S. M. Zaigham Zia
1
, 10.1 Meiosis
Chiasmata
1. Draw a diagram to show chiasmata formed by crossing over
Chiasmata are X-shaped points of attachment between two non-sister chromatids of a homologous pair.
Chiasmata form as a result of crossing over and hence non-sister chromatids should show an exchange
of genetic material. The exchange of genetic material is most easily shown if homologous chromosomes
are differentially colour-coded.
2. Outline the formation of chiasmata during crossing over
During prophase I, chromosomes
shorten and coil. Homologous pairs
of chromosomes come together to
form a bivalent so that maternal
and paternal chromosomes are
next to one another. Homologous
chromosomes contain the same
genes, but since they came from
different parents, they can have
different alleles. As they line up
together, the non-sister chromatids
may touch and break.The two
segments may then re-join at the
corresponding position on the
other chromatid. In this way,
chromatids are formed that are a
mixture of paternal and maternal
alleles.
The region where this happens is
called a chiasma (plural
chiasmata). The point at which a chiasma form is largely random. However, not all chromatids will form
chiasmata and they never occur at all on some chromosomes. In this way, some chromatids will retain
their full complement of paternal or maternal alleles.
S. M. Zaigham Zia
2
1
, 10.1 Meiosis
Chiasmata
1. Draw a diagram to show chiasmata formed by crossing over
Chiasmata are X-shaped points of attachment between two non-sister chromatids of a homologous pair.
Chiasmata form as a result of crossing over and hence non-sister chromatids should show an exchange
of genetic material. The exchange of genetic material is most easily shown if homologous chromosomes
are differentially colour-coded.
2. Outline the formation of chiasmata during crossing over
During prophase I, chromosomes
shorten and coil. Homologous pairs
of chromosomes come together to
form a bivalent so that maternal
and paternal chromosomes are
next to one another. Homologous
chromosomes contain the same
genes, but since they came from
different parents, they can have
different alleles. As they line up
together, the non-sister chromatids
may touch and break.The two
segments may then re-join at the
corresponding position on the
other chromatid. In this way,
chromatids are formed that are a
mixture of paternal and maternal
alleles.
The region where this happens is
called a chiasma (plural
chiasmata). The point at which a chiasma form is largely random. However, not all chromatids will form
chiasmata and they never occur at all on some chromosomes. In this way, some chromatids will retain
their full complement of paternal or maternal alleles.
S. M. Zaigham Zia
2
