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DNA and GenesDNA and Genes• DNA contains genes, sequences of
nucleotide bases• These genes code for polypeptides
(proteins)
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Protein Synthesis (Gene Expression)
Proteins (Review)•Proteins make up all living materials
• Proteins are composed of amino acids – there are 20 different amino acids
• Different proteins are made by combining these 20 amino acids in different combinations
• Amino acid chains are called polypeptides.
• Proteins are manufactured (made) by the ribosomes
•Function of proteins:1.Help fight disease
2.Build new body tissue
3.Enzymes used for digestion and other chemical reactions are proteins (Enzymes speed up the rate of a reaction)
4. Component of all cell membranes
Making a Protein—Transcription•First Step: Copying of genetic information from DNA to RNA called Transcription
Why? DNA has the genetic code for the protein that needs to be made, but proteins are made by the ribosomes—ribosomes are outside the nucleus in the cytoplasm.
DNA is too large to leave the nucleus (double stranded), but RNA can leave the nucleus (single stranded).
The enzyme RNA polymerase binds to DNA and part of DNA temporarily unzips and is used as a template to assemble complementary nucleotides into messenger RNA (mRNA). mRNA copies the template strand
• mRNA then goes through the pores of the nucleus with the DNA code and attaches to the ribosome.
Making a Protein—Translation•Second Step: Decoding of mRNA into a protein/polypeptide chain is called Translation.•Transfer RNA (tRNA) carries amino acids from the cytoplasm to the ribosome.
These amino acids come from the food we eat. Proteins we eat are broken down into individual amino acids and then simply rearranged into new proteins according to the needs and directions of our DNA.
•A series of three adjacent bases in an mRNA molecule codes for a specific amino acid—called a codon.
•A triplet of nucleotides in tRNA that is complementary to the codon in mRNA—called an anticodon.
•Each tRNA codes for a different amino acid.
Amino acid
Anticodon
12copyright cmassengale
• mRNA carrying the DNA instructions and tRNA carrying amino acids meet in the ribosomes.
• Amino acids are joined together to make a protein.
Polypeptide = Protein
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RibosomesRibosomes
• Made of a large and small subunit
• Composed of rRNA (40%) and proteins (60%)
• Have two sites for tRNA attachment --- P and A
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Step 1- InitiationStep 1- Initiation
• mRNA transcript start codon AUG attaches to the small ribosomal subunit
• Small subunit attaches to large ribosomal subunit
mRNA transcript
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RibosomesRibosomes
PSite
ASite
Largesubunit
Small subunit
mRNAmRNA
A U G C U A C U U C G
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Step 2 - Elongation• As ribosome moves, two tRNA with their
amino acids move into site A and P of the ribosome
• Peptide bonds join the amino acids
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InitiationInitiation
mRNAmRNA
A U G C U A C U U C G
2-tRNA
G
aa2
A U
A
1-tRNA
U A C
aa1
anticodon
hydrogenbonds codon
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mRNAmRNA
A U G C U A C U U C G
1-tRNA 2-tRNA
U A C G
aa1 aa2
A UA
anticodon
hydrogenbonds codon
peptide bond
3-tRNA
G A A
aa3
ElongationElongation
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mRNAmRNA
A U G C U A C U U C G
1-tRNA
2-tRNA
U A C
G
aa1
aa2
A UA
peptide bond
3-tRNA
G A A
aa3
Ribosomes move over one codon
(leaves)
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mRNAmRNA
A U G C U A C U U C G
2-tRNA
G
aa1
aa2
A UA
peptide bonds
3-tRNA
G A A
aa3
4-tRNA
G C U
aa4
A C U
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mRNAmRNA
A U G C U A C U U C G
2-tRNA
G
aa1aa2
A U
A
peptide bonds
3-tRNA
G A A
aa3
4-tRNA
G C U
aa4
A C U
(leaves)
Ribosomes move over one codoncopyright cmassengale
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mRNAmRNA
G C U A C U U C G
aa1aa2
A
peptide bonds
3-tRNA
G A A
aa3
4-tRNA
G C U
aa4
A C U
U G A
5-tRNA
aa5
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mRNAmRNA
G C U A C U U C G
aa1aa2
A
peptide bonds
3-tRNA
G A A
aa3
4-tRNA
G C U
aa4
A C U
U G A
5-tRNA
aa5
Ribosomes move over one codoncopyright cmassengale
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mRNAmRNA
A C A U G U
aa1
aa2
U
primaryprimarystructurestructureof a proteinof a protein
aa3
200-tRNA
aa4
U A G
aa5
C U
aa200
aa199
terminatorterminator or stopor stop codoncodon
TerminationTermination
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End Product –The Protein!End Product –The Protein!• The end products of protein synthesis
is a primary structure of a protein• A sequence of amino acid bonded
together by peptide bonds
aa1
aa2 aa3 aa4aa5
aa200
aa199
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Messenger RNA (mRNA)Messenger RNA (mRNA)
methionine glycine serine isoleucine glycine alanine stopcodon
proteinprotein
A U G G G C U C C A U C G G C G C A U A AmRNAmRNA
startcodon
Primary structure of a proteinPrimary structure of a protein
aa1 aa2 aa3 aa4 aa5 aa6
peptide bonds
codon 2 codon 3 codon 4 codon 5 codon 6 codon 7codon 1
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Use one of the codon charts on the next page to find the amino acid sequence coded for by the following mRNA strands.
CAC/CCA/UGG/UGA
___________/___________/___________/____________
AUG/AAC/GAC/UAA
___________/___________/___________/____________
GGUGUG ACC ACU
UAC UUG CUG AUU
CAC/CCA/UGG/UGA
___________/___________/___________/____________Histidine1st
Bas
e
2nd Base
3rd Base
Proline Tryptophan Stop
AUG/AAC/GAC/UAA
___________/___________/___________/____________Methionine Asparagine Aspartic Acid Stop
Protein Synthesis
Protein Synthesis videos
• http://www.youtube.com/watch?v=983lhh20rGY
• http://www.youtube.com/watch?v=erOP76_qLWA
Movie about translation at bottom of webpage. Click on hyperlink in picture above.