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Meiosis

Topic 3
...
3

Biology SL/HL

Meiosis process
➢ Meiosis is divided into several phases
○

Meiosis I
■

Prophase I
●

DNA becomes more compact => chromosomes become
visible

●

Homologues are attracted to each other and pair up

●

Crossing over occurs

●

Spindle fibers made from microtubules form
○

■

Centrioles move to the poles of the cell

Metaphase I
●

The homologues line up across the equator by random
orientation

●
■

The nuclear membrane breaks down

Anaphase I
●

Spindle fibers attach to the chromosomes centromeres

●

They pull the homologous pairs apart to opposite poles of the
cell

■

Telophase I
●

Spindle fiber breaks down

●

DNA is uncoiled (usually)

●

New nuclear membranes are formed (usually)

●

Cytokinesis occurs
○
○

two separated cells are created
they are haploid, but each chromatid still has its
sister chromatid attached to it

●
○

Many plants do not have a telophase I stage

Meiosis II
■

■

Prophase II
●

DNA condenses into chromosomes again

●

New mitotic spindle fibers are produced

Metaphase II
●

Nuclear membranes break down

●

The chromosomes line up along the cell equator by random
orientation

2

Meiosis

Topic 3
...
5-1%)
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Meiosis

Topic 10
...
chiasmata) is the place where two chromatids connect to each other
during crossing over
➢ As segments of DNA are traded during crossing over, a chromosome originally
carrying a recessive allele could end up carrying a dominant one or vice versa
➢ A single bivalent can have several chiasmata producing crossing over in more than
one chromatid
Independent assortment of genes
➢ Gregor Mendel’s law
○

The separation of one pair of alleles between the daughter cells is
independent of the separation of another pair of alleles
■

When an allele is passed on to a gamete, a different allele does not
follow it

➢ Some genes are linked so that if one allele is crossed over the other follows
➢ The random orientation of the chromosomes in meiosis I and II promotes genetic
variation
○

This is why traits are inherited independently from each other

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