MEIOSIS

10.1HL

Meiosis

Meiosis has a number of overall functions• Reduce the chromosome number by half from

diploid(2n) to haploid(n).• Each gamete nuclei will contain one

chromosome from every homologous pair.• Increase genetic diversity.

10.1.1. Describe the behavior of the chromosome sin the phases of meiosis

Interphase• Cell growth and DNA replication

Prophase I• Chromosomes condense• Nucleolous becomes invisible.• Spindle formation takes place• Synapsis: homologous chromosomes pair up side by side ( the pair is now

called a bivalent, actually each of which is made up of two sister chromatid and is a tetrad). The cross over points are called chiasmata.

• The chiasmata are the positions of DNA exchange which is called cross-over.• The four chromatids from each pair of homologous chromosomes are called

a tetrad or bivalents.

Prophase I

• During the condensation of the chromosomes, a point is reached when the homologous pair seems to repel each other except at regions called chiasmata.

• Crossing over increases genetic diversity of the gametes and therefore increases variation in successful fertilizations.

• Nuclear membrane disappears.• In animal cells the centrioles are placed at either pole of the

cell and serves as a focal point for the orientation of the spindle microtubules.

• Microtubules attach to the centromere’s of each pair of homologous chromosomes

Metaphase I

• Homologous pairs are aligned on the equatorial plate.• Homologous pairs have been held together by the chiasmata

but this is not shown for clarity.• Each 'pair of sister chromatids' in a homologous pair is

attached to microtubule. Each half of the spindle is attached to the opposite poles.

Metaphase I:

                                                                                       

Metaphase I

• The centromere of each chromosome have spindle microtubule attached.

• The bivalents line up randomly along the equator of the cell. This is called random orientation.

• Crossing over is terminated and the exchange of DNA is complete, resulting in chromatids from the same chromosome which are usually no longer identical.

Anaphase I

• Homologous pairs split up, one chromosome of each pair goes to each pole. The chromosomes are pulled to opposite poles by the spindle microtubules.

• The result is that there is independent assortment of genes which are not linked

• At this point the chiasmata break down and the exchange of lengths of DNA including alleles of genes is complete.

•

Telophase I

• Chromosomes arrive at poles• Spindle disappears• The genetic material is now organised at the

two pole of the cell.• Each pole contains a pair of 'sister chromatids' ,

one from each homologous pair.• In some species the nuclear envelop re-forms

whilst in others there is an immediate progression to M2 and prophase II.

Prophase II

• Chromosomes condense again if required.

There has been no chromosome replication but each chromosome is represented by a pair of 'sister chromatids'.

• The spindle forms in a new plane for each of the two cells. The positioning of the plane for the spindle is critical for later development.

• Ne spindle is formed at right angles to the previous spindle.• The 'sister chromatids' attach to the spindle microtubule at the

region of their centromere. • The 'pairs' then move around, beginning to align for the

metaphase II.

Metaphase II

• The chromosomes line up at the equator of each cell.• The are now very condensed and at their most visible

stage.• Spindle fibres from opposite poles attach to each of

the sister chromatids at the centromeres.

Anaphase II• The combination of spindle contraction and the centromere 'motor'

pulling on the spindle microtubule, pulls the sister chromatids apart.

• One chromatid from each pair goes to each pole.• Each chromatids can now be referred to as a chromosomes again.• The chromosomes from the pair of 'sister chromatids' are not

identical due to the exchange of DNA during cross-over (not shown in diagram).

Telophase II

• Chromosomes have arrived at poles and form the new haploid buclei.

• Spindle disappears• Nuclear membrane reappears• Nucleolus becomes visible.• Chromosomes become chromatin• The nuclear envelope material that has remained

within the cytoplasm re-forms around the sets of chromosomes.

• There are now two nuclei per cell, each is haploid.

Cytokinesis

• Cell division (cytokinesis), strictly speaking , is not a part of meiosis but is often considered to be the last stage of telophase II. The net result of meiosis is that from one diploid cell, four haploid cells are produced and these are usually sperm or eggs (gametes).

• This enables the chromosomes number of a sexually reproducing species to be kept constant from one generation to the next.

• In the sexually mature human male the process of meiosis and the production of mature motile sperm cells (haploid) takes over a month. In the human female the process begins during embryonic development but then undergoes a period of 'dormancy'. The cell still diploid is held at prophase I until ovulation many years later. The completion of meiosis does not actually end until fertilisation and then in humans this results in a single gametic cell and not the four as described above. (see reproduction unit). Other species have refined this process in different ways as an adaptation to their own ecology.

Outline the formation of Chiasmata in the process of crossing over

Formation of chiasmata

• New combination of genes within the chromosome are possible through a process called crossing over.

• During prophase I, synapsis occurs, the chromatids of the bivalent are close together. It is then possible that parts of two chromatids overpal, break at the chiasmata and reattach to the other chromatid. Chromatid of the (yellow) chromosome are identical and are sister chromatids. The same applies to the sister chromatids of grey chromosomes. The chromosome (yellow) is homologous but not identical to grey chromosome. Each of the chromatids will end up in a gmete.

Formation of chiasmataBefore crossing over, two gametes would have contained the alleles for H and E (e.g. brown hair and brown eyes) and the other two would have contained genetic information h and e (e.g. blond hair and blue eyes)After crossing over is complete, each gamete would contain:Gamete a: H and E ( brown hair, brown eyes)Gamete b: H and e (brown hair, blue eyes)Gamete c: h and E ( blond hair, brown eyes)Gamete d: h and e ( blond hair, blue eyes)Gametes b and c are therefore new combinations. They are called recombinants.

Crossing Over

• The points at which two non-sister chromatids overlap during prophase I of meiosis forms a cross shaped structure. This structure is known as a chiasma. This is the point where crossing over occurs and the segments of the non-sister chromatids will break and reattah to the other chromatid.

• Cells created by meiosis can contain a wide variety of combinations of genes from the father and from the mother. This explains why brothers and sisters from the same parents may have family resemblances but are never identical apart from identical twins. The chances of receiving the same 23 chromosomes from e ach parent are so incredibly small that it is essentially impossible.

10.1.3.Explain how meiosis results in an effectively infinite genetic variety in gametes through crossing over in prophase I and random

orientation in metaphase I

• Crossing over takes place during prophase I. the number of chiasmata may differ. Chiasmata can occur between any non-sister chromatids. It is even possible to have multiple chiasmata on two non sister chromatids.

• The number of different types of gametes produced by random orientation alone is 2n where n= haploid number.

• To this if we add the effect of crossing over the resulting variation is infinite.

Random Orientation

• Random orientation.

Random orientation.

                                                                                                 

• At Metaphase I, homologous pairs aligned on the equatorial plate of the dividing cell. The diploid cells are the centre row. They will divide, as for anaphase I,vertically.

• The homologous pair are both held on the same spindle microtubule (green dashed line).

• Anaphase I will separate the homologous pair and therefore their alleles. • However with this cell (diploid number = 4) then there are two possible

orientations of the homologous pairs on the equatorial plate.• RANDOM orientation means that all orientations are equally possible.• In this example the number of possible gametes is 4.• In general the calculation of the number of possible gametes = 2n • Homo sapiens 2n=46 , n= 23 therefore number gametes = 2n = 8,388,608.• •