Meiosis
10.1.1 Describe the behaviour of the chromosomes in the phases of meiosis
Interphase: Cell growth and DNA replication (duplication of DNA creates sister
chromatid chromosomes)
Meiosis I
 Prophase
I: DNA supercoils and chromosomes condense, nuclear membrane
dissolves, homologous pairs form bivalents, crossing over occurs
 Metaphase I: Spindle fibres from centrioles (at poles) attach to centromeres of
bivalent, bivalents line up along the equator of the cell
 Anaphase I: Spindle fibres contract and split the bivalent, homologous
chromosomes move to opposite poles of the cell
 Telophase I: Chromosomes decondense, nuclear membranes may reform, cell
divides (cytokinesis) forming two haploid daughter cells
Interkinesis: An optional rest period between meiosis I and meiosis II, no DNA
replication occurs in this stage
Meiosis II
 Prophase
II: Chromosomes condense, nuclear membrane dissolves (if
reformed), centrioles move to opposite poles (perpendicular to previous poles)
 Metaphase II: Spindle fibres from centrioles attach to centromeres of
chromosomes, chromosomes line up along the equator of the cell
 Anaphase II: Spindle fibres contract and split the chromosome into sister
chromatids, chromatids (now called chromosomes) move to opposite poles
 Telophase II: Chromosomes decondense, nuclear membrane reforms, cells
divide (cytokinesis) resulting in four haploid daughter cells
Summary
 Meiosis
is the division of a cell to form four haploid gametes, all of which may be
genetically distinct if recombination occurs in prophase I
Overview of Meiosis
10.1.2 Outline the formation of chiasmata in the process of crossing over
 Crossing
over involves the exchange of segments of DNA between homologous
chromosomes during Prophase I of meiosis
 The process of crossing over occurs as follows:
 Homologous chromosomes become connected in a process called
synapsis, forming a bivalent (or tetrad)
 Non-sister chromatids break and recombine with their homologous
partner, effectively exchanging genetic material (crossing over)
 The non-sister chromatids remain connected in an X-shaped structure and
the positions of attachment are called chiasmata
 Chiasma hold homologous chromosomes together as a bivalent until anaphase I
 As a result of crossing over, chromatids may consist of a combination of DNA
derived from both homologues - these are called recombinants
Crossing Over in Prophase I
10.1.3 Explain how meiosis results in an effectively infinite genetic variety in gametes
through crossing over in prophase I and random orientation in metaphase I
 During
anaphase I, homologous chromosomes separate, such that each
resultant daughter cell (and subsequent gametes) contains a chromosome of
either maternal or paternal origin
 The orientation of these homologues in metaphase I is random, such that there is
an equal probability of the daughter cell having either the maternal or paternal
chromosome
 As humans have a haploid number of 23 chromosomes, this means that there is
223 potential gamete combinations (over 8 million combinations)
 Crossing over in prophase I results in entirely new chromosome combinations, as
recombination through gene exchange produces wholly original chromosomes
containing both maternal and paternal DNA, resulting in near infinite genetic
variability
 Other sources of genetic variation include random fertilisations, DNA mutations,
chromosome mutations and non-disjunction
10.1.4 State Mendel's law of independent assortment
Gregor Mendel was a 19th century Moravian monk who demonstrated that the
inheritence of traits (i.e. genes) followed particular laws:
 Law
of Segregation: Each hereditary characteristic is controlled by two alleles,
which segregate and pass into different reproductive cells (gametes)
 Law of Independent Assortment: The separation of alleles for one gene will
occur independently of the separation of alleles for another gene
 According to the law of independent assortment, different allele
combinations should always be equally possible
 However
this law only holds for genes that are on different chromosomes the law of independent assortment does not apply to linked genes
10.1.5 Explain the relationship between Mendel's law of independent assortment and
meiosis
 The
law of independent assortment relates to the random orientation of
homologous chromosomes in metaphase I of meiosis
 Because the orientation of a homologous pair is random, and does not affect the
orientation of any other homologous pair, any one of a pair of alleles on a
chromosome has an equal chance of being paired with, or separated from, any
one of a pair of alleles on another chromosome
 This means the inheritance of two different traits will occur independently of each
other (provided the genes aren't linked)
Linked versus Unlinked Genes