2. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

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

11. 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

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

13. Crossing over and recombination
Chiasmata formation

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

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

22. 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

23. 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

24. 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

25. 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)

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