Animal Cell Meiosis

Sex Cell (Gamete) Production

 

Sexual Reproduction:

Sexual reproduction involves the union of male and female gametes (sex cells; eggs and sperm) through fertilization.  Each gamete contributes half of the genetic makeup of the resulting zygote (fertilized egg).  This insures that the offspring are not genetic clones of their mother or father.  It increases genetic variation in the population and helps the species survive environmental challenges.

 

What does haploid and diploid mean?

Ploidy is a term referring to the number of sets of chromosomes.  Humans have 23 pairs of chromosomes.  The chromosome pairs are numbered 1-23.  These pairs are homologous chromosomes that have the same banding pattern, centromere position, and carry genes for the same traits.  People inherit one member of each homologous pair from their mother (the maternal chromosome) and one member of each homologous pair from their father (the paternal chromosome).  Haploid and diploid are terms referring to the number of sets of chromosomes in a cell.  Most animal cells are said to be diploid because they have pairs of every chromosome.  Di refers to “two.”  We abbreviate diploid as 2n. 

 

Meiosis is a special type of nuclear division which segregates the homologous chromosomes into separate gametes, reducing the chromosome number by half.  A sperm will receive one member of each homologous chromosome pair from the man.  Likewise, an egg will receive one member of each homologous chromosome pair from the woman.  Because these gametes have only one member of each chromosome pair, they are said to be haploid, abbreviated as n.

Meiosis is necessary because when the sperm fertilizes the egg, the resulting zygote will receive one of each type of chromosome from the sperm and one of each type from the egg.  This will produce a baby that is again diploid, with the normal chromosome number.

 

Gametogenesis (Gamete Production):

Spermatogenesis refers to production of sperm, the male gamete.  Oogenesis refers to the production of ovum (eggs), the female gamete.  Notice that meiosis produces four sperm, but only one egg.  Sperm primarily contribute only nuclear DNA to the zygote.  All the organelles, cytoplasm, etc. come from the egg.  Thus, the egg is much larger than the sperm.  (The diagram below illustrates a species in which 2n = 4 chromosomes.)

 

gamete production.gif

 

 

Phases of Meiosis:

Meiosis is actually a series of two cellular divisions (termed meiosis I and meiosis II) that produces four haploid cells.  Meiosis I (also known as reductional division) reduces the ploidy level from 2n to n.  Meiosis II (also known as equational division) divides the remaining sets of chromosomes in a mitosis-like process.  Most of the differences between mitosis (which produces 2 diploid cells for growth and repair) and meiosis (which produces sex cells) occur during prophase I.

 

 Meiosis I (or reductional division):

Below is a diagram illustrating the events that take place during Meiosis I.  Meiosis I begins with a diploid primary sex cell.  Note that in this example, 2n = 6.  The primary sex cell has 3 pairs of homologous chromosomes.  By the conclusion of Meiosis II, two haploid secondary sex cells will have been formed.  The secondary sex cells are n.  They have no homologous pairs of chromosomes.

 

 

Prophase I: 

Prophase I begins with a primary sex cell that is diploid.  Most of what occurs in Prophase I is similar to prophase of mitosis.

  1. The spindle forms and spindle fibers attach to the chromosomes via their centromeres.
  2. The nuclear envelope breaks down as the nuclear chromatin condenses into visible chromosomes.

Prophase I has a couple of unique events.

  1. Homologous chromosomes pair through a process termed synapsis.
    1. The synapsed chromosomes are called bivalents because they consists of 2 (bi = two) chromosomes.
    2. They are also known as tetrads because they consist of 4 (tetra = 4) chromatids.
  2. During synapsis, crossing-over may occur.  Crossing-over is the reciprocal exchange of chromosome arms between homologous chromosomes.  It occurs when the synapsed chromosomes become tightly wound together and break.  Sometimes the broken arms then “heal” (are reattached) to the wrong chromosome.
    1. This process provides for greater genetic variation among a species.

 

Electron Microscope View of Chiasmata

 

Metaphase I:

The homologous pairs of chromosomes line up across from each other at the cell’s equator.  This event explains Mendel’s Law of Independent Assortment, which will be explored further during our genetics lecture.

 

Anaphase I:

The spindle fibers drag the homologous chromosome pairs away from each other, towards opposite poles of the cell.  *Note that no centromeres are broken in this process.

 

Telophase I:

The chromosomes reach opposite poles as the spindle begins to break down.  Cytokinesis, the division of the cell’s cytoplasm between what will be two daughter cells begins with a cleavage furrow.  The daughter cells are known as secondary sex cells.  They are haploid.  This event explains Mendel’s Law of Segregation, which will be explored further during our genetics lecture.

 

Meiosis II (or equational division):

Below is a diagram illustrating the events that take place during Meiosis II.  Meiosis II begins with two haploid secondary sex cells.  By the conclusion of Meiosis II, 4 sperm or 1 egg (the gametes) will be produced.  The events of Meiosis II are very similar to those of mitosis, except that the cells are haploid.

 

Prophase II: 

Prophase I begins with a two secondary sex cells that are haploid.

1.    The spindle forms and spindle fibers attach to the chromosomes via their centromeres.

2.    The nuclear envelope breaks down as the nuclear chromatin condenses into visible chromosomes.

 

Metaphase II:

The spindle fibers drag the chromosomes into a line at the cell’s equator. 

 

Anaphase II:

The centromeres break, allowing the sister chromatids to be dragged toward opposite poles by the spindle fibers.  Once separated, the sister chromatids are termed daughter chromosomes.

 

Telophase II:

The daughter chromosomes reach opposite poles as the spindle begins to break down.  Cytokinesis, the division of the cell’s cytoplasm begins with a cleavage furrow.  The resulting haploid cells are known as sex cells or gametes.