Читать книгу Pet-Specific Care for the Veterinary Team - Группа авторов - Страница 225

DNA is a fabulous device for conserving everything that makes a living being what it is. Whether you believe in evolution or divine intervention, DNA contains a lot more than just the genes that code for all of a living thing's features.

It is the protein‐coding DNA that we are most concerned with as we attempt to understand the genetics of phenes, traits, and diseases. DNA is a molecule composed of two nucleotide chains wrapped around one another in a form known as a double helix. Each nucleotide (or polynucleotide) chain is composed of individual nucleotides, which in turn are composed of four nucleobases (cytosine, guanine, adenine or thymine), a sugar molecule (deoxyribose), and a phosphate group. The nucleotides occur in chains in which the sugar of one nucleotide binds to the phosphate group in the next, and the two separate chains are bound together by the nucleobase pairs, such that cytosine (C) binds to guanine (G) and adenine (A) binds to thymine (T). The structure of the double helix resembles a ladder twisted on its long access, with the nucleobase pairs resembling the rungs and the nucleotide chains resembling the rails or stiles (Figure 3.1.1).

Figure 3.1.1 Double helix structure of DNA.

Source: From Forluvoft, DNA simple2. Wikimedia Commons. Public Domain.

DNA is organized into paired structures, known as chromosomes. Each species has a defined number of chromosomes (23 pairs in humans, 39 pairs in dogs, and 19 pairs in the cat). Two of those chromosomes are known as sex chromosomes (X and Y chromosomes) and the remainder are known as autosomes. Males have one X and one Y chromosome and females have two X chromosomes.

In animals, almost all the DNA is compressed into the nucleus of cells, with a small amount found in mitochondria. The genetic information within an animal's genome is held within its genes and the genetic composition of a given animal for a specific variant is known as its genotype.

Genes represent a region of DNA that influence a particular phene, trait or characteristic. The dog genome consists of about 2.8 billion base pairs of nucleotides that represent about 19 000 protein‐coding genes; cats have about 2.4 billion base pairs of nucleotides that also represent about 19 000 protein‐coding genes. Humans and pets share about 85% of their genes, so it should not be surprising that many diseases found in one species may also be found in others (see 2.19 One Health).

A gene is that portion of DNA that codes for a specific sequence of amino acids, which in turn make proteins, enzymes, or polypeptides. As you might expect, DNA segments resemble but are not exactly like a passenger train, with genes end to end. Leaders and trailers occur before and after genes and within the gene, and some noncoded regions called introns separate coded regions of “expressed sequences” called exons.

For any one genetic character, then, each parent contributes one version of the gene for that character, which is called an allele. The location of a gene on a chromosome is its locus.

Given four bases (G, C, A, T) that can be arranged in groups of three (codon triplets, such as ATG, the triplet codon for the amino acid methionine), there are 64 (4³) combinations possible, but since there are only 20 amino acid products, this provides much opportunity for redundancy in the system (e.g., the amino acid alanine can be formed from the triplet codons GCT, GCC, GCA, GCG, AGA, AGG, CGT, and CGC). The average protein the DNA codes for is about 1000 amino acids long, which also means that it is about 3000 codons in length (three codons to one amino acid). Combined with codons that initiate and complete reading (for example, ATG is a start codon, and TAA, TAG and TGA are stop codons that mark the end of a sequence), the arrangement of codons is called an open reading frame.

Given an alphabet with only four letters (A, G, C, T), it may seem improbable that these nucleotides could account for all the genetic diversity in the world. Given also that these nucleotides code for only 20 amino acids, it seems difficult to forge a plausible argument for all the genetic variations seen. Because those four nucleotides can occupy any position along the DNA sequence, however, even a 10‐nucleotide stretch can form 4¹⁰ (more than 1 million) different combinations. Imagine it as a combination lock: instead of a three‐number sequence, you have an amino acid sequence of nucleotide triplets (e.g., CAT–TAG–GAC–ATT) that can code for an almost endless array of proteins.