Animals cannot synthesize tyrosine from shikimate-3p because they lack epsp synthase. however, tyrosine is listed as a non-essential amino acid in most tables. what is the explanation?

Tyrosine is listed as a non-essential amino acid because it can be synthesised from the essential amino acid phenylalanine.

Explanation:

Amino acids are classified as essential and non essential according to whether they can be synthesized in the body itself or should be supplied by the diet. Essential amino acids cannot be synthesized in the body of the organism and thus can be obtained only from the diet. In the case of human beings, valine, phenylalanine, methionine etc are examples of essential amino acids.

Non essential amino acids can be synthesized within the body of the organism. Thus they need not be supplied by the diet. Alanine, aspartic acid etc are some of the non essential amino acids in human beings. It is given here that animals cannot synthesize tyrosine from shikimate-3p due to the lack of EPSP synthase.

But the amino acid tyrosine can be synthesized from the amino acid phenylalanine which is an essential amino acid. Thus tyrosine will be a non-essential amino acid.

you are certainly talking about this picture:

a: is the label of the white square, it is showing the  replication fork, formed by helicase (d) ==> so it is the site of dna separation and replication 

b: is a new dna brand that is following directly the replication fork, so it is continuous ==> built continuously from 5' to 3

c: dna polymerase cannot replicate dna on its own, it needs the of their proteins, and one of them plays the role of a clamp which fixes  the dna with the dna template and increases  the processivity of the enzyme. ==>   stops polymerase from falling off the  template.

d: is the enzyme responsible for forming the replication fork (helicase) ==> unwinds and separates dna into two strands

e: is the delayed strand, it is built  in the opposite sense of the helicase, so dna will be built in fragments ==> built  fragments from 5' to 3 joins

and finally f: is the enzyme that builds  the new dna strand (dna polymerase) ==>   adds nucleotides to make a new dna helix.

credit: gradman555p98x9p

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