Meiosis is a form of cell division in which the diploid number (2N) of chromosomes is reduced to the haploid number (N) In preparation for meiosis during Interphase, DNA replicates once following which there are two nuclear divisions At the end of meiosis, a single diploid cell gives rise to four haploid cells, each containing one member from each pair of the original homologous chromosomes In sexually reproducing organisms, meiosis occurs at some point in the life cycle to ensure that haploid gametes are produced The haploid gametes fuse at fertilisation and the diploid condition is restored again in the zygote – meiosis is essential for ensuring that the chromosome number does not double with each generation

In humans, the male gametes are manufactured in the walls of the seminiferous tubules in the testes Cells in the walls of the tubules undergo meiosis to give rise to haploid immature sperm called spermatids

Spermatids develop into mature spermatozoa and enter the lumen of the tubules for transport to the epididymis where they are stored Spermatozoa in lumen of tubule

In flowering plants, the male gametes (pollen grains) are produced in the anthers where meiosis takes place

A transverse section through an anther reveals four pollen sacs Specialised cells within each pollen sac undergo meiosis to give rise to the haploid pollen grains

Meiosis is a continuous process, but for convenience of description is divided into the stages of: Prophase Metaphase Anaphase Telophase Each of these stages occurs during Meiosis I and again during Meiosis II Meiosis is therefore a two-stage process although DNA replicates only once during the cycle The following description considers the process of meiosis occurring in a diploid cell with two pairs of chromosomes, i.e. where 2n = 4

During interphase, chromosomes are present as granular material called chromatin; DNA replicates and new macromolecules and organelles are manufactured in preparation for cell division Prophase I is the longest phase of meiosis and begins with condensation and spiralisation of the chromosomes; the chromosomes thus shorten and become visible as distinct threads in the nucleus

Homologous chromosomes come to lie close together in a process called synapsis; the paired chromosomes are called bivalents The bivalents become shorter and thicker by further spiralisation and each chromosome can now be seen to consist of two chromatids held together at the centromere The pairs of identical sister chromatids are the result of DNA replication that occurred during interphase; each group of four chromatids is termed a tetrad Tetrad

A pair of identical sister chromatids resulting from the replication of DNA during Interphase

The four closely associated chromatids, for each of the pairs of homologous chromosomes, form an arrangement known as a tetrad At this stage during meiosis (prophase I), the non-sister chromatids are so closely associated that they become intertwined forming positions of overlap known as chiasmata At these positions of chiasmata, the non-sister chromatids break and exchange sections of genetic material in a process known as crossing over sister chromatids sister chromatids Paternal member of pair Maternal member of pair A Pair of Homologous Chromosomes

The exchange of genetic material between chromatids of maternal and paternal origin is one source of the genetic variation that results from meiosis

Chiasma formation Crossing over and the exchange of genetic material

The photograph below shows chiasmata formation between non-sister chromatids from a pair of homologous chromosomes Positions of chiasmata The longer the chromosome, the greater the number of chiasmata that are likely to form Chiasmata and Crossing Over

Towards the end of Prophase I, the nuclear membrane breaks down and spindle fibres form across the cell During Metaphase I the chromosomes line up along the equator of the spindle in their homologous pairs The orientation of each pair of chromosomes on the spindle is a random event and this provides a second source of the genetic variation that results from the process of meiosis

During Anaphase I spindle activity separates the homologous pairs of chromosomes The homologous pairs of chromosomes migrate to opposite poles of the cell

During Telophase I, the spindle disintegrates and nuclear membranes reform In animal cells, the membrane constricts down the middle of the cell and two daughter cells are formed (cytokinesis) Each daughter cell contains one member from each pair of homologous chromosomes

The two cells entering Prophase II of meiosis possess one member from each pair of homologous chromosomes and are thus described as haploid cells During Late Prophase II the nuclear membranes disintegrate again and spindle fibres begin to form

The chromosomes (each consisting of two sister chromatids) line up independently along the equator of the spindle The purpose of Meiosis II is to separate the sister chromatids and to distribute them into the four products of meiosis

During Anaphase II the centrioles replicate allowing the chromatids to repel one another Spindle activity pulls the chromatids to opposite poles of the cell where they are now described as chromosomes

In animal cells there is constriction across the middle Each cell then divides into two Four haploid daughter cells result, each possessing one member from each pair of original chromosomes Four, genetically different cells are produced as a result of meiosis

The four cells shown here are the products of meiosis as it occurs within the anthers of a flowering plant Each of these four haploid daughter cells separate from one another and develop into the male gametes (pollen grains) of the lily flower

The DNA content of cells varies during the meiotic cycle During the pre-meiotic S Phase (interphase), DNA replicates and hence the DNA content doubles By the end of meiosis I the DNA content is halved as homologous chromosomes separate into different cells At the end of meiosis II, the DNA content is half that of the original cell Meiosis produces haploid cells in which the DNA content has been halved with respect to the original diploid cell

During sexual reproduction, meiosis ensures that the chromosome number of the gametes is halved prior to fertilisation; without meiosis, doubling of chromosome number would occur at each fertilisation Meiosis is an important source of genetic variation through: Chiasmata formation and crossing over The random orientation of the homologous pairs of chromosomes at Metaphase I and their subsequent distribution into the divided cells (Independent Assortment)

Use the provided worksheet to make comparisons between the processes of meiosis and mitosis

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