Meiosis and Sexual Reproduction

Chapter 10

Meiosis—type of nuclear division that occurs as part of sexual reproduction.

Haploid (n) number—cell condition in which one of each type of chromosome is present; half the diploid number.

Diploid (2n) number—cell condition in which two of each type of chromosome are present.

Homologous Pairs of Chromosomes

Member of a pair of chromosomes that are alike and come together in synapsis › Occurs during prophase of first meiotic

division.

› One member of a homologous pair was inherited from the male parent and the other is inherited from the female parent.

Meiosis Overview Meiosis I

› Homologous chromosomes separate.

› Daughter cells have one copy of each kind of chromosome in various combinations.

No replication of DNA is needed between meiosis I and meiosis II.

Meiosis Overview Meiosis II

› Four haploid daughter cells Each chromosome consists of one chromatid.

› Daughter cells mature into gametes; sperm or eggs.

Genetic Recombination Process in which new genetic

information is incorporated into a chromosome or DNA fragment.

Genetic Recombination Crossing-over—is an exchange of

genetic material between nonsister chromatids of a bivalent during meiosis I.

› Bivalent—homologous chromosomes, each having sister chromatids that are joined by a nucleoprotein lattice during meiosis.

Genetic Recombination Independent assortment—homologous

chromosomes separate independently or in a random manner.

4 possible orientation of homologous pairs at metaphase plate

Genetic Recombination Fertilization—fusion of sperm and egg

nuclei, producing a zygote that develops into a new individual.

A bivalent isa) A homologous chromosomeb) The paired homologous chromosomes.c) A duplicated chromosome composed

of sister chromatids.d) The two daughter cells after meiosis I.

Crossing over occurs between

a) Sister chromatids of the same chromosome.

b) Two different kinds of bivalents.c) Two different kinds of chromosomes.d) Nonsister chromatids of a bivalent.e) Two daughter nuclei.

Fertilization a) Is a source of variation during sexual

reproduction.b) Is fusion of the gametesc) Occurs in both animal and plant life

cycles.d) Restores the diploid number of

chromosomes.e) All of these are correct.

Phases of Meiosis Prophase I

› Spindle fibers form as the centrosomes move away from one another.

› Nucleolus disappears.› Homologous chromosomes pair during

synapsis. Crossing-over may occur at this time.

Phases of Meiosis Metaphase I

› Fully formed spindle fibers.› Alignment of the homologous

chromosomes at the metaphase plate.

Phases of Meiosis Anaphase I

› Homologous chromosomes separate and move to opposite poles. Each chromosome still consist of 2

chromatids.

Phases of Meiosis Telophase

› Nuclear envelops re-form and nucleoli appear.

› Each daughter cell has one chromosome from each pair of homologous pair.

Phases of Meiosis Interkinesis

› Similar to interphase between mitotic division. Except DNA replication does not occur.

Chromosomes are already duplicated.

Phases of Meiosis Prophase II

› Cells have one chromosomes from each homologous pair.

Phases of Meiosis Metaphase II

› Chromosomes align at the metaphase plate.

Phases of Meiosis Anaphase II

› Sister chromatids separate. Daughter chromosomes move toward the

poles.

Phases of Meiosis Telophase II

› Spindle fibers disappear.› Nuclei form.› Cytokinesis takes place.

Phases of Meiosis Daughter cells

› Meiosis I Form 2 daughter cells. Diploid number of chromosomes.

› Meiosis II From 4 daughter cells Haploid number of chromosomes.

Comparison Meiosis I

› Prophase I—pairing of homologous chromosomes

› Metaphase—homologous chromosomes align at the metaphase plate.

Mitosis› Prophase—no

pairing of chromosomes.

› Metaphase—duplicated chromosomes at metaphase plate

Comparison Meiosis I

› Anaphase I—homologous chromosome pairs separate and move to separate poles.

› Telophase I—two haploid daughter cells.

Mitosis› Anaphase—sister

chromatids separate, becoming daughter chromosomes that move to the poles.

› Telophase—two daughter cells, identical to the parent cell.

Comparison Meiosis II

› Prophase II—no pairing of chromosomes.

› Metaphase II—haploid number of duplicated chromosomes at metaphase plate.

Mitosis› Prophase—no

pairing of chromosomes.

› Metaphase—duplicated chromosomes at metaphase plate

Comparison Meiosis II

› Anaphase II—sister chromatids separate, becoming daughter chromosomes that move to the poles.

› Telophase—four haploid daughter cells, no genetically identical.

Mitosis› Anaphase—sister

chromatids separate, becoming daughter chromosomes that move to the poles.

› Telophase—two daughter cells, identical to the parent cell.

If a parent cell has twelve chromosomes, then each of the daughter cells following meiosis will have…

a) Forty-eight chromosomesb) Twenty-four chromosomesc) Twelve chromosomesd) Six chromosomese) Any one of these could be correct.

At the metaphase plate during metaphase I of meiosis, there are

a) Chromosomes consisting of one chromatid.

b) Unpaired duplicated chromosomes.c) Bivalents.d) Homologous pairs of chromosomes.e) Both c and d are correct.

At the metaphase plate during metaphase II of meiosis, there are

a) Chromosomes consisting of one chromatid.

b) Unpaired duplicated chromosomes.c) Bivalentsd) Homologous pairs of chromosomes.e) Both c and d are correct.

During which phase of meiosis do homologous chromosomes separate?

a) Prophase IIb) Telophase Ic) Metaphase Id) Anaphase Ie) Anaphase II

During which phase of meiosis do homologous chromosome pairs separate?

a) Prophase IIb) Telophase Ic) Metaphase Id) Anaphase Ie) Anaphase II