Genetics in obstetrics and gynecology / Joe Leigh Simpson [and others].

Date:
[1982], ©1982
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Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Genetics in obstetrics and gynecology / Joe Leigh Simpson [and others]. Source: Wellcome Collection.

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    PRINCIPLES OF HUMAN GENETICS / 3 phosphate residues. The sugar and phosphate residues of opposite sides are connected transversely—like ladder rungs—by various nitrogenous bases called nucleotides. In DNA the nucleotides are composed of the purine adenine (A) or guanine (G) and the pyrimidine thymine (T) or cytosine (C). Each transverse connection consists of one purine and one pyrimidine, joined by hydrogen bonds (base pairing). Adenine is bound to thymine by two hydrogen bonds. Cytosine is bound to guanine by three hydrogen bonds. Thus, the ratio of adenine to thymine, or guanine to cytosine, is always 1:1. The ratio of aden¬ ine-thymine (AT) pairs to guanine-cytosine (GC) pairs varies not only between species but also between different chromosomes in the same species. The genetic information is coded by the sequence of nucleotides. Because of the base pairing described above, the two DNA strands are complementary. For example, if the sequence of bases on one strand is ATTGC (adenine-thymine-thymine-guanine-cytosine), the sequence on the opposite strand is TAACG. A codon, a sequence of three bases, signifies one and only one of the twenty amino acids. The sequence of codons determines the amino acid composition of a polypeptide. However, there are more codon permuta¬ tions, namely 4^ = 64, than there are amino acids; thus, different codons can signify the same amino acid. Some codons initiate protein synthesis, and still others apparently code for nothing (nonsense codons). In humans and other mammals DNA is carried by the chromosomes. Chro¬ mosomes consist of (1) at least two general classes of DNA—one class either moderately or highly repetitive with respect to its nucleotide sequence (repet¬ itive DNA), and another class not repetitive (unique-sequence DNA); (2) histone proteins, which are believed to provide structural integrity for the chromo¬ somes, and (3) nonhistone proteins, whose function is uncertain. The basic structure probably consists of a single long strand of DNA that is tightly coiled and branching. Unique sequence DNA is interspersed with intervening se¬ quences of DNA. Prior to completion of translation (see below), the intervening sequences (introns) must be removed. DNA replication occurs at different times in different chromosomes and can be initiated at several different points along a single chromosome. RNA and Protein Synthesis RNA directs polypeptide synthesis in the cytoplasm (translation). RNA is structurally similar to DNA; however, the sugar in RNA is ribose rather than deoxyribose, and one pyrimidine is uracil rather than thymine. There are sev¬ eral types of RNA: messenger RNA (mRNA), ribosomal RNA (rRNA), transfer RNA (tRNA), and heterodisperse RNA (hRNA). The function of hRNA is not entirely clear, but it appears to be a precursor of mRNA. Messenger RNA con¬ sists of a single-stranded molecule complementary to one of the two strands of the DNA double helix. After its synthesis by complementary pairing from a DNA strand, mRNA moves into the cytoplasm to associate with the ribosomes, structures that consist of protein and a nonspecific high molecular weight RNA (rRNA) (Fig. 1-2). Amino acids are prepared for protein synthesis by the reaction of adenosine triphosphate (ATP) with the a-carboxyl end of an amino acid to form an activated, amino acid-specific tRNA. This amino-acyl tRNA moves toward the surface of the ribosome. Protein synthesis progresses by movement