8/18/2019 BIOL 3301 - Genetics Ch3B - Meiosis St
1/35
Meiosis
• Cell division occur only in germ cell –to form
gamete (sperm or egg cells)- sexual reproduction
• Main goal – to reduce chromosome number in half( that’s why you only get one set from mom and
one from dad)
• hy chromosome number need to be reduced!
"rogeny with unpredictable # chromosomes cause
death of cells$
8/18/2019 BIOL 3301 - Genetics Ch3B - Meiosis St
2/35
8/18/2019 BIOL 3301 - Genetics Ch3B - Meiosis St
3/35
%omologous Chromosomes
Carry &ifferent 'lleles• Cell has two of each chromosome
• ne chromosome in each pair from motherthe other from father
• "aternal and maternal chromosomes carry
different alleles
8/18/2019 BIOL 3301 - Genetics Ch3B - Meiosis St
4/35
Chromosome *umber
• +um total of chromosomes in a cell
• ,erm cells are diploid (n
)
• ,ametes are haploid (n)
• Meiosis halves chromosome number
8/18/2019 BIOL 3301 - Genetics Ch3B - Meiosis St
5/35
+exual .eproduction
+huffles 'lleles
• /hrough sexual reproduction offspring inherit
000000000000000000000000 of alleles whichleads to variations in traits
• ffsprings are not identical
000000000000000000000000 or to the parents
• /his variation in traits is the basis for evolutionary
change
8/18/2019 BIOL 3301 - Genetics Ch3B - Meiosis St
6/35
Meiosis1 /wo &ivisions
• /wo consecutive nuclear divisions
– Meiosis 2
– Meiosis 22
• &*' is not duplicated between divisions (is
duplicated once in interphase before division)
• 3our haploid nuclei form
8/18/2019 BIOL 3301 - Genetics Ch3B - Meiosis St
7/35
Meiosis 2
Each homologue in the
cell pairs with its partner,
then the partners
separate
8/18/2019 BIOL 3301 - Genetics Ch3B - Meiosis St
8/35
Meiosis 22
• /he two sister chromatids of each
duplicated chromosome are separated from
each other
one chromosome
(duplicated)
two chromosomes
(unduplicated)
8/18/2019 BIOL 3301 - Genetics Ch3B - Meiosis St
9/35Figure 2-8 Copyright © 2006 Pearson Prentice Hall, Inc.
8/18/2019 BIOL 3301 - Genetics Ch3B - Meiosis St
10/35
Meiotic divisions
• .eplicated homologous chromosomes pair alongtheir length (synapsis) – form tetrad of four chromatids (or bivalent)
– paired nonsister chromatids may undergo crossing-over
• 't first meiotic division (reduction division)homologous chromosomes segregate to opposite
poles
– spindle attaches only to one side of centromere• 't second meiotic division sister chromatids
segregate to opposite poles
8/18/2019 BIOL 3301 - Genetics Ch3B - Meiosis St
11/35
/erminology
• Chromosome with a sister chromatid – dyad
• hen a dyad is separated in anaphase of
mitosis it forms a monad (or a daughterchromosome)
• hen two dyads synapse in meiosis they
form a tetrad (of chromatids) or a bivalent• 4nivalent – dyad that does not have a
homolog in metaphase 2
8/18/2019 BIOL 3301 - Genetics Ch3B - Meiosis St
12/35
+tages of Meiosis
• &*' is replicated in interphase
• Meiosis 2 – long prophase
• Metaphase 2
• 'naphase 2
• /elophase 2
• /wo cells are formed but each chromosome stillconsists of chromatids – chromosome reduction
8/18/2019 BIOL 3301 - Genetics Ch3B - Meiosis St
13/35
+tages of Meiosis
• Meiosis 22 – cyto5inesis occur but cells do not
separate1
– "rophase 22 – Metaphase 22
– 'naphase 22
– /elophase 22
– Cyto5inesis
6 haploid cells are formed - tetrads
8/18/2019 BIOL 3301 - Genetics Ch3B - Meiosis St
14/35
Meiosis 2 (7)
• "rophase 2 – 8eptonema or leptotene stage (9thin threads:)1 chromosome
coiling
– ;ygonema1 synapsis of homologs
• synaptonemal complex forms between homologs• form bivalent of two chromosomes or tetrad of four chromatids
– "achynema (9thic5 threads:)1 crossing-over occurs
– &iplonema (9two threads:)1 chiasmata become visible
– &ia5inesis (9movement through:)1 terminali;ation of chiasmatachromosomes move to central plane
• Metaphase 21 random alignment of bivalents – results in independent assortment of genes
8/18/2019 BIOL 3301 - Genetics Ch3B - Meiosis St
15/35
Meiosis 2 ()
• 'naphase 21 chromosomes move directionally toopposite poles< chromosome dis=unction(nondis=unction can occur)
– sister chromatids remain attached to centromere• /elophase 2
– cell division
– chromosomes uncoil
– nuclear membrane reforms – not always• /wo haploid cells form
• 2nter5inesis may occur (no &*' synthesis)
8/18/2019 BIOL 3301 - Genetics Ch3B - Meiosis St
16/35
8/18/2019 BIOL 3301 - Genetics Ch3B - Meiosis St
17/35
8/18/2019 BIOL 3301 - Genetics Ch3B - Meiosis St
18/35
Meiosis 22
• "rophase 221 – coiling of sister chromatids attached to centromere
– spindle attaches to 5inetochores on each side of centromere
•Metaphase 221 alignment on e@uatorial plate• 'naphase 22 – centromeres split
– chromosomes segregate to opposite poles
•/elophase 22 – cell division – nuclei and nucleoli reform
www$ johnkyrk.com>meiosis$swf
8/18/2019 BIOL 3301 - Genetics Ch3B - Meiosis St
19/35
Crossing ver
•Each chromosome
becomes zippered to its
homologe
• !ll "or chromatids are
closely aligned
•#onsister chromosomes
e$change segments
8/18/2019 BIOL 3301 - Genetics Ch3B - Meiosis St
20/35
paternal homologue
maternal homologue
Stepped Art
Crossing ver
8/18/2019 BIOL 3301 - Genetics Ch3B - Meiosis St
21/35
Crossing ver
a Both chromosomes shown here
were duplicated during
interphase, before meiosis !hen
prophase " is under wa#, sister chromatids of each chromosome
are positioned so close together
that the# loo$ li$e a single thread
8/18/2019 BIOL 3301 - Genetics Ch3B - Meiosis St
22/35
Crossing ver
b Each chromosome becomes
%ippered to its homologue, so all
four chromatids are tightl# aligned
"f the two se& chromosomes ha'edifferent forms, such as paired
with , the# still get %ippered
together, but onl# in a tin# region
at their ends
8/18/2019 BIOL 3301 - Genetics Ch3B - Meiosis St
23/35
Crossing ver
c !e show the pair
of chromosomes
as if the# alread#
condensed onl# to
gi'e #ou an idea of
what goes on
*he# reall# are in a
tightl# aligned,
threadli$e form
during prophase "
d *he intimate
contact encourages
one crosso'er (and
usuall# more) to
happen at 'arious
inter'als along the
length of nonsister
chromatids
e +onsister chromatids
e&change segments at
the crosso'er sites
*he# continue to
condense into thic$er,
rodli$e forms B# the
start of metaphase ",
the# will be un%ippered
from each other
f rossing o'er
brea$s up old
combinations of
alleles and puts
new ones together
in the cells pairs
of homologous
chromosomes
8/18/2019 BIOL 3301 - Genetics Ch3B - Meiosis St
24/35
Affect of Crossing ver
• 'fter crossing over each chromosome
contains both maternal and paternal
segments
• Creates new allele combination in offspring
8/18/2019 BIOL 3301 - Genetics Ch3B - Meiosis St
25/35
Conse@uences of meiosis
• 3ormation of four haploid cells each with one
complete copy of genome
• .ecombination – crossing-over in prophase 2
– independent assortment of nonhomologous
chromosomes
– results in genetic diversity
• Meiosis occurs at some point in life cycle of all
sexually reproducing organisms
8/18/2019 BIOL 3301 - Genetics Ch3B - Meiosis St
26/35
+ources of ,enetic Bariability 2n
Meiosis• /here are three sources of genetic
variability in sexual reproduction1
– a$ 2ndependent assortment of homologouschromosomes in meiosis
– b$ Crossing over of homologous chromosomesin meiosis
– c$ .andom fertili;ation of an ovum by a sperm(and there are 6 million sperm in the averagehuman e=aculation)
8/18/2019 BIOL 3301 - Genetics Ch3B - Meiosis St
27/35
2ndependent assortment
– 2n humans because there are D pairs of
chromosomes the number of possible
assortments is1• xxxxxxxxxxxxxxxxx
xxxxx E FDFF?FG
– 'ny one of these assortments can combine withany one of the FDFF?F combinations of
his>her partnerG
8/18/2019 BIOL 3301 - Genetics Ch3B - Meiosis St
28/35
.andom 'lignment
• &uring transition between prophase 2 and
metaphase 2 microtubules from spindle
poles attach to 5inetochores ofchromosomes
• 2nitial contacts between microtubules andchromosomes from the parents
8/18/2019 BIOL 3301 - Genetics Ch3B - Meiosis St
29/35
.andom 'lignment
• Aither the maternal or paternal member of a
homologous pair can end up at either pole
• /he chromosomes in a gamete are a mix of
chromosomes from the two parents
8/18/2019 BIOL 3301 - Genetics Ch3B - Meiosis St
30/35
"ossible Chromosome
Combinations
's a result of random alignment the number
of possible combinations of chromosomesin a gamete is1
(n is number of chromosome types)
8/18/2019 BIOL 3301 - Genetics Ch3B - Meiosis St
31/35
or
or
or
% 2 &
combinations possible
"ossible
Chromosome
Combinations
8/18/2019 BIOL 3301 - Genetics Ch3B - Meiosis St
32/35
. 2 /
combinations possible
or
or
or
Alignment at
metaphase "
Stepped Art
8/18/2019 BIOL 3301 - Genetics Ch3B - Meiosis St
33/35
8/18/2019 BIOL 3301 - Genetics Ch3B - Meiosis St
34/35
meiotic
cell di'ision
"ertilization
2n
2n
2n n
n
8/18/2019 BIOL 3301 - Genetics Ch3B - Meiosis St
35/35