The Genetic Code Flashcards
The Central Dogma of Molecular Biology
deals with the detailed residueby-residue transfer of sequential information. It states that such
information cannot be transferred back from protein to either protein or nucleic acid. — Francis
Crick, 1970 (first stated in 1957)
Unit of code specifying a single amino acid is….
CODON
• A codon is a sequence of three DNA or RNA nucleotides that corresponds with a specific
amino acid
• There are 4 nucleotides to specify 20 amino acids If 2 nt per codon: 42 = 16 combinations
(not enough)
• If 3 nt per codon: 4 3 = 64 combinations (more than enough)
• Thus a triplet code (sequence of 3 nucleotides)
What is a codon? How many possible codons are there? Which direction are they read from?
• Codons consist of 3 nucleotides
• 64 possible codons
• Codons are non-overlapping
• Read in 5’ to 3’ direction
• There are 3 possible reading frames (in one direction); only one is correct for a specific protein
• Open reading frame start: initiation codon (AUG;
encodes Met);
• end point defined by termination codon (UAA, UAG or
UGA) not amino-acid
• One codon defines one amino acid
• There are 20 common natural amino acids
Codons are overlapping. True or false?
false
If we have amino acid can we work back to know its codon? Why? Why not?
NO, because few different codon can code for an amino acid,
‘degeneracy”
What is codon degeneracy? And what is its implication?
Degenerate: an amino acid can be specified by > 1 codon
Codon usage: highly expressed genes are optimized for efficient translation
The degeneracy of the genetic code is valuable to living things because it allows for more than one codon to code for one amino acid. It has been suggested that the degeneracy makes the DNA more tolerant to point mutationse.g. a point mutation in CCU codon change it into CCC codon which encodes for the same amino acid. ‘
Is genetic code universal?
genetic codon is universal however there are exceptions
• Mitochondrial protein show variation in codon, e.g. similar codon encodes for different
amino acid in different species
Why each reading frame gives different sequence of codon? Can different reading frame codon for
a similar protein?
- Each reading frame gives a different sequence of codons
- Only one reading frame encodes the correct protein
- Other reading frames unlikely to encode proteins
unambiguous
As well as being degenerate, the genetic code is also referred to as ‘unambiguous’ which means that each possible codon can code for one amino acid only
How does the code work?
The process of protein synthesis is known as TRANSLATION
• Process of matching amino acids with codons is facilitated by:
• carrier molecules (tRNA) that bind both a specific AA and a specific codon
• ribosome that act as a structural framework and a polymerising machine
Crick’s adaptor hypothesis
• Adaptor hypothesis ( t RNA)
• “Wobble” hypothesis
Discovery of transfer RNA, the adaptor by Hoagland & Zamecnik
(1956)
– Discovered that amino acids were “activated” before incorporated
into proteins
– Activation involves attachment of amino acid to an RNA molecule
called transfer RNA (tRNA) to form aminoacyltRNA
– tRNA = 1st adaptor molecule; tRNA synthetases = 2nd adaptor molecule
features of tRNA
• tRNAs are small and chemically-modified RNA
– 75 to 90 nucleotides
– contain unusual bases produced by chemical modification
• They have a characteristic tridimensional shape
– maintained by hydrogen bonds arising from some internal base pairing
– represented as a cloverleaf in diagrams; L shaped once fully folded
• tRNAs have an adaptor function connecting the amino acids, mRNA and ribosome.
– Conformation adapted to this adaptor function
– Attachment of amino acids to the 3’ end
– Recognition of the mRNA through the anticodon sequence
