Lesson 2: Meiosis Flashcards
(14 cards)
Meiosis
Meiosis is a special type of cell division that occurs only in reproductive organs. Meiosis makes reproductive cells called gametes (egg or sperm).
Meiosis serves two functions:
Halving the chromosome number
Producing genetic variety
Chromosome Number
All cells in the human body, except the gametes contain the same number of
chromosomes (46).
This is called the diploid (2n) number of chromosomes.
In gametes that number need to be cut in half in order to produce viable
offspring.
This is called the haploid (n) number of chromosomes.
This means that gametes contain only one copy of each type of chromosome
that the diploid parent cell contains.
Phases of Meiosis
The phases of meiosis are the
same as mitosis: prophase,
metaphase, anaphase,
telophase. However, meiosis
involves two sequences of
these phases:
Meiosis I - reduces
the chromosomes
from diploid to
haploid, referred to
as reduction division
Meiosis II – further
divides the cell
Interphase
Just as in mitosis, during interphase the cell
grows and replicates DNA.
After replication, each chromosome is made
up of a pair of identical sister chromatids held
together by a centromere
Prophase I
Homologous Chromosomes
align forming a bivalent.
Homologous chromosomes
contain the same genes, but
since they come from
different parents, they can
have different alleles.
Nuclear membrane disappears
Spindle fibers begin to form
Crossing over occurs
Crossing Over
Crossing over is the exchange of genetic
material between non sister homologous
chromatids.
During the formation of the bivalent, non
sister chromatids may touch and break. The
two segments may then rejoin at the same
position on the other chromatid.
The region where this occurs is called a
chiasma
This process forms chromatids that are
a mixture of paternal and maternal
alleles.
Crossing over may occur multiple times
and between different chromatids
within the same homologous pair.
Some alleles never form a chiasma.
Metaphase 1
Spindle fibers attach to
the centromere of each
chromosome
Spindle fibers pull each
bivalent to the centre
of the cell
Random assortment
occurs
Random Assortment
The bivalents line up on the equator of the cell randomly.
The paternal chromosomes could both line up together on one side with the
maternal on the other. The end result for the gametes would be that some will
contain maternal chromatids while others will contain paternal chromatids.
The maternal and paternal chromosomes could be on both sides of the
equator. The end result for the gametes would be that all contain a mixture of
maternal and paternal chromatids.
Anaphase 1
The homologous
chromosomes separate
and move to opposite
poles of the cell.
The centromere does
not split as in mitosis
This means that the
sister chromatids are
held together, moving
one chromosome to
each of the poles.
Telophase 1
Does not occur in all cells
When it does occur, the
chromosomes begin to
uncoil and the spindle fibres
disappear. The cytoplasm is
divided and the nuclear
membrane begins to form
around the pairs of
homologous chromosomes.
Because each chromosome
already consists of two
chromatids, DNA replication
does NOT occur between
telophase 1 and prophase 2.
Prophase II
The phases of meiosis II are identical to the phases of
mitosis.
Nuclear membrane disappears and spindle fibres are
formed.
Chromatin coil and become visible.
Metaphase II
Spindle fibres attach to the centromere of each
chromosome
Spindle fibres move the chromosomes to the
middle of the cell
Anaphase II
Spindle fibres pull sister chromatids to opposite
poles of the cell.
Telophase II
The nuclear membrane reappears and spindle fibres disappear
Cytoplasm divides and the new cells are formed.
The result of telophase two are 4 non-identical haploid cells.
