1. Topic 3.5 - Transcription and Translation
Click to view an animation of protein synthesis: http://telstar.ote.cmu.edu/Hughes/HughesArchive/tutorial/polypeptide/tutorial.swf
3.5.1. Compare the structure of RNA and DNA.
DNA RNA
sugar deoxyribose ribose
pyrimidines thymine, cytosine uracil, cytosine
strands double single
3.5.2. Outline the DNA transcription in terms of the formation of an RNA strand complementary
to the DNA strand by RNA polymerase.
A. Initiation: RNA polymerase is an enzyme complex which:
• unwinds and unzips DNA double strand
• attaches to promoter region of gene, which marks the beginning point for
transcription
B. Elongation: RNA polymerase:
• uses DNA anti-sense strand as a template
• synthesizes a complementary RNA strand using base pairing rules
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2. A = U T = A G = C C = G
C. Termination: RNA polymerase :
• reaches termination region of the gene, which marks the end of the
coding sequence
• terminates transcription by releasing both DNA and RNA
3.5.3. Describe the genetic code in terms of codons composed of triplets of bases.
• triplet code = 3 nucleotide bases code for one amino acid
• codon = a group of 3 nucleotide bases is called a codon
• there are 64 different codons (4 x 4 x4 = 64)
Click to view an animation of protein synthesis: http://telstar.ote.cmu.edu/Hughes/HughesArchive/tutorial/polypeptide/tutorial.swf
3.5.4. Explain the process of translation, leading to polypeptide formation.
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3. A. Initiation:
• mRNA binds to the small subunit of the ribosome
• tRNA anticodon binds to mRNA codon by complementary base
pairing
• large ribosomal subunit binds, completing ribosomal structure, and producing two
ribosomal binding sites: P site & A site
B. Elongation:
• tRNA with anticodon complementary to second mRNA codon binds to ribosomal A site,
with appropriate amino acid attached to tRNA
• enzymes in ribosome catalyse formation of peptide bond between 1st, P site, and 2nd, A
site, amino acids
• P site tRNA, now separated from amino acid, exits ribosome
• ribosome moves one codon (3 nucleotides) along the mRNA, thus shifting previous A-site
tRNA to P-site, and opening A-site
• tRNA with anticodon complementary to A-site mRNA codon binds to ribosomal A-site,
with appropriate amino acid attached to tRNA terminal
• enzymes in ribosome catalyse formation of peptide bond between 2nd and 3rd amino acids
• P site tRNA, now separated from its amino acid, exits ribosome
• ribosome moves one codon (3 nucleotides) along the mRNA, thus shifting previous A-site
tRNA to P-site, and opening A-site
• repetition of process until stop codon is reached
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4. C. Termination:
• when ribosomal A-site reaches a stop codon, no tRNA has a complementary anticodon
• release factor protein binds to ribosomal A-site stop codon
• polypeptide and mRNA are released
• large and small ribosomal subunits separate
• start codon: the mRNA triplet codon AUG is universally the start codon used to mark the
beginning of the coding sequence of a gene
• stop codon: there are three stop codons in the genetic code; none of these have a
corresponding tRNA
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5. 3.5.5 Discuss the relationship between one gene and one polypeptide.
One gene corresponds to one polypeptide. It does not, however,
always code for a protein, because many proteins consist of
more than one polypeptide.
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6. 3.5.5 Discuss the relationship between one gene and one polypeptide.
One gene corresponds to one polypeptide. It does not, however,
always code for a protein, because many proteins consist of
more than one polypeptide.
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