Chapter 27
Type
Materials
Reviewed
Status Not started
Chapter 27 - Genetic Code
Three Major advances in Understanding Protein
synthesis
1. Proteins synthesized at ribosomes
2. Amino acids activated for synthesis by attachment to tRNA via aminoacyl-tRNA
synthetases
3. tRNA acts as an “adapter” to translate mRNA into protein
Adapter (tRNA) brings amino acids to mRNA
tRNA: adaptor that translates nucleotide sequence into amino acid sequence
Chapter 27 1
, Amino acid is covalently bound at the 3’ end of the tRNA
A specific nucleotide triplet (anticodon) interacts with a triplet codon in mRNA
by hydrogen bonding of complementary bases
The Genetic Code
There are 20 common, genetically encoded amino acids
There are 4 different nucleotide bases in the genetic code
A four letter code in groups of 2 per codon
4^2= 16 combinations (to few)
A four letter code in groups of 3 per codon
4^3 = 64 combinations (sufficient)
mRNA code is written in 5’-3’ direction
The 5 rules of the genetic code
1. Triplet code:
A codon of 3 nucleotides specifies the amino acid inserted within protein
2. Non-overlapping code:
Specific first codon sets a reading frame, no nucleotides shared by codons
3. Commaless code:
The reading frame contains no punctuation, sequence is contiguous.
4. Degenerate code:
There are 4^3 = 64 codons
1 Initiation codon (AUG)
3 Termination codons (UAA, UAG, UGA)
4 more than one codon may exist for an amino acid
5. Universal code:
All organisms use the same code (nearly, mitochondria are exceptions)
Triplet Code & Nonoverlapping Code
Chapter 27 2
, A codon is a triplet of nucleotides that codes for a specific amino acid
In translation triplet code are read in a successive nonoverlapping fashion
The first codon establishes reading frame
Nonoverlapping Code & Commaless Code
The amino acid sequence of a protein defined by a linear sequence of
contiguous triplets
If reading frame is thrown off by 1 or 2 bases, all subsequent codons are out of
order
Shown by effects of deletions and insertions
Chapter 27 3
Type
Materials
Reviewed
Status Not started
Chapter 27 - Genetic Code
Three Major advances in Understanding Protein
synthesis
1. Proteins synthesized at ribosomes
2. Amino acids activated for synthesis by attachment to tRNA via aminoacyl-tRNA
synthetases
3. tRNA acts as an “adapter” to translate mRNA into protein
Adapter (tRNA) brings amino acids to mRNA
tRNA: adaptor that translates nucleotide sequence into amino acid sequence
Chapter 27 1
, Amino acid is covalently bound at the 3’ end of the tRNA
A specific nucleotide triplet (anticodon) interacts with a triplet codon in mRNA
by hydrogen bonding of complementary bases
The Genetic Code
There are 20 common, genetically encoded amino acids
There are 4 different nucleotide bases in the genetic code
A four letter code in groups of 2 per codon
4^2= 16 combinations (to few)
A four letter code in groups of 3 per codon
4^3 = 64 combinations (sufficient)
mRNA code is written in 5’-3’ direction
The 5 rules of the genetic code
1. Triplet code:
A codon of 3 nucleotides specifies the amino acid inserted within protein
2. Non-overlapping code:
Specific first codon sets a reading frame, no nucleotides shared by codons
3. Commaless code:
The reading frame contains no punctuation, sequence is contiguous.
4. Degenerate code:
There are 4^3 = 64 codons
1 Initiation codon (AUG)
3 Termination codons (UAA, UAG, UGA)
4 more than one codon may exist for an amino acid
5. Universal code:
All organisms use the same code (nearly, mitochondria are exceptions)
Triplet Code & Nonoverlapping Code
Chapter 27 2
, A codon is a triplet of nucleotides that codes for a specific amino acid
In translation triplet code are read in a successive nonoverlapping fashion
The first codon establishes reading frame
Nonoverlapping Code & Commaless Code
The amino acid sequence of a protein defined by a linear sequence of
contiguous triplets
If reading frame is thrown off by 1 or 2 bases, all subsequent codons are out of
order
Shown by effects of deletions and insertions
Chapter 27 3
