Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
a interphase nucleus b meiosis I, early prophase I:
chromosomes condensing and
becoming visible
c prophase I: homologous chromosomes have paired up, forming
bivalents, and crossing over of chromatids is occurring; members of
each pair of chromosomes are repelling each other but are still held
at the crossing-over points (chiasmata)
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d metaphase I: bivalents line up across the
equator of the spindle; the spindle is not
visible in the photo; e anaphase I: homologous
chromosomes move to opposite poles of the
spindle
e anaphase I: homologous chromosomes
move to opposite poles of the spindle
f telophase I and cytokinesis
g meiosis II, metaphase II:
single chromosomes line up
across the equator of a new
spindle
h anaphase II: chromatids
separate and move to opposite
poles of the new spindle
i late anaphase II j telophase II
Figure 16.9 Stages of meiosis in an animal cell (locust) (× 950). Interphase (not part of meiosis) is also shown.
QUESTIONS
16.2 Name the stage of meiosis at which each of the
following occurs. Remember to state whether the stage
you name is during division I or division II.
a Homologous chromosomes pair to form bivalents.
b Crossing over between chromatids of homologous
chromosomes takes place.
c Homologous chromosomes separate.
d Centromeres split and chromatids separate.
e Haploid nuclei are first formed.
16.3 A cell with three sets of chromosomes is said to be
triploid, 3n. A cell with four sets of chromosomes is
said to be tetraploid, 4n. Could meiosis take place in a
3n or a 4n cell? Explain your answer.