Transcript Meiosis II

MEIOSIS
Process of reductional division in
which the number of chromosomes
per cell is cut in half.
Meiosis
• This Cell division occurs in the gametes,
giving rise to the sperm and egg.
• This is the Characteristic of eukaryotes only:
not in prokaryotes.
• Normal cells are diploid: 2n copies of every
gene.
• Diploidy is useful because there are 2 copies
of every gene, that means there is a backup
copy, if one gets mutated.
• Mutations are very frequent in the cells of
large organisms.
• Gametes are haploid: 1 copy of every gene.
• Meiosis produces 4 haploid cells.
• ***Mitosis produces 2 diploid cells.
• Why do we have sexual reproduction?
Shuffling of alleles between parents and
offspring leads to new combinations. Bad
combinations die without reproducing; good
combinations survive and reproduce more
offspring.
• Two successive nuclear divisions occur,
Meiosis I (Reduction) and Meiosis II
(Division).
• Meiosis I reduces 2n to n , while
Meiosis II divides the remaining set of
chromosomes in a mitosis-like process
(division).
• Most of the differences between the
processes occur during Meiosis I.
• Meiosis is a series of two nuclear divisions
--- meiosis I and meiosis II.
• These two divisions are each divided into
further phases:
- Prophase
– Metaphase
– Anaphase
– Telophase
Meiosis I
Meiosis I encompasses four stages:
– Prophase I
– Metaphase I
– Anaphase I
– Telophase I
• The stages are similar to the stages in
mitosis but the largest differences occur in
prophase I.
• In most cases, at the end of meiosis I, two
daughter cells are produced.
• Each of these cells are haploid, having one
homologous chromosome with two
chromatids.
Meiosis II
Meiosis II also encompasses four stages:
– Prophase II
– Metaphase II
– Anaphase II
– Telophase II
• At the end of meiosis II, four daughter cells
are produced.
• Each of these resulting daughter cells is
haploid(n) having one chromatid making
up the chromosome.
• Start with a diploid cell, with 2 copies of each chromosome,
one from each parent.
• The two copies are called homologues.
• Chromosomes each has 2 chromatids attached at the
centromere.
 Use 2 cell divisions:
• Meiosis 1. First separate the homologous chromosomes
Meiosis 2. Then separate the 2 chromatids.
• The stages of meiosis have the same names as in mitosis:
prophase 1, metaphase 1, anaphase 1, telophase 1.
• Each of the 2 cell divisions has all of these stages.
Prophase 1
• Its one of the most important stages of meiosis.
• During this stage, many crucial events occur.
• The chromatid threads begin to twist and condense, creating
chromosomal structures which are visible to the microscope.
• Each chromosome then actively seeks out its homologous
chromosome.
• After the homologous chromosomes pair, the structure is
referred to as a tetrad (four chromatids).
• The point at which two non-sister chromatids intertwine is
known as a chiasma.
• Sometimes a process known as “crossing over "occurs at this
point.
• This is where two non-sister chromatids exchange genetic
material. This exchange does not become evident, however,
until the two homologous pairs separate.
Types of Prophase I
• Prophase I has so many processes happening that it is
usually separated into five stages.
• Leptonema ( leptotene) .
• Zygonema (Zygotene).
• Pachynema (Pachytene) .
• Diplonema ( Diplotene).
• Diakinesis .
Leptonema
• During this stage, the chromosomes begin to condense
and become visible. Its also believed that homologous
pair searching begins also at this stage.
Zygonema
• The chromosomes continue to become denser. The
homologous pairs have also found each other and begin
to initially align with one another, referred to as 'rough
pairing'. Lateral elements also form between the two
homologous pairs, forming a synaptonemal complex.
Pachynema
• Coiling and shortening continues as the
chromosomes become more condense. A
synapsis forms between the pairs, forming
a tetrad.
Diplonema
• The sister chromatids begin to separate
slightly, revealing points of the chiasma.
This is where genetic exchange occurs
between two non-sister chromatids, a
process known as “crossing over”.
• Two important events occur:
– crossing over in prophase, and
– the pairing of homologues in
metaphase.
• Crossing over: Homologues
break at identical locations, then
rejoin opposite partners.
• This creates new combinations of
the alleles on each chromosome.
• Occurs randomly several times on
every chromosome.
• Results in mixing of the genes you
inherited from your parents.
• A chiasma (chiasmata), in genetics, is thought to be the
point where two homologous non-sister chromatids
exchange genetic material during chromosomal
crossover during meiosis (sister chromatids also form
chiasmata between each other, but because their
genetic material is identical, it does not cause any
change in the resulting daughter cells).
• The chiasmata becomes visible during the diplotene
stage of prophase I of meiosis, but the actual "crossingover" of genetic material is thought to occur during the
previous pachytene stage.
• When each bivalent, which is composed of two pairs of
sister chromatids, begins to split, the only points of
contact are at the chiasmata.
Diakinesis
• The chromosomes continue to pull apart, but non-sister
chromatids are still loosely associated via the chiasma.
The chiasma begin to move toward the ends of the
tetrad as separation continues. This process is known as
terminalization. Also during diakinesis, the nuclear
envelope breaks down and the spindle fibers begin to
interact with the tetrad.
Metaphase 1
• The chromosomes begin
their migration to the
equator.
• The main event in this is the
pairing and separation of the
homologues ‘C’.
• At metaphase, the pairs of
homologous chromosomes
line up side by side.
• This does not happen in
meiosis 2, but only in meiosis
1.
• Metaphase I
Metaphase I
Anaphase I
• Its very similar to anaphase in mitosis.
• The following changes occur:
• At anaphase1, the pairs of homologues ‘C’ are
pulled to opposite poles by the spindle.
• Note: the centromeres do NOT divide; the
chromosomes remain in the 2-chromatid Xshaped state, or sister chromatids remain
together.
• Anaphase I
Anaphase I
Telophase I
• The following changes occur:
• The spindles continue to move the homologous
chromosomes to the poles.
• Once movement is complete, each pole has a haploid
number of chromosomes.
• In most cases, cytokinesis occurs at the same time as
telophase I.
• At the end of telophase I and cytokinesis, two
daughter cells are produced, each with one half the
number of chromosomes of the original parent cell.
• Telophase I
Telophase I
Meiosis 2
• Meiosis II is the second part of the meiotic process.
Much of the process is similar to mitosis and meiosis I.
• The following changes occur:
• Prophase II If needed, the nuclear membrane and nuclei
break up while the spindle "network" appears and the
chromosomes begin migrating to the metaphase II plate
(at the cell's equator).
• Metaphase II
The chromosomes line up at the equatorial plate at the
cell's center. The kinetochores ( centromere) of the sister
chromatids point toward opposite poles.
• Anaphase II
The sister chromatids separate and move toward the
opposite cell poles.There is longitudinal splitting of the
centromere.
• Telophase II
Distinct nuclei form at the opposite poles and cytokinesis
occurs.
Result of Meiosis 1
• Go from 1 cell to 2 cells.
• Each daughter cell contains 1
copy of each chromosome:
they are haploid, with the
chromosomes still having 2
chromatids each.
• For humans: start with one
cell containing 46
chromosomes (23 pairs) to 2
cells containing 23
chromosomes.
• As a result of crossing over,
each chromosome is the
mixture of the original
homologues.
Meiosis 2
• Meiosis 2 is just like mitosis
• No replication of DNA between
meiosis 1 and meiosis 2.
• Chromosomes line up
individually on the equator of the
spindle at metaphase.
• At anaphase the centromeres
divide, splitting the 2 chromatids.
• The one-chromatid
chromosomes are pulled to
opposite poles.
Summary of Meiosis
• 2 cell divisions.
• Start with 2 copies of each
chromosome (homologues), each
with 2 chromatids.
• In meiosis 1, crossing over in
prophase mixes alleles between
the homologues.
• In metaphase of meiosis 1,
homologues pair up, and in
anaphase the homologues are
separated into 2 cells.
• Meiosis 2 is just like mitosis. The
centromeres divide in anaphase,
giving rise to a total of 4 cells,
each with 1 copy of each
chromosome, and each
chromosome with only 1
chromatid.
Chromosomal
Abnormalities
Deletion
• A deletion can happen in every chromosome and
exhibit every size.
• The consequences of a deletion depends on the
size of the missing segment and which genes are
found on it.
• A partial deletion on the short arm (p) of
chromosome 5, for example, is responsible for
the "cri du chat" syndrome.
Duplication
• A chromosome duplication is
the doubling of a chromosome
piece.
• A duplication is sometimes
termed a "partial trisomy".
• If a duplication is present, the
person is equipped with 3
copies of the genes in the
associated chromosome
segment.
• This means that extra directions
(genes) are present, leading to
congenital abnormalities or
developmental problems.
Inversion
• If chromosome pieces that have been broken
out become inserted again, but reversed, an
inversion has occurred.
• The phenotype of this disorder is usually
unobtrusive, since the entire chromosomal
information is still present.
• When the interchanged region includes the
centromere, one refers to it as a pericentric
inversion, otherwise to a paracentric
inversion.
• Normal chromosome
Chromosome with
paracentric inversion
• Normal chromosome
Chromosome with
pericentric inversion
Insertion
• If chromosome pieces are reinserted
somewhere else, this is referred to as an
insertion.
• Carriers of such insertions can be
phenotypically inconspicuous because no
information has been lost.
• 2 normal
chromosome pairs
Insertion of a
broken off piece on
another
chromosome
Reciprocal Translocation
• In a reciprocal translocation two broken off
chromosome pieces of non-homologous
chromosomes are exchanged.
• This is a relatively frequent anomaly.
• One finds it with an incidence of 1:500
newborns.
• Reciprocal translocations are frequently
balanced because the entire genetic material
is present.
• Problems occur, though, in gamete formation.
• Two nonhomologous
chromosome
pairs
Reciprocal
translocation