Meiosis
Mitosis
+
making identical copies of diploid cells
Meiosis
+
making haploid gametes from a diploid precursor
Some terminology
Diploid – containing two copies of the genome per cell
Haploid - containing one copy of the genome per cell
Gametes – cells that are the haploid cells
combined during fertilization
Haploid gametes (n = 23)
Key
Haploid stage (n)
Diploid stage (2n)
n
Egg cell
n
Sperm cell
Meiosis
Ovary
Fertilization
Testis
2n
Multicellular
diploid adults
(2n = 46)
Mitosis and
development
Diploid
zygote
(2n = 46)
Learning Outcomes
Describe the steps in meiosis
Compare and contrast mitosis and meiosis
Distinguish between haploid and diploid,
sister chromatids and nonsister chromatids
Describe how genetic variability is generated
through meiosis (independent assortment of
homologous chromosomes and crossing over)
Function of Meiosis
Create haploid cells from diploid precursors
Ensure that each gamete has
one complete copy of the genome
Generate genetic variation
How does meiosis produce genetic variation?
Genes occur in pairs on
homologous chromosomes.
The members of each pair of
genes may be identical, or they
may differ slightly, as alleles.
Many genes have multiple alleles in the
population, so new combinations of alleles
are produced when gametes combine
to form a new individual.
Karyotypes show Replicated Chromosomes
Centromere
Sister
chromatids
Pair of
homologous
chromosomes
Sex chromosomes
Overview of Meiosis
Overview of Meiosis
Meiosis 1 –
homologous chromosomes separate
Meiosis 2 –
sister chromatids separate
Meiosis I -Homologous chromosomes separate
PROPHASE 1
Tetrads form and
crossing over
takes place
Meiosis I -Homologous chromosomes separate
METAPHASE 1
Tetrads line up
on the metaphase plate
Meiosis I -Homologous chromosomes separate
ANAPHASE 1
Homologous chromosomes
move to opposite ends
of the spindle.
Meiosis I -Homologous chromosomes separate
TELOPHASE 1
(and cytokinesis)
Two haploid cells with
replicated chromosomes
are formed.
Meiosis II -Sister chromatids separate
PROPHASE 2 –
Replicated chromosomes
condense and the spindle starts
to form.
Meiosis II -Sister chromatids separate
METAPHASE 2Replicated chromosomes
line up independently
at the metaphase plate
Meiosis II -Sister chromatids separate
ANAPHASE 2 –
Sister chromatids are separated
and move to opposite poles of
the spindle.
Meiosis II -Sister chromatids separate
TELOPHASE 2
chromosomes decondense
and cytokinesis occurs.
Meiosis
Meiosis 1 – homologous chromosomes separate
Meiosis 2 – sister chromatids separate
Meiosis leads to genetic variation
Meiosis leads to genetic variation
1) homologous chromosomes assort independently
in humans, with 23 pairs –
223 = 8,000,000 combinations
2) chromosomes can undergo crossing over
generates more new combinations of genes
Crossing Over Occurs during Prophase I
When tetrads form, non-sister chromatids
can exchange corresponding pieces of DNA.
Crossing over generates new combinations of genes
Crossing Over Occurs during Prophase I
multiple cross overs can occur between
non-sister chromatids in a given tetrad
centromere
crossover
crossover
Non-disjunction in Meiosis
Metaphase I
Anaphase I
Telophase I
Metaphase II
Anaphase II
Telophase II
Non-disjunction events can occur
either in Meiosis 1 or Meiosis 2
from Chapter 14
Human Trisomy Conditions result in
severe developmental problems
Only trisomy 21 produces viable offspring, all the others are lethal.
Trisomy 21 (Down syndrome)
Trisomy 18 (Edwards syndrome)
Trisomy 13 (Patau syndrome)
Trisomy 9
Trisomy 8 (Warkany syndrome 2)
Trisomy 22
In contrast, extra X or Y chromosomes are usually well tolerated.
XX = normal female, but XXX and XO are normal also
XY = normal male, but XXY and XYY are viable also. YO is not viable.
Polyploidy