Chapter 11: Meiosis and Sexual Reproduction

Sexual reproduction

Combines haploid (n) gametes through fertilization

• Define sexual reproduction and its role in increasing genetic diversity. (exam)

Sexual reproduction combines genetic material from two gametes during fertilization. It increases genetic diversity through crossing over, independent assortment, and random fertilization, producing offspring with unique combinations of genes.

Sexual reproduction combines haploid gametes through ______

fertilization

Fertilization

restores the diploid (2n) chormosome number

Sexual reproduction produces ______ ___ ___

Genetically unique offspring

Mitosos is cell division for ______, Meiosis is cell division for _______

body cells; sex cells

Process of meiosis - reduces number

Reduces the number of chromosomes (2n) by half

2n → n (to 23)

One set inherited from each parent

Fertilization does not double the chromosome number every generation, it ____

recodes the number of chromosomes by ½ to produce haploid (1n) cells

Chromosomes and DNA Packaging (EXAM)

• Distinguish between:

  • Somatic cells (__) – ______, ___ set of chromosomes

  • Gametes (__) – ______, ___ set of chromosomes.

Somatic cells (2n) – diploid, two sets of chromosomes.

Gametes (1n) – haploid, one set of chromosomes.

Diploid Chromsomes (EXAM)

2n, Two sets of chromosomes (one from each parent)

Haploid Chromsomes (EXAM)

1n, one set of chromosomes (gametes)

Two rounds of division in meiosis (which happens after DNA replication): (EXAM)

Meiosis I and Meiosis II

Meiosis I Goal : (EXAM)

Goal: Separate homologous chromosomes; reduce chromosome number by half. (2n → n)

Meiosis I steps

Proceded by normal interphase (G1, S, G2), where DNA replication occurs ( during s phase), which produces sister chromatids

There is NO second round of DNA before meiosis II

Sister chromatids remain attached at the centromere

• Prophase I:

  • Homologous chromosomes pair (synapsis) to form tetrads (pairs of four sister chromatids)

  • Crossing over at chiasmata exchanges genetic material, increasing diversity.

• Prometaphase I: Spindle fibers attach; nuclear membrane disintegrates.

• Metaphase I: Tetrads align randomly (independent assortment).

• Anaphase I: Homologous chromosomes separate; sister chromatids remain attached.

• Telophase I and Cytokinesis:

  • Chromosomes reach poles; cells divide into two haploid cells.

  • Cleavage furrow (animals) or cell plate (plants) forms.

________ is the location, _______ is the handle on the chromosome

centromere, kinetochore

Interphase in Meiosis one

DNA replication occurs (s phase of interphase) ; sister chromatids form.

Prophase I (EXAM)

• Homologous chromosomes pair (synapsis) to form tetrads (four sister chromatids paired)

• Crossing over at chiasmata exchanges genetic material, increasing diversity.

Paired homologs form a tetrad (four sister chromatids paired)

Crossing over occurs between non sister chromatids

Visible structure at cross over points are called chiasmata

What happens in crossing over (during prophase I)

Crossover exchanges genetic material (DNA) between non sister chromatids of homologous chromosomes

Prometaphase I (EXAM)

Spindle fiber microtubules attach to the kentochore proteins at the centromeres

Each homologous is connected toward an opposite pole

nuclear membrane disintegrates.

Metaphase I (EXAM)

Homologus chromosomes are arranged at the cell equator with kinetochores facing opposite poles

Maternal and paternal homologous chromsomes ( 4 tetrads) orient randomly (Independent assortment)

This Creates different combinations of maternal and paternal chromosomes in gametes

Anaphase I (EXAM)

Homologous chromosomes seperate and move to opposite poles

Chiasmata are broken, but sister chromatids remain attached

Telophase I and Cytokinesis (EXAM)

AFTER Meiosis I: n, but chromosomes are still duplicated

• Chromosomes reach poles; cells divide into two haploid cells.

• Cleavage furrow (animals) or cell plate (plants) forms.

Result: Two HAPLOID cells

Each chromosome still consist of two sister chromatids

Meosis II - Separation of Sister Chromatids: (EXAM)

Goal: separate Sister chromatids (similar to mitosis), final result: four genetically distinct haploid (n) cells

Meiosis II resembles mitosis - how?

Cell begin haploid (n), sister chromatids separate, no DNA replication occurs between I and II

Beginning Meiosis II

Chrosomes condense if necessary; a new spindle forms (Prophase II)

Spindle Microtubules attack to kinetochores of sister chromatids from opposite poles (Prometaphase II)

Prophase II (EXAM)

Sister chromatids condense. A new spindle begins to form. The nuclear envelops start to fragments

Prometaphase II (EXAM)

The nuclear envelope disappears, and the spindle fibers (microtubules) attach to the the individual kinetochores on the sister chromatids

Chromosome alignment difference between meiosis I and meiosis II

Meios I: Homologous chromosomes separate

Meiosis II: Sister chromatids separate

Meios I:_______ separate

Homologous chromosomes

Meiosis II: _______ separate

Sister chromatids

Metaphase II and Anaphase II

Chromosomes align individually at metaphase plate.

Sister chromatids separate during anaphase II.

This looks like mitosis

Metaphase II (EXAM)

Chromosomes align at metaphase plate

Anaphase II (EXAM)

Sister chromatids are pulled apart to opposite poles by the shortening of the kinetochore microtubules. Non kinetochore microtubules lengthen the cell

Telophase II and Cytokinesis

chromosomes decondense; nuclei reform; four haploid cells result.

One diploid (2n) parent cell → FOUR genetically unique haploid (n) cells

Telophase II and Cytokenisis

Chormosomes arrive at the poles of the cell and decondense. Nuclear envelopes surround the four nuclei. Cleavage furrows divide the two cells into four haploid cells

Comparing meiosis to Mitosis (EXAM Q?)

Meiosis

— 2 divisions

— homologs pair in Prophase I

— Crossing over + independent assortment — only in meiosis

— 4 genetically unique haploid (n) cells

— Ploidy reduced

Mitosis

— 1 division

— homologs do not pair

— 2 genetically similar daughter cells

• Mitosis _____ chromosome number; meiosis _____ chromosome number (EXAM)

maintains, reduces

Sexual Reproduction: Cost

Requires combining genomes/mating and is less efficient than cloning

Sexual Reproduction: Benefit

Produces genetically variable offspring, which can help populations responds to changing environments, parasites, predators

3 Main categories of life cycles in multicellular organisms (EXAM)

Diploid dominant → animals — multicellular organism is diploid; gametes are haploid.

Haploid dominant → fungi/algae - multicellular organism is haploid; zygote is diploid but undergoes meiosis immediately.

Alternation of generations → plants/some algae — both haploid (gametophyte) and diploid

Diploid dominant life cycle (Animals) (EXAM)

Diploid adult → meiosis → haploid gametes (1n) → fertilization → diploid zygote → mitosis → diploid adult. (2n)

Only the gametes are haploid; there is no multicellular haploid stage

Overall definition: multicellular organism is diploid; gametes are haploid.

Haploid dominant life cycle (fungi, some algae)(EXAM)

Haploid multicellular organism → haploid cells fuse → diploid zygote → meiosis immediately → haploid spores → mitosis → haploid organisms

The zygote is the only diploid stage

Overall definition: multicellular organism is haploid; zygote is diploid but undergoes meiosis immediately

Alternation of Generations (plants, some algae) (EXAM)

Both the haploid and diploid stages are multicellular

Gametophyte (n) produces gametes by MITOSIS

Sporophyte (2n) → MEIOSIS → spores (n) → MITOSIS → gametophyte (n) → MITOSIS → gametes (n) → FERTILZATION → zygote (2n) → MITOSIS → sporophyte

Overall definition: both haploid (gametophyte) and diploid (sporophyte) multicellular stages alternate.

Understand that ________ restores diploidy (2n) by fusing two haploid gametes. (EXAM)

fertilization