Meiosis does more than halve chromosome number
Meiosis is a form of nuclear division but it is normally described in the context of the associated cell division.
The cells produced - often known as daughter cells - are genetically different from one another, and the parent.
There are actually two nuclear divisions in meiosis, and as a result 4 haploid cells - each with n
chromosomes - can be produced, although some may not be continued to maturity.
In particular, the production of female gametes often involves the discarding of a number of cells
, or at least their chromosomes. Presumably this is in order to maintain a certain amount of cytoplasm, making the female zygote larger (and less able to move) whereas male gametes need to be more numerous and able to move.
As in mitosis, chromosomes move out of the nucleus and become attached to a spindle, to be moved to different ends of a cell before it divides. The same names are given to the stages in meiosis - prophase, metaphase, anaphase and telophase, but they are numbered I and II because of the two divisions.
Quite a lot happens during prophase I, and it may be subdivided into several sections.
Initially chromosomes condense
so they appear to become shorter and thicker. In fact they have undergone DNA replication during the S-phase, and they consist of two chromatids
, but this is more obvious at later stages.
In these diagrams only 2 pairs of chromosomes are shown, and the colours denote maternal/paternal origin
Each chromosome lines up with its homologous partner
Each chromosome becomes associated with its partner chromosome
: in humans there are are 2 copies of chromosomes 1-22 which are described as homologous pairs
, and of course they can be traced back to the two gametes which fused at fertilisation, so they can be described as being of paternal and maternal origin
They line up in pairs along their whole length, but X and Y are only similar for a short section so they only pair up in this region. These pairings are known as bivalents
. There are 4 strands alongside one another (each of which contains a double strand of DNA), held together by protein strands (the 'synaptonemal complex').
Crossing over causes a chiasma (arrowed) to form
Since they are closely grouped, sometimes two of these strands may break and re-join, but with their partner strand. These are also known as non-sister homologous chromatids, and the mutual transfer of genetic material is known as crossing over
. This means that the combination of genetic material (effectively the sequence of alleles) on these chromatids differs from the original chromatid. Genetic recombination
has taken place.
It is possible for this to take place at several points, producing more variation. Crossing over can also occur between sister chromatids, but the result is not significally different, unless unequal amounts of genetic information are exchanged.
As the homologous chromosomes separate, the points at which exchange of genetic material has taken place can be seen as cross-shapes where they overlap, and these chiasmata
(singular: chiasma) gradually move to the ends of the chromosomes as they separate.
Chromosomes become attached to the spindle via the centromere region from which kinetochore fibres radiate.
Chromosomes - each still consisting of a pair of chromatids - line up across the middle of the cell - the 'equator'
Each chromosome still consists of a pair of chromatids
Chromosomes move out along the spindle fibres towards opposite ends of the cell
- opposite 'poles'. They appear to be pulled apart, and they have a characteristic V-shape.
A nuclear membrane forms around the chromosomes.
The cytoplasm divides, forming two cells, or it proceeds directly to the next stage, with the spindles typically at right angles to the previous ones.
Division II of meiosis
This is practically identical to mitosis, resulting in the separation of chromatids
to separate nuclei inside four daughter cells
Each cell has only half the original number of chromosomes.
Crossing over has resulted in new combinations of alleles.
Meiosis results in genetic variation
In the production of sex cells, only half the genetic information of an ordinary body cell is passed on to the gametes: egg cells (ova) in females, sperm cells (spermatozoa) in males.
This may seem to ask a question:
Which (genetic information) is kept and which is thrown away (or at least passed on to another cell)?
The answer appears to involve several degrees of randomness.
In the first division of meiosis, homologous chromosomes are separated into two groups. But each group consists of a different combination of chromosomes.
In humans, each resulting cell can have either
the paternal or the maternal version of chromosomes 1-22 etc (as well as either one of the X chromosomes in females), and either
a (paternal) Y or the (maternal) X chromosome in males.
The randomness of this separation into different groups means that the likelihood of daughter cells containing the same full set of chromosomes (and alleles) that they inherited from one of their parents is low.
In fact the possible number of different combinations of 23 chromosomes following meiosis is 223
. This is 1 in 8,388,608.
This is ignoring the variation that may be introduced by crossing over.
You could be asked to suggest a formula using the expression 2n (also written as 2^n) (where n is the number of pairs of homologous chromosomes) to calculate such a number.
Random fertilisation introduces another degree of variation
Since both the male and female gametes show the same variation in combinations of chromosomes, the number of different combinations in the fertilised egg (zygote) are the product of the two i.e the square of this figure.
The possible number of different combinations of chromosomes following random fertilisation of two gametes described above (ignoring crossing over) would be
, which is 7.037 ×1013
Sources of variation in meiosis
The separation of homologous chromosomes in division I
is called independent assortment or random segregation of chromosomes
or genetic material. As the chromosome number is halved, the two sets of chromosomes that are dealt out contain one or the other of each homologue - which has its own, probably unique, combination of alleles.
introduces another degree of difference between daughter cells, providing different base sequences and combinations of alleles between the two chromatids.
Separation of chromatids in division II
results in the separation of similar nuclei, allowing the new combinations introduced by crossing over to be seen.
Sperm cell production
Human sperm cells are produced in the testes at the astonishing rate of 1500 per second, and the (maturing) process takes about 65 days!
How many of the resulting (4) sperms contain an X chromosome?
> 2 - half of them
Egg cell production .. plus
Human egg cells are usually produced singly following division within follicles of the ovary, and they are the largest cells in the body. Some cells ('polar bodies') do not develop, and development of the main cell stops at certain stages.
In many organisms, meiosis is involved in the production of reproductive cells
(sperm and eggs in animals, nuclei within pollen and ovules in flowering plants).
These are haploid
(n) - they have half the number of chromosomes of an ordinary cell which is diploid (2n).
In life-cycles there is usually a stage involving the combination of haploid cells (or at least their nuclei, containing their chromosomes). This is generally described as fertilisation
, and it results in a diploid cell.
Meiosis, involving halving of chromosome number, needs to alternate with fertilisation, which doubles it.
By combining the features of one organism with those of another, sexual reproduction provides raw material for evolution
which is another way of looking at the next generation! And of course for the sake of equality only half of each parent's genetic material can be involved, and the selection of that half is subject to a shuffling process thanks to meiosis.
Both haploid cells and diploid cells can be increased in number by the process of mitosis. In the production of sperms, cell division by mitosis is a continuous process to provide cells which undergo meiosis.
In some organisms the diploid stage is the predominant one, and the haploid stage is temporary, but in other organisms the reverse is true.
Many plants show alternation of generations
: They alternate between multicellular haploid sexual gametophyte
stages and multicellular diploid asexual sporophyte
Haploid spores produced by meiosis in the mature sporophyte germinate and grow into a haploid gametophyte, which grows and produces gametes by mitosis. Fusion of two gametes (syngamy
) results in a diploid zygote.
This is the case in mosses and ferns. See below
The life cycle of a fern
This shows alternation of generations between the sporophye and the gametophyte stages
The gametophyte stage is also photosynthetic - it is just in blue to show it is haploid.
You should be able to able to name the processes occuring at the points marked A-G above.
A > meiosis
B > mitosis
C > mitosis
D > mitosis
E > fertilisation
F > mitosis
G > mitosis
Which stage in the life-cycle requires the following environmental conditions?
dry/windy > spore dispersal
wetness > sperms swimming to meet egg
What is the significance of the sexual organs maturing at different times?
> It favours cross fertilisation - which increases genetic diversity
Flowering plants also show alternation of generations, but it is not so obvious, and there are several mitotic divisions accompanying the division by meiosis.
In the ovule inside the ovary of flowers, female megaspore mother cells undergo meiosis but only one of the four haploid cells continues to progress. In this respect it is similar to oogenesis in mammals, resulting in a large female gamete. The resulting megaspore nucleus divides three times by mitosis so the embryo sac contains 8 nuclei. One remaining at the edge near to the micropyle acts as a female gamete and will eventually fuse with a male nucleus coming down a pollen tube which enters at the micropyle, and two in the centre will also fuse with another male nucleus. The other nuclei appear to have guidance functions.
In regions of sporogenous tissue within the male anthers, microspore mother cells undergo meiosis, producing four haploid nuclei which enter separate pollen cells. Pollen itself is not a gamete but it contains haploid nuclei (produced by mitosis) and these function as male gametes
. They are transferred to the female parts of the flower by pollinating insects or by being blown by the wind. One nucleus fuses with the female nucleus to form a diploid embryo and the other fuses with two female nuclei to form triploid tissue which develops into the seed's nutritive endosperm tissue.
This is called double fertilisation
In fact these are preceded by another 'pollen tube nucleus' which controls the growth of the pollen tube from the stigma down the style and into the embryo sac.
Conversation between the attractive danseuse Isadora Duncan and Nobel prize winning poet, journalist, and novelist Anatole France, who were discussing eugenics, and the possibility of them producing a child together
Isadora: "Imagine a child with my beauty and your brains!"
Anatole: "Yes, but imagine a child with my beauty and your brains!"
OK both of these features are likely to be polygenic, i.e. caused by a number of genes/alleles so the child would probably be in the middle somewhere, but a few dominant alleles could swing things somewhat!