Download meiotic cell division - CAPE Biology Unit 1 Haughton XLCR 2013

MEIOTIC CELL DIVISION
JANUARY 17, 2013
MEIOSIS
• Meiosis is the cell division which occurs
only in reproductive organs during gamete
production.
• Meiosis results in the formation of four
genetically unidentical cells.
• Each of these cells contain half the
number of chromosomes as the parent cell
(the haploid or n number).
• Meiosis ensures that:
• Each daughter cell has the haploid
number of chromosomes so that the
diploid number (and no more) can be
restored within the zygote at fertilization.
• Meiosis ensures that:
• Each daughter cell has a different
combination of genes which leads to
variation amongst offspring.
• Please remember that gametes must be
different from regular body cells in that
they have half the chromosome number
(haploid or n) so that when the mother and
father gametes meet, they produce a
zygote with the correct diploid number (2n)
of a normal body cell.
• Meiosis (meio, “ reduce”) is a form of
nuclear division in which the
chromosome number is halved from
the diploid (2n) to the haploid number
(n).
• Like mitosis, it involves DNA replication
during interphase in the parent cell, but
this is followed by two cycles of nuclear
divisions and cytoplasmic divisions, known
as meiosis I (the first meiotic division) and
meiosis II (the second meiotic division).
• Thus a single diploid cell gives rise to four
haploid cells.
• Meiosis occurs during the formation of
sperms and eggs (gametogenesis) in
animals and during spore formation in
plants.
• Like mitosis, meiosis is a continuous
process but is conveniently divided into
two prophase, two metaphase, two
anaphase and two telophase.
• These stages occur in the first meiotic
division and again in the second meiotic
division.
CHROMOSOMES AND MEIOSIS
HOMOLOGOUS
CHROMOSOMES
MEIOSIS
PROPHASE I
• The longest phase.
• Homologous chromosomes pair up in a
process called synapsis.
• One of the pair comes from the father and
the other from the mother.
PROPHASE I
CHIASMATA FORMATION
• Homologous chromosomes pair up.
• Chromatids now visible.
• Chromosomes are joined at several places
along their length (chiasmata).
• Each chiasma is the site of an exchange
between chromatids.
• It is produced by the breakage and reunion
between any two of the four strands present at
each site.
• As a result, genes from one chromosome may
swap with genes from the other chromosome
leading to new gene combinations in the
resulting chromatids.
• This is called crossing over (slides 33-36).
METAPHASE I
ANAPHASE I
TELOPHASE I
INTERPHASE II
• This stage is present usually only in
animal cells and varies in length.
• No futher DNA replication occurs.
PROPHASE II
METAPHASE II
ANAPHASE II
TELOPHASE II
MEIOSIS IN ACTION !
MEIOSIS IN SUMMARY
Recall: MITOSIS SUMMARY
RELATIONSHIP BETWEEN MITOSIS AND
MEOSIS
SIGNIFICANCE OF MEIOSIS
• Sexual reproduction
• Genetic variation
– Independent assortment of chromosomes
– Crossing over
GENETIC VARIATION
• ..\..\..\Documents\ImTOO\Download
YouTube Video\GENETIC DIVERSITY.flv
INDEPENDENT ASSORTMENT
• Bivalent arrange themselves randomly on
the equator of the spindle fibres.
• Independent assortment refers to the fact
that the bivalents line up independently
and so the chromosomes in each bivalent
separate (assort) independently of those in
other bivalents during anaphase I.
ANAPHASE I
CROSSING OVER
• Chromosomal crossover (or crossing over) is
an exchange of genetic material
tween homologous chromosomes. It is one of
the final phases of genetic recombination, which
occurs during prophase I of meiosis (pachytene)
in a process called synapsis.
• Crossover usually occurs when matching
regions on matching chromosomes break and
then reconnect to the other chromosome.
Consequences of cross-over
• In most eukaryotes, a cell carries two copies of
each gene, each referred to as an allele. Each
parent passes on one allele to each offspring.
• An individual gamete inherits a complete haploid
complement of alleles on chromosomes that are
independently selected from each pair
of chromatids lined up on the metaphase plate.
• Without recombination, all alleles for those
genes linked together on the same chromosome
would be inherited together.
• Meiotic recombination allows a more
independent selection between the two alleles
that occupy the positions of single genes, as
recombination shuffles the allele content
between homologous chromosomes.
• Rarely, crossovers can also occur
between similarities in sequence
at other regions.
• The result is mismatched
alignments.
• These processes are called
unbalanced recombination.
• Severe problems can arise if a gamete
containing unbalanced recombinants
becomes part of a zygote.
• The result can be a local duplication of
genes on one chromosome and
a deletion of these on the other,
a translocation of part of one chromosome
onto a different one, or an inversion.
COMPARISON OF MITOSIS
AND MEIOSIS
MITOSIS AND MEIOSIS
• Mitosis is necessary
for:
• Meiosis is necessary
for:
• Growth of cells
• Repair of cells
• Asexual reproduction
• The production of male
and female gametes for
sexual reproduction.
• These are the egg and
sperm cells.
• (Involves only one
division of genetic
material).
• (Involves twp divisions
of genetic material).