Key Differences Between Meiosis I and Meiosis II
Meiosis is split into meiosis I and meiosis II which are further divided into Karyokinesis I and Cytokinesis I and Karyokinesis II and Cytokinesis II respectively. The preparatory steps that initiate meiosis are identical in pattern and name to interphase of the mitotic cell cycle. Interphase is split into three phases: Growth 1 (G1) phase: during this very active phase, the cell synthesizes its vast array of proteins, including the enzymes and structural proteins it'll need for growth. In G1, each of the chromosomes consists of one linear molecule of DNA. In the Synthesis (S) phase: each of the cell's chromosomes duplicates to become two identical sister chromatids attached at a centromere and the genetic material is replicated.
History & Discovery
Meiosis was discovered and described for the primary time in echinoderm eggs in 1876 by the German biologist Oscar Hertwig. It was described again in 1883, at the extent of chromosomes, by the Belgian zoologist Edouard Van Beneden, in Ascaris roundworm eggs. In 1890 a German biologist August Weismann described the significance of meiosis for copy and inheritance, he noted that if the number of chromosomes had to be maintained then two cell divisions were compulsory to transform one diploid cell into four haploid cells. In 1911, the American geneticist Thomas Hunt Morgan detected crossovers in meiosis within the pomace fly drosophila, which helped to determine that genetic traits are transmitted on chromosomes. The term "meiosis" (originally spelled "miosis") springs from the Greek word μείωσις, meaning 'lessening'. It was introduced to biology by J.B. Farmer and J.E.S. Moore in 1905:We propose to use the terms Meiosis or Meiotic phase to hide the entire series of nuclear changes included within the two divisions that were designated as Heterotypic and Homotype by Flemming.
Meiosis I
Homologous chromosomes are segregated during Meiosis I, which are joined as tetrads (2n, 4c), producing two haploid cells (n chromosomes, 23 in humans) which each contain chromatid pairs (1n, 2c). Because the ploidy is reduced from diploid to haploid, ‘meiosis I’ is mentioned as a reductional division. During Meiosis II the sister chromatids are segregated, creating four haploid daughter cells (1n, 1c), meiosis II is an equational division analogous to mitosis.
Prophase I
Typically the longest phase of meiosis is the prophase I. Homologous chromosomes pair exchange genetic information (homologous recombination) during prophase I. This often results in the chromosomal crossover. This process facilitates pairing between homologous chromosomes and hence accurate segregation of the chromosomes at the first meiosis division. The new combinations of DNA created during crossover are a big source of genetic variation and end in new combinations of alleles, which can be beneficial. The paired and replicated chromosomes are called bivalents or tetrads, which have two chromosomes and 4 chromatids, with one chromosome coming from each parent. The process of pairing the homologous chromosomes is called synapsis, non-sister chromatids might get cross-over at points called chiasmata (plural; singular chiasma) during this phase. Prophase I have historically been divided into a series of substages that are named according to the appearance of chromosomes.
Leptotene
The first stage of prophase is the leptotene stage, also referred to as leptonema, from Greek words meaning "thin threads". during this stage of prophase I, individual chromosomes — each consisting of two sister chromatids — become "individualized" to make visible strands within the nucleus.the 2 sister chromatids closely associate and are visually indistinguishable from each other. Lateral elements of the synaptonemal complex assemble during this phase. Leptotene is of very short duration and progressive condensation and coiling of the chromosome, fibers take place.
Did You Know?
Meiosis occurs in eukaryotic life cycles involving amphimixis, consisting of the constant cyclical process of meiosis and fertilization. This takes place alongside normal mitotic cell division. In multicellular organisms, there's an intermediary step between the diploid and haploid transition where the organism grows. Germ cells produce gametes at certain stages of the life cycle. Somatic cells structure the body of the organism and aren't involved in gamete production.
Cycling meiosis and fertilization events produce a series of transitions back and forth between alternating haploid and diploid states. Either during the diploid state (diplontic life cycle), during the haploid state (haplontic life cycle), or both (haplodiplontic life cycle, the organism phase of the life cycle can occur in which there are two distinct organism phases, one during the haploid state and therefore the other during the diploid state). In this sense there are three sorts of life cycles that utilize amphimixis, differentiated by the situation of the organism phase(s).
In the diplontic life cycle (with pre-gametic meiosis), of which humans are a neighborhood, the organism is diploid, grown from a diploid cell called the zygote. The organism's diploid germ-line stem cells undergo meiosis to make haploid gametes that fertilize to make the zygote.
FAQs on Meiosis Phases: Step-by-Step Guide for Students
1. What is meiosis, and what is its primary purpose in living organisms?
Meiosis is a specialised type of cell division that reduces the chromosome number in a parent cell by half to produce four genetically unique daughter cells. Its primary purpose is to produce haploid gametes (sperm and eggs) for sexual reproduction. This reduction in chromosome number is crucial to ensure that when gametes fuse during fertilisation, the resulting zygote has the correct diploid number of chromosomes, maintaining the species' standard chromosome count across generations.
2. What are the main stages of meiosis in the correct sequence?
Meiosis is divided into two main, consecutive rounds of division: Meiosis I and Meiosis II. The correct sequence of the phases is as follows:
- Meiosis I: Prophase I, Metaphase I, Anaphase I, Telophase I. This is the reductional division where homologous chromosomes are separated.
- Meiosis II: Prophase II, Metaphase II, Anaphase II, Telophase II. This is the equational division where sister chromatids are separated, similar to mitosis.
3. What are the five sub-stages of Prophase I and their key events?
Prophase I is the longest and most complex phase of meiosis, divided into five sub-stages:
- Leptotene: Chromosomes begin to condense and become visible as thin threads.
- Zygotene: Homologous chromosomes pair up in a process called synapsis to form bivalents.
- Pachytene: The paired chromosomes become shorter and thicker. Crossing over, the exchange of genetic material between non-sister chromatids, occurs here.
- Diplotene: The homologous chromosomes begin to separate but remain attached at points called chiasmata, which are the sites of crossing over.
- Diakinesis: Chromosomes reach maximum condensation, the chiasmata terminalise (move to the ends), and the nuclear envelope breaks down.
4. What is the fundamental difference between Anaphase I and Anaphase II?
The fundamental difference lies in what is being separated and pulled to opposite poles of the cell. In Anaphase I, the homologous chromosomes separate, but the sister chromatids remain attached at their centromeres. In contrast, in Anaphase II, the centromeres divide, and the sister chromatids separate, moving to opposite poles. This separation in Anaphase II is what leads to the formation of four haploid cells.
5. How does meiosis contribute to genetic variation in offspring?
Meiosis is a major source of genetic variation, which is essential for evolution and adaptation. It achieves this through two main mechanisms:
- Crossing Over: During Prophase I, non-sister chromatids of homologous chromosomes exchange segments of DNA. This creates new combinations of alleles on the chromosomes.
- Independent Assortment: During Metaphase I, the orientation of each pair of homologous chromosomes at the metaphase plate is random and independent of other pairs. This leads to a vast number of possible combinations of paternal and maternal chromosomes in the resulting gametes.
6. Why is a second meiotic division (Meiosis II) necessary?
A second meiotic division is necessary because at the end of Meiosis I, the two daughter cells are haploid in terms of chromosome number, but each chromosome still consists of two sister chromatids. Meiosis II functions to separate these sister chromatids. Without Meiosis II, the gametes would have chromosomes with duplicated DNA, and upon fertilisation, the resulting organism would have an incorrect amount of genetic material. Therefore, Meiosis II is essential to produce true haploid cells, where each chromosome is a single chromatid.
7. What are the key differences between meiosis and mitosis?
While both are forms of cell division, meiosis and mitosis differ significantly in their process and outcome:
- Number of Divisions: Mitosis involves one round of division, while meiosis involves two rounds (Meiosis I and Meiosis II).
- Daughter Cells: Mitosis produces two genetically identical diploid (2n) daughter cells. Meiosis produces four genetically unique haploid (n) daughter cells.
- Genetic Variation: Meiosis introduces genetic variation through crossing over and independent assortment. Mitosis produces genetically identical clones.
- Chromosome Pairing: Homologous chromosomes pair up (synapsis) during Prophase I of meiosis, which does not happen in mitosis.
- Purpose: Mitosis is for growth, repair, and asexual reproduction. Meiosis is exclusively for producing gametes for sexual reproduction.
8. What is the significance of the reduction in chromosome number during Meiosis I?
The reduction of the chromosome number from diploid (2n) to haploid (n) during Meiosis I is the most critical event of the entire process. Its significance lies in maintaining the constancy of the chromosome number across generations in sexually reproducing species. By halving the chromosome number to create haploid gametes, it ensures that when two gametes fuse during fertilisation, the normal diploid state is restored in the offspring. Without this reduction, the chromosome number would double in each successive generation, leading to genetic instability and inviability.
