biol 3301 - genetics ch3b - meiosis st

8/18/2019 BIOL 3301 - Genetics Ch3B - Meiosis St

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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$


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%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


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Chromosome *umber

• +um total of chromosomes in a cell

• ,erm cells are diploid (n

)

• ,ametes are haploid (n)

• Meiosis halves chromosome number


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+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


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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


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Meiosis 2

Each homologue in the

cell pairs with its partner,

then the partners

separate


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Meiosis 22

• /he two sister chromatids of each

duplicated chromosome are separated from

each other

one chromosome

(duplicated)

two chromosomes

(unduplicated)


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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


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/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


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+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


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+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


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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


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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)


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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


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Crossing ver

•Each chromosome

becomes zippered to its

homologe

• !ll "or chromatids are

closely aligned

•#onsister chromosomes

e$change segments


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paternal homologue

maternal homologue

Stepped Art

Crossing ver


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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


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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


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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


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Affect of Crossing ver

• 'fter crossing over each chromosome

contains both maternal and paternal

segments

• Creates new allele combination in offspring


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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


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+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)


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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


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.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


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.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


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"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)


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or

or

or

% 2 &

combinations possible

"ossible

Chromosome

Combinations


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. 2 /

combinations possible

or

or

or

Alignment at

metaphase "

Stepped Art


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meiotic

cell di'ision

"ertilization

2n

2n

2n n

n


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biol 3301 - genetics ch3b - meiosis st

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