Does Dna Replicate Itself Again for Meiosis 11 Does Dna Replicate Itself Again for Meiosis 2

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

Fundamental Points

• Sexual reproduction is the production of haploid cells and the fusion of two of those cells to form a diploid jail cell.
• Before sexual reproduction tin can occur, the number of chromosomes in a diploid prison cell must decrease by half.
• Meiosis produces cells with half the number of chromosomes as the original prison cell.
• Haploid cells used in sexual reproduction, gametes, are formed during meiosis, which consists of ane circular of chromosome replication and two rounds of nuclear sectionalisation.
• Meiosis I is the start round of meiotic division, while meiosis Ii is the 2nd circular.

Key Terms

• haploid: of a jail cell having a single set of unpaired chromosomes
• gamete: a reproductive cell, male (sperm) or female (egg), that has just half the usual number of chromosomes
• diploid: of a cell, having a pair of each type of chromosome, ane 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 feature of all living things. In kind ways that the offspring of any organism closely resemble their parent or parents. Sexual reproduction requires fertilization: the union of two cells from two individual organisms. Haploid cells incorporate 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 cycle is to go on, so the diploid cell must somehow reduce its number of chromosome sets before fertilization can occur again 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 any organism closely resemble their parent or parents. The hippopotamus gives birth to hippopotamus calves (a). Joshua trees produce seeds from which Joshua tree seedlings sally (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 unmarried, unique diploid cell called a zygote. All animals and almost plants produce these gametes, or eggs and sperm. In most plants and animals, through tens of rounds of mitotic prison cell division, this diploid cell volition develop into an adult organism.

Haploid cells that are function of the sexual reproductive wheel are produced by a type of cell division called meiosis. Meiosis employs many of the same mechanisms equally mitosis. However, the starting nucleus is e'er diploid and the nuclei that issue at the end of a meiotic cell segmentation are haploid, so the resulting cells take half the chromosomes every bit the original. To reach this reduction in chromosomes, meiosis consists of 1 round of chromosome duplication and two rounds of nuclear sectionalisation. Considering the events that occur during each of the sectionalisation stages are analogous to the events of mitosis, the same stage names are assigned. However, because there are two rounds of segmentation, the major procedure and the stages are designated with a "I" or a "2." Thus, meiosis I is the first round of meiotic division and consists of prophase I, prometaphase I, and and then on. Meiosis II, the 2nd round of meiotic division, includes prophase II, prometaphase II, and so on.

Meiosis I

In meiosis I, the first round of meiosis, homologous chromosomes exchange Deoxyribonucleic acid and the diploid prison cell is divided into two haploid cells.

Learning Objectives

Describe the stages and results of meiosis I

Key Takeaways

Key Points

• Meiosis is preceded by interphase which consists of the 1000₁ phase (growth), the South phase ( DNA replication), and the G₂ phase.
• During prophase I, the homologous chromosomes condense and go visible every bit the x shape we know, pair up to course a tetrad, and commutation genetic fabric by crossing over.
• During prometaphase I, microtubules adhere at the chromosomes' kinetochores and the nuclear envelope breaks down.
• In metaphase I, the tetrads line themselves upward at the metaphase plate and homologous pairs orient themselves randomly.
• In anaphase I, centromeres break down and homologous chromosomes separate.
• In telophase I, chromosomes move to opposite poles; during cytokinesis the cell separates into two haploid cells.

Central Terms

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

Meiosis I

Meiosis is preceded by an interphase consisting of three stages. The G₁ phase (also called the get-go gap phase) initiates this stage and is focused on cell growth. The S phase is next, during which the Deoxyribonucleic acid of the chromosomes is replicated. This replication produces 2 identical copies, called sis chromatids, that are held together at the centromere by cohesin proteins. The centrosomes, which are the structures that organize the microtubules of the meiotic spindle, besides replicate. Finally, during the Thousand_two phase (besides chosen the 2d gap phase), the prison cell undergoes the final preparations for meiosis.

Prophase I

During prophase I, chromosomes condense and go visible inside the nucleus. Equally the nuclear envelope begins to break down, homologous chromosomes movement closer together. The synaptonemal complex, a lattice of proteins betwixt the homologous chromosomes, forms at specific locations, spreading to cover 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 complex also supports the substitution of chromosomal segments between non-sister homologous chromatids in a process called crossing over. The crossover events are the commencement source of genetic variation produced by meiosis. A single crossover effect between homologous non-sister chromatids leads to an exchange of DNA between chromosomes. Post-obit crossover, the synaptonemal complex breaks down and the cohesin connection between homologous pairs is likewise removed. At the end of prophase I, the pairs are held together only at the chiasmata; they are called tetrads considering the four sister chromatids of each pair of homologous chromosomes are now visible.

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

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

Prometaphase I

The key event in prometaphase I is the formation of the spindle fiber appliance where spindle fiber microtubules attach to the kinetochore proteins at the centromeres. Microtubules grow from centrosomes placed at contrary 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 one homologous chromosome facing each pole. In addition, the nuclear membrane has broken down entirely.

Metaphase I

During metaphase I, the tetrads move to the metaphase plate with kinetochores facing reverse 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 up a set. In that location are ii possibilities for orientation at the metaphase plate. The possible number of alignments, therefore, equals 2n, where due north is the number of chromosomes per gear up. Given these two mechanisms, information technology is highly unlikely that any ii haploid cells resulting from meiosis volition have the same genetic composition.

Meiosis I ensures unique gametes: Random, independent assortment during metaphase I can be demonstrated by considering a cell with a set of two chromosomes (due north = 2). In this example, there are two possible arrangements at the equatorial airplane in metaphase I. The full possible number of different gametes is 2n, where due north equals the number of chromosomes in a gear up. In this example, there are 4 possible genetic combinations for the gametes. With n = 23 in human cells, there are over viii meg possible combinations of paternal and maternal chromosomes.

Anaphase I

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

Telophase I and Cytokinesis

In telophase I, the separated chromosomes make it at opposite poles. In some organisms, the chromosomes decondense and nuclear envelopes form around the chromatids in telophase I. So cytokinesis, the physical separation of the cytoplasmic components into two daughter cells, occurs without reformation of the nuclei. In most all species of animals and some fungi, cytokinesis separates the prison cell contents via a cleavage furrow (constriction of the actin ring that leads to cytoplasmic partitioning). In plants, a cell plate is formed during cell cytokinesis by Golgi vesicles fusing at the metaphase plate. This cell plate will ultimately lead to the germination of cell walls that split up the two girl cells.

2 haploid cells are the stop result of the offset meiotic sectionalisation. The cells are haploid because at each pole there is only i of each pair of the homologous chromosomes. Therefore, only i full set of the chromosomes is nowadays. Although there is simply 1 chromosome set, each homolog still consists of ii sister chromatids.

Meiosis Ii

During meiosis Ii, the sister chromatids within the two girl cells separate, forming iv new haploid gametes.

Learning Objectives

Draw the stages and results of Meiosis 2

Key Takeaways

Key Points

• During prophase II, chromsomes condense once again, centrosomes that were duplicated during interphase I motility away from each other toward reverse poles, and new spindles are formed.
• During prometaphase II, the nuclear envelopes are completely broken down, and each sister chromatid forms an private kinetochore that attaches to microtubules from reverse poles.
• During metaphase II, sister chromatids are condensed and aligned at the equator of the cell.
• During anaphase Ii sister chromatids are pulled apart past the kinetochore microtubules and motility toward opposite poles.
• During telophase II and cytokinesis, chromosomes arrive at opposite poles and begin to decondense; the 2 cells divide into four unique haploid cells.

Key Terms

• meiosis Ii: the second role of the meiotic process; the cease issue is production of four haploid cells from the two haploid cells produced in meiosis I

Meiosis II

Meiosis II initiates immediately subsequently cytokinesis, usually earlier the chromosomes have fully decondensed. In contrast to meiosis I, meiosis Two resembles a normal mitosis. In some species, cells enter a brief interphase, or interkinesis, earlier entering meiosis 2. Interkinesis lacks an Southward phase, so chromosomes are not duplicated. The two cells produced in meiosis I go through the events of meiosis Ii together. During meiosis Two, the sis 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 just one set of homologous chromosomes.

Prophase II

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

Prometaphase 2

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

Metaphase II

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

Anaphase II

The sis chromatids are pulled apart by the kinetochore microtubules and move toward opposite poles. Not-kinetochore microtubules elongate the prison cell.

Meiosis I vs. Meiosis II: The process of chromosome alignment differs between meiosis I and meiosis II. In prometaphase I, microtubules attach 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 II, microtubules attach to the kinetochores of sis chromatids, and the sister chromatids are arranged at the midpoint of the cells in metaphase Ii. In anaphase 2, the sis chromatids are separated.

Telophase II and Cytokinesis

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

Consummate Stages of Meiosis: An brute cell with a diploid number of four (2n = 4) proceeds through the stages of meiosis to grade four haploid daughter cells.

Comparing Meiosis and Mitosis

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

Learning Objectives

Compare and dissimilarity mitosis and meiosis

Key Takeaways

Key Points

• For the most function, in mitosis, diploid cells are partitioned into two new diploid cells, while in meiosis, diploid cells are partitioned into iv new haploid cells.
• In mitosis, the girl cells have the same number of chromosomes as the parent cell, while in meiosis, the daughter cells accept one-half the number of chromosomes as the parent.
• The daughter cells produced by mitosis are identical, whereas the daughter cells produced past meiosis are dissimilar because crossing over has occurred.
• The events that occur in meiosis just non mitosis include homologous chromosomes pairing up, crossing over, and lining upwards forth the metaphase plate in tetrads.
• Meiosis II and mitosis are not reduction partition like meiosis I considering the number of chromosomes remains the same; therefore, meiosis Ii is referred to as equatorial segmentation.
• When the homologous chromosomes carve up and move to opposite poles during meiosis I, the ploidy level is reduced from two to one, which is referred to as a reduction partitioning.

Key Terms

• reduction division: the starting time of the 2 divisions of meiosis, a type of cell partitioning
• ploidy: the number of homologous sets of chromosomes in a jail cell
• equatorial division: a procedure of nuclear segmentation in which each chromosome divides equally such that the number of chromosomes remains the same from parent to daughter cells

Comparing Meiosis and Mitosis

Mitosis and meiosis are both forms of division of the nucleus in eukaryotic cells. They share some similarities, but too exhibit distinct differences that lead to very different outcomes. The purpose of mitosis is cell regeneration, growth, and asexual reproduction,while the purpose of meiosis is the production of gametes for sexual reproduction. Mitosis is a unmarried nuclear division that results in ii nuclei that are usually partitioned into 2 new daughter cells. The nuclei resulting from a mitotic division are genetically identical to the original nucleus. They have the aforementioned number of sets of chromosomes, one set in the case of haploid cells and ii sets in the instance of diploid cells. In most plants and all creature species, information technology is typically diploid cells that undergo mitosis to form new diploid cells. In contrast, meiosis consists of two nuclear divisions resulting in four nuclei that are usually partitioned into four new haploid daughter cells. The nuclei resulting from meiosis are not genetically identical and they contain one chromosome gear up merely. 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 i round of DNA replication; however, meiosis includes ii nuclear divisions. The iv girl cells resulting from meiosis are haploid and genetically distinct. The daughter cells resulting from mitosis are diploid and identical to the parent cell.

The principal differences between mitosis and meiosis occur in meiosis I. In meiosis I, the homologous chromosome pairs go associated with each other and are bound together with the synaptonemal circuitous. Chiasmata develop and crossover occurs between homologous chromosomes, which and so line upwards along the metaphase plate in tetrads with kinetochore fibers from contrary 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 broken upwardly and the homologous chromosomes movement to opposite poles, the ploidy level is reduced from two to ane. For this reason, meiosis I is referred to as a reduction division. There is no such reduction in ploidy level during mitosis.

Meiosis 2 is much more similar to a mitotic partitioning. In this case, the duplicated chromosomes (only 1 fix, every bit the homologous pairs have now been separated into two different cells) line up 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 every bit chromosomes, are pulled to opposite poles. The 2 daughter cells of mitosis, withal, are identical, dissimilar the daughter cells produced by meiosis. They are different because there has been at least one crossover per chromosome. Meiosis Two is not a reduction sectionalisation considering, although there are fewer copies of the genome in the resulting cells, there is still ane set of chromosomes, as there was at the terminate of meiosis I. Meiosis II is, therefore, referred to as equatorial division.

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

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