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Lefebvre's hints part 2.docx

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BIOL 102
Daniel Lefebvre

Lefebvre's Hints Part Two Figure 14.16 Formation of a bivalent and crossing over during meiosis I.At the beginning of meiosis, homologous chromosomes pair up to form a bivalent, usually with a synaptonemal complex between them. Crossing over occurs between homologous, nonsister chromatids within the bivalent. During this process, homologues exchange segments of chromosomes. For simplicity the two homologous chromosomes are shown side by side, with two nonsister chromatids close and two much farther apart. In real cells, however, the homologous chromosomes actually lie on top of each other; hence, both nonsister chromatids of homologous chromosomes are equivalent distances apart. ------------ Figure 14.17 The stages of meiosis in a plant cell.The top portion of each panel shows micrographs of nuclei of the Easter lily (2n = 24) progressing through the indicated stages of meiosis. The lower panel gives a diagrammatic representation of the comparable event in a simpler nucleus (2n = 6). ------- Figure 15.24 Epistasis in the sweet pea.The colour of the sweet pea flower is controlled by two genes, each with a dominant and a recessive allele. Each of the dominant alleles (C and P) encodes an enzyme required for the synthesis of purple pigment. A plant that is homozygous recessive for either gene (cc or pp) cannot synthesize the pigment and will have white flowers. • epistatic genes --------- --------- Figure 15.30 Random X-chromosome inactivation in a calico cat.(a) A calico cat. (b) X inactivation during embryonic development. The calico pattern is due to random X-chromosome inactivation in a female that is heterozygous for the X-linked gene with black and orange alleles. The cells at the top of this figure represent a small mass of cells making up the very early embryo. In these cells, both X chromosomes are active. At an early stage of embryonic development, one X chromosome is randomly inactivated in each cell. The initial inactivation pattern is maintained in the descendents of each cell as the embryo matures into an adult. The pattern of orange and black fur in the adult cat reflects the pattern of X inactivation in the embryo. • a short region on the X chromosome called the X-inactivation centre (Xic) is known to play a critical role. Eeva Therman and Klaus Patau identified Xic from its key role in X inactivation. • The counting of human X chromosomes is accomplished by counting the number of Xics. The Xic on each X chromosome is necessary for inactivation to occur. • in cells with two X chromosomes, if one of them is missing its Xic because of a chromosome mutation, neither X chromosome will be inactivated. This is a lethal condition for a human female embryo. -------- Figure 18.2 Step 2 of gene cloning: The actions of a restriction enzyme and DNA ligase to produce a recombinant vector.The restriction enzyme binds to a specific sequence in both the chromosomal and the vector DNA. It then cleaves the DNA backbones, producing DNA fragments. The complementary single-s
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