Question
This amino acid is ____.
Answer
This amino acid is Tyrosine, TYR, Y: Barely Polar, Aromatic Uncharged.
Polar Neutral
Aromatic
Tyrosine is the precursor to catecholamines.
Caution: Tyrosine is to be lumped into the polar category, it can be trippy since Kaplan classified tyrosine as hydrophobic.
Important Discrepencies To Keep In Mind Amongst Some Amino Acids
Reasoning: TL/DR: these are borderline, complicated cases. There is no broad consensus on whether cysteine and tyrosine should be considered hydrophobic or polar. Proline is clearly nonpolar though.
The reason for the confusion is that are there several ways to define and measure polarity and hydrophobicity for amino acids. First, the polarity and hydrophobicity of the free amino acid is very different from that of the amino acid side chain in the context of a peptide (protein). I assume you are interested in the latter.
Proline is a bit special in that it does not have an -NH3 group; the amine nitrogen binds to the side chain -- it is more of a "loop" than a chain -- and the peptide bond is different. But this "side loop" clearly has no polar group, and I think there is good agreement that proline should be considered nonpolar. The Khan Academy page is probably mistaken on this one. Proline is intermediate in hydrophobicity.
Tyrosine: As mentioned, polarity of the tyrosine side chain is unclear since it is quite large and has both polar and nonpolar groups. Tyrosine is somewhat hydrophobic, but among the amino acids, it places itself near the middle on most hydrophobicity scales. So tyrosine is a borderline case, which explains why different sources disagree.
Cysteine is special in the context of proteins, since cysteine residues often pair up via disulfide bonds, and this changes its properties. By itself, the -SH group is polar (although less so than an -OH group) so the cysteine side chain is somewhat polar. It is intermediate in hydrophobicity as measured by solvent partitioning. Cysteine was considered hydrophobic based on the observation that cysteine is often found in the interior of proteins, away from the water solvent; but this is largely due to its ability to form disulfide bonds.
Note: Though the following visual is helpful, it is misleading to label methionine under the 'polar, uncharged' category. Also do not condone the 'ass***' mnemonic.
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A ____ sense RNA strand is itself an mRNA and can be translated directly into DNA.
A positive sense RNA strand is itself an mRNA and can be translated ____ into DNA.
This amino acid is ____.
This amino acid is Glycine, GLY, G: Nonpolar Aliphatic.
This amino acid is ____.
This amino acid is Alanine, ALA, A: Nonpolar Aliphatic.