Porth's Essentials of Pathophysiology, 4e

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Genetic Control of Cell Function and Inheritance

C h a p t e r 5

Chromosomes Most genetic information of a cell is organized, stored, and retrieved in discrete bundles of DNA called chro- mosomes. Although the chromosomes are visible only in dividing cells, they retain their integrity between cell divisions. The chromosomes are arranged in pairs: one member of the pair is inherited from the mother, the other from the father. The maternal and paternal chro- mosomes of a pair are called homologous chromosomes (homologs). Humans have 23 pairs of chromosomes (46 total). Of the 23 pairs, 22 are called autosomes . These have the same appearance in all individuals, males and females, and each has been given a numeric des- ignation for classification purposes (Fig. 5-6). The sex chromosomes, which make up the 23rd pair of chro- mosomes, determine the sex of a person. All males have an X and Y chromosome (i.e., an X chromosome from the mother and a Y chromosome from the father); all females have two X chromosomes (i.e., one from each parent). The much smaller Y chromosome contains the male-specific region (MSY) that determines sex. This region comprises more than 90% of the length of the Y chromosome. Only one X chromosome in the female is active in controlling the expression of genetic traits; however, both X chromosomes are activated during gametogen- esis. In the female, the active X chromosome is invisible, but the inactive X chromosome can be visualized with appropriate nuclear staining. This inactive chromatin mass is called a Barr body . The genetic sex of a child can be determined by microscopic study of cell or tissue samples for the presence of a Barr body. For example, the cells of a normal female have one Barr body and therefore a total of two X chromosomes. A normal male has no Barr bodies. Males with Klinefelter syndrome (one Y, an inactive X, and an active X chromosome) exhibit one Barr body.

■■ Genetic information is stored in a stable macromolecule called deoxyribonucleic acid (DNA). Genes transmit information contained in the DNA molecule as a triplet code consisting of an arrangement of the nitrogenous bases of the four nucleotides (i.e., adenine, guanine, thymine [or uracil in RNA], and cytosine). ■■ Gene mutations represent accidental errors in duplication, rearrangement, or deletion of parts of the genetic code. Fortunately, most mutations are corrected by DNA repair mechanisms in the cell. ■■ The production of proteins requires both DNA and a second type of macromolecule called ribonucleic acid (RNA).The process is accomplished by (1) the transcription of the DNA code onto messenger RNA, (2) the synthesis of proteins by ribosomal RNA, and (3) the translation of messenger RNA code by transfer RNA, which delivers the amino acids needed for protein synthesis to ribosomal RNA of ribosomes located in the cytoplasm. ■■ The degree to which a gene or particular group of genes is active is called gene expression. Gene expression involves a set of complex interrelationships among different levels of control including RNA transcription and posttranslational processing. ■■ Posttranslational processing involves the proper folding of the newly synthesized polypeptide chain into its unique three-dimensional conformation. Posttranslational processing may also involve the combination of polypeptide chains from the same or an adjacent chromosome, the binding of small cofactors, or enzyme modification.

FIGURE 5-6. Karyotype of a normal male (xy). (From National Cancer Institute Visuals. No. AV-9700-4394.)