In preparation for meiosis during Interphase, DNA replicates 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

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

Each of these stages occurs during Meiosis I and 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

Prophase I is the longest phase of During interphase, material in the nucleus is called chromatin as individual chromosomes cannot be seen; DNA replicates and new organelles are manufactured in preparation for cell division Prophase I is the longest phase of meiosis and begins with condensation of the chromosomes; the chromosomes shorten and become visible in the nucleus

They become shorter and thicker and each chromosome can now be seen to consist of two chromatids held together at the centromere Centromere Chromatid Homologous chromosomes come to lie close together. The pairs of identical sister chromatids are the result of DNA replication that occurred during interphase

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

A Pair of Homologous Chromosomes Paternal member of pair Maternal member At this stage during meiosis (prophase I), the chromatids are so closely associated that they become intertwined forming positions of overlap known as chiasmata At these positions, the chromatids break and exchange sections of genetic material in a process known as crossing over sister chromatids

Recombination of chromatids is a major source of the genetic variation that results from meiosis

Crossing over and recombination Chiasmata formation

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

The nuclear membrane breaks down and spindle fibres form across the cell The chromosomes line up along the equator of the spindle in their pairs The orientation of each pair of chromosomes on the spindle is random and this provides a second source of genetic variation – often referred to as independent segregation

They migrate to opposite poles of the cell During Anaphase I spindle shortening separates the pairs of chromosomes They migrate to opposite poles of the cell

During Telophase I, the spindle disintegrates and nuclear membranes reform Each daughter cell contains one member from each pair of homologous chromosomes, some of which have exchanged genes

The two cells entering Prophase II possess one member from each pair of homologous chromosomes and are thus described as haploid cells 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

Spindle activity pulls the chromatids to opposite poles of the cell where they are now described as daughter chromosomes

Each cell then divides into two Four haploid daughter cells result, each possessing one of each pair of original chromosomes Genetically different cells are produced as a result of meiosis

The four cells shown here are the products of meiosis 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 again 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

Meiosis is an important source of genetic variation through: 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 assortment of the homologous pairs of chromosomes at Metaphase I (Independent Segregation)

Use the provided worksheet to make comparisons between the processes of meiosis and mitosis (chapter 11)