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Meiosis Part I

Notes

Meiosis Part I

Sections

HUMAN CELLS

  1. Somatic cells: majority of the body's cells
  • 46 chromosomes – diploid (2n)
  1. Reproductive cells: sperm and egg cells (gametes)
  • 23 chromosomes – haploid (1n)
  • Diploid germ line cells: precursors for reproductive cells; undergo meiosis

FERTILIZATION

  • 1 egg and 1 sperm fuse to form zygote (2n)
  • Followed by repeated cycles of mitosis to produce multicellular organism (2n)

INTERPHASE

  • Parent cell (2n): two sets of 23 chromosomes
  • Homologous chromosomes: contain the same genes in the same order, each from a different parent (contain different alleles)
  • S-phase: each set of 23 chromosomes duplicates (92 chromosomes total), sister chromatids pair at the centromere

PROPHASE I

  • >90% of meiosis
  • Chromosomes condense
  • Tetrad forms via synapsis: each gene aligns with its homologue (4 chromatids)
  • Synaptonemal complex: zipper-like structure holds chromosomes together until crossing over occurs
  • Crossing over: paternal chromosome crosses over to maternal and vice versa
  • Chiasma (site of crossing over) holds tetrad together after synaptonemal complex disassembles

Other features of this phase:

  • Nuclear envelope fragments
  • Nucleolus disperses
  • Centrosomes move to opposite poles
  • Microtubules form spindle & attach kinetochores of homologous chromosomes

METAPHASE I

  • Tetrads align on metaphase plate
  • Sister chromatids face same pole
  • Homologous chromosomes face opposite poles

ANAPHASE I

  • Homologous chromosomes separate

TELOPHASE I AND CYTOKINESIS

  • Two haploid daughter cells: 1 tetrad in each

PROPHASE II

  • Each cell has one duplicated set of 23 chromosomes

METAPHASE II

  • Sister chromatids line up on metaphase plate and face opposite poles

ANAPHASE II

  • Sister chromatids separate

TELOPHASE II

  • Nuclear envelope reforms
  • Nucleolus reappears
  • Mitotic spindles depolymerize
  • Cleavage furrow

CYTOKINESIS

  • 4 haploid daughter cells
  • Daughter cells genetically distinct from each other and parent cells
  • Each develops into reproductive cell (egg or sperm cells)

CLINICAL CORRELATION

Down's Syndrome (Trisomy 21): aneuploid gametes

  • Nondisjunction: chromosome 21 fails to separate properly during meiosis I
  • 2 daughter cells with extra chromosome 21 copy
  • 2 daughter cells missing chromosome 21
  • Trisomy 21: gamete with extra chromosome fuses with normal gamete during fertilization = zygote with 3 copies of chromosome 21

Full-Length Text

  • Here we will learn meiosis, a specialized cell cycle that produces sperm and egg cells in humans.
    • This is part I of a two-part tutorial.
    • In this tutorial, we will introduce gametes and learn the key features of prophase I, the longest phase in meiosis.
    • In part II, we complete the meiotic cycle.
  • To begin, start a table to learn the two types of human cells.
    • Other eukaryotes, such as yeast, undergo meiosis as well, but we will use human reproductive cells as our model.
  • Denote that they include:
    • Somatic cells, which describes the majority of the body's cells.
    • Reproductive cells, which are sperm and egg cells aka gametes.
  • These two types of cells contain different amounts of chromosomes.
  • Denote that somatic cells contain 46 chromosomes total and that they are diploid because they contain two sets of 23 chromosomes, one set inherited from each parent.
  • Denote that reproductive cells contain 23 chromosomes, and that they are therefore haploid.

In this tutorial we describe reproductive (germ) cell division: meiosis.

Before we begin, let's specifically track the distribution of chromosomes through the process of meiosis, fertilization and finally mitosis.

  • First, draw two diploid germ line cells; they are precursor cells that divide to produce reproductive cells.
    • Label one mother and the other father.
  • Within each cell, show that there are 46 chromosomes; they have double the chromosomes of a haploid cell (2n = 2 times 23 chromosomes).
  • We use different colors in each of the cells to distinguish maternal and paternal chromosomes.
  • Now, draw an egg cell and a sperm cell below the parent cells.
  • And in each cell, show that there are 23 chromosomes; indicate that that these reproductive cells are haploid (n).
    • In reality, each of these cells has 23 chromosomes.
  • Next, show that via meiosis, the diploid cells produce haploid reproductive cells. (We will illustrate this process in detail shortly.)
  • Finally, draw a zygote below the haploid gametes.
  • Show that it contains one set of 23 chromosomes from the haploid egg cell and one set of 23 chromosomes from the haploid sperm cell.
  • Indicate that it is diploid (2n); it contains two sets of 23 chromosomes.
    • One set from the mother and the second set from the father.
  • Next, use arrows to demonstrate that the successful fertilization of one egg cell by one sperm cell produces this diploid cell.
  • Indicate that via repeated cycles of mitosis, our diploid zygote becomes a diploid, multicellular organism.
    • Mitosis is the division of somatic cells to produce more diploid somatic cells.

Now, let's take a closer look at meiosis, which involves two separate cell divisions – meiosis I and meiosis II. Before we begin, though, we need to establish the key constituents of the cell in interphase (which precedes meiosis).

  • Write that the parent cell is 2n, 46 chromosomes; (it has two sets of 23 chromosomes: 2 x 23 = 46).
  • Draw a diploid parent cell.
  • Show that it has an intact nucleus and nucleolus.
  • Draw a centrosome with two centrioles.
  • Within the nucleus, draw two chromatin fibers (uncondensed chromosomes) in two different colors to represent the 46 chromosomes (two pairs of 23 chromosomes).
  • Indicate that these chromosomes are homologous, which means that they both contain the same genes in the same order, each from a different parent (thus, they contain different alleles).

Next, we'll show it transition to the S-phase of interphase: its duplication phase.

  • Write that, here, the parent cell is still 2n, but has duplicated its DNA and now contains 46 x 2 chromosomes. (It has 2 sets of 23 chromosomes that are duplicated: 2 x 23 x 2 = 92).
  • Redraw this cell with an intact nucleus. We'll leave out the nucleolus for clarity.
  • This time, draw two pairs of chromatin fibers.
  • Show that each pair contains two identical sister chromatids that connect at the centromere.
  • Finally, draw a centrosome and its duplicate.
  • Show that each contains two centrioles.
  • Indicate that DNA duplicates in the S phase of interphase. So far, this process is identical to mitosis.

Now, we're ready for meiosis I.

  • Indicate that we begin with prophase I
    • Write that here the cell is 2n, with 46 x 2 chromosomes.
  • List some key features of prophase I:
    • It accounts for more than 90% of meiosis.
    • Chromosomes condense.
    • Tetrad forms via synapsis, in which each gene aligns with its homologue.
    • A synaptonemal complex, which is a zipper-like structure, holds the chromosomes together until crossing over occurs.
  • Draw a cell.
  • Show that in this phase,
    • The nuclear envelope fragments.
    • The nucleolus disperses.
    • The centrosomes begin moving to opposite poles of the cell.
    • The microtubules form a mitotic spindle.

Now, let's add our chromosomes.

  • Draw a chromosomal tetrad, which comprises two homologous pairs of sister chromatids (four chromosomes); they are condensed, so we show them as thicker than in the prior cells.
  • There are 23 tetrads in the cell in this phase – all of which are visible via microscope; they remain held together by the zipper-like synaptonemal complex until crossing over occurs.

Finally, let's address the process of crossing over, which occurs in prophase I.

  • Draw an expanded view of our tetrad, but indicate that each homologue is missing a small section.
  • Then, fill those portions with the opposite chromosome.
    • Portions of the paternal chromosome "crosses over" to the maternal chromosome and vice versa.
  • Label this site chiasma; it holds the tetrad together after the synaptonemal complex diassembles.
    • Every tetrad in a meiotic cell has one or more chiasmata.
  • As a final point, write that microtubules attach to the kinetochores of homologous chromosomes at the end of prophase I, which allows the tetrads to begin moving towards the metaphase plate in preparation for metaphase I.

In part II, we will learn the rest of meiosis I and meiosis II. We will also address the mechanisms that generate genetic diversity in the four daughter cells that are produced at the end of the cycle.

References:

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