meiosis is a form of cell division in which the diploid number (2n) of chromosomes is reduced to the...
TRANSCRIPT
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
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
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)