The Nucleus Divides in Meiosis I and Again in Meiosis Ii True or False
Recap: What is Meiosis?
Meiosis is how eukaryotic cells (plants, animals, and fungi) reproduce sexually. It is a process of chromosomal reduction, which means that a diploid jail cell (this ways a cell with 2 complete and identical chromosome sets) is reduced to course haploid cells (these are cells with just i chromosome set). The haploid cells produced by meiosis are germ cells, also known as gametes, sex cells or spores in plants and fungi. These are essential for sexual reproduction: ii germ cells combine to class a diploid zygote, which grows to form another functional adult of the same species.
The procedure of chromosomal reduction is important in the conservation of the chromosomal number of a species. If chromosome numbers were not reduced, and a diploid germ cell was produced by each parent, then the resulting offspring would take a tetraploid chromosome gear up: that is, information technology would have iv identical sets of chromosomes. This number would keep increasing with each generation. This is why the chromosomal reduction is vital for the continuation of each species.
Meiosis occurs in two singled-out phases: meiosis I and meiosis II. At that place are many similarities and differences between these phases, with each phase producing different products and each phase beingness equally crucial to the production of viable germ cells.
What Happens Before Meiosis?
Before meiosis, the chromosomes in the nucleus of the cell replicate to produce double the amount of chromosomal material. After chromosomal replication, chromosomes split up into sister chromatids. This is known every bit interphase, and tin be further broken down into two phases in the meiotic cycle: Growth (M), and Synthesis (Due south). During the Thou stage proteins and enzymes necessary for growth are synthesized, while during the S phase chromosomal material is doubled.
Meiosis is then split up into ii phases: meiosis I and meiosis Two. In each of these phases, there is a prophase, a metaphase, and anaphase and a telophase. In meiosis I these are known as prophase I, metaphase I, anaphase I and telophase I, while in meiosis Two they are known as prophase II, metaphase II, anaphase Ii and telophase 2. Unlike products are formed by these phases, although the basic principles of each are the same. Also, meiosis I is preceded in interphase by both G phase and S phase, while meiosis 2 is only preceded past S phase: chromosomal replication is non necessary again.
The Phases of Meiosis I
After Interphase I meiosis I occurs subsequently Interphase I, where proteins are grown in K phase and chromosomes are replicated in South phase. Following this, four phases occur. Meiosis I is known every bit reductive partitioning, as the cells are reduced from being diploid cells to being haploid cells.
1. Prophase I
Prophase I is the longest phase of meiosis, with 3 main events occurring. The starting time is the condensation of chromatin into chromosomes that can be seen through the microscope; the second is the synapsis or physical contact between homologous chromosomes; and the crossing over of genetic material between these synapsed chromosomes. These events occur in v sub-phases:
- Leptonema– The first prophase consequence occurs: chromatin condenses to form visible chromosomes. Condensation and coiling of chromosomes occur.
- Zygonema– Chromosomes line upwardly to form homologous pairs, in a process known as the homology search. These pairs are also known as bivalents. Synapsis happens when the homologous pairs join. The synaptonemal circuitous forms.
- Pachynema– The 3rd master issue of prophase I occurs: crossing over. Nonsister chromatids of homologous chromosome pairs exchange parts or segments. Chiasmata form where these exchanges have occurred. Each chromosome is now different to its parent chromosome but contains the aforementioned amount of genetic material.
- Diplonema– The synaptonemal circuitous dissolves and chromosome pairs begin to separate. The chromosomes uncoil slightly to permit DNA transcription.
- Diakinesis – Chromosome condensation is furthered. Homologous chromosomes separate further but are still joined by a chiasmata, which moves towards the ends of the chromatids in a process referred to every bit terminalization. The nuclear envelope and nucleoli disintegrate, and the meiotic spindle begins to form. Microtubules attach to the chromosomes at the kinetochore of each sister chromatid.
2. Metaphase I
Homologous pairs of chromosomes align on the equatorial airplane at the eye of the cell. Independent assortment determines the orientation of each bivalent merely ensures that half of each chromosome pair is oriented to each pole. This is to ensure that homologous chromosomes exercise not finish upward in the aforementioned cell. The arms of the sister chromatids are convergent.
3. Anaphase I
Microtubules begin to shorten, pulling i chromosome of each homologous pair to opposite poles in a process known every bit disjunction. The sis chromatids of each chromosome stay connected. The prison cell begins to elongate in preparation for cytokinesis.
4. Telophase I
Meiosis I ends when the chromosomes of each homologous pair arrive at opposing poles of the jail cell. The microtubules disintegrate, and a new nuclear membrane forms around each haploid set of chromosomes. The chromosomes uncoil, forming chromatin once again, and cytokinesis occurs, forming two non-identical daughter cells. A resting phase known as interkinesis or interphase II happens in some organisms.
The Phases of Meiosis 2
Meiosis Ii may begin with interkinesis or interphase Ii. This differs from interphase I in that no Due south phase occurs, as the Deoxyribonucleic acid has already been replicated. Thus only a K stage occurs. Meiosis Two is known as equational division, equally the cells begin as haploid cells and end as haploid cells. There are again four phases in meiosis II: these differ slightly from those in meiosis I.
1. Prophase II
Chromatin condenses to form visible chromosomes again. The nuclear envelope and nucleolus disintegrate, and spindle fibers begin to appear. No crossing over occurs.
ii. Metaphase II
Spindle fibers connect to the kinetochore of each sister chromatid. The chromosomes align at the equatorial plane, which is rotated ninety° compared to the equatorial plane in meiosis I. One sister chromatid faces each pole, with the arms divergent.
3. Anaphase II
The spindle fibers continued to each sister chromatid shorten, pulling 1 sister chromatid to each pole. Sis chromatids are known as sis chromosomes from this point.
4. Telophase II
Meiosis II ends when the sister chromosomes accept reached opposing poles. The spindle disintegrates, and the chromosomes recoil, forming chromatin. A nuclear envelope forms around each haploid chromosome set, before cytokinesis occurs, forming two girl cells from each parent cell, or four haploid daughter cells in full.
Effigy one. The phases of meiosis I and meiosis II, showing the formation of four haploid cells from a unmarried diploid cell.
How is Meiosis I Different from Meiosis II?
Meiosis is the production of 4 genetically various haploid daughter cells from one diploid parent jail cell. Meiosis tin only occur in eukaryotic organisms. It is preceded by interphase, specifically the Grand phase of interphase. Both Meiosis I and II have the aforementioned number and arrangement of phases: prophase, metaphase, anaphase, and telophase. Both produce two girl cells from each parent prison cell.
Nonetheless, Meiosis I begins with i diploid parent cell and ends with two haploid girl cells, halving the number of chromosomes in each prison cell. Meiosis II starts with two haploid parent cells and ends with four haploid daughter cells, maintaining the number of chromosomes in each cell. Homologous pairs of cells are present in meiosis I and separate into chromosomes before meiosis II. In meiosis Two, these chromosomes are further separated into sister chromatids. Meiosis I includes crossing over or recombination of genetic material between chromosome pairs, while meiosis 2 does not. This occurs in meiosis I in a long and complicated prophase I, split into five sub-phases. The equatorial plane in meiosis II is rotated 90° from the alignment of the equatorial aeroplane in meiosis I.
The tabular array beneath summarizes the similarities and differences between meiosis I and meiosis Two.
Table 1. The similarities and differences between meiosis I and meiosis II.
| Meiosis I | Meiosis 2 |
Similarities | |
| Tin can only occur in eukaryotes | |
| G stage of interphase usually occurs first | |
| Production of daughter cells based on parent cell's genetic material | |
| Ways of sexual reproduction in plants, animals, and fungi | |
| 4 phases occur: prophase, metaphase, anaphase, telophase | |
Differences | |
| Starts equally diploid; ends every bit haploid | Starts as haploid; ends equally haploid |
| Reductive division | Equational division |
| Homologous chromosome pairs separate | Sister chromatids separate |
| Crossing over happens | Crossing over does non happen |
| Complicated sectionalization process | Simple division process |
| Long duration | Short duration |
| Preceded by S-phase and G-phase | Preceded just by 1000-phase |
| Sister chromatids in prophase have convergent arms | Sister chromatids in prophase take divergent arms |
| Equatorial plane is centered | Equatorial plane is rotated 90° |
| Prophase split into 5 sub-phases | Prophase does non have sub-phases |
| Ends with ii daughter cells | Ends with 4 daughter cells |
Why is Meiosis Of import?
Meiosis is essential for the sexual reproduction of eukaryotic organisms, the enabling of genetic diverseness through recombination, and the repair of genetic defects.
The crossing over or recombination of genes occurring in prophase I of meiosis I is vital to the genetic diverseness of a species. This provides a buffer against genetic defects, susceptibility to disease and survival of possible extinction events, equally there will e'er be certain individuals in a population better able to survive changes in environmental condition. Recombination further allows genetic defects to be masked or even replaced past good for you alleles in offspring of diseased parents.
Meiosis I and Meiosis Ii Biology Review
We now know that meiosis is the process of the production of haploid daughter cells from diploid parent cells, using chromosomal reduction. These daughter cells are genetically distinct from their parent cells due to the genetic recombination which occurs in meiosis I. This recombination is essential for genetic diversity inside the population and the correction of genetic defects.
Meiosis I and II are similar in some aspects, including the number and organisation of their phases and the production of two cells from a single cell. However, they also differ greatly, with meiosis I being reductive division and meiosis Ii existence equational partitioning. In this fashion, meiosis Two is more than similar to mitosis. Both stages of meiosis are important for the successful sexual reproduction of eukaryotic organisms.
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