Transcription & Translation
Section 5, Chapter 4
3 RNA Molecules • Messenger RNA (mRNA): • Transcribed from DNA in nucleus • Transfer RNA (tRNA): •Translates a codon of MRNA into an amino acid •Carries amino acids to mRNA •Anticodons on tRNA are complimentary to codons of mRNA • • Ribosomal RNA (rRNA): • Provides structure and enzyme activity for ribosomes 4
mRNA Molecules Messenger RNA (mRNA): •Delivers genetic information from nucleus to the cytoplasm
DNA DNA
S
P
Direction of “reading” code
• RNA nucleotides are complementary to DNA nucleotides (exception – no thymine in RNA; replaced with uracil)
A
U
T
A
G
C
P
S
• Single polynucleotide chain •Formed beside a strand of DNA
mRNA
S
P
P
S S
P
P
S S
P
C
G
G
C
P
S S
P S
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P
Step 1. RNA Polymerase attaches to DNA strands & breaks Hydrogen bonds Strand 1
A C T A C T A A C G G A T G A T C RNA Polymerase
T G A T G A T T G C C T A C T A G Strand 2
Step 2. Strands Separate T A C C G G A T G A T C RNA Polymerase
A C T A A
A U G G C C U A C U A G mRNA
T G A T T A T G G C C T A C T A G Replication bubble
Step 3. RNA Polymerase synthesizes mRNA using DNA strand as a template
T A C C G G A T G A T C A C T A A
A U G G C C U A C U A G mRNA
T G A T T A T G G C C T A C T A G
Step 4. RNA Polymerase releases mRNA & DNA resumes original structure
RNA Polymerase
A C T A A T A C C G G A T G A T C
T G A T T A T G G C C T A C T A G
A U G G C C U A C U A G mRNA
Step 5. mRNA is undergoes further processing & leaves nucleus
• Codon = 3 letter sequence that encodes for an amino acid • All mRNA begin with AUG “Start Codon”
Start Codon A U G G C C U A C U A G mRNA
Note: • Codons are redundant - Each amino acid corresponds to more than one codon • e.g. UCU, UCC, and UCA all encode for Serine
•Start Codon (AUG) initiates translation •Stop Codons terminate translation
Protein Synthesis
The codon sequence of mRNA determines the amino acid sequence of a protein.
Figure 4.23
tRNA
2. Amino acid binding site
Clover-leaf shape RNA with 2 important regions
1. Anticodon
Ribosomes • Small particle of protein & ribosomal RNA (rRNA) • Ribosomes have 2 subunits • Small subunit binds to mRNA • Large subunit holds tRNA & amino acids • Small subunit has 2 binding sites for adjacent mRNA codons • Ribosomes link amino acids by peptide bonds
Ribosomes Peptide bond forming
large subunit anticodons
small subunit Binding sites with codons
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
Anchors polypeptide.
tRNA released
Figure 4.23 TRANSLATION
TRANSCRIPTION
After translation Chaperone proteins fold protein into its configuration
Enzymes may further modify proteins after translation = post-translational modification • Phosphorylation – adding a phosphate to the protein • Glycosylation – adding a sugar to the protein
End of Section 5, Chapter 4