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