Meiosis

Each meiotic division (known as meiosis I and II) involves the formation of a meiotic spindle and the same sequence of prophase, prometaphase, metaphase, anaphase, and telophase (Figure).

Stages of meiosis

One set of chromosomes is from the father and another from the mother. Meiosis is often extended process, in different organisms can last for months or even years, mainly due to lengthy prophase of meiosis I, prophase I. Prophase I differs from mitotic prophase in that the paternal and maternal chromosomes (each composed of two chromatids) line up side by side. Prophase I is the hardest stage of meiosis. By morphological changes of chromosomal material it is divided into leptotene, zygotene, pachytene, diplotene and diakinesis.

Leptotene begins with appearance in meiotic cell of chromosomal material as thin filaments, which coil and begin to shrink.

Zygotene begins with pairs of homologous chromosomes conjugation with exceptional accuracy and orderliness. Each point of one chromosome is located closely with morphologically identical point of corresponding homologous chromosome. These pairs are bivalents.

Pachitene is characterized by bivalent coiling continuing.

There are two longitudinal (продольный) grooves in each chromosome associate. One of them is a gap that has emerged due to the convergence of two-chromosome homologues. It is reduction groove. Along this fissure during meiotic anaphase I division of each bivalent may occur. Perpendicular to the reduction slit of bivalent there is another gap. It is equation groove. In the second meiotic division each chromosome of two chromatids may be divided into two daughter chromosomes along this groove. It may be opposite situation – in the first division chromatids may be divided, and in the second one – chromosomes may be divided.

Continued bivalent coiling is accompanied by chromomere formation. A chromomere (idiomere) is one of the serially aligned beads or granules of eukaryotic chromosome, resulting from local coiling of continuous DNA thread. Any change in the structure of chromosomes causes a change in chromomeres location on bivalent.

Diplotene starts with homologous chromosomes in bivalents reciprocal repulsion. However, their isolation from one another is impossible because of happened crossing-over between chromatids. So, along each bivalent x-like formations appear which are called chiasmas. It is believed that the number of chiasmas corresponds to the number of crossings that occurred between chromatids of homologous chromosomes.

Diakinesis is characterized by chiasmas terminalization. It starts in centromeric parts of chromosomes. Chiasmas gradually move to terminal areas of bivalent. By the end of diakinesis bivalents remain united among themselves only by terminal chiasmas. Therefore bivalent take the form of circulary structures. Meiotic spindle is formed, nucleolus and nuclear membrane disappear.

As the cell progresses to metaphase I, the maternal/paternal chromosome pairs line up along the metaphase plate. Centromeric parts of paired chromosomes are directed to the opposite poles.

At anaphase I, the homologous pairs separate (complete chiasmas terminalization), but the paired chromatids remain attached and journey together to the poles. дочірні клітини, які утворились внаслідок такого поділу, матимуть лише половинну, тобто – гаплоїдну кількість хромосом.

Telophase I starts after chromosomes achieve the poles. Chromosomes are compact, partially despiralized. The two progeny nuclei and nucleoli are formed. Cytokinesis occurs.

Two newborn cells are located next to one another (dyad) and almost immediately enter meiosis II. Interkinesis is very short-term, has not stages.

Prophase II is often so brief as to be undetectable. Meiotic spindle is formed, nucleolus and nuclear membrane disappear.

Metaphase II and anaphase II are similar to mitotic.

In metaphase II chromosomes actively move in the equatorial plane of the cell, and then their centromeres connected with spindle threads.

In anaphase II chromatids finally separate to give haploid gametes (sperm or eggs) and move to opposite poles. Each gamete contains only one copy, derived from either the father or the mother. Тепер кожна дочірня хроматида набуває статуту хромосоми

Telophase II is the last meiotic stage. Chromosomes despiralized completely. Nucleus and nucleolus form, centrioles doubles. Except nuclear membrane, cell walls form that separate protoplasm of daughter cells.

There are some important distinctions of male and female meiosis in animals. In males meiosis produces four equal sized haploid cells called spermatids, each subsequently developing into a spermatozoon. In females, both meiotic divisions are asymmetric, resulting in one large cell that survives, the oocyte, and three small cells, called polar bodies, that are discarded.

5. Comparison of mitosis and meiosis

Both somatic cells and premeiotic germ cells have two copies of each chromosome (2 n), one maternal and one paternal. In mitosis, the replicated chromosomes, each composed of two sister chromatids, align at the cell center in such a way that both daughter cells receive a maternal and paternal homolog of each chromosome.

During the first meiotic division each replicated chromosome pairs with its homologous partner at the cell center; this pairing off is referred to as synapsis. One replicated chromosome of each morphologic type then goes into one daughter cell, and the other goes into the other cell in a random fashion. The resulting cells undergo a second division without intervening DNA replication, with the sister chromatids of each morphologic type being apportioned to the daughter cells. Each diploid cell that undergoes meiosis produces four haploid (1 n) cells.

Fig. Distribution of genetic material during mitosis

Fig. Distribution of genetic material during meiosis

6. Types of sexual reproduction

Sexual reproduction is a process that creates a new organism by combining the genetic material of two organisms. It occurs both in eukaryotes and in prokaryotes. A key similarity between them is that DNA originating from two different individuals (parents) join so that homologous sequences are aligned with each other, and this is followed by exchange of genetic information (a process called genetic recombination). After the new recombinant chromosomes are formed they are passed on to progeny.

There are two main processes during sexual reproduction in eukaryotes: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the restoration of the original chromosomes number. During meiosis, the chromosomes of each pair usually cross over to achieve homologous recombination.

Fertilization (also known as conception, fecundation and syngamy) is the fusion of gametes to produce a new organism. In animals, the process involves the fusion of an ovum with a sperm, which eventually leads to the development of an embryo. Depending on the animal species, the process can occur within the body of the female in internal fertilization, or outside (external fertilization). The entire process of development of new individuals is called reproduction.

The autogamy, the production of gametes by the division of a single parent cell,is the fertilization between two daughter gametes of the same gametocyte. Typically the nucleus of the diploid or tetraploid gametocyte divides without DNA synthesis and complete cytokinesis (separation of daughter cells). The resulting cell then behaves like a zygote resulting from fertilization. Autogamy may be found in species where cross pollination cannot be assured.

Self-fertilization or endogamy occurs in bisexual organisms, including most flowering plants, numerous protozoans, and many invertebrates. Fusion of female and male gametes derived from genetically similar sources, usually the same species.

Parthenogenesis is a form of asexual reproduction in which growth and development of embryos occur without fertilization, development of unfertilized eggs into new individuals. The word parthenogenesis comes from the Greek parthenos, meaning "virgin", and genesis, meaning "birth". The offspring are clones of the mother and hence are usually female. Parthenogenesis occurs naturally in many plants, some invertebrate animal species and a few vertebrates. This type of reproduction has been induced artificially in a few species including fish and amphibians.

Some species reproduce exclusively by parthenogenesis while others can switch between sexual reproduction and parthenogenesis. This is called facultative parthenogenesis or cyclical parthenogenesis. Facultative parthenogenesis occurs when a species that normally reproduces sexually undergoes asexual reproduction. This is in contrast to obligate parthenogenesis, where the females reproduce exclusively by asexual means.

Facultative parthenogenesis is a response to a lack of a viable male. A female may undergo facultative parthenogenesis if a male is absent from the habitat or if it is unable to produce viable offspring. This behavior has been documented in sharks, komodo dragons and a variety of domesticated birds. Facultative parthenogenesis has also been seen in crustaceans and decapods in nature. In fact, parthenogenesis has been documented in over 70 different species.

Obligate parthenogenesis is the process in which organisms exclusively reproduce through asexual means. There are over 80 species of unisex reptiles, amphibians and fishes in nature, for which males are no longer a part of the reproductive process; a male is not needed to provide sperm to fertilize the egg. A female will produce an ovum with a full set (two sets of genes) provided solely by the mother.

This form of asexual reproduction is thought to be a serious threat in some cases to biodiversity for the subsequent lack of gene variation and potentially decreased fitness of the offspring.

7. Apoptosis

An adult human is made up of about 30 trillion (3·10¹³) cells, all of which originate from a single fertilized egg. If this first cell divides into 2, the 2 progeny cells into 4, and so on, it would take only about 45 rounds of division to produce the number of cells required to make an adult human. In fact, cell division occurs constantly through our lifetimes, such that we generate a new complete set of 3·10¹³ cells every 2 weeks. The reason that multicellular organisms do not become infinitely large is because the proliferation of cells is balanced by cell death. Cells die for two quite different reasons. One is accidental, the result of mechanical trauma or exposure to some kind of toxic agent, and often referred to as necrosis. This is the only type of death seen in unicellular organisms. The other type of death is deliberate, the result of an built-in suicide mechanism known as apoptosis or programmed cell death. The two types of cell death are quite distinct. In cells that are injured, ATP concentrations fall so low that the Na⁺/K⁺ ATP-ase can no longer operate, and therefore ion concentrations are no longer controlled. This causes the cells to swell and then burst. The cell contents then leak out, causing the surrounding tissues to become inflamed. Cells that die by suicide on the other hand shrink, and their cell contents are packaged into small membrane-bound packets called blebs. The nuclear DNA becomes chopped up into small fragments, each of which becomes enclosed in a portion of the nuclear envelope. The dying cell modifies its plasma membrane, signaling to macrophage, which respond by engulfing the blebs and the remaining cell fragments and by secreting cytokines that inhibit inflammation. The changes that occur during apoptosis are the result of hydrolysis of cellular proteins by a family of proteases called caspases (short for cysteine-containing, cleaving at aspartate). All the cells of our body contain caspases, but they are normally locked in an inactive form by an integral inhibitory domain of the protein. Proteolysis cleaves the inhibitory domain off, releasing the active caspases. The advantage to the cell of this strategy is that no protein sythesis is required to activate the apoptotic pathway–all the components are already present. Thus, for example, if a virus infects a cell and takes over all protein synthesis, the cell can still commit suicide and hence prevent viral replication (Figure).

The complex control systems that regulate the decision to die or survive