Cell Structure and Genetic Control 81Table 3.3 | Stages of Meiosis
Stage Events
First Meiotic Division
Prophase I Chromosomes appear double-stranded.
Each strand, called a chromatid, contains
duplicate DNA joined together by a structure
known as a centromere.
Homologous chromosomes pair up side by side.
Metaphase I Homologous chromosome pairs line up at equator.
Spindle apparatus is complete.
Anaphase I Homologous chromosomes separate; the two
members of a homologous pair move to
opposite poles.
Telophase I Cytoplasm divides to produce two haploid cells.
Second Meiotic Division
Prophase II Chromosomes appear, each containing two
chromatids.
Metaphase II Chromosomes line up single file along equator as
spindle formation is completed.
Anaphase II Centromeres split and chromatids move to
opposite poles.
Telophase II Cytoplasm divides to produce two haploid cells from
each of the haploid cells formed at telophase I.Figure 3.31 Crossing-
over. ( a ) Genetic variation results
from the crossing-over of tetrads,
which occurs during the first meiotic
prophase. ( b ) A diagram depicting the
recombination of chromosomes that
occurs as a result of crossing-over.(a) First meiotic prophase Chromosomes pairing Chromosomes crossing-over(b) Crossing-overEpigenetic inheritance occurs by a variety of mecha-
nisms, including (1) posttranslational modifications of his-
tone proteins (the basic proteins that regulate the degree of
compaction of the chromatin; see fig. 3.15 ); and (2) methyla-
tion (the addition of single-carbon methyl groups) of cytosine
bases in DNA (specifically cytosines that precede guanines).
DNA methylation is generally associated with decreased
genetic transcription and gene silencing. Through these
means, only one allele (gene) of a pair (from the maternal or
paternal chromosomes) may be expressed, and only one X
chromosome of the two Xs in a female is active. Acetylation
(the addition of a two-carbon acetyl groups) to lysine amino
acids in histone proteins has the opposite effect, increasing
gene activity (genetic transcription; see fig. 3.15 ). These and
other epigenetic changes can be induced by environmental
factors, such as disturbances in energy metabolism caused by
starvation or a high-fat diet. Because of epigenetic changes
(as well as the effects of “jumping genes,” previously dis-
cussed), even identical twins can have differences in gene
expression.
Cancers generally show a reduction in DNA methylation,
the epigenetic mechanism for long-term gene silencing. This is
associated with activation of genes and instability of chromo-
some structure in the cells of a tumor. By contrast, extensive
methylation of cytosine bases in “islands” of DNA has also
been seen in some cancers, and seems to be associated with
repression of specific tumor suppressor genes.