treatment with antimyosin antibodies in vitro. The contractile ring pulls the
plasma membrane of the telophase cell into the cleavage furrow. Kineto-
chore microtubules, which attach the kinetochores to the spindle appara-
tus, shorten and pull the chromatids to opposite poles in anaphase A
(answer a).Growth of polar microtubules results in the separation of the
spindle poles in anaphase B (answer c).M-cdk complex (MPF) regulates
metaphase events through phosphorylation (answer d).Antitubulin anti-
bodies block movement of chromosomes (answer e).The events that
occur during each stage of the cell cycle are shown in the table below.
Cell Biology: Nucleus Answers 147Interphase (G 1 , S, Duplication of centrioles and DNA synthesis (S phase)
and G 2 phases)
Prophase Nucleolus disappears
Prometaphase Nuclear envelope breaks down
Metaphase Alignment of chromosomes in metaphase plate
Anaphase Separation of sister chromatids; initiation of cytokinesis
Telophase Nuclear envelope reforms; completion of cytokinesis(Reproduced, with permission, from McKenzie JC, Klein, RM:Basic Concepts in Cell Biology
and Histology.New York: McGraw-Hill, 2000.)
Phase of
Cell Cycle Defining Event(s)63.The answer is a.(Alberts, pp 207–211. Junqueira, pp 54–55.)The
patient in the scenario suffers from systemic lupus erythematosus (SLE), a
chronic autoimmune disease. Nucleosomes are the basic structural packag-
ing unit of chromatin. Chromatin strands that have been treated to unpack
the chromatin structure have the appearance of beads on a string in elec-
tron micrographs. The beads are formed by a core of histones as an octamer
(i.e., two of each of the four nucleosomal histones: H2A, H2B, H3, and H4)
plus two turns of DNA. The nucleosome beads plus the DNA between
beads (i.e., linker DNA) constitute the nucleosome. There are additional
orders of chromosome packing, including nucleosomal packing. The tran-
scription of DNA is carried out by RNA polymerases I, II, and III, which are
responsible for transcription of different types of genes (answer b).The
nuclear pores are perforations in the nuclear envelope, each composed of a
nuclear pore complex (answer c).The nuclear matrix is the intranuclear
cytoskeleton and forms the scaffolding for nuclear structures (answer d).
Chromatids are held together at the centromere (answer e).