Protein synthesis 2
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- The sequence of events for transcription. (6)
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1) RNA polymerase binds to promoter region of DNA
2) DNA double helix unwinds; one strand used as template for mRNA synthesis (Sense Strand)
3) RNA poly. moves from 3' -> 5' of DNA chain; free ribonucleotides taken up and matched to template by complementary base-paring; mRNA strand grows 5' -> 3'
4) A with U, T with A, G with C, C with G; adjacent ribonucleotides joined by phosphodiester bonds
5) RNA poly. reaches end of gene; mRNA released; DNA rewinds.
6) mRNA leaves through nuclear pores - What is required for transcription?
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1) RNA polymerase
2) Free ribonucleotides:
uracil, adenine, guanine, cytosine - Where does transcription occur?
- In the nucleus
- What's required for translation?
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1) mRNA
2) rRNA
3) tRNA (anticodon made up of a specific triplet base sequence)
4) Free amino acids
5) Ribosomes - Where does translation occur?
- In cytoplasm
- Describe the sequence of events for amino acid activation.
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1) Enzymes involved are aminoacyl-tRNA synthetase
2) Amino acid reacts with ATP to form a.a-AMP
3) a.a.-AMP is then attached to specific tRNA molecule, to form amino acid-tRNA complex
4) Amino acid attached to the hydroxyl group of C3 of ribose of last nucleotide (A) of tRNA.
5) Aminoacyl-tRNA synthetase is specific to each of 20 amino acids - Describe the main stages of translation.
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1) Binding of mRNA to ribosome
2) Initiation of polypeptide chain
3) Chain elongation
4) Chain termination - Describe the sequence of events of translation.
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1) Binding of mRNA to ribosome
-Information processing region of 40S subunit binds to mRNA at 5'
-Ribosome covers 2 codons at a time
2) Polypeptide chain initiation
-Amino acid-tRNA complex with anticodon UAC binds to start codon AUG on mRNA
-a.a-tRNA complex held in 'P' site of 60S subunit
-tRNA with UAC always carries methionine; it is always the first amino acid
3) Chain elongation
-2nd a.a-tRNA complex with anticodon complementary to the second codon of mRNA binds to the latter; complex held at 'A' site of 60S
-peptide bond between first and second a.a. is formed; energy from ATP hydrolysis; catalysed by catalytic region in 60S
-Ribosome moves along mRNA to next codon
-First tRNA is released and recycled
-Second a.a-tRNA complex moves from 'A' to 'P', leaving 'A' empty for next a.a-tRNA complex with anticodon complementary to 3rd mRNA codon
-Process continues until stop codon is reached
4) Chain termination
-Polypeptide is released from the ribosome; secondary and tertiary structures assumed
-may undergo modification at Golgi
-Ribosome disassembles to subunits
-tRNA re-attached to respective a.a. - What is the role of mRNA in protein synthesis?
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mRNA:
-Genetic message for synthesis of polypeptide found on DNA
-mRNA is formed from transcription of gene coding for polypeptide
-mRNA conveys genetic message from nucleus to the ribosomes
-Each mRNA has a sequence of nucleotides complementary to that of gene
-Each triplet codes for one amino acid
-mRNA codons specify the order in which amino acids are sequenced to form a polypeptide - What is the role of tRNA in protein synthesis?
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tRNA:
-The CCA site at 3' end allows for attachment of an amino acid
-tRNA carries amino acid to ribosome
-Each tRNA molecule is specific—has specific anticodon, and carries corresponding amino acid
-Every codon on mRNA has a corresponding anticodon on tRNA, allows for correct sequencing of amino acids in polypeptide - What is the role of rRNA in protein synthesis?
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rRNA:
-surrounds part of mRNA during protein synthesis
-holds mRNA, amino acid-tRNA complex & enzymes in precise orientation - What is the role of ribosomes in protein synthesis?
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Ribosomes
-Important in translation
-Small subunit binds to mRNA
-Large subunit has 2 sites:
*'P' site for recognition of start codon, initiation of chain formation, and for formation of peptide bonds
*'A' site for holding amino acid-tRNA complex next in sequence
-Catalytic site on 60S catalyses formation of peptide bond b/w amino acids
-Many ribosomes may move along Ribosomes
-Important in translation
-Small subunit binds to mRNA
-Large subunit has 2 sites:
*'P' site for recognition of start codon, initiation of chain formation, and for formation of peptide bonds
*'A' site for holding amino acid-tRNA complex next in sequence
-Catalytic site on 60S catalyses formation of peptide bond b/w amino acids
-Many ribosomes may move along mRNA at same time to increase rate of translation