Genes determine hereditary traits through the provision of precise instructions for cellular activity. Genes are both the functional and physical unit of heredity passed from parent to offspring. A gene is composed of a linear segment of DNA that encodes instructions for the synthesis of RNA molecules, which in turn provide the instructions for the synthesis of proteins. Each DNA molecule contains from tens to thousands of genes.
The nucleic acid sequence of a DNA molecule is encoded by four repeating nucleotide bases: adenine (A), cytosine (C), guanine (G), and thymine (T) (Figure 4.1). A DNA molecule is composed of two strands of nucleo-tides with the nucleic acids facing inward in an antiparallel fashion and the sugar-phosphate backbone forming a ladder-like structure which twists for stability Nucleotide bases on each side of the ladder are linked by hydrogen bonds to form a base pair, with adenine coupled with thymine and guanine coupled with cytosine.
DNA molecules are wound around histone protein complexes that provide structural support and regulatory functions. This structure permits a remarkable amount of compaction, which results in condensed superstructures called chromosomes. Nucleated cells of humans have 23 pairs of morphologically distinct chromosomes, with one chromosome of a pair inherited from each parent. There are 22 autosomes and a pair of sex chromosomes. The chromosomes that form each pair are termed homologs, and with the exception of a set of genes harbored in the sex chromosomes, each chromosome pair provides two copies of each gene. The two copies of the gene are called alleles. The two alleles are referred to as homozygous if their sequence is the same and heterozygous if each allele's sequence is different. Chromosomes can be isolated from cells, stained and visualized by microscopy with the total chromosomal set of a cell termed a karyotype (Figure 4.2). Publication of the human genome sequence in 2000 provided more precise estimates of the position of genes and has largely superseded the use of karyotype analysis (i.e. chromosome banding). However, gross chromosomal abnormalities such as extra, missing, or broken chromosomes are still commonly identified by examining the karyotype.
In addition to the nuclear genome, DNA is contained in mitochondria. The mitochondrial genome is a compact circular DNA molecule and exists in multiple copies within each mitochondrion. The number of mitochondria found in each cell type is dependent on the energy requirements of that cell. Neurons are among the most mitochondrion-rich cell types. The human mitochondrial genome is composed of 37 genes, including 24 genes that encode RNA end-products (2 ribosomal RNAs (rRNA) and 22 transfer RNAs (tRNA)). Mitochondrial genomes are transmitted matrilineally to offspring as the only gamete that contains both cytoplasm and organelles is the human egg.
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