protein by
bonding amino acids
together.
Translation: What Happens?
1. tRNA translates the mRNA using “anticodons.” (complementary to the codon)
– Each tRNA molecule has an anticodon on one end and an amino acid on the other.
2. tRNA transfers the amino acids to the ribosome in the correct sequence.
3. rRNA forms peptide bonds between the amino acids. This links them together to build the protein.
Translation: Where does it go?
The protein will do one of 2 things
• Stay inside the cell to carry out functions
for that cell
OR
• Be exported to other cells
– Rough ER Golgi Exocytosis
Messenger RNA
Messenger RNA is transcribed in the nucleus.
Transfer RNA
The mRNA then enters the cytoplasm and attaches
to a ribosome. Translation begins at AUG, the start
codon. Each transfer RNA has an anticodon
whose bases are complementary to a codon on the
mRNA strand. The ribosome positions the start
codon to attract its anticodon, which is part of the
tRNA that binds methionine. The ribosome also
binds the next codon and its anticodon.
mRNA Start codon
Ribosome
Methionine
Phenylalanine tRNALysine
Nucleus
mRNA
The Polypeptide “Assembly Line”
The ribosome joins the two amino acids—
methionine and phenylalanine—and breaks the
bond between methionine and its tRNA. The
tRNA floats away, allowing the ribosome to bind
to another tRNA. The ribosome moves along
the mRNA, binding new tRNA molecules and
amino acids.
mRNA
Ribosome
Translation direction
LysinetRNA
tRNA
Ribosome
Growing polypeptide chain
mRNA
Completing the Polypeptide
The process continues until the ribosome reaches
one of the three stop codons. The result is a growing
polypeptide chain.