Download Meiosis NOTES

MEIOSIS
The great gene
shuffling machine
© 2007 Paul Billiet ODWS
Copyright Dr. Michael Knee
Source: http://hcs.osu.edu/hcs300/
Widow’s Peak
Bent
Bent Pinkies
Hitchhiker’s thumb
Red-Green Colorblindness
Attached Earlobes
Longer Second Toe
Gene
heritable factor that controls a
specific characteristic
Gene vs. Allele vs. Genome
Allele = one specific form of a gene; has the
same locus, its code is just different - making a
different protein
Genome = the whole of the genetic information
of the organism
homologous
chromosomes:
chromosomes in a
diploid cell which
contain the same
sequence of
genes, but are
derived from
different parents
Karyotype
locus = the particular position on homologous
chromosomes of a gene
Meiosis performs two functions
 It
halves the number of chromosomes to make
_________ gametes (sex cells)
 It
shuffles the genes to produce new
combinations (recombinations)
© 2007 Paul Billiet ODWS
Meiosis and sexual reproduction
 Meiosis
is needed to produce sex cells (gametes)
with unpaired sets of chromosomes (haploid)
 Sex
cells are used in fertilization
 At
fertilization two sets of genes come together to
form a hybrid with a set of paired chromosomes
(diploid)
 The
hybrid, whilst similar to the parents, is unique
© 2007 Paul Billiet ODWS
Haploid and Diploid

Karyotypes of somatic cells show paired
sets of chromosomes

The origin of the pairs are the maternal
and paternal chromosomes of the egg
and the sperm

The number of types of chromosomes of
a species
is constant = n

So the diploid (paired set) = 2n
© 2007 Paul Billiet ODWS
Organism
Human
Chromosome
numbers
46
Chimpanzee
48
House Mouse
40
Maize
20
Goldfish
© 2007 Paul Billiet ODWS
The sexual reproduction life cycle
Diploid (2n)
(Somatic Cells)
Fertilization
Meiosis
Haploid (n)
(Gametes – Sex Cells)
© 2007 Paul Billiet ODWS
Meiosis a two step process
 Meiosis

cells are now haploid
 Meiosis

© 2007 Paul Billiet ODWS
1 is the _______ division
2 resembles mitosis
just the division of the sister chromatids
Meiosis 1: Early Prophase 1
 Chromosomes
 Homologous
condense
pairs linked by
_______
 ____________
between
non-sister chromatids occurs
© 2007 Paul Billiet ODWS
Homologous Chromosomes
eye color
locus
hair color
locus
----
----
---- ----
Paternal
Maternal
eye color
locus
hair color
locus
Crossing Over - variation
nonsister chromatids
chiasmata: site of
crossing over
Tetrad
variation
Crossing over
 During
prophase 1, not only do the
homologous pairs link
 The
chromosomes exchange genetic
material
 The
alleles on each chromosome may not
be identical, they may be different

Alleles are different versions of a gene

e.g. Ear shape gene has two alleles the ear
lobe allele and the no ear lobe allele
© 2007 Paul Billiet ODWS
Meiosis 1: Late Prophase 1
Spindle
fibres form and
spread out between the
centrioles
© 2007 Paul Billiet ODWS
Meiosis 1: Metaphase 1
 The
pairs of chromosomes
line up on the equator
 The
orientation of the
maternal and the paternal
chromosomes is random
© 2007 Paul Billiet ODWS
Meiosis 1: Metaphase 1 revisited
 The
pairs of chromosomes
could orientate in different
ways
 This means there
n
2 combinations
 In
are
humans this means
or over ________
combinations
© 2007 Paul Billiet ODWS
23
2
Meiosis 1: Anaphase 1
 Maternal
and paternal
chromosomes segregate
(pulled separate on the
spindle)
 They
move to opposite
poles
© 2007 Paul Billiet ODWS
Meiosis: Anaphase 1
 So
when the pairs are
separated, the alleles of the
genes on the same
chromosome are recombined
(reshuffled in terms of maternal
vs. paternal genes/alleles)
Genes on the same
chromosome are called _____
genes
© 2007 Paul Billiet ODWS
Meiosis 1: Anaphase 1 revisited
 Resulting
in different
combinations of
chromosomes
 This means there
n
2 combinations
 In
are
humans this means
or over 8 million
combinations
© 2007 Paul Billiet ODWS
23
2
Meiosis 1: Telophase 1
 Now
the cells are haploid
 The
chromosomes do not
decondense at the end of
meiosis 1
 Each
chromosome still has
two “identical” chromatids
(although they may be
different due to crossing over)
© 2007 Paul Billiet ODWS
Meiosis 2: Prophase 2
 Spindles
© 2007 Paul Billiet ODWS
form again
Meiosis 2: Prophase 2
 Each
cell is haploid (n)
 The
sister chromatids are no
longer identical copies
© 2007 Paul Billiet ODWS
Meiosis 2: Metaphase 2
 The
chromosomes line up
on the spindle equator
independently
© 2007 Paul Billiet ODWS
Meiosis 2: Anaphase 2

The sister chromatids separate
on the spindle

Each cell will receive a single
copy of each chromosome type
(i.e. it will receive n chromatids,
but they are all different)
© 2007 Paul Billiet ODWS
Meiosis 2: Anaphase 2
 At
anaphase 2 the
chromatids segregate
(separate) randomly
 Even
greater variation is
achieved in the sex cells
© 2007 Paul Billiet ODWS
Meiosis 2: Telophase 2
 Four
haploid sex cells are
produced
© 2007 Paul Billiet ODWS
Meiosis 2: Telophase 2
 An
infinite variety of sex cells is
possible
n
(2 combinations PLUS the
effects of crossing over)
 Combined
with random mating
between males and females an
infinite variety of individuals is
conceived at fertilisation
© 2007 Paul Billiet ODWS
Spermatogenesis
n=23
human
sex cell
sperm
n=23
n=23
2n=46
haploid (n)
n=23
diploid (2n)
n=23
n=23
meiosis I
meiosis II
The inheritance of gender
Is it going to be a boy or a girl?
© University of New Mexico
Female Karyotype
 Male
Karyotype
38
The inheritance of gender
Mother
Father
XX
XY
Meiosis
Sex cells
Fertilisation
X
X
X
Y
Possible
children
Chance of a girl 50%
Chance of a boy 50%
© 2007 Paul Billiet ODWS
Sex chromosomes
 The
sex of many animals is determined by
genes but on chromosomes called ___
chromosomes
 The
other chromosomes are called _______
 One
sex is homogametic
 The
other sex is heterogametic
© 2007 Paul Billiet ODWS
Sex determination in different animals
HOMOGAMETIC SEX
HETEROGAMETIC SEX
Female XX
Male XY
Male ZZ
Female ZW
Female XX
Male Xo
© 2007 Paul Billiet ODWS
SEX DETERMINATION
Presence of Y-chromosome =
maleness (mammals and fish)
Presence of second Xchromosome = femaleness
(Drosophila, the fruit fly)
Birds, amphibians, reptiles,
butterflies, moths.
Grasshoppers