Meiosis. Objectives 4.2.1 – State that meiosis is a reduction division of a diploid nucleus to form haploid nuclei. 4.2.2 – Define homologous chromosomes.

MeiosisMeiosis

ObjectivesObjectives4.2.1 – State that meiosis is a reduction division of a diploid nucleus to form haploid nuclei.4.2.2 – Define homologous chromosomes.4.2.3 – Outline the process of meiosis, including pairing of homo-logous chromosomes and crossing over, followed by

two divisions, which results in four haploid cells.4.2.4 – Explain that non-disjunction can lead to changes in chromosome number, illustrated by reference to Down syndrome (trisomy 21).4.2.5 – State that, in karyotyping, chromosomes are arranged in

pairs according to their size and structure.4.2.6 – State that karyotyping is performed using cells collected by chorionic villus sampling or amniocentesis, for pre-

natal diagnosis of chromosome abnormalities.4.2.7 – Analyze a human karyotype to determine gender and

whether non-disjunction has occurred.

4.2.1 – State that meiosis is a reduction division of a diploid nucleus to form haploid nuclei.4.2.2 – Define homologous chromosomes.4.2.3 – Outline the process of meiosis, including pairing of homo-logous chromosomes and crossing over, followed by

two divisions, which results in four haploid cells.4.2.4 – Explain that non-disjunction can lead to changes in chromosome number, illustrated by reference to Down syndrome (trisomy 21).4.2.5 – State that, in karyotyping, chromosomes are arranged in

pairs according to their size and structure.4.2.6 – State that karyotyping is performed using cells collected by chorionic villus sampling or amniocentesis, for pre-

natal diagnosis of chromosome abnormalities.4.2.7 – Analyze a human karyotype to determine gender and

whether non-disjunction has occurred.

Remember mitosis: stages in the cell cycleRemember mitosis: stages in the cell cycle

Interphase: 3 sub-phases:G1 phase (1st gap) – centers on growth.S phase (synthesis) – when chromosomes are copied.G2 phase (2nd gap) – the cell completes preparations for cell division.

Organelles multiply.Chomosomes form from chromatin.Centrosomes are duplicated.Microtubles form.

Then the cell divides (M).

Stages in the cell cycle Stages in the cell cycle Mitosis is broken into 4 sub-

phases:

Interphase Prophase Metaphase Anaphase Telophase

Genetically identical nucleiGenetically identical nuclei

By this method, each daughter cell ends up with one copy ofevery chromosome.

An abnormal number of chromosomes in a cell is usually fatal.

Definition of meiosisDefinition of meiosisMeiosis is a reduction division of a diploid (2n)

nucleus to form haploid (1n) nuclei.Eukaryotic species have a characteristic number of chromosomes in the nucleus.

Human somatic (body) cells have 46 chromosomes.Human gametes (sperm & eggs) have 23 chromosomes.

Meiosis is a reduction division of a diploid (2n) nucleus to form haploid (1n) nuclei.

Eukaryotic species have a characteristic number of chromosomes in the nucleus.

Human somatic (body) cells have 46 chromosomes.Human gametes (sperm & eggs) have 23 chromosomes.

Blue & red represent chromosomes from mother & father.

Sperm has an uneven mix of red & blue, and some that arecombinations.

Homologous chromosomesHomologous chromosomesHomologous chromosomes are chromosome

pairs of the same length, centromere position, and staining pattern that possess genes for the same characters at corre-sponding loci. One homologous chromosome is inherited from the father and one from the mother.

Homologous chromosomes are chromosome pairs of the same length, centromere position, and staining pattern that possess genes for the same characters at corre-sponding loci. One homologous chromosome is inherited from the father and one from the mother.

A set of homologouschromosomes pro-duces a karyotype.

Homologous chromosomesHomologous chromosomesHomologous chromosomes are chromosome

pairs of the same length, centromere position, and staining pattern that possess genes for the same characters at corre-sponding loci.

Homologous chromosomes are chromosome pairs of the same length, centromere position, and staining pattern that possess genes for the same characters at corre-sponding loci.

Homologous pairsof chromosomes

Mitosis vs. meiosisMitosis vs. meiosisMeiosis reduces chromosome number from 2n to

1n.Chromosomes replicate before either mitosis or meiosis.

In mitosis 1 cell division produces 2 diploid daughter cells.In meiosis: 2 cell divisions produce 4 haploid daughter cells.

Meiosis reduces chromosome number from 2n to 1n.

Chromosomes replicate before either mitosis or meiosis.

In mitosis 1 cell division produces 2 diploid daughter cells.In meiosis: 2 cell divisions produce 4 haploid daughter cells.

Process of meiosisProcess of meiosisMeiosis reduces chromosome number from 2n to

1n.In interphase chromosomes replicate and form genetically identical sister chromatids, joined at the centromere.


In interphase chromosomes replicate and form genetically identical sister chromatids, joined at the centromere.


1n.In prophase I homologous chromosomes pair up and exchange segments – called crossing over – a major event.


In prophase I homologous chromosomes pair up and exchange segments – called crossing over – a major event.

Note tetrads

Process of meiosisProcess of meiosisCrossing over – non-sister chromatids of

homologous chromosomes exchange genetic information: the genomes of the mother & father of this parent mix.

Crossing over – non-sister chromatids of homologous chromosomes exchange genetic information: the genomes of the mother & father of this parent mix.

From parent’s Crossing Haploid (1n)Mother Father over gametes

No sperm or eggsare identical.

The crossing points within the tetrads are called chiasmata.

Tetrad: the 4homologouschromosomes

One parent’s DNA

Process of meiosisProcess of meiosisCrossing over & independent assortment* introduce

variation, as does the random joining of a unique sperm and egg at fertilization.

*There are 23 chromosome pairs in human

germ cells, therefore they can line up in

223 = 8,388,608 different ways. Sperm and egg can produce

> 64 trillion unique individuals.

Crossing over & independent assortment* introduce variation, as does the random joining of a unique sperm and egg at fertilization.

*There are 23 chromosome pairs in human

germ cells, therefore they can line up in

223 = 8,388,608 different ways. Sperm and egg can produce

> 64 trillion unique individuals.

This is the advantageof sexual reproductionover asexual.

mix in 223

different ways


1n.In metaphase 1 homologs line up at metaphase plate.In anaphase 1 homologous chromosomes separate.


In metaphase 1 homologs line up at metaphase plate.In anaphase 1 homologous chromosomes separate.


1n.After telophase 1, a 2nd cell division separates the sister chromatids. Each gamete has only 1 copy of DNA.


After telophase 1, a 2nd cell division separates the sister chromatids. Each gamete has only 1 copy of DNA.

Non-disjunctionNon-disjunctionNon-disjunction: an accident of meiosis or mitosis

in which the members of a pair of homologous chromosomes or sister chromatids fail to move apart properly.

Some gametes get 2 of one chromosome; others get none.

Ex: Down syndrome (trisomy 21)

Non-disjunction: an accident of meiosis or mitosis in which the members of a pair of homologous chromosomes or sister chromatids fail to move apart properly.

Some gametes get 2 of one chromosome; others get none.

Ex: Down syndrome (trisomy 21)

Non-disjunctionNon-disjunctionDown syndrome (trisomy 21)

3 copies of chromosome 21 Down syndrome (trisomy 21)

3 copies of chromosome 21

The karyotype

KaryotypingKaryotypingIn karyotyping, chromosomes are arranged in

pairs according to their size and structure.In karyotyping, chromosomes are arranged in

pairs according to their size and structure.

A picture of one’s chromosomes

KaryotypingKaryotypingKaryotyping is performed using cells collected by

chorionic villus sampling or amniocentesis, for pre-natal diagnosis of chromosome abnormalities.

Karyotyping is performed using cells collected by chorionic villus sampling or amniocentesis, for pre-natal diagnosis of chromosome abnormalities.

The chorion is a membrane forming the placenta.

KaryotypingKaryotypingSocial and ethical issues of karyotyping unborn

fetusesAbortion of fetuses with abnormalitiesAbortion based on gender or superficial characteristics

Eye color?

Social and ethical issues of karyotyping unborn fetuses

Abortion of fetuses with abnormalitiesAbortion based on gender or superficial characteristics

Eye color?

KaryotypingKaryotypingAnalyze a human karyotype to determine gender

and whether non-disjunction has occurred.

What is the sex?

Analyze a human karyotype to determine gender and whether non-disjunction has occurred.

What is the sex?


and whether non-disjunction has occurred.XYY syndrome - Affected individuals are usually very

tall and thin. Many experience severe acne during adolescence. Additional symptoms may include antisocial or behavioral problems and learning disabilities. Intelligence is usually normal, although IQ,

on average, is 10 to 15 points lower than

siblings.


XYY syndrome - Affected individuals are usually very tall and thin. Many experience severe acne during adolescence. Additional symptoms may include antisocial or behavioral problems and learning disabilities. Intelligence is usually normal, although IQ,

on average, is 10 to 15 points lower than

siblings.


and whether non-disjunction has occurred.Trisomy 18 is a disorder that shows symptoms as

soon as the child is born. About 25% of Edward's syndrome victims die before they are one month old, and only 10% live for one year.


Trisomy 18 is a disorder that shows symptoms as soon as the child is born. About 25% of Edward's syndrome victims die before they are one month old, and only 10% live for one year.

Meiosis. Objectives 4.2.1 – State that meiosis is a reduction division of a diploid nucleus to form haploid nuclei. 4.2.2 – Define homologous chromosomes.

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