Biology  5/e   Raven/Johnson
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Cell division: Most cells divide by binary fission or mitosis. In binary fission, cells duplicate their circular DNA and simply divide in half. In mitotic cell division, DNA is packaged into chromosomes. These chromosomes are replicated, and then sorted into the two new cells, in such a way that each cell has a complete genome.

Meiosis: Meiosis differs from mitosis in several ways. Cells must divide twice to complete one meiotic division. There is no replication of chromosomes between the two divisions. Genes are exchanged between homologous chromosomes before the chromosomes are sorted into the new cells. The end product is a set of cells with half the number of chromosomes that were in the original cells and of different composition than that of other cells.

Mapping genes: The location of genes on a chromosome can be mapped if their frequency of crossing over is known. The farther apart two genes are on a chromosome, the more likely it is that a cross over will occur between them.

DNA and genes: DNA contains the heredity information of cells. DNA is composed of two strands of nucleotide wrapped around each other in a double helix. Hydrogen bonds hold the two strands together, A paring with T and G pairing with C. A gene is a segment of DNA whose nucleotide sequence specifies a specific protein, or a region controlling the transcription of one or more genes.

Gene expression: Genes are expressed through processes called transcription and translation. The nucleotide sequences of DNA are copied (transcribed) into complementary sequences in mRNA. The nucleotide sequences of mRNA are translated by ribosomes into nucleic acid sequences forming the specified polypeptide.

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