spindle fibers move homologous chromosomes to opposite sides in what phase

Introduction to Meiosis

Meiosis is the nuclear division of diploid cells into haploid cells, which is a necessary step in sexual reproduction.

Learning Objectives

Describe the importance of meiosis in sexual reproduction

Key Takeaways

Primal Points

• Sexual reproduction is the production of haploid cells and the fusion of two of those cells to form a diploid cell.
• Before sexual reproduction can occur, the number of chromosomes in a diploid prison cell must subtract past half.
• Meiosis produces cells with half the number of chromosomes every bit the original cell.
• Haploid cells used in sexual reproduction, gametes, are formed during meiosis, which consists of 1 round of chromosome replication and 2 rounds of nuclear division.
• Meiosis I is the start round of meiotic partitioning, while meiosis 2 is the second round.

Cardinal Terms

• haploid: of a cell having a single set of unpaired chromosomes
• gamete: a reproductive cell, male (sperm) or female (egg), that has only half the usual number of chromosomes
• diploid: of a cell, having a pair of each type of chromosome, one of the pair being derived from the ovum and the other from the spermatozoon

Introduction: Meiosis and Sexual Reproduction

The ability to reproduce in kind is a bones characteristic of all living things. In kind means that the offspring of whatever organism closely resemble their parent or parents. Sexual reproduction requires fertilization: the union of two cells from two individual organisms. Haploid cells contain one set of chromosomes. Cells containing two sets of chromosomes are called diploid. The number of sets of chromosomes in a cell is called its ploidy level. If the reproductive bike is to keep, then the diploid jail cell must somehow reduce its number of chromosome sets earlier fertilization tin occur once more or in that location volition be a continual doubling in the number of chromosome sets in every generation. Therefore, sexual reproduction includes a nuclear division that reduces the number of chromosome sets.

Offspring Closely Resemble Their Parents: In kind ways that the offspring of whatever organism closely resemble their parent or parents. The hippopotamus gives birth to hippopotamus calves (a). Joshua trees produce seeds from which Joshua tree seedlings emerge (b). Developed flamingos lay eggs that hatch into flamingo chicks (c).

Sexual reproduction is the production of haploid cells (gametes) and the fusion (fertilization) of two gametes to form a single, unique diploid prison cell called a zygote. All animals and near plants produce these gametes, or eggs and sperm. In about plants and animals, through tens of rounds of mitotic cell division, this diploid cell volition develop into an adult organism.

Haploid cells that are part of the sexual reproductive cycle are produced by a type of jail cell division chosen meiosis. Meiosis employs many of the same mechanisms as mitosis. However, the starting nucleus is always diploid and the nuclei that result at the stop of a meiotic cell division are haploid, so the resulting cells take half the chromosomes every bit the original. To achieve this reduction in chromosomes, meiosis consists of one round of chromosome duplication and 2 rounds of nuclear division. Because the events that occur during each of the division stages are analogous to the events of mitosis, the same stage names are assigned. Nevertheless, considering there are two rounds of division, the major process and the stages are designated with a "I" or a "Two." Thus, meiosis I is the first circular of meiotic sectionalization and consists of prophase I, prometaphase I, then on. Meiosis Ii, the second round of meiotic division, includes prophase II, prometaphase Ii, and then on.

Meiosis I

In meiosis I, the get-go round of meiosis, homologous chromosomes exchange Dna and the diploid cell is divided into two haploid cells.

Learning Objectives

Describe the stages and results of meiosis I

Cardinal Takeaways

Cardinal Points

• Meiosis is preceded by interphase which consists of the 1000₁ stage (growth), the S stage ( Dna replication), and the G₂ phase.
• During prophase I, the homologous chromosomes condense and become visible every bit the 10 shape nosotros know, pair upwards to form a tetrad, and exchange genetic cloth past crossing over.
• During prometaphase I, microtubules attach at the chromosomes' kinetochores and the nuclear envelope breaks down.
• In metaphase I, the tetrads line themselves up at the metaphase plate and homologous pairs orient themselves randomly.
• In anaphase I, centromeres intermission downward and homologous chromosomes carve up.
• In telophase I, chromosomes motion to contrary poles; during cytokinesis the prison cell separates into ii haploid cells.

Cardinal Terms

• crossing over: the exchange of genetic material betwixt homologous chromosomes that results in recombinant chromosomes
• tetrad: two pairs of sis chromatids (a dyad pair) aligned in a sure way and often on the equatorial plane during the meiosis process
• chromatid: either of the two strands of a chromosome that separate during meiosis

Meiosis I

Meiosis is preceded by an interphase consisting of 3 stages. The M₁ phase (also called the starting time gap phase) initiates this stage and is focused on cell growth. The South phase is next, during which the Deoxyribonucleic acid of the chromosomes is replicated. This replication produces two identical copies, called sister chromatids, that are held together at the centromere past cohesin proteins. The centrosomes, which are the structures that organize the microtubules of the meiotic spindle, also replicate. Finally, during the 1000₂ phase (as well chosen the second gap phase), the jail cell undergoes the final preparations for meiosis.

Prophase I

During prophase I, chromosomes condense and become visible inside the nucleus. As the nuclear envelope begins to break down, homologous chromosomes move closer together. The synaptonemal complex, a lattice of proteins between the homologous chromosomes, forms at specific locations, spreading to comprehend the entire length of the chromosomes. The tight pairing of the homologous chromosomes is chosen synapsis. In synapsis, the genes on the chromatids of the homologous chromosomes are aligned with each other. The synaptonemal circuitous also supports the exchange of chromosomal segments between non-sister homologous chromatids in a process called crossing over. The crossover events are the first source of genetic variation produced by meiosis. A single crossover event betwixt homologous non-sis chromatids leads to an exchange of DNA betwixt chromosomes. Following crossover, the synaptonemal complex breaks downward and the cohesin connection betwixt homologous pairs is also removed. At the cease of prophase I, the pairs are held together only at the chiasmata; they are chosen tetrads because the four sister chromatids of each pair of homologous chromosomes are now visible.

Crossover betwixt homologous chromosomes: Crossover occurs betwixt non-sister chromatids of homologous chromosomes. The event is an exchange of genetic fabric between homologous chromosomes.

Synapsis holds pairs of homologous chromosomes together: Early on in prophase I, homologous chromosomes come together to form a synapse. The chromosomes are bound tightly together and in perfect alignment past a protein lattice chosen a synaptonemal complex and by cohesin proteins at the centromere.

Prometaphase I

The central result in prometaphase I is the formation of the spindle fiber apparatus where spindle fiber microtubules attach to the kinetochore proteins at the centromeres. Microtubules grow from centrosomes placed at opposite poles of the cell. The microtubules move toward the centre of the jail cell and attach to one of the two fused homologous chromosomes at the kinetochores. At the end of prometaphase I, each tetrad is attached to microtubules from both poles, with i homologous chromosome facing each pole. In addition, the nuclear membrane has cleaved down entirely.

Metaphase I

During metaphase I, the tetrads move to the metaphase plate with kinetochores facing opposite poles. The homologous pairs orient themselves randomly at the equator. This event is the 2nd mechanism that introduces variation into the gametes or spores. In each cell that undergoes meiosis, the arrangement of the tetrads is different. The number of variations is dependent on the number of chromosomes making upward a set. There are two possibilities for orientation at the metaphase plate. The possible number of alignments, therefore, equals 2n, where n is the number of chromosomes per gear up. Given these two mechanisms, it is highly unlikely that any two haploid cells resulting from meiosis will accept the aforementioned genetic composition.

Meiosis I ensures unique gametes: Random, independent assortment during metaphase I can exist demonstrated by considering a cell with a gear up of 2 chromosomes (due north = 2). In this case, there are ii possible arrangements at the equatorial airplane in metaphase I. The total possible number of different gametes is 2n, where due north equals the number of chromosomes in a set. In this case, there are four possible genetic combinations for the gametes. With due north = 23 in man cells, there are over eight one thousand thousand possible combinations of paternal and maternal chromosomes.

Anaphase I

In anaphase I, the microtubules pull the fastened chromosomes apart. The sister chromatids remain tightly bound together at the centromere. The chiasmata are broken in anaphase I as the microtubules fastened to the fused kinetochores pull the homologous chromosomes apart.

Telophase I and Cytokinesis

In telophase I, the separated chromosomes arrive at reverse poles. In some organisms, the chromosomes decondense and nuclear envelopes class around the chromatids in telophase I. And then cytokinesis, the physical separation of the cytoplasmic components into two daughter cells, occurs without reformation of the nuclei. In nearly all species of animals and some fungi, cytokinesis separates the jail cell contents via a cleavage furrow (constriction of the actin ring that leads to cytoplasmic division). In plants, a cell plate is formed during cell cytokinesis past Golgi vesicles fusing at the metaphase plate. This cell plate volition ultimately atomic number 82 to the formation of jail cell walls that separate the two daughter cells.

Two haploid cells are the finish effect of the showtime meiotic division. The cells are haploid because at each pole at that place is just one of each pair of the homologous chromosomes. Therefore, only one full set of the chromosomes is present. Although there is only i chromosome fix, each homolog still consists of ii sis chromatids.

Meiosis II

During meiosis II, the sister chromatids within the two daughter cells divide, forming four new haploid gametes.

Learning Objectives

Describe the stages and results of Meiosis Ii

Cardinal Takeaways

Fundamental Points

• During prophase Ii, chromsomes condense once again, centrosomes that were duplicated during interphase I movement abroad from each other toward opposite poles, and new spindles are formed.
• During prometaphase Two, the nuclear envelopes are completely broken down, and each sister chromatid forms an individual kinetochore that attaches to microtubules from contrary poles.
• During metaphase Ii, sister chromatids are condensed and aligned at the equator of the jail cell.
• During anaphase Two sister chromatids are pulled apart by the kinetochore microtubules and move toward opposite poles.
• During telophase Ii and cytokinesis, chromosomes arrive at opposite poles and begin to decondense; the two cells divide into 4 unique haploid cells.

Central Terms

• meiosis II: the second part of the meiotic procedure; the finish upshot is production of 4 haploid cells from the two haploid cells produced in meiosis I

Meiosis 2

Meiosis II initiates immediately after cytokinesis, usually earlier the chromosomes accept fully decondensed. In contrast to meiosis I, meiosis Ii resembles a normal mitosis. In some species, cells enter a cursory interphase, or interkinesis, earlier entering meiosis II. Interkinesis lacks an S phase, so chromosomes are non duplicated. The two cells produced in meiosis I go through the events of meiosis II together. During meiosis II, the sister chromatids within the two daughter cells separate, forming four new haploid gametes. The mechanics of meiosis II is similar to mitosis, except that each dividing cell has only one set of homologous chromosomes.

Prophase Ii

If the chromosomes decondensed in telophase I, they condense again. If nuclear envelopes were formed, they fragment into vesicles. The centrosomes that were duplicated during interphase I move away from each other toward contrary poles and new spindles are formed.

Prometaphase II

The nuclear envelopes are completely broken downwards and the spindle is fully formed. Each sister chromatid forms an private kinetochore that attaches to microtubules from opposite poles.

Metaphase II

The sister chromatids are maximally condensed and aligned at the equator of the jail cell.

Anaphase II

The sister chromatids are pulled apart by the kinetochore microtubules and move toward contrary poles. Non-kinetochore microtubules elongate the cell.

Meiosis I vs. Meiosis Two: The procedure of chromosome alignment differs between meiosis I and meiosis II. In prometaphase I, microtubules adhere to the fused kinetochores of homologous chromosomes, and the homologous chromosomes are arranged at the midpoint of the cell in metaphase I. In anaphase I, the homologous chromosomes are separated. In prometaphase 2, microtubules adhere to the kinetochores of sister chromatids, and the sis chromatids are bundled at the midpoint of the cells in metaphase Two. In anaphase II, the sister chromatids are separated.

Telophase 2 and Cytokinesis

The chromosomes arrive at opposite poles and begin to decondense. Nuclear envelopes form effectually the chromosomes. Cytokinesis separates the two cells into four unique haploid cells. At this signal, the newly-formed nuclei are both haploid. The cells produced are genetically unique because of the random assortment of paternal and maternal homologs and because of the recombining of maternal and paternal segments of chromosomes (with their sets of genes) that occurs during crossover.

Complete Stages of Meiosis: An brute cell with a diploid number of four (2n = 4) gain through the stages of meiosis to form four haploid girl cells.

Comparison Meiosis and Mitosis

Mitosis and meiosis share some similarities, but also some differences, most of which are observed during meiosis I.

Learning Objectives

Compare and contrast mitosis and meiosis

Key Takeaways

Cardinal Points

• For the most part, in mitosis, diploid cells are partitioned into 2 new diploid cells, while in meiosis, diploid cells are partitioned into four new haploid cells.
• In mitosis, the daughter cells accept the aforementioned number of chromosomes equally the parent cell, while in meiosis, the daughter cells have half the number of chromosomes equally the parent.
• The girl cells produced past mitosis are identical, whereas the girl cells produced past meiosis are different because crossing over has occurred.
• The events that occur in meiosis only non mitosis include homologous chromosomes pairing up, crossing over, and lining up along the metaphase plate in tetrads.
• Meiosis 2 and mitosis are not reduction division like meiosis I considering the number of chromosomes remains the same; therefore, meiosis II is referred to as equatorial partition.
• When the homologous chromosomes separate and movement to reverse poles during meiosis I, the ploidy level is reduced from ii to one, which is referred to equally a reduction division.

Central Terms

• reduction division: the showtime of the two divisions of meiosis, a type of prison cell partitioning
• ploidy: the number of homologous sets of chromosomes in a cell
• equatorial partition: a procedure of nuclear segmentation in which each chromosome divides as such that the number of chromosomes remains the same from parent to daughter cells

Comparison Meiosis and Mitosis

Mitosis and meiosis are both forms of division of the nucleus in eukaryotic cells. They share some similarities, but also exhibit singled-out differences that atomic number 82 to very different outcomes. The purpose of mitosis is jail cell regeneration, growth, and asexual reproduction,while the purpose of meiosis is the production of gametes for sexual reproduction. Mitosis is a single nuclear division that results in two nuclei that are usually partitioned into two new girl cells. The nuclei resulting from a mitotic division are genetically identical to the original nucleus. They have the aforementioned number of sets of chromosomes, 1 prepare in the case of haploid cells and 2 sets in the case of diploid cells. In about plants and all animal species, it is typically diploid cells that undergo mitosis to form new diploid cells. In dissimilarity, meiosis consists of 2 nuclear divisions resulting in 4 nuclei that are usually partitioned into iv new haploid daughter cells. The nuclei resulting from meiosis are not genetically identical and they contain one chromosome set just. This is half the number of chromosome sets in the original cell, which is diploid.

Comparing Meiosis and Mitosis: Meiosis and mitosis are both preceded past one round of Deoxyribonucleic acid replication; nonetheless, meiosis includes two nuclear divisions. The four daughter cells resulting from meiosis are haploid and genetically distinct. The daughter cells resulting from mitosis are diploid and identical to the parent jail cell.

The main differences between mitosis and meiosis occur in meiosis I. In meiosis I, the homologous chromosome pairs become associated with each other and are spring together with the synaptonemal circuitous. Chiasmata develop and crossover occurs between homologous chromosomes, which so line up along the metaphase plate in tetrads with kinetochore fibers from opposite spindle poles attached to each kinetochore of a homolog in a tetrad. All of these events occur only in meiosis I.

When the tetrad is cleaved upwardly and the homologous chromosomes move to contrary poles, the ploidy level is reduced from two to 1. For this reason, meiosis I is referred to as a reduction division. There is no such reduction in ploidy level during mitosis.

Meiosis Two is much more similar to a mitotic division. In this case, the duplicated chromosomes (but one prepare, equally the homologous pairs have now been separated into two different cells) line upwards on the metaphase plate with divided kinetochores attached to kinetochore fibers from opposite poles. During anaphase II and mitotic anaphase, the kinetochores divide and sister chromatids, now referred to as chromosomes, are pulled to contrary poles. The ii daughter cells of mitosis, withal, are identical, unlike the daughter cells produced past meiosis. They are unlike considering there has been at least one crossover per chromosome. Meiosis II is not a reduction partition because, although at that place are fewer copies of the genome in the resulting cells, in that location is nevertheless one gear up of chromosomes, as there was at the terminate of meiosis I. Meiosis II is, therefore, referred to as equatorial division.

reuterunclachis.blogspot.com

Source: https://courses.lumenlearning.com/boundless-biology/chapter/the-process-of-meiosis/

Share this post