Meiosis

Objectives – State that meiosis is a reduction division of a diploid nucleus to form haploid nuclei – Define homologous chromosomes – Outline the process of meiosis, including pairing of homo- logous chromosomes and crossing over, followed by two divisions, which results in four haploid cells – Explain that non-disjunction can lead to changes in chromo- some number, illustrated by reference to Down syndrome (trisomy 21) – State that, in karyotyping, chromosomes are arranged in pairs according to their size and structure – State that karyotyping is performed using cells collected by chorionic villus sampling or amniocentesis, for pre- natal diagnosis of chromosome abnormalities – Analyze a human karyotype to determine gender and whether non-disjunction has occurred – State that meiosis is a reduction division of a diploid nucleus to form haploid nuclei – Define homologous chromosomes – Outline the process of meiosis, including pairing of homo- logous chromosomes and crossing over, followed by two divisions, which results in four haploid cells – Explain that non-disjunction can lead to changes in chromo- some number, illustrated by reference to Down syndrome (trisomy 21) – State that, in karyotyping, chromosomes are arranged in pairs according to their size and structure – State that karyotyping is performed using cells collected by chorionic villus sampling or amniocentesis, for pre- natal diagnosis of chromosome abnormalities – Analyze a human karyotype to determine gender and whether non-disjunction has occurred.

Remember mitosis: stages in the cell cycle Interphase: 3 sub-phases: G1 phase (1 st gap) – centers on growth. S phase (synthesis) – when chromosomes are copied. G2 phase (2 nd 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 Mitosis is broken into 4 sub-phases: Interphase Prophase Metaphase Anaphase Telophase

Genetically identical nuclei By this method, each daughter cell ends up with one copy of every chromosome. An abnormal number of chromosomes in a cell is usually fatal.

Definition of meiosis 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. 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 are combinations.

Homologous chromosomes 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 homologous chromosomes pro- duces a karyotype.

Homologous chromosomes 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 pairs of chromosomes

Mitosis vs. meiosis 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. 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 meiosis Meiosis reduces chromosome number from 2n to 1n. In interphase chromosomes replicate and form genetically identical sister chromatids, joined at the centromere. Meiosis reduces chromosome number from 2n to 1n. In interphase chromosomes replicate and form genetically identical sister chromatids, joined at the centromere.

Process of meiosis Meiosis reduces chromosome number from 2n to 1n. In prophase I homologous chromosomes pair up and exchange segments – called crossing over – a major event. Meiosis reduces chromosome number from 2n to 1n. In prophase I homologous chromosomes pair up and exchange segments – called crossing over – a major event. Note tetrads

Process of meiosis 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 eggs are identical. The crossing points within the tetrads are called chiasmata. Tetrad: the 4 homologous chromosomes One parent’s DNA

Process of meiosis Crossing over & independent assortment* introduce variation, as does the random joining of a unique sperm and egg at fertilization. * There are 23 chromo- some pairs in human germ cells, therefore they can line up in 2 23 = 8,388,608 dif- ferent 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 chromo- some pairs in human germ cells, therefore they can line up in 2 23 = 8,388,608 dif- ferent ways. Sperm and egg can produce > 64 trillion unique individuals. This is the advantage of sexual reproduction over asexual. mix in 2 23 different ways

Process of meiosis Meiosis reduces chromosome number from 2n to 1n. In metaphase 1 homologs line up at metaphase plate. In anaphase 1 homologous chromosomes separate. Meiosis reduces chromosome number from 2n to 1n. In metaphase 1 homologs line up at metaphase plate. In anaphase 1 homologous chromosomes separate.

Process of meiosis Meiosis reduces chromosome number from 2n to 1n. After telophase 1, a 2 nd cell division separates the sister chromatids. Each gamete has only 1 copy of DNA. Meiosis reduces chromosome number from 2n to 1n. After telophase 1, a 2 nd cell division separates the sister chromatids. Each gamete has only 1 copy of DNA.

Non-disjunction 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-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 Down syndrome (trisomy 21) 3 copies of chromosome 21 Down syndrome (trisomy 21) 3 copies of chromosome 21 The karyotype

Karyotyping In karyotyping, chromosomes are arranged in pairs according to their size and structure. A picture of one’s chromosomes

Karyotyping 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.

Karyotyping Social and ethical issues of karyotyping unborn fetuses Abortion of fetuses with abnormalities Abortion based on gender or superficial characteristics Eye color? Social and ethical issues of karyotyping unborn fetuses Abortion of fetuses with abnormalities Abortion based on gender or superficial characteristics Eye color?

Karyotyping Analyze 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?

Karyotyping Analyze a human karyotype to determine gender 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. Analyze a human karyotype to determine gender 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.

Karyotyping Analyze a human karyotype to determine gender 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. Analyze a human karyotype to determine gender 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.