Download Meiosis - TGHSLevel1Science

Meiosis
Meiosis
• Meiosis is the cell division which
produces the sex cells or gametes.
• Meiosis occurs in the cells of the
reproductive organs:
– Sperm cells – testes
– Ova (eggs) – ovaries
– Pollen - anthers
Meiosis
• Gametes are produced during sexual
reproduction.
• Sexual reproduction involves
fertilization – the fusion of 2 gametes,
one male and one female.
• Sexual reproduction produces
genetically different individuals.
Meiosis I
• In meiosis, the chromosome replicates
prior to cell division.
• The chromosomes have shortened and
thickened and appear as two chromatids
joined by a centromere.
• The nuclear membrane disappears and
the spindle begins to form.
• Homologous pairs of chromosomes come
together.
Meiosis I
• Homologous pairs of chromosomes line
up on the equator of the cell.
• Spindle fibres attach to the members
of each pair.
• The spindle fibres contract separating
the pairs of chromosomes.
• The nuclear membrane reforms
temporarily and the cell divides.
Meiosis II
• The nuclear membrane disappears again.
• The chromosomes (consisting of 2
chromatids) line up in the centre of the
cell.
• Spindle fibres attach to each
chromatid.
• The fibres contract separating and
pulling the chromatids to each end of
the cell.
Meiosis II
• The nuclear membrane forms around
each half set of chromosomes.
• The cell divides.
• Four daughter cells are produced, each
with half the number of chromosomes
as the parent cell.
• These cells become gametes.
• Gametes are haploid (n).
Sexual Reproduction and
Genetic Variation
• Sexual reproduction produces many
different individuals (a lot of genetic
variation).
• This is a result of three processes:
Independent Assortment
 Gametes are produced with many
combinations of chromosomes from
each parent.
Sexual Reproduction and
Genetic Variation
Random Fertilisation
 Random gametes combine at
fertilisation
Crossing Over
 As gametes are produce portions of the
parent cell’s two copies of each
chromosome are swapped around
Independent Assortment
• When testes cells produce sperm and
ovarian cells produce eggs the
chromosome number is halved and the
gametes contain different combinations
of the chromosomes.
• Humans can produce 223 or 8,388,608
different gametes through the
independent assortment of
chromosomes.
Independent Assortment
Male Gametes
Testis Cell
Female Gametes
Ovarian
Cell
Gametes are
produced with
many
combinations
of
chromosomes
from each
parent
Random Fertilization
• Random gametes combine at fertilisation,
any of the many different male gametes
can fertilise any of the many different
female gametes
• Through random fertilisation, each male
and female human can produce 223 x 223 =
246 or 70,368,744,177,664 genetically
unique zygotes.
Random Fertilization
Male
Female Gametes
Gametes
Zygotes
Random
gametes
combine
at
fertilisation
Crossing Over
• During meiosis, two chromosomes of a
homologous pair (one maternal, one
paternal) can cross over and exchange
segments, producing genetic variations in
the resulting gametes.
• This is in addition to the variation created
by independent assortment and random
fertilisation.
Crossing Over
Crossing Over
Testis Cell
Ovarian Cell
Male Gametes
As gametes
are produced
portions of the
parent cell’s
Female Gametes two copies of
each
chromosome
are swapped
around
Mutation
• Some times when chromosomes are being
copied during mitosis mutation occurs
and the copy is not exact
• Most often these mutations have little
effect or cause fatality
• Some time the mutation results in a new
variant of a characteristic (trait).
Comparing Mitosis and Meiosis
• Mitosis
– cell division for growth and
repair (used for reproduction only in
simple organisms such as bacteria)
– Synapsis (crossing over) does not
occur
– one division
– two identical daughter cells
– chromosome number is
maintained (two copies of each
chromosome)
Comparing Mitosis and Meiosis
• Meiosis
– cell division for gamete production
– synapsis (crossing over) occurs
– two divisions
– four unique daughter cells
– chromosome number is reduced by
half (one copy of each chromosome)