Meiosis and Sexual Life Cycles

Chapter 13: Meiosis and Sexual Life Cycles

• Reading and Notes due Monday 1/13

Introduction to Meiosis

• Meiosis is a process that produces daughter nuclei.

Meiosis Overview

• Meiosis in 3 minutes…

Diploid vs. Haploid Cells

• Diploid Cell (2n): Contains two copies of each chromosome (one maternal and one paternal).

• Haploid Cell (n): Contains one copy of each chromosome.

• Example:

  • $2n = 8$

  • $n = 4$

Why Meiosis?

• Meiosis produces haploid gametes.

• Without meiosis, chromosome number would double with each fertilization.

• Fertilization involves the fusion of sperm (1n) and egg (1n) to produce a zygote (2n).

• Meiosis reduces the chromosome number, offsetting the doubling effect of fertilization.

Chromosome Terminology

• Single Chromosome

• Duplicated Chromosome

• Homologous Pair (before replication)

• Homologous Pair (after replication)

Karyotypes

• Karyotype: A picture of an organism's chromosomes arranged in pairs by length, centromere position, and banding pattern.

• Humans have 23 pairs (46 total chromosomes) in their somatic (body) cells.

  • Pairs 1-22 are called autosomes (code for most traits).

  • The 23rd pair are called sex chromosomes (XX = Female; XY = Male).

Sexual Reproduction

• Sexual Reproduction: Involves two parents; offspring are genetically different from parents and other offspring.

• Key terms:

  • Haploid gametes (n = 23)

  • Diploid (2n)

  • Egg (n)

  • Sperm (n)

  • Fertilization

  • Diploid zygote (2n = 46)

  • Multicellular diploid adults (2n = 46)

• Gametogenesis: Generation of gametes.

  • Oogenesis (occurs in the ovary)

    • Produces 1 egg and 3 polar bodies

  • Spermatogenesis (occurs in the testis)

    • Produces 4 sperm

Fertilization Details

• Fertilization: Haploid (n) sperm + Haploid (n) egg = Diploid (2n) baby!

Egg vs. Sperm

• Egg:

  • One egg participates per menstrual cycle.

  • Largest diameter cell in the human body: ~$0.12mm$

  • Haploid, but meiosis II uncompleted.

  • Contains 100,000-200,000 mitochondria.

• Sperm:

  • ~250-280 million sperm participate per ejaculation.

  • ~$50\mu m$ long (10,000x smaller than the egg).

  • Haploid: 23 chromosomes.

  • Contains 75-100 mitochondria.

Cortical Reaction

• Fusion triggers the cortical reaction: Release of calcium ions causes cortical granules inside the egg to fuse with the plasma membrane.

• Granules release their contents outside the cell, toward the remains of the zona pellucida.

• Enzymes digest the zona pellucida, making it unable to bind more sperm, while other molecules create a new protective layer.

• The cortical reaction prevents polyspermy (fertilization of a single egg by multiple sperms).

Diploid State

• Mom is diploid (2n) and so is the baby!

Gametes and Fertilization

• Female gamete (egg) (n) + Male gamete (sperm) (n) = Zygote (2n) = Diploid offspring contains homologous pair of chromosomes

Meiosis vs. Mitosis

• Meiosis:

  • Meiosis produces gametes (eggs or sperm) for reproduction.

  • $2n \rightarrow n$

• Mitosis:

  • Mitosis produces cells for growth and repair.

  • Zygote $2n \rightarrow 2n$

Homologous Chromosomes and Sister Chromatids

• Homologous pair of chromosomes consists of a paternal and a maternal chromosome.

• Replication produces sister chromatids.

Chromosome Structure

• Sister chromatids are made during duplication.

• Nonsister chromatids

• Chromosome components:

  • Centromere

  • Kinetochore

  • Paternal chromosome

  • Maternal chromosome

  • Homologous pair

Alleles and Loci

• Locus: Location for a gene (e.g., flower-color gene).

• Alleles: Different versions of a gene (e.g., allele for purple flowers, allele for white flowers).

Stages of Meiosis

• Interphase

• Meiosis I:

  • Prophase I

  • Metaphase I

  • Anaphase I

  • Telophase I

  • Cytokinesis

• Meiosis II:

  • Prophase II

  • Metaphase II

  • Anaphase II

  • Telophase II

  • Cytokinesis

• Interkinesis occurs between Meiosis I and Meiosis II.

Meiosis and Chromosome Number

• Interphase 1 of Meiosis: Germline cell with homologous pair of chromosomes in diploid parent cell ($2n=2$).

• Chromosomes replicate, resulting in homologous pair of replicated chromosomes with sister chromatids ($2n=2$).

• Meiosis I: Homologous chromosomes separate ($2n \rightarrow n$).

• Meiosis II: Sister chromatids separate ($n \rightarrow n$).

• Haploid cells with replicated chromosomes ($n=1$).

• Haploid cells with unreplicated chromosomes ($n=1$).

• During meiosis I, homologous chromosomes separate.

• During meiosis II, sister chromatids separate (just like mitosis).

Crossing Over

• Synapsis: Pairing of two homologous chromosomes (forms a tetrad).

• Crossing Over: Exchange of genetic material between homologous partners.

• Chiasma (pl. chiasmata): Point of cross over (makes an "x" shape).

• Crossing over significance: Variety!

• Only happens during Prophase I.

Synapsis and Recombination

• Synapsis between nonsister chromatids of homologues.

• Exchange of chromatids.

• Recombinant daughter chromosomes result after exchange.

• During prophase of meiosis I, homologous pairs snuggle-up and exchange chromatid chunks!

Visualizing Chiasmata

• Chiasmata are visible points of crossing over between homologous chromosomes.

Bivalents and Crossing Over

• Nucleoprotein lattice forms during crossing over.

• Sister chromatids of a chromosome and its homologue.

• Bivalent formation.

• Chiasmata of nonsister chromatids (1 and 3).

• Daughter chromosomes after crossing over.

Genetic Significance of Crossing Over

• Crossing over is a good adaptation because it increases genetic variation.

Tetrads, Synapsis, Chiasma, and Homologous Chromosomes

• Tetrad, synapsis, chiasma, and homologous chromosomes are important concepts in meiosis.

Law of Independent Assortment

• Independent assortment occurs in Metaphase I

• Possibility 1 vs Possibility 2: Two equally probable arrangements of chromosomes at metaphase I

• Maternal set of chromosomes vs. Paternal set of chromosomes

• Metaphase II

• Combination 1, Combination 2, Combination 3, Combination 4

Importance of Independent Assortment

• The law of independent assortment increases genetic variation.

Law of Segregation

• Law of Segregation: The pair of alleles segregate from each other during meiosis cell division (gamete formation) so that only one allele will be present in each gamete.

Fertilization

• Fertilization is adaptive, as it increases genetic variation.

Mechanisms Increasing Genetic Variation

• Independent Assortment of Chromosomes

• Crossing Over

• Random Fertilization

Meiosis Summary

• Interphase

• Meiosis I (Prophase I, Metaphase I, Anaphase I, Telophase I):

  • Crossing over

  • Law of Independent Assortment (L.O.I.A.)

  • Law of Segregation (L.O.S.)

• Meiosis II (Prophase II, Metaphase II, Anaphase II, Telophase II)

• Cytokinesis.

Oogenesis vs. Spermatogenesis

• Oogenesis:

  • Primary oocyte (2n)

  • First separation of Homologous Chromosomes (Meiosis I) results in a secondary oocyte and a polar body (n).

  • Second separation of sister chromatids (Meiosis II) in Ootid which becomes Ovum (n).

  • One ovum is produced; the first polar body also divides into 2 new polar bodies ($n$).

• Spermatogenesis:

  • Primary spermatocyte (2n)

  • First separation of Homologous Chromosomes (Meiosis I) results in a secondary spermatocyte ($n$).

  • Second separation of sister chromatids (Meiosis II) forms Spermatids (n) which become Sperm (n).

  • Four sperms are produced.

  • Fertilization results in a Zygote (2n).

• Two different strategies are used.

Meiosis in Males vs. Females

• DNA replication is the first stage in both processes.

• Oogenesis:

  • Oogonium undergoes DNA replication to form Primary oocyte.

  • Crossing over occurs.

  • Primary oocyte undergoes Meiosis I to form a first polar body and a Secondary oocyte.

  • The secondary oocyte undergoes Meiosis II to produce an Ovum. ( ) indicates chromatids

• Spermatogenesis:

  • Spermatogonium undergoes DNA replication to form Primary spermatocyte.

  • Crossing over occurs.

  • Primary spermatocyte undergoes Meiosis I to form Secondary spermatocytes.

  • The secondary spermatocytes undergo Meiosis II to produce Spermatids (functional sperm cell).

Mitosis vs. Meiosis Stages

• Interphase: chromosome duplication.

• Meiosis I:

  • Prophase I: Chromosomes have duplicated. Homologous chromosomes pair during synapsis and crossing-over occurs.

  • Metaphase I: Homologous pairs align independently at the metaphase plate.

  • Anaphase I: Homologous chromosomes separate and move toward the poles.

• Laws of Independent Assortment & Segregation (Mendel).

Alternation of Generations

• Diploid sporophyte undergoes meiosis to produce haploid spores.

• Haploid gametophytes undergo mitosis to produce gametes.

• Gametes fuse during fertilization to form a diploid zygote.
  Isomorphic Life Cycle (Ulva lactuca):

• The diploid sporophytes and haploid gametophytes look alike
  *Haploid (n) gametes undergo fusing to DIPLOID (2n)

• Ulva male and female gametes look the same-Ulva is isogamous

  • Mitosis occurs by gametophyte and sporophyte
    *Asexual reproduction occur on HAPLOID (n)

Double Fertilization in Plants

• Double fertilization: egg + sperm and polar nuclei + sperm

Meiosis I and II

• Meiosis I: Pairing of homologous pairs, during synapsis. Called a bivalent or tetrad.

  • Centrioles & nucleolus

  • Sister chromatids

  • Synapsis

  • Centromere

  • Chromosome duplication

  • $2n=4$

• Meiosis II: Sister chromatids separate, becoming daughter chromosomes.

  • Four haploid daughter cells ($n=2$).

Chromosomal Arrangements

• Possibility 1 vs Possibility 2: Two equally probable arrangements of chromosomes at metaphase I

• Combination 1, Combination 2, Combination 3, Combination 4

Telophase I and Interkinesis

• Telophase I: Daughter cells have one chromosome from each homologous pair ($n = 2$).

• Chromosomes still consist of two chromatids.

Meiosis II Stages

• Prophase II: Cells have one chromosome from each homologous pair ($n=2$).

• Metaphase II: Chromosomes align at the metaphase plate.

• Anaphase II: Sister chromatids separate and become daughter chromosomes.

• Telophase II: Spindle disappears, nuclei form, and cytokinesis takes place.

Results of Meiosis II

• Meiosis results in four haploid daughter cells ($n=2$).

Review Questions

• What is the diploid number of this organism?

• What is the haploid number of this organism?

• How many homologous pairs does this organism have in one of its diploid cells?

Scramble Review

• What is the overall purpose of meiosis?

• Meiosis begins with one (diploid/haploid) cell and ends with 4 (diploid/haploid) cells.

• What are homologous chromosomes?

• What are some of the sources of genetic variation that occur during meiosis (think of things that occurred with the chromosomes)?

• What are the Two biggest differences between meiosis I and meiosis II? (think in terms of what is going on with chromosomes)

Highlight the Stage

• Highlight the stage where Chromatids separate: Anaphase II

• Highlight the stage where Homologous pairs are in the middle: Metaphase I

• Highlight the stage where It forms a total of two cells: Telophase I

• Highlight the stage where Crossing-over occurs: Prophase I

• Highlight the stage where Homologous pairs separate: Anaphase I

• Highlight the stage where Chromosomes become visible for the first time: Prophase I

• Highlight the stage where It is forming four haploid cells: Telophase II

Comparison of Meiosis and Mitosis

• How are Meiosis I and Mitosis the same? How are they different? *Meiosis:

  • Prophase 1: Synapsis and crossing-over occur.

  • Metaphase I: Homologues align independently

  • Anaphase I: Homologues separate

  • Telophase I: Daughter cells form & Daughter nuclei are not genetically identical to parental cell.
    *Mitosis:

  • Prophase: No synapsis occur

  • Metaphase: Chromosomes align on the metaphase plate

  • Anaphase: Sister Chromatids separate.

  • Telophase: Daughter cells form & Daughter nuclei are genetically identical to parental cell.

Meiosis I Compared to Mitosis

Meiosis II Compared to Mitosis

Comparison of Meiosis I with Mitosis

Animal Life Cycle

• Organism is always diploid, meiosis forms haploid gametes

• Meiosis

  • Egg fusion (fertilization) Sperm

  • Adults created by growth (mitosis) as juvenile

Alternation of Generations

• Mitosis: sperm and egg archegonium produced on egg

• Zygote occurs in sporophyte

• Fertilization occurs by diploid 2n mitosis:
  ** Germination happens in gametophyte
  **Spores occur by antheridium

• Mitosis occurs by diploid 2n:

  • Sporangia, egg and sperm is produced

• Haploid In MEIOSIS sporophyte occur

Chromosome Transmission

• Explain how meiosis results in the transmission of chromosomes from one generation to the next (IST-1.F).

  • Meiosis is a process that ensures the formation of haploid gamete cells in sexually reproducing diploid organisms.

  • Meiosis results in daughter cells with half the number of chromosomes of the parent cell.

  • Meiosis involves two rounds of a sequential series of steps (meiosis I and meiosis II).

• Describe similarities and/or differences between the phases and outcomes of mitosis and meiosis (IST-1.G).

  • Mitosis and meiosis are similar in the way chromosomes segregate but differ in the number of cells produced and the genetic content of the daughter cells.

Genetic Diversity

• Explain how the process of meiosis generates genetic diversity (IST-1.H).

  • Separation of the homologous chromosomes in meiosis I ensures that each gamete receives a haploid (1n) set of chromosomes that comprises of both maternal and paternal chromosomes.

  • During meiosis I, homologous chromatids exchange genetic material via a process called