THE PRINCIPAL FEATURES OF THE GENETIC CODE-STELLA ANGELI
THE PRINCIPAL FEATURES OF
THE GENETIC CODE
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THE PRINCIPAL FEATURES OF THE GENETIC CODE-STELLA ANGELI
The principal features of the genetic code
Have you ever wondered about the importance of the genetic code in the
continuity of life? The fact is that a possible destruction of the code would be a
total disaster for all living species, while is essential for the “building” of
polypeptide chains, which have specific functions that are vital for all the
organisms. The revealing of the genetic code is based on a complicate process:
DNA replication, transcription and translation, called the central dogma of the
molecular biology (Francis Crick 1957).
The Central Dogma:
DNA encodes the information to make RNA.........and RNA molecules function
together to make protein
Figure 1: the central dogma of the molecular biology
All the genetic information is “hidden” in the DNA molecule. Therefore,
it has to unwind and copy the two parent strands for the production of two
identical complementary strands, which is vital for the perpetuation of the
genetic information. When the two complementary strands are synthesized,
transcription is the next step. Transcription is the process of RNA synthesis by
using the DNA strand as a template and the product is the mature mRNA, which
includes the genetic code that is ready to be translated. The final process is
translation, the synthesis of a polypeptide chain using the mature mRNA. All
the flowing of information from the genes to proteins depends on the triplet
code. The sequence of three bases is a codon (Crick 1958), which specifies an
amino acid. The whole process is based on the translation of the base language
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THE PRINCIPAL FEATURES OF THE GENETIC CODE-STELLA ANGELI
to the amino acids language. Small and large subunits of the ribosome and the
tRNA work together in order to create a polypeptide chain.
Figure 2: DNA replication
Figure 3: transcription
Figure 4: mature mRNA
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Figure 5: translation
THE PRINCIPAL FEATURES OF THE GENETIC CODE-STELLA ANGELI
Figure 6: The genetic code
The genetic code was discovered by Francis Crick in 1953 when DNA
molecule was already proposed. The principal features of the code are:
1. The code is triplet, unpunctuated and nonoverlapping
Francis Crick tried to find out how the genetic code works by
making experiments. It is a fact that there are 20 amino acids and only 4
bases that specify them. He made the hypothesis that two bases specify
one amino acid, therefore, while the bases are 4, the result is 4 2 =16
combinations. That meant the combinations were not enough for all the
amino acids. The final estimation was the right one: three bases specify
one amino acid (43 =64 possible combinations) and the triplets are called
codons.
While Francis Crick was doing experiments using bacteriophage
crosses to indicate in vitro that the genetic code was actually a triplet
code, another scientist, named Marshall Nirenberg, who used poly-U as a
control RNA template in his experiment, discovered that UUU was the
codon for phenylalanine and after that he could solve the whole code.
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THE PRINCIPAL FEATURES OF THE GENETIC CODE-STELLA ANGELI
Francis Crick comments on Nirenbergs discovery: "What he had discovered
was that he could add an artificial message to a test-tube system that synthesized
proteins and get it to direct some synthesis. In detail, he had added poly U -- the RNA
message consisting almost entirely of a sequence of uracils -- to the system and it had
synthesized phenylalanine. This suggested that UUU (assuming a triplet code) was a
codon for phenylalanine (one of the "magic twenty" amino acids), as indeed it is...
Nevertheless it was an epoch-making discovery, after which there was no looking
back." (Francis Crick in "What Mad Pursuit" on a visit to the Biochemical
Congress in Moscow in 1961). Crick’s comment indicated the importance
of that discovery because after that everything was clear for the solving of
the genetic code.
Figure 7: Francis Crick
Figure 8: The
experiment which used
uracil (U) as a template
produced a protein
entirely made up of the
amino acid
phenylalanine (F). The
first letter of the genetic
code was hence
identified.( Lotta
Fredholm, Science
Journalist)
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THE PRINCIPAL FEATURES OF THE GENETIC CODE-STELLA ANGELI
Apart from a triplet code, the genetic code is unpunctuated, that
means that there are no gabs between the triplets, and nonoverlapping,
mmmiwhich means that triplets are read in order as a result to have diversity
hhhhhand each nucleotide belongs to only one codon. If it was overlapping,
gggggithere would be less combinations of the amino acids.
Figure 9: several possibilities how triplets might code for amino acids
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THE PRINCIPAL FEATURES OF THE GENETIC CODE-STELLA ANGELI
2. The Genetic Code is Unambiguous
The code is unambiguous and that means that each codon
specifies a specific amino acid and only that. For example, GCU
codes for the amino acid alanine and no other amino acid.
(Nature,192 1227; 1961)|
3. The code is degenerate
Degeneration is one of the most important features of the
genetic code because it has a protective role in all the organisms.
Most of the amino acids can be specified by more than one codon
(e.g. glutamic acid is specified by GAA and GAG codons), which
differ in the third (wobble) position. The codons that specify the
same amino acid are called synonyms. Only two amino acids are
specified by only one codon: methionine (AUG) and tryptophan
(UGG). Degeneracy is a huge property because it gives protection
against mutation. For example, if a codon is influenced by
mutation in the third position, there would not be a change in the
synthesized polypeptide chain because there are other codons that
specify that specific amino acid. David Tyler said in his article,
"Optimal features of the genetic code": "…the assignment of amino
acids to codons appears to be optimal for minimizing the effect of
translational misread errors". This is because that kind of errors
do not influence the translated protein.
4. There is one start and three stop codons
The start codon in all the organisms is AUG and specifies
methionine. The stop codons are UAG, UGA and UAA. The
presence of these codons on the mRNA molecule leads to the
termination of the polypeptide chain synthesis.
5. The Genetic Code is nearly Universal
The genetic code is nearly universal because it is the same
worldwide .All the organisms have the same genetic code, thus, the
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THE PRINCIPAL FEATURES OF THE GENETIC CODE-STELLA ANGELI
mRNA from any organism can be translated into plant, animal or
bacteria cell extract in vitro and produce the exact same protein.
On the other hand there are some exceptions in the genetic code of
mitochondria and protozoan species, as figure 10 and 11 show
(Nature, vol.367, 1994)
Figure 10 : the genetic code in nucleus
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Figure 11: the genetic code in mitochondria
THE PRINCIPAL FEATURES OF THE GENETIC CODE-STELLA ANGELI
Finally, by having an overview, the genetic code includes all the
information, which is going to be translated for the protein synthesis. Therefore
the importance of its features is vital for the survival of all organisms, but there
are still so many unknown secrets of the code that are ready to be revealed.
Once, Tobias Bollenbach, wrote in his article: "Looking deeper into the
structure of the code, we wonder what other remarkable properties it may bear.
While our understanding of the genetic code has increased substantially over
the last decades, it seems that exciting discoveries are waiting to be made."
(Tobias Bollenbach, Kalin Vetsigian, and Roy Kishony, 2007, Genome
Research).
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REFERENCES
1. Ambrogelly A. and Palioura S., (2007), Natural Chemical Biology 3, 29-35
2. Ansary Z. A., (2007), Chemical crosshairs on the central dogma, Natural Chemical
Biology, vol,3, numb1, pp.1-7
3. Bollenbanch T., Vetsigian K., Kishony R.,(2007) Genome research, 17:401-404
4. Campbell and Reece, The molecular basis of inheritance, pp.305-316
5. Campbell and Reece, From gene to protein, pp.328-331
6. Hayes B., (2004), Genome biology: on the genetic code, science week,93:494
7. Itzkovitz S. and Alon U,(2007), The genetic code is nearly optimal for allowing
additional information within protein-coding sequences, Genome research
8. Maddox J. (2004), The genetic code by numbers, Nature, vol.367, pp.111
9. Mathews, Holde. Van,(2000) aherrn , third edition, pp.104-107 Ahern’s
Biochemisrtey
10. Novozhilov S. A., wolf I.Y., Koonin V,E, Education of the genetic code: partial
optimization of a random code for robustuess to translation error in a ragged fitness
landscape, Biology direct (2007).
11. Stryer L. Chapter 5, Flow of the genetic information, Biochemistry, pp. 103-111
12. Stryer L. Chapter 34, Protein synthesis, Biochemistry, pp. 886-7
13. http://www.mun.ca/biochem/courses/3107/Topics/Genetic_code.html
14.
15.
16.
17.
http://en.wikipedia.org/wiki/Genetic_code
http://www.brighthub.com/education.aspx
http://nobelprize.org/educational_games/medicine/gene-code/history.html
http://www.arn.org/blogs/index.php/literature/2007/03/01/optimality_features_in_the_ge
netic_code
18. http://www.bio.iupui.edu/biocourses/N100/imag...
19. http://www.accessexcellence.org/RC/VL/GG/genetic.php
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THE PRINCIPAL FEATURES OF THE GENETIC CODE-STELLA ANGELI
20. http://www.emunix.emich.edu/~rwinning/genetics/code.htm
IMAGES
Figure 1 taken from: http://www.accessexcellence.org/RC/VL/GG/genetic.php
Figure 2 taken from:
http://cellbiology.med.unsw.edu.au/units/images/DNA_replication_fork.png
Figure 3 taken from:
http://porpax.bio.miami.edu/~cmallery/150/gene/c7.17.7b.transcription.jpg
Figure 4 taken from: http://genome.wellcome.ac.uk/assets/GEN10000674.jpg
Figure 5 taken from:
http://nobelprize.org/educational_games/medicine/dna/a/translation/pics/translation2.
gif
Figure 6 taken from: http://www.emc.maricopa.edu/faculty/farabee/BIOBK/code.gif
Figure 7 taken from: http://www.skeptic.com/eskeptic/04-07-30images/crick.jpg
Figure 8 taken from: http://nobelprize.org/educational_games/medicine/genecode/history.html
Figure 9 taken from: http://bass.bio.uci.edu/~hudel/bs99a/lecture20/lecture1_3.html
Figure 10,11 taken from:
http://www.mun.ca/biochem/courses/3107/images/Nuclear_variants.GIF
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