stages of meiosis 1 and 2 pdf Monday, March 8, 2021 2:47:06 PM

Stages Of Meiosis 1 And 2 Pdf

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Walker, Michelle Y. Hawley, R.

Skip navigation. Meiosis, the process by which sexually reproducing organisms generate gametes sex cells , is an essential precondition for the normal formation of the embryo. As sexually-reproducing, diploid, multicellular eukaryotes, humans rely on meiosis to serve a number of important functions, including the promotion of genetic diversity and the creation of proper conditions for reproductive success.

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Meiosis is the nuclear division of diploid cells into haploid cells, which is a necessary step in sexual reproduction. The ability to reproduce in kind is a basic characteristic of all living things. In kind means 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 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 cycle is to continue, then the diploid cell must somehow reduce its number of chromosome sets before fertilization can occur again or there will 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 means 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 emerge b. Adult 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 cell called a zygote.

All animals and most plants produce these gametes, or eggs and sperm. In most plants and animals, through tens of rounds of mitotic cell division, this diploid cell will develop into an adult organism. Haploid cells that are part of the sexual reproductive cycle are produced by a type of cell division called meiosis. Meiosis employs many of the same mechanisms as mitosis.

However, the starting nucleus is always diploid and the nuclei that result at the end of a meiotic cell division are haploid, so the resulting cells have half the chromosomes as the original. To achieve this reduction in chromosomes, meiosis consists of one round of chromosome duplication and two 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. In meiosis I, the first round of meiosis, homologous chromosomes exchange DNA and the diploid cell is divided into two haploid cells.

Meiosis is preceded by an interphase consisting of three stages. The G 1 phase also called the first gap phase initiates this stage and is focused on cell growth. The S phase is next, during which the DNA of the chromosomes is replicated. This replication produces two identical copies, called sister 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, also replicate. Finally, during the G 2 phase also called the second gap phase , the cell undergoes the final preparations for meiosis.

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 cover the entire length of the chromosomes. The tight pairing of the homologous chromosomes is called synapsis. In synapsis, the genes on the chromatids of the homologous chromosomes are aligned with each other. The synaptonemal complex 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 between homologous non-sister chromatids leads to an exchange of DNA between chromosomes. Following crossover, the synaptonemal complex breaks down and the cohesin connection between homologous pairs is also removed. At the end of prophase I, the pairs are held together only at the chiasmata; they are called tetrads because the four sister chromatids of each pair of homologous chromosomes are now visible.

Crossover between homologous chromosomes : Crossover occurs between non-sister chromatids of homologous chromosomes. The result 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 form 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.

The key event 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 middle of the 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. 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 second 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. 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 set.

Given these two mechanisms, it is highly unlikely that any two haploid cells resulting from meiosis will have the same genetic composition.

In this case, there are two possible arrangements at the equatorial plane in metaphase I. The total possible number of different gametes is 2n, where n equals the number of chromosomes in a set. In this example, there are four possible genetic combinations for the gametes. In anaphase I, the microtubules pull the attached chromosomes apart. The sister chromatids remain tightly bound together at the centromere. The chiasmata are broken in anaphase I as the microtubules attached to the fused kinetochores pull the homologous chromosomes apart.

In telophase I, the separated chromosomes arrive at opposite poles. In some organisms, the chromosomes decondense and nuclear envelopes form around the chromatids in telophase I. 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 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 by Golgi vesicles fusing at the metaphase plate.

This cell plate will ultimately lead to the formation of cell walls that separate the two daughter cells. Two haploid cells are the end result of the first meiotic division.

The cells are haploid because at each pole there is just one of each pair of the homologous chromosomes. Therefore, only one full set of the chromosomes is present. Although there is only one chromosome set, each homolog still consists of two sister chromatids. During meiosis II, the sister chromatids within the two daughter cells separate, forming four new haploid gametes.

Meiosis II initiates immediately after cytokinesis, usually before the chromosomes have fully decondensed. In contrast to meiosis I, meiosis II resembles a normal mitosis. In some species, cells enter a brief interphase, or interkinesis, before entering meiosis II. Interkinesis lacks an S phase, so chromosomes are not duplicated. The two cells produced in meiosis I go through the events of meiosis II together.

The mechanics of meiosis II is similar to mitosis, except that each dividing cell has only one set of homologous chromosomes. 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 opposite poles and new spindles are formed. 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 opposite poles. The sister chromatids are pulled apart by the kinetochore microtubules and move toward opposite poles. Non-kinetochore microtubules elongate the cell. Meiosis I vs. 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 sister chromatids, and the sister chromatids are arranged at the midpoint of the cells in metaphase II. In anaphase II, the sister chromatids are separated.

The chromosomes arrive 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 because of the recombining of maternal and paternal segments of chromosomes with their sets of genes that occurs during crossover.

Mitosis and meiosis share some similarities, but also some differences, most of which are observed during meiosis I. Mitosis and meiosis are both forms of division of the nucleus in eukaryotic cells.

They share some similarities, but also 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.

Meiosis in Humans

If you're seeing this message, it means we're having trouble loading external resources on our website. To log in and use all the features of Khan Academy, please enable JavaScript in your browser. Donate Login Sign up Search for courses, skills, and videos. Fertilization terminology: gametes, zygotes, haploid, diploid. Chromosomal crossover in meiosis I.

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This learning material is about the life cycle of a cell and the series of stages by which two successive nuclear and cellular divisions (Meiosis I and Meiosis II).


Meiosis in Humans

Sexual reproduction requires the production of haploid gametes sperm and egg with only one copy of each chromosome; fertilization then restores the diploid chromosome content in the next generation. This reduction in genetic content is accomplished during a specialized cell division called meiosis, in which two rounds of chromosome segregation follow a single round of DNA replication. In preparation for the first meiotic division, homologous chromosomes pair and synapse, creating a context that promotes formation of crossover recombination events. These crossovers, in conjunction with sister chromatid cohesion, serve to connect the two homologs and facilitate their segregation to opposite poles during the first meiotic division. During the second meiotic division, which is similar to mitosis, sister chromatids separate; the resultant products are haploid cells that become gametes.

The different stages of meiosis occurred in various frequencies in individual anthers. When anthers reach a certain stage of development the range of meiotic stages increases markedly. It is suggested that the rate of meiosis is altered, and variable among individual microsporocytes. The multipolar spindles were generally tripolar.

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1 Comments

Lafgumddenxy 09.03.2021 at 02:06

Meiosis is part of the sexual process because gametes (sperm, eggs) have one half the chromosomes as diploid (2N) individuals. Phases of Meiosis. There are.

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