Is It True That Nucleus Divides in Meiosis I and Again in Meiosis Ii
What Is Meiosis?
All cells arise from other cells through the process of jail cell division. Meiosis is a specialized class of jail cell division that produces reproductive cells, such as constitute and fungal spores and sperm and egg cells.
In general, this procedure involves a "parent" cell splitting into two or more "daughter" cells. In this way, the parent cell can pass on its genetic textile from generation to generation.
Eukaryotic cells and their chromosomes
Based on the relative complexity of their cells, all living organisms are broadly classified as either prokaryotes or eukaryotes. Prokaryotes, such as bacteria, consist of a unmarried cell with a uncomplicated internal structure. Their DNA floats freely within the cell in a twisted, thread-like mass called the nucleoid.
Animals, plants and fungi are all eukaryotes. Eukaryotic cells have specialized components called organelles, such as mitochondria, chloroplasts and the endoplasmic reticulum. Each of these performs a specific role. Unlike prokaryotes, eukaryotic DNA is packed within a central compartment called the nucleus.
Inside the eukaryotic nucleus, long double-helical strands of DNA are wrapped tightly effectually proteins called histones. This forms a rod-like structure called the chromosome.
Cells in the human body have 23 pairs of chromosomes, or 46 in total. This includes 2 sex chromosomes: two Ten chromosomes for females and one 10 and one Y chromosome for males. Because each chromosome has a pair, these cells are chosen "diploid" cells.
On the other hand, homo sperm and egg cells accept just 23 chromosomes, or one-half the chromosomes of a diploid cell. Thus, they are called "haploid" cells.
When the sperm and egg combine during fertilization, the total chromosome number is restored. That'south considering sexually reproducing organisms receive a set of chromosomes from each parent: a maternal and paternal fix. Each chromosome has a respective pair, orhomolog.
Mitosis vs. meiosis
Eukaryotes are capable of ii types of cell division: mitosis and meiosis
Mitosis allows for cells to produce identical copies of themselves, which means the genetic material is duplicated from parent to daughter cells. Mitosis produces two girl cells from one parent prison cell.
Single-celled eukaryotes, such as amoeba and yeast, employ mitosis to reproduce asexuallyand increment their population. Multicellular eukaryotes, similar humans, use mitosis to abound or heal injured tissues.
Meiosis, on the other mitt, is a specialized class of cell division that occurs in organisms that reproduce sexually. As mentioned higher up, it produces reproductive cells, such as sperm cells, egg cells, and spores in plants and fungi.
In humans, special cells called germ cells undergo meiosis and ultimately give rising to sperm or eggs. Germ cells contain a complete set of 46 chromosomes (23 maternal chromosomes and 23 paternal chromosomes). By the end of meiosis, the resulting reproductive cells, or gametes, each have 23 genetically unique chromosomes.
The overall process of meiosis produces four daughter cells from 1 unmarried parent cell. Each daughter cell is haploid, because it has half the number of chromosomes as the original parent prison cell.
"Meiosis is reductional," said Yard. Andrew Hoyt, a biologist and professor at Johns Hopkins University.
Unlike in mitosis, the daughter cells produced during meiosis are genetically various. Homologous chromosomes substitution bits of Deoxyribonucleic acid to create genetically unique, hybrid chromosomes destined for each daughter prison cell.
A closer look at meiosis
Before meiosis begins, some important changes take identify within the parent cells. First, each chromosome creates a copy of itself. These duplicated chromosomes are known every bit sis chromatids. They are fused together and the signal where they are joined is known as the centromere. Fused sister chromatids roughly resemble the shape of the letter "X."
Meiosis occurs over the course of two rounds of nuclear divisions, chosen meiosis I and meiosis II, according to Nature Education's Scitable. Furthermore, meiosis I and Ii are each divided into four major stages: prophase, metaphase, anaphase and telophase.
Meiosis I is responsible for creating genetically unique chromosomes. Sister chromatids pair upwardly with their homologs and exchange genetic cloth with one another. At the end of this sectionalisation, one parent prison cell produces two girl cells, each conveying i set of sister chromatids.
Meiosis 2 closely resembles mitosis. The two daughter cells move into this phase without any further chromosome duplication. The sis chromatids are pulled apart during this partition. A total of four haploid girl cells are produced during the grade of meiosis Ii.
Meiosis I
The four stages of meiosis Iare as follows, co-ordinate to "Molecular Biology of the Prison cell." (Garland Science, 2002):
Prophase I: At this stage, chromosomes go compact, dense structures and are easily visible under the microscope. The homologous chromosomes pair together. The two sets of sister chromatids resemble two X's lined up side by side to each other. Each set exchanges bits of Dna with the other and recombines, thus creating genetic variation. This process is known equally crossing over, or recombination.
Even though in humans the male person sexual activity chromosomes (10 and Y) are not verbal homologs, they tin still pair together and exchange Dna. Crossing over occurs inside simply a small region of the 2 chromosomes.
By the end of prophase I, the nuclear membrane breaks downwardly.
Metaphase I: The meiotic spindle, a network of poly peptide filaments, emerges from two structures called the centrioles, positioned at either end of the jail cell. The meiotic spindle latches onto the fused sister chromatids. Past the finish of metaphase I, all the fused sister chromatids are tethered at their centromeres and line upwards in the heart of the jail cell. The homologs nonetheless look similar two 10'southward sitting close together.
Anaphase I: The spindle fibers showtime to contract, pulling the fused sister chromatids with them. Each 10-shaped complex moves away from the other, toward reverse ends of the prison cell.
Telophase I: The fused sister chromatids attain either end of the cell, and the cell body splits into 2.
Meiosis I results in 2 girl cells, each of which contains a set up of fused sister chromatids. The genetic makeup of each daughter cell is distinct because of the DNA commutation betwixt homologs during the crossing-over procedure.
Meiosis Two
"Meiosis II looks like mitosis," Hoyt told Alive Scientific discipline. "Information technology's an equational sectionalisation."
In other words, by the end of the procedure, the chromosome number is unchanged between the cells that enter meiosis Two and the resulting daughter cells.
The four stages of meiosis 2 are as follows, according to "Molecular Biology of the Cell, 4th edition."
Prophase 2: The nuclear membrane disintegrates, and meiotic spindles brainstorm to form once over again.
Metaphase II: The meiotic spindles latch onto the centromere of the sister chromatids, and they all line up at the center of the cell.
Anaphase II: The spindle fibers start to contract and pull the sister chromatids apart. Each individual chromosome now begins to moves to either terminate of the jail cell.
Telophase II: The chromosomes achieve opposite ends of the cell. The nuclear membrane forms once more, and the prison cell body splits into two
Meiosis Two results in 4 haploid girl cells, each with the same number of chromosomes. However, each chromosome is unique and contains a mix of genetic information from the maternal and paternal chromosomes in the original parent cell.
Why is meiosis of import?
Proper "chromosomal segregation," or the separation of sister chromatids during meiosis I and II is essential for generating good for you sperm and egg cells, and by extension, healthy embryos. If chromosomes fail to segregate completely, it's chosen nondisjunction and tin result in the formation of gametes that take missing or actress chromosomes, according to "Molecular Biology of the Cell, 4th edition."
When gametes with abnormal chromosome numbers fertilize, near of the resulting embryos don't survive. However, not all chromosomal abnormalities are fatal to the embryo. For example, Down's syndrome occurs as a issue of having an actress copy of chromosome 21. And, people with Klinefelter syndrome are genetically male but take an extra X chromosome.
The most significant bear upon of meiosis is that it generates genetic diversity, and that'southward a major advantage for species survival.
"Shuffling the genetic information allows you to discover new combinations which will perhaps be more fit in the real world," Hoyt said.
Source: https://www.livescience.com/52489-meiosis.html
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