IB Biology 2015

1. 10.1 Meiosis
Essential idea: Meiosis leads to independent assortment of
chromosomes and unique composition of alleles in daughter cells.
http://upload.wikimedia.org/wikipedia/c
ommons/4/46/Dividing_Cell_Fluorescenc
e.jpg

2. Understandings
Statement Guidance
10.1 U.1 Chromosomes replicate in interphase before meiosis.
10.1 U.2 Crossing over is the exchange of DNA material between non-sister
homologous chromatids.
10.1 U.3 Crossing over produces new combinations of alleles on the
chromosomes of the haploid cells.
10.1 U.4 Chiasmata formation between non-sister chromatids can result in
an exchange of alleles.
10.1 U.5 Homologous chromosomes separate in meiosis I.
10.1 U.6 Sister chromatids separate in meiosis II.
10.1 U.7 Independent assortment of genes is due to the random
orientation of pairs of homologous chromosomes in meiosis I.

3. Applications and Skills
Statement Guidance
10.1 S Drawing diagrams to show chiasmata formed by crossing over.

4. 10.1 U1 Chromosomes replicate in interphase before meiosis.
• During the S phase of
the cell cycle, so that
each chromosome has a
copy of itself and
consists of two sister
chromatids.
• During meiosis I,
chromosomes condense
and synapse to form
bivalents (homologous
chromosomes are
aligned next to each
other).
http://www.columbia.edu/cu/biology/co
urses/c200507/images/meiosis_19.gif

5. 10.1 U2 Crossing over is the exchange of DNA material between
non-sister homologous chromatids.

6. 10.1 U3 Crossing over produces new combinations of alleles on the
chromosomes of the haploid cells.
http://www.indiana.edu/~oso/lessons/Genetics/
figs/sheep/2_fXe_sp2.jpg

7. 10.1 U4 Chiasmata formation between non-sister chromatids can result
in an exchange of alleles.
• Chiasmata are points where two
homologous non-sister chromatids exchange
genetic material during crossing over in meiosis.
• Chromosomes intertwine and break at the exact
same positions in non-sister chromatids.
• The two chromosomes are now attached at
the same corresponding position on the non-
sister chromatid.
• Once attached the non-attached portions of the
chromatids actually repel each other.
• Chiasmata refer to the actual break of the
phosphodiester bond during crossing over.
• The chiasmata are separated during anaphase 1
which can result in an exchange of
alleles between the non-sister chromatids from
the maternal and paternal chromosomes.

8. 10.1 U5 Homologous chromosomes separate in meiosis I.
• During meiosis I, unlike mitosis
homologous chromosomes separate
to opposite poles; however, their
sister chromatids remain attached
to each other
• Homologous chromosomes can
exchange material in a process
called crossing over
• Meiosis I is considered reduction
division because the chromosome
number is reduced by half (2n -> n
in humans)
http://i.ytimg.com/vi/RfRtVDkABPI/hqdefault.jpg

9. 10.1 U6 Sister chromatids separate in meiosis II.
• During meiosis II sister
chromatids separate (some are
non-identical sister chromatids
due to crossing over
• This type of separation is very
similar to mitosis as the
chromatids are separated from
each other

10. 10.1 U7 Independent assortment of genes is due to the random
orientation of pairs of homologous chromosomes in meiosis I.
• Independent assortment is an essential
component in explaining how
chromosomes align themselves during
meiosis.
• It also explains how unlinked genes are
passed on from generation to generation.
• When homologues line up along the
equatorial plate in metaphase I, the
orientation of each pair of is random;
meaning the maternal or paternal
homologue can orient towards either pole.
• The orientation of how one set of
homologues line up has no effect on how
any of the other homologues line up.
• Two young women to the right are non
identical twin sisters, who because of
independent assortment have different
hair and skin coloring http://hellogiggles.com/bi-racial-twins/

11. 10.1 U7 Independent assortment of genes is due to the random
orientation of pairs of homologous chromosomes in meiosis I.

12. 10.1 S.1 Drawing diagrams to show chiasmata formed by crossing
over.
http://www.thestudentroom.co.uk/showthread.php?t=1561992
• The centromere is a section of
DNA on the chromosome where
the chromatids are closest, it's
generally tightly packed, the DNA
typically doesn't have a defined
sequence and is often satellite
DNA, non-coding. It's the point at
which the mitotic spindle attaches
itself.
• Chiasmata are places where the
chromosomes cross over, and
sometimes exchanges of DNA take
place. This is usually looser
packed, often functional DNA