1. The genetic code
© 2016 Paul Billiet ODWS

2. The tRNA molecule tRNA molecules do the final translating
A specific amino acid is attached at one end by a tRNA activating enzyme
= first part of translating
An anticodon is at the other end, complementary to the mRNA codons
= second part of translation.
© 2016 Paul Billiet ODWS

3. The 3-D structure of a tRNA
library.thinkquest.org/.../content/_thb_tRNA.jpg
© 2016 Paul Billiet ODWS

4. The genetic code Made of 64 triplets of bases (codons).
© 2016 Paul Billiet ODWS

5. 1st position 2nd position U C A G 3rd position ↓ Phe Ser Tyr Cys Leu
STOP
Trp
Pro
His
Arg
Gln
Ile
Thr
Asn
Lys
Met
Val
Ala
Asp
Gly
Glu

6. Genetic code: characteristics
Only 61 triplets or codons code for amino acids
3 stop codons (aka nonsense codons or terminator codons) UUA UAG UGA.
© 2016 Paul Billiet ODWS

7. 1st position U C A G 2nd position 3rd position Phe Ser Tyr Cys Leu
STOP
Trp
Pro
His
Arg
Gln
Ile
Thr
Asn
Lys
Met
Val
Ala
Asp
Gly
Glu

8. The degenerate genetic code
A few amino acids are coded for by a single codon
Most are coded for by more than one codon
Some are coded for by up to six codons
This is degeneracy in the code.
© 2016 Paul Billiet ODWS

9. 1st position 2nd position U C A G 3rd position ↓ Phe Ser Tyr Cys Leu
STOP
Trp
Pro
His
Arg
Gln
Ile
Thr
Asn
Lys
Met
Val
Ala
Asp
Gly
Glu
© 2016 Paul Billiet ODWS

10. The degenerative code Several codons code for the same amino acid
The first two letters seem to be the most important
The third one tends to be interchangeable
Codon
Amino acid
UUU
UUA
Phenylalanine
Leucine
UUC
UUG
U & C are both pyrimidines
A & G are both
purines
© 2016 Paul Billiet ODWS

11. 1st position 2nd position U C A G 3rd position ↓ Phe Ser Tyr Cys Leu
STOP
Trp
Pro
His
Arg
Gln
Ile
Thr
Asn
Lys
Met
Val
Ala
Asp
Gly
Glu
Acidic
Basic
Uncharged Polar
Non-polar

12. Similar amino acids have similar codons
Example Aspartic acid codons GAU and GAC Glutamic acid codons GAA and GAG Both are acidic amino acids.
© 2016 Paul Billiet ODWS

13. 1st position U C A G 2nd position 3rd position Phe Ser Tyr Cys Leu
STOP
Trp
Pro
His
Arg
Gln
Ile
Thr
Asn
Lys
Met
Val
Ala
Asp
Gly
Glu

14. Grammar in the code?
Three codons are nonsense codons; they represent the end of the information = STOP
The codon for methionine, found at the beginning of the information to be transcribed, means START
The methionine amino acid is usually removed from the finished protein.
© 2016 Paul Billiet ODWS

15. Punctuation? There is no punctuation between each codon
The reading frame is set at the beginning of the gene
Frame shift mutations can be caused by the ADDITION or DELETION of only one or two bases Everything downstream is misread.
© 2016 Paul Billiet ODWS

16. Reading the code
mRNA is always read in the same direction 5’ to 3’ (same as transcription and replication)
The polypeptide chain is constructed from the amino end to the carboxyl end.
© 2016 Paul Billiet ODWS

17. A universal code The code is used by all organisms
So it is very ancient
Permits investigations into common ancestry
Permits genetically transformed organisms.
© 2016 Paul Billiet ODWS

18. Well almost universal Mitochondrial gene expression is a bit different
Mitochondria use UGA to code for tryptophan rather than as a stop codon
Animal mitochondria use AUA for methionine instead of isoleucine
All vertebrate mitochondria use AGA and AGG as stop codons
Yeast mitochondria use all codons beginning with CU for threonine instead of leucine.
© 2016 Paul Billiet ODWS

19. 20 is the limit
Some amino acids are chemically altered AFTER translation
e.g. In collogen proline is converted to hydroxyproline
Therefore the total number of amino acids found in proteins is greater than 20 but the total used in translation is only 20.
© 2016 Paul Billiet ODWS

20. Well almost 20
There are two codons that operate under exceptional circumstances
The stop codon UGA codes for selenocysteine
The stop codon UAG codes for pyrrolysine in species of archaea and bacteria.
© 2016 Paul Billiet ODWS