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FIGURE 38–1. Basic structure of nucleic acids.Nucleotides are three-part structures composed of a nitrogen-containing base, a five-carbon sugar, and a phosphate group. Chains of nucleic acid are formed by phosphate bonds between the 5' and 3' carbons of alternating nucleotide sugar molecules.

FIGURE 38–2. Chromosome structure.During cell division, chromosomal material becomes tightly compacted and individual chromosomes become visible as four-armed structures joined at a common centromere. Between cell divisions, chromosomal material is dispersed within the nucleus in a seemingly disorganized mass of DNA and protein.

FIGURE 38–3. Mitosis and meiosis.In mitosis, somatic cells proceed through stages of prophase, metaphase, anaphase, and telophase resulting in two identical daughter cells. In meiosis, germ cells initially follow the same stages of division resulting in two identical daughter cells, followed by a second meiotic division, during which stages of prophase, metaphase, anaphase, and telophase are repeated but result in four haploid germ cells.

FIGURE 38–4. Crossing over and recombination.During prophase of the first meiotic division, homologous arms of chromosome pairs become intertwined, in a process known as crossing over, and can undergo recombination, or exchange of DNA segments. Recombination serves to increase the genetic variability within chromosomes prior to segregation of homologous pairs into separate haploid daughter cells.

FIGURE 38–5. A "typical" gene and steps in polypeptide synthesis.DNA transcription is initiated at the 5' end of the gene and proceeds in a 3' direction, beginning at the promoter region. Transcribed precursor mRNA undergoes subsequent modifications into mature RNA prior to release in the cytoplasm where it serves to translate cell proteins.

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