Ward's World + McGraw Hill's AccessScience

35569_Ward's World+MGH Human Genetics

Issue link: https://wardsworld.wardsci.com/i/1435968

Contents of this Issue


Page 2 of 8

A gene initiates synthesis of a protein by transcription of the DNA into messenger ribonucleic acid (mRNA), which is a single-stranded molecule complementary to the DNA. After processing within the cell nucleus, the mRNA moves into the cytoplasm, where it binds to ribosomes. The translation of RNA into protein takes place on the ribosomes. In addition, a typical gene is not a simple uninterrupted length of DNA. Most genes are made up of coding sequences (exons) separated by noncoding regions (intervening sequences, or introns). Tran- scription of the gene into RNA begins at an initiation site in advance of the first coding sequence and terminates beyond the end of the last sequence. After the entire DNA segment has been transcribed into mRNA, the intervening sequences are re- moved, and the coding sequences are spliced together before the mRNA is translated into a polypeptide (Fig. 4). Any gene occupies a specific chromosomal position, or locus. The alternative genes at a particular locus are said to be alleles. If a pair of alleles are identical, the individual is homozygous; if they are different, the individual is heterozygous. Mutation Genetic variation has its origin in mutation. Although any change in DNA is a mutation (whether it is a microscopically detectable change in the structure of a chromosome or a single base change in the genetic code), the term is usually applied to a stable change in DNA that alters the genetic code and thus leads to synthesis of an altered protein. Mutation can occur in reproductive cells or somatic cells, but the genetically signifi- cant ones are those that occur in reproductive cells and can be transmitted to future generations. Natural selection acts upon the genetic diversity generated by mutation to preserve benefi- cial mutations and eliminate deleterious ones. A very large amount of genetic variation exists in the hu- man population. Everyone carries many mutations; some are newly acquired, but others are inherited through innumerable generations. Although the exact number is unknown, it is likely that everyone is heterozygous at numerous loci, perhaps as many as 20%. Single-gene inheritance The patterns of inheritance of characteristics determined by single genes or gene pairs depend on two conditions: (1) whether the gene in question is on an autosome (autosomal inheritance) or on the X chromosome (X-linked inheritance); and (2) whether the gene is dominant (expressed in heterozy- gotes, when it is present on only one member of a chromosom- Human Genetics (continued) + ward ' s science Fig. 3 The double-helix structure of a DNA molecule. (Credit: Darryl Leja/National Human Genome Research Institute) Fig. 4 Simplified diagram showing (a) the structure of a gene, (b) the mature messenger RNA (mRNA) after the two intervening sequences have been spliced out and the "cap" and "tail" added, and (c) the polypeptide translated from the mRNA. (Copyright © McGraw Hill)

Articles in this issue

Links on this page

Archives of this issue

view archives of Ward's World + McGraw Hill's AccessScience - 35569_Ward's World+MGH Human Genetics