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

Meiosis is a continuous process, but for convenience of description is divided into the stages of:

ProphaseMetaphaseAnaphaseTelophase

Each of these stages occurs during Meiosis I andagain 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, chromosomesare present as granular materialcalled chromatin; DNA replicates

and new macromolecules and organelles are manufactured in

preparation for cell division

Prophase I is the longest phase ofmeiosis and begins with condensationand spiralisation of the chromosomes;

the chromosomes thus shorten and becomevisible as distinct threads in the nucleus

Homologous chromosomes come tolie close together in a process calledsynapsis; the paired chromosomes

are called bivalents

The bivalents become shorter and thickerby further spiralisation and each

chromosome can now be seen to consistof two chromatids held together at the centromere

The pairs of identical sister chromatidsare the result of DNA replication that

occurred during interphase; each group of four chromatids is termed a tetrad

Tetrad

A pair of identical sister chromatidsresulting from the replication of

DNAduring Interphase

The four closely associatedchromatids, for each of the pairs

of homologous chromosomes, form an arrangement

known as a tetrad

At this stage during meiosis(prophase I), the non-sisterchromatids are so closely

associated that they becomeintertwined forming positions

of overlap known as chiasmata

At these positions of chiasmata,the non-sister chromatids break

and exchange sections ofgenetic material in a process

known as crossing over

sisterchromatids

sisterchromatids

Paternal memberof pair

Maternal memberof 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 numberof 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 upindependently 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 allowingthe 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 developinto the male gametes

(pollen grains) of the lily flower

The DNA content of cells varies during the meiotic cycle

During the pre-meioticS Phase (interphase),

DNA replicates and hencethe 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

Acknowledgements

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