[erma79]

Various texts give various answers, why?

[Furanone]

Histadine is conditionally essential. Adults can produce it on their own, but infants need it supplied in their diet. Therefore for human adults there are eight essential amino acids, and for infants there are nine essential amino acids. The texts would have clarified that somewhere.

[Babcock_Hall]

Arginine is essential for infant rats but not for adult rats (Zubay, Biochemistry, p. 599).  A couple of amino acids can be made from others (Cys, Tyr); IMO this category falls into a grey area.

[writer]

Older lists used to have 8 essential AA, but newer tend to have 9 (histidne being the ninth).
http://www.nutrientsreview.com/proteins/amino-acids/histidine.html

These lists contain only what we have found so far. There were only 20 proteinogenic amino acids in humans known for a long time, but now are 22 (selenocysteine being the 22nd).

[erma79]

These lists contain only what we have found so far. There were only 20 proteinogenic amino acids in humans known for a long time, but now are 22 (selenocysteine being the 22nd).

I didn't know that. Thanks for the replies, all 

[writer]

Oops, I should say 21 proteinogenic amino acids in humans are known so far.

[Babcock_Hall]

The 64 codons are all accounted for.  I would not go looking for additional proteinogenic amino acids.

[erma79]

The 64 codons are all accounted for.  I would not go looking for additional proteinogenic amino acids.

Oops, I should say 21 proteinogenic amino acids in humans are known so far.

So how do I reconcile these two statements?

[Borek]

Simple - more than one codon for some of the aminoacids.

[Babcock_Hall]

Yes, exactly.  The genetic code is degenerate (more than one codon usually codes for an amino acid), but it is not ambiguous (one codon specifies only one amino acid, not more).  The last statement does not apply to the stop codons.

[erma79]

I know that the Genetic code is degenerate (although the term I was taught was "redundant" -in French, mind you)

The versions of it I am used to seeing show only 20 proteinogenic amino acids. That's why I was surprised to read Writer's claim that there were 21 of them.

Babcock seems to claim that this is false.
I don't claim anything, maybe Writer knows something I don't...

[Yggdrasil]

For the most part, humans encode 20 amino acids.  The one exception is selenocysteine which is present in just 25 of the ~ 30,000 proteins encoded by the human genome.  Selenocysteine gets incorporated into proteins in place of a stop codon when the stop codon is embeded within a specific RNA structure.  It is present in human proteins and it gets incorporated by the ribosome, so it technically counts as a proteinogenic amino acid.  But, it's present so rarely that most would say that there are only 20 proteinogenic amino acids.

[Babcock_Hall]

Degenerate and redundant mean the same thing in this context. 

Selenocysteine is an unusual amino acid.  I have an older copy of Voet and Voet's textbook which discusses it with respect to E. coli.  Its tRNA bears the anticodon UCA, which pairs with the codon UGA (which is ordinarily the Opal stop codon).  Therefore, I would say that this is a partial exception to the rule that the genetic code is unambiguous.  I used the word "partial" because the alternatives are an amino acid and a stop codon, not two amino acids.  Could there be another amino acid like selenocysteine?  Yes, but I am not aware of any evidence that there is one.  Here is a discussion of whether or not selenocysteine should be called the 21st amino acid:  http://jnci.oxfordjournals.org/content/96/7/504.full

There are a few other examples of stop codons which do not always mean stop.  In ciliated protozoa UAA and UAG are translated as Gln.  However from Voet and Voet's treatment (which makes it sound as if there is only one true stop codon in these organisms), it sounds as if this example illustrates that the genetic code is not quite universal, not an exception to the rule with respect to its being unambiguous.

[erma79]

I see. Thanks for the replies! I like going to bed a little less ignorant than when I got up.