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# Protection and Survival ## Gene Expression Although all nucleated cells (except gametes) have an identical set of genes, each cell type uses only those genes related directly to its own particular function. For example, the only cell type containing hemoglobin is the red blood cell, although all...

# Protection and Survival ## Gene Expression Although all nucleated cells (except gametes) have an identical set of genes, each cell type uses only those genes related directly to its own particular function. For example, the only cell type containing hemoglobin is the red blood cell, although all body cells carry the hemoglobin gene. This selective gene expression is controlled by various regulatory substances, and the genes not needed by the cell are kept switched off. ### Spot Check 1. If the base sequence of a portion of DNA is ATTGAC, what will the base sequence be on the corresponding RNA? 2. What is the name given to a triplet sequence of three bases on an RNA molecule, which codes for a specific amino acid? ## Cell Division ### Learning Outcomes After studying this section, you should be able to: - Explain the mechanism of DNA replication. - Compare and contrast the processes of mitosis and meiosis. - Describe the basis of genetic diversity from generation to generation. Most body cells are capable of division, even in adulthood. Cell division usually leads to the production of two identical diploid daughter cells by the process of mitosis and is important in body growth and repair. Production of gametes is different in that the daughter cells have only half the normal chromosome number: 23 instead of 46, i.e., they are haploid. Gametes are produced by a form of cell division called meiosis. DNA replication takes place before mitosis and meiosis. ### DNA Replication DNA is the only biological molecule capable of self-replication. Mistakes in copying may lead to production of non-functioning or poorly functioning cells, or cells that do not respond to normal cell controls (this could lead to the development of a tumor). Accurate copying of DNA is therefore essential. The initial step in DNA replication is the unfolding of the double helix and the unzipping of the two strands to expose the bases, as happens in transcription. Both strands of the parent DNA molecule are copied. The enzyme responsible for DNA replication moves along the base sequence on each strand, reading the genetic code and adding the complementary base to the newly forming chain. This means that each strand of opened bases becomes a double strand and the end result is two identical DNA molecules (Fig. 17.7). As each new double strand is formed, other enzymes cause it to twist and coil back into its normal highly folded form. ## Mitosis This is described on page 47. ## Meiosis Meiosis is the final step in gamete production. On fertilisation, when the male gamete (sperm cell) and the female gamete (ovum) unite, the resulting zygote is diploid because each gamete was haploid. Unlike mitosis, meiosis involves two distinct cell divisions rather than one. Additionally, meiosis produces four daughter cells, not two, all different from the parent cells and from each other. This is the basis of genetic diversity and the uniqueness of each human individual. --- **Figure 17.7**: DNA replication.