7.2 Meiosis

The multicellular "body" of the organisms is haploid.

Two individuals join specialized haploid cells to form a diploid zygote.

The third life-cycle type is called alternation of generations.

Both haploid and diploid multicellular organisms are part of the life cycle of these species.
The organisms that produce gametes are already haploid and Meiosis is not involved.
A diploid zygote is formed by fertilization between the gametes.
Haploid spores will be produced by specialized cells of the sporophyte.
The gametophytes will develop from the spores.

The resulting cell has two sets of chromosomes if the two cells contain one set of chromosomes.

ploidy level is the number of sets of chromosomes in a cell.
One set of chromosomes is contained in haploid cells.
Cells with two sets of chromosomes are called diploid.
If the reproductive cycle is to continue, the diploid cell must reduce its number of chromosomes before fertilization can occur again, or there will be a continual doubling in the number of chromosomes in every generation.
Sexual reproduction includes a nuclear division that reduces the number of chromosomes.

Somatic cells are sometimes referred to as body cells.

Matching pairs of Homologous chromosomes have the same genes in the same locations.
Diploid organisms have a full set of chromosomes from each parent.
Animals have haploid cells with a single copy of each chromosomes.
A diploid cell can be produced by gametes and another haploid gamete.

haploid cells are formed by the nuclear division that is related to meiosis.

In a cell reproduction cycle, identical daughter nuclei that are also genetically identical to the original parent nucleus are part of the process.
The parent and daughter nuclei have the same number of chromosomes.
Many of the same mechanisms are employed by meiosis.
The starting nucleus is always diploid and the end of a meiotic cell division is haploid.
Meiosis consists of one round of chromosome duplication and 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 number of chromosomes is reduced from two to one.
During this division, the genetic information is mixed to create unique chromosomes.

The G1, S, and G2 phases are nearly identical to the phases preceding meiosis.

The first phase of interphase is focused on cell growth.
The chromosomes are replicated in the S phase.
The final preparations for meiosis take place in the G2 phase.

During meiosis II, each of the chromosomes becomes composed of two identical copies that are held together at the centromere until they are pulled apart.

The centrosomes that organize the microtubules of the meiotic spindle also replicate in an animal cell.
The cell is prepared for the meiotic phase.

The chromosomes can be seen clearly.

The pair are close to each other as the nuclear envelope breaks down.
The genes on the chromatids are aligned with each other in synapsis.

As prophase I progresses, the close association between chromosomes begins to break down, and the chromosomes continue to condense.

The first source of genetic variation is the crossover events.

The exchange of equivalent DNA between a maternal and a paternal chromosomes is caused by a single event between non-sister chromatids.
When the sister chromatid is moved into a gamete, it will carry some genetic material from one parent to the other.

The blue and red chromosomes came from the father and mother of the individual.

There are two non-sister chromatids of the same chromosomes.
The result is an exchange of genes.
The chromosomes that have a mixture of maternal and paternal sequence are called non-recombinant.

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

The middle of the cell is where the microtubules from the opposite poles of the cell grow.

At the end of prometaphase I, each tetrad is attached to a microtubule from one pole and another from the other.

At chiasmata, the chromosomes are still held together.
The nuclear membrane has broken down.

The kinetochores are facing opposite poles in the center of the cell during metaphase I.

The orientation of the chromosomes at the center of the cell is random.

The basis for the generation of the second form of genetic variation in offspring is randomness.

There are two separate sets of chromosomes in a sexually reproducing organisms.
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 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.

The maternally inherited chromosomes may face either pole.
The paternally inherited chromosomes may face either pole.
The orientation of each tetrad is not related to the orientation of the other 22.

The arrangement of the tetrads is different in each cell that undergoes meiosis.

The number of variations depends on the number of chromosomes.
There are over eight million possibilities due to the fact that humans have 23 chromosomes.
The variability previously created in the sister chromatids is not included in this number.

The maternal and paternal genes are recombined by events occurring on each pair during prophase I, and the random assortment of tetrads at metaphase produces a unique combination of maternal and paternal chromosomes.

The upper cell of each panel shows two possible arrangements at the equatorial plane in metaphase I.

There are two possible orientations that lead to the production of genetically different gametes.

There are more possible arrangements with more chromosomes.

In anaphase I, the linked chromosomes are pulled apart.

At the centromere, the sister chromatids are tightly bound together.
The chiasma connections are broken in anaphase I as the fibers attached to the kinetochores arrive at opposite poles.
Depending on the species, the remainder of the typical telophase events may or may not occur.
The chromosomes decondense and nuclear envelopes are found in some organisms.

The separation of the cytoplasmic components into two daughter cells is called cytokinesis.

In almost all species, cytokinesis separates the cell contents by either a cell plate or a cleavage furrow, which leads to the formation of cell walls that separate the two daughter cells.
Only one full set of the chromosomes is present at each pole.
The cells are considered haploid because there is only one set of chromosomes, even though there are duplicate sets.
Although the sister chromatids were once duplicate of the same chromosome, they are no longer.

Four haploid cells will be formed in meiosis II, with the connected sister chromatids remaining in the haploid cells.

During interkinesis, chromosomes are not duplicated.

I go through the events of meiosis II in chronological order.
The division of a haploid cell is similar to meiosis II.

If the chromosomes condense in telophase I, they do so again in prophase II.

Nuclear envelopes fragment into small objects.
The centrosomes duplicated during interkinesis move away from each other.
The nuclear envelopes are completely broken down in prometaphase II.
The kinetochore that each sister forms is attached to the microtubules.
The chromatids are aligned at the center of the cell in metaphase II.
In anaphase II, the sister chromatids are pulled apart by the spindle fibers.

Microtubules attach to the kinetochores of the homologous chromosomes.

The chromosomes are separated in anaphase I. Microtubules attach to individual kinetochores of sisters.
The sisters are separated in anaphase II.

The chromosomes arrive at opposite poles in telophase II.

Nuclear envelopes are around the chromosomes.
The two cells are separated into four different types of cells.
The cells that have been produced are haploid and only have one copy of the single set of chromosomes.
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.

There are similarities between meiosis and meosis, but also distinct differences that lead to different outcomes.

A single nuclear division results in two nuclei being partitioned into two new cells.
The nuclei are identical to the original.
There are two sets of chromosomes in the case of haploid cells and two in the case of diploid cells.
Two nuclear divisions result in four nuclei, which are usually partitioned into four new cells.

This is half the number of the original cell, which was diploid, because the nuclei resulting from meiosis are never genetically identical.

There are differences in the behavior of the chromosomes that affect the outcomes of meiosis.

Most of the differences in the processes occur in meiosis I, which is a very different nuclear division.
In meiosis I, the chromosomes are bound together, experience chiasmata, and line up along the metaphase plate with spindle fibers from opposite poles attached to each kinetochore.
All of the events occur in meiosis I.

During meiosis I, the number of sets of chromosomes in each nucleus is reduced from two to one.

There is no reduction inploidy level.

Meiosis II is more similar to a division.

The duplicated chromosomes line up at the center of the cell with divided kinetochores attached to the opposite poles.
The kinetochores divide and one sister is pulled to one pole and the other sister is pulled to the other pole during anaphase II.
The two products of each meiosis II division would be the same, but they are different because there has always been at least one crossover per chromosomes.
Although there are fewer copies of the genome in the resulting cells, there is still one set of chromosomes.

Different parts of the body will function as a part of growth or as a replacement for dead or damaged cells.

Some organisms may be involved in asexual reproduction.

Sexual reproduction is only possible in cells produced by meiosis in a diploid-dominant organisms.