8885d_c24_920-9472/11/041:36 PM Page927mac76mac76:385 24.1 Chromosomal Elements ∥中 °p FIGURE 24-5 Eukaryotic chromosomes. (a) A pair of linked and condensed sister chromatids from a human chromosome. Eukaryotic chromosomes an in this state after replication and at metaphase during mitosis. (b)A complete set of chromosomes from a leukocyte from one of the authors. There are 46 chromosomes in every normal human somatic cell. Eukaryotic cells also have organelles, mitochondria (Fig. 24-6)and chloroplasts, that contain DNA. Mito- chondrial DNA (mtDNA) molecules are much smaller than the nuclear chromosomes In animal cells. mtdNA contains fewer than 20,000 bp(16, 569 bp in human mtDNA) and is a circular duplex. Each mitochondrion typically has two to ten copies of this mtDNA molecule and the number can rise to hundreds in certain cells when an embryo is undergoing cell differentiation. In a few organisms(trypanosomes, for example) each mito- chondron contains thousands of copies of mtDNA, or- ganized into a complex and interlinked matrix known as a kinetoplast. Plant cell mtDNA ranges in size from 200,000 to 2,500,000 bp Chloroplast DNA (CpDNA)als exists as circular duplexes and ranges in size from 120,000 to 160,000 bp. The evolutionary origin of mito- chondrial and chloroplast dnAs has been the subject of much speculation. A widely accepted view is that they FIGURE 24-6 A dividing mitochondrion. Some mitochondrial are vestiges of the chromosomes of ancient bacteria that proteins and RNAs are encoded by one of the copies of the mito- gained access to the cytoplasm of host cells and became chondrial DNA (none of which are visible here). The DNA(mtDNA) the precursors of these organelles(see is replicated each time the mitochondrion divides, before cell division.Eukaryotic cells also have organelles, mitochondria (Fig. 24–6) and chloroplasts, that contain DNA. Mito￾chondrial DNA (mtDNA) molecules are much smaller than the nuclear chromosomes. In animal cells, mtDNA contains fewer than 20,000 bp (16,569 bp in human mtDNA) and is a circular duplex. Each mitochondrion typically has two to ten copies of this mtDNA molecule, and the number can rise to hundreds in certain cells when an embryo is undergoing cell differentiation. In a few organisms (trypanosomes, for example) each mito￾chondrion contains thousands of copies of mtDNA, or￾ganized into a complex and interlinked matrix known as a kinetoplast. Plant cell mtDNA ranges in size from 200,000 to 2,500,000 bp. Chloroplast DNA (cpDNA) also exists as circular duplexes and ranges in size from 120,000 to 160,000 bp. The evolutionary origin of mito￾chondrial and chloroplast DNAs has been the subject of much speculation. A widely accepted view is that they are vestiges of the chromosomes of ancient bacteria that gained access to the cytoplasm of host cells and became the precursors of these organelles (see Fig. 1–36). 24.1 Chromosomal Elements 927 (a) (b) FIGURE 24–6 A dividing mitochondrion. Some mitochondrial proteins and RNAs are encoded by one of the copies of the mito￾chondrial DNA (none of which are visible here). The DNA (mtDNA) is replicated each time the mitochondrion divides, before cell division. FIGURE 24–5 Eukaryotic chromosomes. (a) A pair of linked and condensed sister chromatids from a human chromosome. Eukaryotic chromosomes are in this state after replication and at metaphase during mitosis. (b) A complete set of chromosomes from a leukocyte from one of the authors. There are 46 chromosomes in every normal human somatic cell. 8885d_c24_920-947 2/11/04 1:36 PM Page 927 mac76 mac76:385_reb: