What Is The Genetic Makeup Of Meiosis Summary About The Genetic Makeup Of Meiosis

Chapter 7: The Cellular Basis of Inheritance

Meiosis

Learning Objectives

By the stop of this section, you will be able to:

• Describe the behavior of chromosomes during meiosis
• Describe cellular events during meiosis
• Explain the differences between meiosis and mitosis
• Explain the mechanisms within meiosis that generate genetic variation among the products of meiosis

Sexual reproduction requires fertilization, a union of 2 cells from two individual organisms. If those two cells each incorporate one set of chromosomes, and so the resulting jail cell contains two sets of chromosomes. The number of sets of chromosomes in a cell is chosen its ploidy level. Haploid cells contain i set of chromosomes. Cells containing two sets of chromosomes are chosen diploid. If the reproductive cycle is to continue, the diploid jail cell must somehow reduce its number of chromosome sets earlier fertilization can occur over again, or there will be a continual doubling in the number of chromosome sets in every generation. And so, in addition to fertilization, sexual reproduction includes a nuclear division, known as meiosis, that reduces the number of chromosome sets.

Near animals and plants are diploid, containing two sets of chromosomes; in each somatic cell (the nonreproductive cells of a multicellular organism), the nucleus contains two copies of each chromosome that are referred to every bit homologous chromosomes. Somatic cells are sometimes referred to as "body" cells. Homologous chromosomes are matched pairs containing genes for the same traits in identical locations forth their length. Diploid organisms inherit one re-create of each homologous chromosome from each parent; all together, they are considered a full set of chromosomes. In animals, haploid cells containing a single copy of each homologous chromosome are found merely inside gametes. Gametes fuse with some other haploid gamete to produce a diploid cell.

The nuclear division that forms haploid cells, which is called meiosis, is related to mitosis. As you accept learned, mitosis is part of a cell reproduction cycle that results in identical daughter nuclei that are likewise genetically identical to the original parent nucleus. In mitosis, both the parent and the daughter nuclei contain the same number of chromosome sets—diploid for about plants and animals. Meiosis employs many of the same mechanisms as mitosis. Nonetheless, the starting nucleus is always diploid and the nuclei that upshot at the end of a meiotic prison cell division are haploid. To achieve the reduction in chromosome number, meiosis consists of one round of chromosome duplication and 2 rounds of nuclear division. Because the events that occur during each of the segmentation stages are analogous to the events of mitosis, the same stage names are assigned. However, considering at that place are ii rounds of division, the stages are designated with a "I" or "II." Thus, meiosis I is the first circular of meiotic partitioning and consists of prophase I, prometaphase I, and so on. Meiosis I reduces the number of chromosome sets from ii to 1. The genetic information is also mixed during this division to create unique recombinant chromosomes. Meiosis Two, in which the second round of meiotic division takes identify in a way that is similar to mitosis, includes prophase II, prometaphase II, then on.

Interphase

Meiosis is preceded by an interphase consisting of the G₁, S, and M₂ phases, which are virtually identical to the phases preceding mitosis. The G₁ phase is the get-go stage of interphase and is focused on cell growth. In the Due south stage, the Deoxyribonucleic acid of the chromosomes is replicated. Finally, in the 1000_ii stage, the prison cell undergoes the final preparations for meiosis.

During DNA duplication of the Due south stage, each chromosome becomes equanimous of two identical copies (called sis chromatids) that are held together at the centromere until they are pulled autonomously during meiosis 2. In an animal prison cell, the centrosomes that organize the microtubules of the meiotic spindle also replicate. This prepares the cell for the first meiotic phase.

Meiosis I

Early in prophase I, the chromosomes can exist seen clearly microscopically. Equally the nuclear envelope begins to pause down, the proteins associated with homologous chromosomes bring the pair close to each other. The tight pairing of the homologous chromosomes is called synapsis. In synapsis, the genes on the chromatids of the homologous chromosomes are precisely aligned with each other. An commutation of chromosome segments between non-sister homologous chromatids occurs and is chosen crossing over. This process is revealed visually after the exchange as chiasmata (atypical = chiasma) ([Figure 1]).

As prophase I progresses, the close clan between homologous chromosomes begins to pause down, and the chromosomes keep to condense, although the homologous chromosomes remain attached to each other at chiasmata. The number of chiasmata varies with the species and the length of the chromosome. At the end of prophase I, the pairs are held together only at chiasmata ([Figure 1]) and are called tetrads considering the four sis chromatids of each pair of homologous chromosomes are now visible.

The crossover events are the first source of genetic variation produced by meiosis. A single crossover event between homologous not-sister chromatids leads to a reciprocal exchange of equivalent Dna between a maternal chromosome and a paternal chromosome. Now, when that sister chromatid is moved into a gamete, information technology will bear some DNA from ane parent of the individual and some DNA from the other parent. The recombinant sister chromatid has a combination of maternal and paternal genes that did non exist earlier the crossover.

Figure 1: In this analogy of the furnishings of crossing over, the blue chromosome came from the individual'southward father and the red chromosome came from the individual's mother. Crossover occurs betwixt non-sis chromatids of homologous chromosomes. The result is an exchange of genetic cloth between homologous chromosomes. The chromosomes that take a mixture of maternal and paternal sequence are called recombinant and the chromosomes that are completely paternal or maternal are chosen non-recombinant.

The key event in prometaphase I is the attachment of the spindle fiber microtubules to the kinetochore proteins at the centromeres. The microtubules assembled from centrosomes at opposite poles of the prison cell grow toward the heart of the prison cell. At the end of prometaphase I, each tetrad is attached to microtubules from both poles, with one homologous chromosome attached at one pole and the other homologous chromosome attached to the other pole. The homologous chromosomes are still held together at chiasmata. In addition, the nuclear membrane has cleaved downwardly entirely.

During metaphase I, the homologous chromosomes are bundled in the center of the jail cell with the kinetochores facing opposite poles. The orientation of each pair of homologous chromosomes at the center of the jail cell is random.

This randomness, chosen contained assortment, is the concrete ground for the generation of the second form of genetic variation in offspring. Consider that the homologous chromosomes of a sexually reproducing organism are originally inherited as two dissever sets, one from each parent. Using humans as an case, 1 set of 23 chromosomes is present in the egg donated by the mother. The father provides the other set of 23 chromosomes in the sperm that fertilizes the egg. In metaphase I, these pairs line upwards at the midway signal between the 2 poles of the prison cell. Because there is an equal chance that a microtubule cobweb will meet a maternally or paternally inherited chromosome, the arrangement of the tetrads at the metaphase plate is random. Any maternally inherited chromosome may face either pole. Any paternally inherited chromosome may besides face up either pole. The orientation of each tetrad is independent of the orientation of the other 22 tetrads.

In each cell that undergoes meiosis, the organization of the tetrads is unlike. The number of variations depends on the number of chromosomes making up a gear up. At that place are 2 possibilities for orientation (for each tetrad); thus, the possible number of alignments equals two ⁿ where n is the number of chromosomes per prepare. Humans have 23 chromosome pairs, which results in over eight million (2²³) possibilities. This number does non include the variability previously created in the sister chromatids by crossover. Given these two mechanisms, it is highly unlikely that any two haploid cells resulting from meiosis will have the aforementioned genetic composition ([Figure 2]).

To summarize the genetic consequences of meiosis I: the maternal and paternal genes are recombined past crossover events occurring on each homologous pair during prophase I; in addition, the random assortment of tetrads at metaphase produces a unique combination of maternal and paternal chromosomes that volition make their way into the gametes.

Figure 2: To demonstrate random, independent assortment at metaphase I, consider a cell with due north = 2. In this case, at that place are two possible arrangements at the equatorial airplane in metaphase I, as shown in the upper cell of each panel. These two possible orientations atomic number 82 to the production of genetically different gametes. With more chromosomes, the number of possible arrangements increases dramatically.

In anaphase I, the spindle fibers pull the linked chromosomes apart. The sis chromatids remain tightly bound together at the centromere. It is the chiasma connections that are broken in anaphase I every bit the fibers fastened to the fused kinetochores pull the homologous chromosomes apart ([Figure 3]).

In telophase I, the separated chromosomes go far at reverse poles. The rest of the typical telophase events may or may not occur depending on the species. In some organisms, the chromosomes decondense and nuclear envelopes grade around the chromatids in telophase I.

Cytokinesis, the physical separation of the cytoplasmic components into two daughter cells, occurs without reformation of the nuclei in other organisms. In virtually all species, cytokinesis separates the cell contents by either a cleavage furrow (in animals and some fungi), or a cell plate that will ultimately lead to formation of cell walls that split up the two daughter cells (in plants). At each pole, there is just one fellow member of each pair of the homologous chromosomes, so only one full set of the chromosomes is present. This is why the cells are considered haploid—there is merely one chromosome set, even though there are indistinguishable copies of the prepare because each homolog still consists of two sister chromatids that are even so attached to each other. All the same, although the sister chromatids were in one case duplicates of the same chromosome, they are no longer identical at this stage because of crossovers.

Review the procedure of meiosis, observing how chromosomes align and migrate, at this site.

Meiosis Two

In meiosis Ii, the continued sister chromatids remaining in the haploid cells from meiosis I will be split up to grade four haploid cells. In some species, cells enter a brief interphase, or interkinesis, that lacks an Southward phase, before entering meiosis II. Chromosomes are non duplicated during interkinesis. The two cells produced in meiosis I go through the events of meiosis Two in synchrony. Overall, meiosis II resembles the mitotic partitioning of a haploid cell.

In prophase Two, if the chromosomes decondensed in telophase I, they condense again. If nuclear envelopes were formed, they fragment into vesicles. The centrosomes duplicated during interkinesis motion away from each other toward contrary poles, and new spindles are formed. In prometaphase 2, the nuclear envelopes are completely broken downwardly, and the spindle is fully formed. Each sister chromatid forms an individual kinetochore that attaches to microtubules from opposite poles. In metaphase II, the sister chromatids are maximally condensed and aligned at the center of the cell. In anaphase Ii, the sister chromatids are pulled apart past the spindle fibers and movement toward reverse poles.

Figure 3: In prometaphase I, microtubules attach to the fused kinetochores of homologous chromosomes. In anaphase I, the homologous chromosomes are separated. In prometaphase Two, microtubules attach to individual kinetochores of sister chromatids. In anaphase II, the sister chromatids are separated.

In telophase II, the chromosomes make it at opposite poles and brainstorm to decondense. Nuclear envelopes form around the chromosomes. Cytokinesis separates the two cells into four genetically unique haploid cells. At this indicate, the nuclei in the newly produced cells are both haploid and have only one copy of the single prepare of chromosomes. The cells produced are genetically unique because of the random array of paternal and maternal homologs and considering of the recombination of maternal and paternal segments of chromosomes—with their sets of genes—that occurs during crossover.

Comparing Meiosis and Mitosis

Mitosis and meiosis, which are both forms of division of the nucleus in eukaryotic cells, share some similarities, but also exhibit distinct differences that pb to their very different outcomes. Mitosis is a single nuclear division that results in 2 nuclei, unremarkably partitioned into ii new cells. The nuclei resulting from a mitotic division are genetically identical to the original. They have the same number of sets of chromosomes: ane in the instance of haploid cells, and 2 in the case of diploid cells. On the other paw, meiosis is 2 nuclear divisions that consequence in iv nuclei, usually partitioned into four new cells. The nuclei resulting from meiosis are never genetically identical, and they contain i chromosome set only—this is half the number of the original prison cell, which was diploid ([Figure four]).

The differences in the outcomes of meiosis and mitosis occur because of differences in the behavior of the chromosomes during each process. Most of these differences in the processes occur in meiosis I, which is a very different nuclear division than mitosis. In meiosis I, the homologous chromosome pairs go associated with each other, are bound together, experience chiasmata and crossover between sis chromatids, and line up along the metaphase plate in tetrads with spindle fibers from opposite spindle poles attached to each kinetochore of a homolog in a tetrad. All of these events occur only in meiosis I, never in mitosis.

Homologous chromosomes move to opposite poles during meiosis I so the number of sets of chromosomes in each nucleushoped-for is reduced from 2 to one. For this reason, meiosis I is referred to equally a reduction division. There is no such reduction in ploidy level in mitosis.

Meiosis II is much more analogous to a mitotic segmentation. In this case, duplicated chromosomes (just one set of them) line up at the center of the cell with divided kinetochores attached to spindle fibers from opposite poles. During anaphase II, every bit in mitotic anaphase, the kinetochores dissever and one sis chromatid is pulled to 1 pole and the other sister chromatid is pulled to the other pole. If it were non for the fact that there had been crossovers, the two products of each meiosis 2 partitioning would be identical every bit in mitosis; instead, they are dissimilar because there has always been at least one crossover per chromosome. Meiosis II is non a reduction division because, although in that location are fewer copies of the genome in the resulting cells, there is still one set of chromosomes, every bit there was at the end of meiosis I.

Cells produced by mitosis will role in different parts of the trunk every bit a function of growth or replacing dead or damaged cells. They may even be involved in asexual reproduction in some organisms. Cells produced by meiosis in a diploid-dominant organism such as an animate being will but participate in sexual reproduction.

Figure iv: Meiosis and mitosis are both preceded by one round of Dna replication; however, meiosis includes ii nuclear divisions. The four girl cells resulting from meiosis are haploid and genetically distinct. The daughter cells resulting from mitosis are diploid and identical to the parent cell.

Section Summary

Sexual reproduction requires that diploid organisms produce haploid cells that can fuse during fertilization to form diploid offspring. The procedure that results in haploid cells is called meiosis. Meiosis is a series of events that arrange and carve up chromosomes into daughter cells. During the interphase of meiosis, each chromosome is duplicated. In meiosis, there are 2 rounds of nuclear division resulting in 4 nuclei and usually iv haploid daughter cells, each with half the number of chromosomes as the parent cell. During meiosis, variation in the girl nuclei is introduced because of crossover in prophase I and random alignment at metaphase I. The cells that are produced past meiosis are genetically unique.

Meiosis and mitosis share similarities, but have distinct outcomes. Mitotic divisions are single nuclear divisions that produce daughter nuclei that are genetically identical and have the same number of chromosome sets equally the original cell. Meiotic divisions are two nuclear divisions that produce iv daughter nuclei that are genetically different and take one chromosome set rather than the two sets the parent cell had. The main differences betwixt the processes occur in the commencement division of meiosis. The homologous chromosomes separate into different nuclei during meiosis I causing a reduction of ploidy level. The second segmentation of meiosis is much more similar to a mitotic division.

For an animation comparison mitosis and meiosis, get to this website.

Multiple Option

Meiosis produces ________ girl cells.

1. ii haploid
2. two diploid
3. four haploid
4. four diploid

[reveal-answer q="249061″]Prove Answer[/reveal-answer]
[hidden-answer a="249061″]3[/hidden-respond]

At which stage of meiosis are sis chromatids separated from each other?

1. prophase I
2. prophase Two
3. anaphase I
4. anaphase 2

[reveal-respond q="958677″]Show Respond[/reveal-answer]
[hidden-respond a="958677″]4[/subconscious-answer]

The role of meiosis that is similar to mitosis is ________.

1. meiosis I
2. anaphase I
3. meiosis Ii
4. interkinesis

[reveal-respond q="77444″]Show Answer[/reveal-respond]
[hidden-answer a="77444″]three[/hidden-answer]

If a muscle cell of a typical organism has 32 chromosomes, how many chromosomes will be in a gamete of that aforementioned organism?

1. 8
2. xvi
3. 32
4. 64

[reveal-answer q="587145″]Show Answer[/reveal-answer]
[subconscious-respond a="587145″]two[/hidden-reply]

Free Response

Explicate how the random alignment of homologous chromosomes during metaphase I contributes to variation in gametes produced by meiosis.

Random alignment leads to new combinations of traits. The chromosomes that were originally inherited past the gamete-producing individual came equally from the egg and the sperm. In metaphase I, the duplicated copies of these maternal and paternal homologous chromosomes line up across the center of the cell to form a tetrad. The orientation of each tetrad is random. In that location is an equal hazard that the maternally derived chromosomes volition exist facing either pole. The aforementioned is truthful of the paternally derived chromosomes. The alignment should occur differently in almost every meiosis. As the homologous chromosomes are pulled autonomously in anaphase I, any combination of maternal and paternal chromosomes will motility toward each pole. The gametes formed from these two groups of chromosomes will have a mixture of traits from the individual's parents. Each gamete is unique.

In what ways is meiosis II similar to and dissimilar from mitosis of a diploid prison cell?

The ii divisions are similar in that the chromosomes line up along the metaphase plate individually, significant unpaired with other chromosomes (every bit in meiosis I). In addition, each chromosome consists of two sister chromatids that will exist pulled autonomously. The 2 divisions are dissimilar because in meiosis Two there are half the number of chromosomes that are nowadays in a diploid prison cell of the aforementioned species undergoing mitosis. This is because meiosis I reduced the number of chromosomes to a haploid country.

Glossary

chiasmata

(singular = chiasma) the structure that forms at the crossover points after genetic fabric is exchanged

crossing over

(also, recombination) the exchange of genetic material between homologous chromosomes resulting in chromosomes that incorporate genes from both parents of the organism forming reproductive cells

fertilization

the union of two haploid cells typically from two individual organisms

interkinesis

a period of rest that may occur between meiosis I and meiosis II; in that location is no replication of Deoxyribonucleic acid during interkinesis

meiosis I

the outset circular of meiotic cell partitioning; referred to as reduction division considering the resulting cells are haploid

meiosis II

the second round of meiotic cell division following meiosis I; sis chromatids are separated from each other, and the event is iv unique haploid cells

recombinant

describing something equanimous of genetic textile from two sources, such equally a chromosome with both maternal and paternal segments of DNA

reduction sectionalisation

a nuclear division that produces daughter nuclei each having one-half as many chromosome sets as the parental nucleus; meiosis I is a reduction sectionalisation

somatic jail cell

all the cells of a multicellular organism except the gamete-forming cells

synapsis

the germination of a shut association between homologous chromosomes during prophase I

tetrad

two duplicated homologous chromosomes (4 chromatids) jump together past chiasmata during prophase I

Source: https://opentextbc.ca/conceptsofbiologyopenstax/chapter/meiosis/

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