11.1 The Process of Meiosis

If the reproductive cycle is to continue for any sexually reproducing species, then the diploid cell must somehow reduce its number of chromosomes to produce haploid gametes; otherwise, the number of chromosomes will double with every future round of fertilization.

Nuclear division reduces the number of chromosomes by half.

The majority of animals and plants are diploid and have two sets of chromosomes.

Matching pairs of Homologous chromosomes have the same genes in the same places.
One copy of each parent's chromosomes is given to diploid organisms.

Most multicellular animals have the same "ploidy level"--diploid in the case of the parent and daughter nuclei.

Both haploid and diploid cells are used by plants to grow as sporophytes and to produce eggs and sperm as gametophytes.
The starting nucleus is always diploid and the daughter nucleus is haploid.
Meiosis consists of one round of chromosome replication followed by two rounds of nuclear division.

The same stage names are assigned because the events that occur during each of the division stages are similar to the events of mitosis.

The interphase consisting of G1, S, and G2 is almost identical to the phases preceding meiosis.

The first gap phase is focused on cell growth.
During the second phase of interphase, the cell copies or replicates the DNA of the chromosomes.
The final preparations for meiosis take place in the second gap phase.

Before the chromosomes can be seen with a microscope, they are attached at their tips to the nuclear envelope.

The pair are closer together as the nuclear envelope breaks down.
The chromosomes do not pair together.

synapsis is the tight pair of chromosomes.

Even though the X and Y sex chromosomes are not completely homologous, there is a small region of homology that allows the X.
There is a partial synaptonemal complex.

In prophase I, the chromosomes come together.

The chromosomes are bound tightly and in perfect alignment by the synaptonemal complex and cohesin proteins at the centromere.

A new connection is made between the nonsister chromatids after the double-stranded DNA of each chromatid is cleaved.

The synaptonemal complex begins to break down as prophase I progresses.
The chromosomes are attached to each other at the centromere and chiasmata when the synaptonemal complex is gone.
The number of chiasmata depends on the species and the length of the chromosomes.

There must be at least one chiasma per chromosomes for the proper separation of the homologous chromosomes during meiosis I.

The synaptonemal complex breaks down and the cohesin connection between pairs is removed.

The first source of genetic variation in the nuclei is the crossover events.

The exchange of equivalent DNA between a maternal and a paternal chromosomes is caused by a single event.
When a sister chromatid is moved into a gamete cell, it will carry some genes from one parent and some from the other.
The maternal and paternal genes that were not present before the crossover are present in the recombinant chromatid.
There are events that can occur along the length of the chromosomes.
Different cells undergoing meiosis will have different combinations of maternal and parental genes.
The same effect can be achieved by exchanging segments of DNA in an arm of the chromosomes.

There are two chromatids of the same chromosomes.

The result is an exchange of genes.

The kinetochore proteins at the centromeres is the key event in prometaphase I.

There are multiprotein complexes that bind the centromeres of a chromosomes to the microtubules.
Microtubule-organizing centers are where microtubule grow.
The centrosomes are located at opposite poles of the cell.
The microtubules from each pole move towards the middle of the cell and attach to one of the kinetochores.
In the next phase, the microtubule extending from opposite poles of the cell can pull the homologous pair apart.
A kinetochore microtubule is a fiber attached to a kinetochore.
Each tetrad is attached to a microtubule from one pole and another from the other.
At the chiasmata, the chromosomes are still held together.
The nuclear membrane has broken down.

The kinetochores are facing opposite poles in the middle of the cell.

The pairs are at the equator.
If the two members of chromosome 1 are labeled a and b, the chromosomes could line up a-b or b-a.
The genes carried by a gamete will only receive one of the two chromosomes.

Much of the genetic variation in the offspring is due to the randomness in the alignment of recombined chromosomes at the metaphase plate.

The egg donated by the mother contains one set of 23 chromosomes.

The father gives the sperm thatfertilizes the egg a set of 23 chromosomes.

The multicellular offspring have copies of the original two sets of chromosomes.
The tetrads are formed by the homologous chromosomes.
The metaphase plate is formed at the midway point between the two poles of the cell.
The arrangement of the tetrads at the metaphase plate is random because there is an equal chance that a microtubule fiber will encounter a maternal or paternally inherited chromosome.
Any maternally inherited chromosome may face either pole.
Any paternally inherited chromosome can face either pole.
The orientation of each tetrad is not related to the orientation of the other 22.

The random assortment of homologous chromosomes at the metaphase plate is the second mechanism that introduces variation into the gametes.

The arrangement of the tetrads is different in each cell that undergoes meiosis.
The number of variations depends on the number of chromosomes.
The number of alignments equates to 2n in a diploid cell, where n is the number of chromosomes per haploid set.
The random alignment of chromosomes at the metaphase plate results in over eight million possible genetically distinct gametes.
The variability that was previously produced by crossing over between the nonsister chromatids is not included in this number.
It is highly unlikely that any two haploid cells will have the same genetic composition.

I create genetically diverse gametes in two ways.

During prophase I, events between the nonsister chromatids of each pair of chromosomes cause new combinations of maternal and paternal genes.
There are unique combinations of maternal and paternal chromosomes that will make their way into the gametes.

There are two possible arrangements at the plane.

The total number of different gametes is 2n, which is the number of chromosomes in a set.
There are four possible combinations for the gametes.
There are over eight million possible combinations of paternal and maternal chromosomes.

The linked chromosomes are pulled apart by the microtubules.

At the centromere, the sister chromatids are tightly bound together.
The chiasmata are broken in anaphase I as the microtubules attached to the kinetochores pull the chromosomes apart.

The separated chromosomes arrive at opposite poles in telophase.

Depending on the species, the remainder of the typical telophase events may or may not occur.
In some organisms, the chromosomes "decondense" and nuclear envelopes form around the separated sets of chromatids.
The separation of the components into two daughter cells does not involve the reformation of the nucleus.
In almost all species of animals and some fungi, cytokinesis separates the cell contents via a cleavage furrow.
In plants, a cell plate is formed when Golgi vesicles fusion at the metaphase plate.
The formation of cell walls that separate the two daughter cells is the result of this cell plate.

The first meiotic division of a diploid cell resulted in two haploid cells.

The cells are haploid because there are only one pair of chromosomes at each pole.
Only one full set of the chromosomes is present.
The cells are considered haploid because there is only one set of chromosomes.
Sister chromatids are only duplicate of one of the two chromosomes, except for changes that occurred during crossing over.
Four haploid daughter cells will be created in meiosis II.

Interkinesis does not have an S phase.

I go through the events of meiosis II in chronological order.
Four new haploid gametes were formed when the sister chromatids within the two daughter cells separated.
The mechanics of meiosis II are the same as those of mitosis, except that each dividing cell has only one set of chromosomes.
Each cell has half the number of sister chromatids to separate out as a diploid cell.
Haploid cells in G2 are similar to cells at the start of meiosis II.

In telophase I, the chromosomes condense.

Nuclear envelopes fragment into small objects.
During interkinesis, the duplicated MTOCs move away from each other towards opposite poles, and new spindles are formed.

The nuclear envelopes have been broken down.

The kinetochore that each sister forms is attached to the microtubules.

The chromatids are aligned at the equator.

The sister chromatids are pulled apart by the kinetochore microtubules.

The cell has nonkinetochore microtubules.

In metaphase I, the fused kinetochores of the homologous chromosomes are arranged at the midline of the cell.

The homologous chromosomes are separated in anaphase I.
In metaphase II, the kinetochores of sister chromatids are arranged at the center of the cells.
The sisters separate in anaphase II.

The chromosomes arrive at different poles.

Nuclear envelopes are around the chromosomes.
If the parent cell was diploid, then cytokinesis separates the two cells into four different types of cells.
The cells produced are unique because of the random assortment of paternal and maternal homologs and because of the recombination of maternal and paternal segments of chromosomes.

Four haploid daughter cells are formed from an animal cell with a diploid number of four.

There are two forms of division of the nucleus in cells.

They have some similarities, but also have a number of important and distinct differences that lead to very different outcomes.
A single nuclear division results in two nuclei being partitioned into two new cells.
The nucleus of the original nucleus is identical to the nucleus resulting from a mitotic division.
There are two sets of chromosomes in the case of haploid cells and one set in the case of diploid cells.
Meiosis consists of two nuclear divisions resulting in four different types of cells.
The four nuclei produced during meiosis are not the same as one another.

The nuclear division in meiosis I is very different from the one in mitosis.

The chromosomes develop chiasmata after this.

The chromosomes line up along the metaphase plate with kinetochore fibers attached to each kinetochore.
All of the events occur in meiosis I.

The ploidy level, the number of sets of chromosomes in each future nucleus, has been reduced when the chiasmata resolve and the tetrad is broken up.

There is no reduction in ploidy level.

Meiosis II is similar to a division.

The duplicated chromosomes line up on the metaphase plate with the kinetochores attached to the opposite poles.
The kinetochores divide and one sister chromatid are pulled to one pole while the other sister is pulled to the other pole during anaphase II.
The two products of each individual meiosis II division would be the same.
They are different because of the fact that there has always been one.
Although there are fewer copies of the genome in the resulting cells, there is still one set of chromosomes.

Two nuclear divisions are included in meiosis, which is preceded by one cycle of DNA replication.

The four daughter cells are haploid.