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Section 5, Chapter 4
Transcription & Translation
Transcription
RNA synthesis from DNA template
RNA molecule
1. single-stranded nucleic acid
2. sugar phosphate backbone
3. sugar = ribose
4. Uracil (U) replaces Thymine (T)U & A = complimentary base pair
Properties of RNA
4
There are several kinds of RNA
Transfer RNA (tRNA):Transfers amino acids to the ribosomes during translation.
Ribosomal RNA (rRNA):Provides structure and enzyme activity for ribosomes
Messenger RNA (mRNA): Conveys genetic information from DNA to the ribosomes
Messenger RNA (mRNA):
Delivers genetic information from the DNA inside the nucleus to the cytoplasm
mRNA is formed beside a strand of DNA
RNA nucleotides are complementary to DNA nucleotides with one exception – no thymine in RNA; replaced with uracil)
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DNA
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mRNA
Transcription
1. Synthesis of messenger RNA (mRNA) from DNA template
2. Occurs in nucleus
3. Only 1 of the DNA strands is transcribed
4. Transcription is catalyzed by RNA Polymerase
5. mRNA undergoes further processing & leaves the nucleus
Properties of mRNA
A Codon is the 3 letter nucleotide sequence of mRNA that encodes for 1 amino acid.
AUG is the first codon in protein synthesis, so it’s it’s called the start codon
The codon sequence of mRNA determines the amino acid sequence of a protein.
Figure 4.23
Protein Synthesis
Examples of Codons
amino acid
The start codon marks the site at which translation into protein sequence begins, and the stop codon marks the site at which translation ends.
Translation = Ribosomes synthesize proteins by reading the codons of the mRNA transcript.
1. Occurs on Ribosomes in cytoplasm
2. transfer RNA (tRNA) transports amino acids to mRNA
3. anticodons on tRNA align with codons on mRNA
tRNA
Anticodon
Amino acid binding site
Clover-leaf shape RNA with 2 important regions
Small particle of protein & ribosomal RNA (rRNA)
Ribosomes have 2 subunitsLarge subunit holds tRNA & amino acidsSmall subunit binds to mRNA
Small subunit has 2 binding sites for adjacent mRNA codons
Ribosomes link amino acids by peptide bonds
Ribosomes
large subunit
small subunit
Binding sites with codons
anticodons
Peptide bond forming
Ribosomes
1. mRNA binds to the small subunit of a Ribosome.
2. The ribosome ‘reads’ the mRNA sequence
3. tRNA brings amino acids to the ribosomes, aligning their anticodons with mRNA codons
4. The Ribosome links the amino acids together
5. Polypeptide chain lengthens
Sequence of Translation
Translation- Figure 4.24
Anchors polypeptide.
Translation
tRNA released
TRANSCRIPTION
Figure 4.23 overview of protein synthesis
Once translation is complete chaperone proteins fold the protein into its configuration
enzymes may further modify proteins after translation phosphorylation – adding a phosphate to the protein
glycosylation – adding a sugar to the protein
post-translational modification
End of Chapter 4