Topic 10 Genetics and Evolution (AHL)
10.1 Meiosis
Understandings:
Chromosomes replicate in interphase before meiosis
Homologous chromosomes separate in meiosis I
Sister chromatids separate in meiosis II
Independent assortment of genes is due to the random orientation of pairs of
homologous chromosomes in meiosis I
Chiasmata formation between non-sister chromatids can result in an exchange
of alleles
Crossing over is the exchange of DNA material between non-sister homologous
chromatids
Crossing over produces new combinations of alleles on the chromosomes of the
haploid cells
Skills:
Drawing diagrams to show chiasmata formed by crossing over
Interphase
Interphase is an active period that precedes meiosis and involves
key events needed to prepare the cell for successful division
DNA is replicated during the S phase of interphase, resulting in chromosomes that
contain two identical DNA strands
These genetically identical strands are called sister chromatids and are held
together by a central region called the centromere
These chromatids separate during meiosis II, becoming independent
chromosomes each made of a single DNA strand
If DNA replication did not occur prior to
meiosis there would be no need for a
2nd meiotic division (meiosis I =
diploid → haploid)
The fact that DNA replication does occur
suggests that meiosis evolved from mitosis
(where initial DNA replication is necessary)
One benefit of the duplication of
chromatids is that it increases the
potential for genetic recombination
to occur (more variation)
Stages of Meiosis
Meiosis consists of two divisions that follow the same stages as Mitosis
o Prophase
o Metaphase
o Anaphase
o Telophase
Meiosis is preceded by interphase in which DNA is replicated to produce
chromosomes consisting of two sister chromatids.
Meiosis I
The first Meiotic division is a reduction division (diploid to haploid)
Homologous chromosomes are separated
Prophase I:
Chromosomes condense, nuclear membrane dissolves, homologous chromosomes
form bivalents, crossing over occurs.
Bivalent: two homologous chromosomes joined together by synapsis in Prophase I
Metaphase I:
, Spindle fibres from opposing centrosomes connect to bivalents (at centromeres) and
align them along the middle of the cell
Centromere: A centromere is a constricted region of a chromosome that separates it
into a short arm (p) and a long arm (q).
Centrosome: The organelle that divides and migrates to opposite sides of the cell
during mitosis, it is involved in the formation of the mitotic spindle, assembly of
microtubules, and regulation of cell cycle progression.
Anaphase I:
Spindle fibres contract and split the bivalent, homologous chromosomes move to
opposite poles of the cell
Telophase I:
Chromosomes decondense, nuclear membrane may reform, cell divides (cytokinesis)
to form two haploid daughter cells
Meiosis II
Prophase II:
Chromosomes condense, nuclear membrane dissolves, centrosomes move to opposite
poles (perpendicular to before)
Metaphase II:
Spindle fibres from opposing centrosomes attach to chromosomes (at centromere)
and align them along the cell equator
Anaphase II:
Spindle fibres contract and separate the sister chromatids, chromatids (now called
chromosomes) move to opposite poles
Telophase II:
Chromosomes decondense, nuclear membrane reforms, cells divide (cytokinesis) to
form four haploid daughter cells
The final outcome of meiosis is the production of four haploid daughter cells
o These cells may all be genetically distinct if crossing over occurs in prophase I
(causes recombination of sister chromatids)
Random Assortment
10.1 Meiosis
Understandings:
Chromosomes replicate in interphase before meiosis
Homologous chromosomes separate in meiosis I
Sister chromatids separate in meiosis II
Independent assortment of genes is due to the random orientation of pairs of
homologous chromosomes in meiosis I
Chiasmata formation between non-sister chromatids can result in an exchange
of alleles
Crossing over is the exchange of DNA material between non-sister homologous
chromatids
Crossing over produces new combinations of alleles on the chromosomes of the
haploid cells
Skills:
Drawing diagrams to show chiasmata formed by crossing over
Interphase
Interphase is an active period that precedes meiosis and involves
key events needed to prepare the cell for successful division
DNA is replicated during the S phase of interphase, resulting in chromosomes that
contain two identical DNA strands
These genetically identical strands are called sister chromatids and are held
together by a central region called the centromere
These chromatids separate during meiosis II, becoming independent
chromosomes each made of a single DNA strand
If DNA replication did not occur prior to
meiosis there would be no need for a
2nd meiotic division (meiosis I =
diploid → haploid)
The fact that DNA replication does occur
suggests that meiosis evolved from mitosis
(where initial DNA replication is necessary)
One benefit of the duplication of
chromatids is that it increases the
potential for genetic recombination
to occur (more variation)
Stages of Meiosis
Meiosis consists of two divisions that follow the same stages as Mitosis
o Prophase
o Metaphase
o Anaphase
o Telophase
Meiosis is preceded by interphase in which DNA is replicated to produce
chromosomes consisting of two sister chromatids.
Meiosis I
The first Meiotic division is a reduction division (diploid to haploid)
Homologous chromosomes are separated
Prophase I:
Chromosomes condense, nuclear membrane dissolves, homologous chromosomes
form bivalents, crossing over occurs.
Bivalent: two homologous chromosomes joined together by synapsis in Prophase I
Metaphase I:
, Spindle fibres from opposing centrosomes connect to bivalents (at centromeres) and
align them along the middle of the cell
Centromere: A centromere is a constricted region of a chromosome that separates it
into a short arm (p) and a long arm (q).
Centrosome: The organelle that divides and migrates to opposite sides of the cell
during mitosis, it is involved in the formation of the mitotic spindle, assembly of
microtubules, and regulation of cell cycle progression.
Anaphase I:
Spindle fibres contract and split the bivalent, homologous chromosomes move to
opposite poles of the cell
Telophase I:
Chromosomes decondense, nuclear membrane may reform, cell divides (cytokinesis)
to form two haploid daughter cells
Meiosis II
Prophase II:
Chromosomes condense, nuclear membrane dissolves, centrosomes move to opposite
poles (perpendicular to before)
Metaphase II:
Spindle fibres from opposing centrosomes attach to chromosomes (at centromere)
and align them along the cell equator
Anaphase II:
Spindle fibres contract and separate the sister chromatids, chromatids (now called
chromosomes) move to opposite poles
Telophase II:
Chromosomes decondense, nuclear membrane reforms, cells divide (cytokinesis) to
form four haploid daughter cells
The final outcome of meiosis is the production of four haploid daughter cells
o These cells may all be genetically distinct if crossing over occurs in prophase I
(causes recombination of sister chromatids)
Random Assortment
