BIOB11H3 Lecture Notes - Lecture 6: Polynucleotide Phosphorylase, Sickle-Cell Disease, Stop Codon
BIOB11
LEC 6&7: Translation (Protein Synthesis)
January 19, 2017
→ Properties of the Genetic Code
● Each aa in a polypeptide encoded by 3 sequential nucleotides
● Code words (codons) for aa → nucleotide triplets
● 20 different aa (words) specified ; codons must contain at least 3 successive nucleotides (letters)
● Triplet code is nonoverlapping → each nucleotide along the mRNA would be part of only one codon
→ Figure 2.24 Amino Acid Structure
● Contains
○ Side chain (R)
○ Amino group
○ Carboxyl group
● Formation of peptide bond occurs through condensation of 2 aa
○ Condensation occurs on a ribosome as the aa is transferred from the carrier (tRNA) onto
the end of the growing polypeptide chain
→ Sickle Cell Anemia
● Mutant hemoglobin responsible
● Mutant protein found to have a single aa substitution
● A triplet code can specify 64 different aa but there are only 20 aa to be specified
○ At least some of the aa would be specified by more than one codon → degenerate
● 3/64 codons that don’t specify an aa have a special “punctuation” function
○ Recognized by ribosome as termination codons and cause the reading of the message to stop
→ Identifying the Codons
● Nirenberg and Matthaei
○ Used enzyme polynucleotide phosphorylase to synthesize their own artificial genetic messages
■ Then determine what kind of protein they encoded
○ First message tested was polyribonucleotide consisting exclusively of uridine
■ Message : poly(U)
■ When added to test tube w// bacterial extract w/ all 20 aa and materials needed for protein
synthesis (ribosomes + various soluble factors)
● System followed the artificial messenger’s instructions → manufactured a
polypeptide
■ Assembled polypeptide found to be phenylalanine → codon UUU (polyU) specifies
phenylalanine
● clustering shows the similarity in codons that specify the same aa
○ Spontaneou mutations causing single base changes in a gene won’t affect the aa sequence of the
corresponding protein
○ Synonymous: change in nucleotide sequence that doesn’t affect aa sequence
■ These changes are much less likely to change an organism’s phenotype more than
nonsynonymous changes
○ Nonsynonymous: change that causes an aa substitution
■ More likely to be selected for or against by natural selection
● Greatest similarities b/w aa-related codons occur in the first two nucleotides of the triplet
find more resources at oneclass.com
find more resources at oneclass.com
● Greatest variability occurs in the third nucleotide
→ Figure 11.40 2-D structure of tRNAs
● Aa becomes linked to 3’ end of tRNA
● Opposite end has the anticodon
● contains 10 modified bases
● The greatest variability among tRNAs occurs in the V (variable) arm
→ Decoding the Codons: The Role of Transfer RNAs
● Nucleic acids and proteins are like two languages written with different types of letters
○ Why protein synthesis is called translation → info encoded in nucleotide sequence of an mRNA is
decoded
○ Used to direct the sequential assembly of aa into polypeptide chain
● Decoding info in mRNA done by tRNA (act as adaptors)
○ Each tRNA linked to specific aa → aa-tRNA
○ That same tRNA able to recognize codons in mRNA
○ Interaction b/w successive codons in mRNA and specific aa-tRNAs → synthesis of polypeptide w/
ordered sequence of aa
→ Structure of tRNAs
● All tRNAs roughly the same length → b/w 73 and 93 nucleotides
○ All had significant % of unusual bases → result from enzymatic modifications of one of A,U,C,G
after it had been incorporated in RNA chain (posttranscriptionally)
● All tRNAs had sequences of nucleotides in one part of the mol that were complementary to sequences
located in other parts of the mol → become folded like a cloverleaf shape
○ Base-paired stems
○ Unpaired loops
● Unusual bases concentrated in the loops disrupt H-bonding in these regions
○ Serve as potential binding sites for proteins
● Mature tRNAs have triplet sequence CCA at their 3’ end
○ This sequence is added enzymatically in eukaryotes
● Fold into unique and defined tertiary structure
● tRNAs translate a sequence of mRNA codons into a sequence of aa residues
● Info in mRNA is decoded through base pairing b/w complementary sequences in tRNAs and mRNAs
●
Anticodon →
part of tRNA that complementarily pairs with the codon of mRNA
○ Stretch of 3 sequential nucleotides
○ Sits at opposite end (from where the aa is attached)
● Interchangeability can occur of the base of the third position
○ Crick proposed : the same tRNA can recognize more than one codon → Wobble Hypothesis
■ The bonding b/w anticodon of tRNA and codon of mRNA is very strict for the first two
positions but more flexible at third position
● U of anticodon : A/G of mRNA
● G of anticodon : U or C of mRNA
● I (derived from Guanine in original tRNA mol) of anticodon : U, C, or A of mRNA
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Each aa in a polypeptide encoded by 3 sequential nucleotides. Code words (codons) for aa nucleotide triplets. Triplet code is nonoverlapping each nucleotide along the mrna would be part of only one codon. 20 different aa (words) specified ; codons must contain at least 3 successive nucleotides (letters) Formation of peptide bond occurs through condensation of 2 aa. Condensation occurs on a ribosome as the aa is transferred from the carrier (trna) onto the end of the growing polypeptide chain. Mutant protein found to have a single aa substitution. A triplet code can specify 64 different aa but there are only 20 aa to be specified. At least some of the aa would be specified by more than one codon degenerate. 3/64 codons that don"t specify an aa have a special punctuation function. Recognized by ribosome as termination codons and cause the reading of the message to stop. Used enzyme polynucleotide phosphorylase to synthesize their own artificial genetic messages.
