Genetics Basics - Lesson 5: Meiosis


5.1   Meiosis Defined



5.2   Meiosis Terminology—Homolog



5.3   Meiosis Terminology—Chromatid



Click here to see an animation of the steps of meiosis I

5.4   The Two Parts of Meiosis—Meiosis I



5.4.1   Introduction

Most cells reproduce to form identical copies, or daughter cells, for the purpose of growth or repair.  This process is called mitosis. However, parental gametes – the sperm and egg cells – are unique from all other cells in that they reproduce through a process called meiosis.  The purpose of meiosis is to shuffle genetic information and cut the cellular chromosome number in half, from 46 chromosomes to 23 chromosomes.  In this way, when an egg and sperm cell combine during fertilization, the resulting embryo will inherit the appropriate amount of unique genetic information from each parent.

If sperm and egg cells were to divide and keep the same number of chromosomes that other cells do, then the resulting embryo would have twice the normal number of chromosomes. For example, if a human egg and sperm each contained 46 chromosomes, then the resulting embryo would contain 92 chromosomes– a chromosome number not compatible with normal development or survival.

The process of meiosis involves two divisions of the genetic material. The first division is called the reduction division – or meiosis I – because it reduces the number of chromosomes from 46 chromosomes or 2n to 23 chromosomes or n (n describes a single chromosome set). In humans, the reduction division results in two sperm cells each with 23 chromosomes or an egg cell with 23 chromosomes and one discarded polar body. The second division – or meiosis II – is called equational division.  The two sperm cells split again producing four sperm cells, each with 23 or n chromosomes. In oogenesis, the egg cell and polar body divide to form the ovum and two additional polar bodies.  In this way, the offspring are assured to get one half of the chromosomes from the mother and one half from the father.



1.  Metabolically active cell with a diploid nucleus (2n=6).
2.  DNA replication occurs.

5.4.2   Interphase I

A cell that is not dividing is said to be in interphase. It contains 23 pairs of homologous or matching chromosomes – one that was inherited from the person’s mother and one that was inherited from the person’s father. When referring to one of a pair of homologous chromosomes it is common to refer to this as a homolog, for short. During Interphase I, the cell duplicates its nuclear material. Each homolog replicates, forming two identical sister chromatids. At this point in time, there is the equivalent of 4 copies of each chromosome in the cell. In reality, chromosomes are not yet condensed enough to be visible until the end of prophase 1.



1. Chromosomes become shorter and thicker.
2. The nuclear envelope begins to disintegrate.

5.4.3   Early Prophase I

In prophase I, the chromosomes become shorter and thicker and more easily stained. They can now be viewed underneath a light microscope. The sister chromatids are joined at the centromere(see diagram).   The nuclear envelope begins to disappear.



1. Chromosomes have become fully shortened and thickened.
2. Homologous chromosomes lie side by side (in a process called synapsis).
3. The nuclear envelope disappears.
4. Spindle fibers begin to form.

5.4.4   Late Prophase I

Later in prophase I, homologous chromosomes – for example, the maternal chromosome #1 and the paternal chromosome #1 – line up next to one another in a process called synapsis. An aggregate of RNA and protein holds the paired chromosomes together. Homologous chromosomes switch material in a process known as recombination or crossing over, which is mediated by the formation of chiasmata.  The two chromatids attached at the chiasma exchange parts. In this way some of the genes originally on the father’s chromosome are now on the mother’s chromosome and vice versa. This shuffling process helps to create and maintain variation in a species.     The nuclear envelope begins to disappear and spindle fibers begin to form at the poles of the cell.



1.  Recombination occurs.

5.4.5   Recombination

Homologous chromosomes switch materials in a process known as recombination, or ‘crossing over’, which is mediated by the formation of chiasmata.
The two chromatids attached to the chiasmata exchange parts.  In this way, some of the genes originally on the father’s chromosomes are now on the mother’s chromosomes, and vice versa.  This shuffling process helps to create and maintain variation in a species.



1. Pairs of homologous chromosomes migrate to the equatorial plane of the cell.

5.4.6   Metaphase I

In metaphase I, the pairs migrate to the equatorial plane of the cell.  The spindle fully forms and attaches to the centromeres.
“The pattern in which homologs align during metaphase I determine the combination of maternally and paternally derived chromosomes in the progeny cells.
Each chromosome moves independently of all the others – a phenomenon called independent assortment. For example, the copy of chromosome 1 that a gamete receives in no way influences if the same gamete will receive the maternally-derived or paternally-derived chromosome 4.



1. Homologous chromosomes (each consisting of two chromatids held
together by a centromere) move to opposite poles.

5.4.7   Anaphase I

During anaphase I, the homologous chromosomes - each consisting of two sister chromatids held together by a centromere – separate and begin to move to the opposite poles of the cell, pulled by the shrinking spindle fibers.



1. Movement of chromosomes to the poles is completed.  (Note: Although there is one of each
type of chromosome at each pole, there is a mixture of maternal and paternal chromosomes.)
2. The spindle disappears.
3. Cell division starts (a process called cytokenesis).

5.4.8   Telophase I

In telophase I, the movement of the homologous chromosomes to the poles is completed. Remember that although there is one of each type of chromosome at each pole, there is a mixture of maternal and paternal chromosomes due to the recombination that occurred during prophase I. The spindle now disappears. Cytokinesis, or cell division occurs, producing two haploid (n) daughter cells, each with only one set of chromosomes (1n).  However, the chromosomes are still in their replicated state and consist of sister chromatids. The cells move back into interphase, marking the end of meiosis I.



Click here to see an animation of the steps of meiosis II

5.5   The Two Parts of Meiosis-Meiosis II



1.  The chromosomes uncoil.
2.  The nuclear envelope reappears, resulting in two haploid cells (n=3).
3.  In contrast to regular cell division (mitosis), this interphase is briefer.
There is no DNA synthesis between the first and second meiotic divisions.

5.5.1   Interphase

In contrast to regular cell division, or mitosis, this second interphase takes less time. The chromosomes uncoil into very thin threads and the nuclear envelope reappears. Most importantly, although proteins are manufactured during this interphase period, the genetic material is NOT replicated a second time.



1.  Chromosomes again become thicker and shorter.
2.  The nuclear envelope begins to disintegrate.

5.5.2   Prophase II

Prophase II marks the beginning of the second meiotic division. Meiosis II is similar to mitosis in that each chromosome consists of two sister chromatids attached at the centromere.  The goal of meiosis II is to separate the sister chromatids.



3.  The chromosomes (consisting of two chromatids -- remember, the homologous
chromosomes separated during Meiosis I) begin to move to the equatorial plane.
4.  Spindle fibers begin to form.

5.5.3   Prophase II

In prophase II, the chromosomes again become condensed. The nuclear envelope begins to disintegrate. The chromosomes, consisting of the two sister chromatids only, begin to move to the equatorial plane of the cell. Spindle fibers once again begin to form at the poles.



1.  The chromosomes become aligned on the equatorial plane.
2.  Fibers of the spindle are reformed.

5.5.4   Metaphase II

During metaphase II the chromosomes become aligned on the equatorial plane and spindle fibers attach to the centromeres.



1.  Centromeres divide, separating the chromatids, which move to the
opposite poles as chromosomes. 

5.5.5   Anaphase II

In anaphase II, the centromeres divide, effectively separating the sister chromatids, which move to the opposite poles as they are pulled by the spindle fibers.



1.  The movement of the chromosomes to the poles is complete.

5.5.6   Telophase II

In telophase II, the movement of the chromosomes to the poles is completed and the spindle disappears. In spermatogenesis, nuclear envelopes form around the four nuclei, which then separate into four individual spermatids. In oogenesis, the ultimate result is one ovum and 3 polar bodies, which are discarded.



1.  The movement of the chromosomes to the poles is complete.
2.  The spindle disappears.
3.  Cell division starts.

5.5.7   Telophase II

The result of meiosis II is the formation of four unique cells, each carrying a new assortment of genes and chromosomes, and each with half the number of original chromosomes.



1.  The chromosomes uncoil and resume interphase appearance.
2.  The nuclear envelope reappears, resulting in four haploid nuclei (n=3).
3.  The second cell division completes, resulting in four haploid cells or gametes.

5.5.8   Summary

In summary, meiosis is a form of cell division that results in half the normal number of chromosomes in sex cells, or gametes, thus maintaining the chromosome number of a species from generation to generation. Additionally, genetic variability results from the recombination and independent assortment of the genetic material.



5.6   Review of the Genetic Consequences of Meiosis