2024-01-23 23:00:00
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Certain white or cream cats have a very interesting pattern. Their legs, ears, tail, or the area around the nose are darker in color that can even reach black. Cats with this coloration are normally called Siamese, although neither the breed of cat nor even the species has anything to do with its characteristic shade, called colorpoint.
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The most curious thing about this shade is that it apparently varies with the seasons. Animals with colorpoint are lighter in summer and slightly darken their fur during winter. The season really doesn’t have much to do with their color, but rather they suffer from a type of temperature-dependent albinism. The central area of the body is whiter because the temperature in that area is higher, while the extremities, the ears, and the area around the snout, where colder air enters, is darker.
Look of a cat of the modern Siamese breed. You can see the characteristic triangular shape of its head, its blue eyes and the colorpoint pattern.
The precious biochemistry that explains the process
The mutation that colorpoint animals present has to do with the production of a protein called tyrosinase. Tyrosinase is a oxidase, that is, it is responsible for add oxygen to certain compoundss that the body needs to guarantee its correct functioning. This enzyme is found in animals, plants and fungi, and oxidizes a wide variety of compounds, but mainly an amino acid called tyrosine.
In mammals, tyrosinase is active in the melanocytes, cells found in the skin that have special organelles called melanosomes. These organelles are responsible for producing melanin, a pigment made up of different compounds that colors our eyes, our hair, and makes us brown in summer. The main function of melanin is not to provide color itself, but rather it is responsible for absorb UV rays from the sun to prevent damage to DNA. In addition, recent studies show that melanin also has antioxidant properties, and new drugs based on this molecule are being developed to combat aging or cancer.
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Representation of a melanocyte. Melanosomes (in yellow) move along cellular branches called dendrites to deliver melanin to epithelial cells.
Melanin production is a fascinating biochemical journey. Inside the melanosome, the enzyme tyrosinase adds oxygen to tyrosine. This oxidation transforms tyrosine into dihydroxyphenylalanine, known as DOPA. DOPA It can be further oxidized to form eumelanin (the most common form of melanin) or a cysteine can be added before further chemical reactions that produce pheomelanin.
There are other types of melanin, such as pyomelanin, allomelanin and neuromelanin that come from similar chemical pathways, all generated from DOPA. The mixture of the different forms of melanin produces the pigment that we all knowalthough the most common is eumelanin.
Tyrosinase problems
The entire process that we have described is dependent on the enzyme tyrosinase. Therefore, Any alteration in this enzyme will affect the transformation of tyrosine into DOPA. and, consequently, that melanocytes can create melanin. That is, if a mutation occurs in the enzyme tyrosinase that completely prevents it from carrying out its activity, the body cannot produce melanin. The loss in melanin production causes albinism of the oculocutaneous type, when melanin is not produced in the skin or eyes. However, there is a special mutation that does not completely deactivate the enzyme, but rather makes it more sensitive to temperature.
The mutation, characterized in 2005, is known as “Himalayan mutation” although scientifically it is written as C975del. That is, in cellular DNA, the gene that produces tyrosinase the nucleotide cytosine is lost at position 975. Due to the way the cell reads DNA, the loss of this nucleotide causes a decisive change in the production of tyrosinase. To understand the reason for this change, it is necessary to take a look at the DNA reading mechanism that occurs inside the cell.
If we have a DNA molecule with the 4 nucleotides ordered – for example – in the following way: ATGCATCGA[…], the cell will create a protein by reading the nucleotides, dividing them 3 by 3 in what are known as codons. Therefore, first the cell will transcribe this DNA into messenger RNA, which will leave the nucleus and reach the ribosomes. And then, the ribosome will read the transcribed RNA that contained the information (ATG) (CAT) (CGA) […] and each of those triplets will be translated into amino acids. So, Every 3 nucleotides, the ribosome will add an amino acid and assemble the protein of interest. Now, a deletion mutation means that a nucleotide is removed. Specifically in the case of tyrosinase, a C is eliminated.
Therefore, continuing with the previous example but eliminating the first C, the DNA would look like this: ATGATCGA[…], and the cell reading would be the following (ATG) (ATC) (GA…). That is to say, Although the first part of the protein is correct, after the mutation a change in the reading pattern occurs and its amino acids change completely, which alters the final protein.
In the specific case of tyrosinase, the c975del mutation destabilizes the enzyme. This lack of stability means that it can only operate at full capacity when the temperature is below 33°C. At higher temperatures, the enzyme is unable to oxidize tyrosinase to DOPA and, therefore, Melanin is only produced in places where the skin is below 33°C. In the specific case of cats, the extremities, ears and snout usually have temperatures slightly lower than body temperature, which is why they end up darkening. However, if the cat is exposed to the elements in a cold climate, or if the hair in some area has to be cut for surgery, the skin temperature may drop and darken. This is why the color change.
Colorpoint is not just a cat thing
Colorpoint is normally associated with Siamese cats, but this breed of cat has other characteristics that make them unique. Among the features of these felines, the triangular-shaped head, a slender neck and body, and the shape of the tails and legs stand out. However, The colorpoint pattern does not only occur in Siamese cats, but can also occur in virtually any other breed.
Furthermore, colorpoint mutations are not just black and white, but There is a whole range of colors from brown to reddish. This is because the tyrosinase enzyme can suffer other mutations, such as G715T or G940A, where one nucleotide is replaced by another and, therefore, the consequences can be milder, giving a darker tone. Finally, colorpoint also appears in other mammals such as rats, mice, rabbits, ferrets, guinea pigs, dogs and sheep.
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Rabbit showing colorpoint pattern
The beauty of science lies in finding the explanation or why of things. In this biological case, temperature physics, biochemistry and genetics come together to show in detail what happens in colorpoint patterns. It is impressive how these animals walk in their fur, without knowing it, a map that indicates your skin temperature.
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