Reproduction and Chromosome Transmission

Reproduction and Chromosome Transmission - Reproduction and...

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1 Reproduction and Chromosome Transmission General Features of Chromosomes General information The cellular structures that contain the genetic material are the chromosomes. The structure of prokaryotic and eukaryotic chromosomes differs considerably, although both are comprised of long chains of DNA. Prokaryotic cells, such as the bacteria, typically have a single, circular chromosome located within an area of the cell called the nucleoid (Figure 3.1a). Prokaryotic cells also possess a cell wall. Eukaryotic cells (fungi, protists, plants, animals) contain internal compartments, called organelles, which have specialized functions (Figure 3.1b). The nucleus of eukaryotic cells contains the majority of the DNA. Other organelles, such as the mitochondria and chloroplast, contain small amounts of DNA. Eukaryotic chromosomes are examined cytologically to yield a karyotype. The field of cytogenetics involves the microscopic examination of chromosomes. In actively dividing cells, the chromosomes are condensed, allowing an easier examination of their structure and number. To prepare human chromosomes for viewing (Figure 3.2a): Somatic cells are obtained from the blood. The cells are exposed to drugs that stimulate cell division. The cells are placed in a hypotonic solution that makes them swell, but not burst. The cells are exposed to a fixative to prevent movement. A dye that binds to the chromosomes is applied to the cells. The cells are then placed on a microscope slide and viewed (Figure 3.2b). At this point the chromosomes may be photographed, and a karyotype prepared to aide in the analysis of the chromosomes (Figure 3.2c).
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2 Eukaryotic chromosomes are inherited in sets. The majority of eukaryotic species are diploid; thus, all somatic cells have two sets of chromosomes. Pairs of the same chromosomes are called homologues. Homologous chromosomes are very similar in sequence and have the same genes, but may contain different alleles of those genes (Figure 3.3). Sex chromosomes are not homologous. Genes on homologous chromosomes have the same location, or loci. Cell Division General information Asexual reproduction in unicellular organisms involves the division of a preexisting cell (an organism) to form two new cells (two organisms). This type of asexual reproduction (i.e., one cell divides to produce two) is similar in bacteria and some unicellular eukaryotic species (yeast, amoeba) Cell division is necessary for the formation of a multicellular organism from a fertilized egg. Bacteria reproduce asexually by binary fission. Prokaryotic organisms typically live as single cells. Some bacteria, such as E. coli , can divide every 20-30 minutes. Prior to cell division the bacterium duplicates its chromosome. Division, called binary fission, occurs by forming a septum down the center of the
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Reproduction and Chromosome Transmission - Reproduction and...

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