Eukaryota

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The Eukaryota (Latin:Eukaryotes) An organism composed of eukaryotic cell. All animals, plants, fungi, and many unicellular organisms are eukaryotes. They constitute a major group of living things, along with the two groups of prokaryotes, the Bacteria and the Archaea. The fundamental difference between eukaryotes and prokaryotes is that the cells of the former have a nucleus bounded by a nuclear membrane, so this type of cell is named after eukaryotes. Many eukaryotic cells also contain other organelles, such as mitochondria, chloroplasts, and Golgi apparatus.

Compared with prokaryotes, eukaryotes have a nucleus, and the cell size is relatively large, and the growth rate is fast. Eukaryotes are usually heterotrophic microorganisms, which can derive a variety of organic acids during the growth and reproduction process, and are easy to form complexes with metal ions during the leaching process, which is beneficial to the leaching of valuable metals. There are many eukaryotic organisms that can grow in an acidic environment, but more researches are mainly on the use of fungi for leaching ore, the main genera are Aspergillus (Aspergillus) and Penicillium (Penicillium), and the types of minerals leached are mainly laterite nickel ore, Chalcopyrite, manganese ore, oceanic nodules, etc.

A eukaryote is defined as any organism that is chiefly characterized by a cell with one or more nuclei at least once in its lifetime as opposed to a prokaryote that has a cell lacking a well-defined nucleus and with a nucleoid only. Eukaryotic cells are cells that contain a nucleus and organelles, and are enclosed by a plasma membrane. Organisms with eukaryotic cells are grouped into the biological domain Eukaryota (also sometimes called Eukarya). The other two domains of life, Archaea and Bacteria, have prokaryotic cells, which are simpler and lack organelles except for ribosomes, which make proteins.

According to whether the cells have a nucleus bounded by the nuclear membrane, the cells are divided into two types, eukaryotic cells and prokaryotic cells; organisms composed of eukaryotic cells become eukaryotes; on the contrary, organisms composed of prokaryotic cells are called prokaryotes.

what are eukaryotic cells

Organisms made up of eukaryotic cells are called eukaryotes. In the nucleus of eukaryotic cells, DNA and proteins such as histones together form the chromosome structure, and nucleoli can be seen in the nucleus. In the cytoplasmic inner membrane system is very developed, there are organelles such as endoplasmic reticulum, Golgi apparatus, mitochondria and lysosome, which perform specific functions respectively.

Eukaryotes include familiar animals and plants as well as tiny protozoa, single-cell algae, fungi, mosses, etc. Eukaryotic cells have one or more nuclei surrounded by double membranes, and genetic material is contained in the nucleus and exists in the form of chromosomes.

Chromosomes are composed of a small amount of histones and certain basic proteins rich in arginine and lysine. Eukaryotes reproduce sexually and undergo mitosis. There are also some eukaryotic cells that can undergo amitosis, such as red blood cells in frogs and liver cells in humans.

Mitosis and Meiosis

The cells of eukaryotes divide by mitosis and meiosis. While a single cell that undergoes mitosis gives rise to two daughter cells, in meiosis, one cell gives rise to four daughter cells. The cells from meiosis will be haploid after two consecutive divisions. In males, the haploid cell will grow into a spermatozoon (sperm cell) whereas, in females, it could develop into an ovum (egg cell). These two gametes could come together in a union via fertilization and give rise to a diploid zygote.

Meiosis is essential as it is one of the major sources of genetic variations by way of genetic recombinations and chromosomal assortment.

Cellular organization

In multicellular eukaryotes, the zygote divides by a series of mitoses to give rise to stem cells that can develop and differentiate later into specialized cells that carry out a particular function and assemble into tissues, organs, and biological systems.

In humans, there are several cell types: myocytes, adipocytes, blood cells, neurons, hepatocytes, osteocytes, macrophages, etc.

Some eukaryotes are single-celled. The cell is an entire organism capable of performing all the fundamental functions (e.g. ingestion, respiration, excretion, osmoregulation, homeostasis, etc.) that different systems do in a multicellular organism. These single-celled organisms are exemplified by protists.

Eukaryote vs. Prokaryote

Cell size. Prokaryotic cells are considerably smaller than eukaryote cells. They also have a greater surface area to volume ratio and therefore have greater metabolic rates.

Taxonomic domains. Eubacteria and archaea are two prokaryotic cells that share these features albeit belonging to separate domains, i.e. Domain Bacteria and Domain Archaea, respectively. Domain Eukarya (Eucarya) includes all eukaryotes.

Genetic material. The DNAs inside the nucleus are complexed with histone proteins forming chromatin. During cell division, the chromatin condenses into a chromosome. The chromosomes are linear strands of DNA as opposed to the chromosomes of prokaryotes which are mostly circular.

Nucleus vs nucleoid. Both eukaryotes and prokaryotes have genetic information stored in their genes. Nevertheless, prokaryotes have genes but they are not bound by an internal membrane system.  Prokaryotes lack a nucleus. The region in the cytoplasm where the prokaryotic genes and DNAs are found is referred to as the nucleoid. Eukaryotes have a nucleus that contains nuclear DNA. The nucleus has a nuclear envelope (nuclear membrane), which is a lipid bilayer membrane perforated with nuclear pores. 

Other DNA sites. Another eukaryotic trait is their extranuclear DNA. They have DNA in organelles such as mitochondria and chloroplasts. These DNAs are particularly called mitochondrial DNA and chloroplast DNA, respectively. Prokaryotes, in contrast, do not have mitochondrial and chloroplast. As already mentioned, they have a nucleoid, which is simply a region in the cytoplasm where their DNA is found.

Metabolic energy. The main source of metabolic energy for both eukaryotes and prokaryotes is ATP and they generate energy via cellular respiration. Those that make use of molecular oxygen in cellular respiration (aerobic-type) are eukaryotes and aerobic prokaryotes. Prokaryotes make use of their cytoplasm and cell membrane for aerobic cellular respiration whereas eukaryotes use the cytoplasm as the initial site and then use the mitochondria where the process culminates. Anaerobes, in turn, are generally those that generate energy via metabolic processes that do not utilize oxygen. Examples of such cellular processes are fermentation and anaerobic respiration. The human body resorts to fermentation when the oxygen supply becomes limited, such as during strenuous exercise, and then reverts to aerobic respiration when the oxygen level goes back to normal.

Protein synthesis. Both of them also have ribosomes that assist during protein synthesis. However, the ribosomes of eukaryotes are 80S. In prokaryotes, the ribosomes are 70S. Both prokaryotic and eukaryotic ribosomes are made up of two ribosomal subunits. The prokaryotic ribosome (70S) is made up of 50S (large subunit) and 30S (small subunit). The eukaryotic ribosome (80S) consists of 60S (large subunit) and 40S (small subunit). [N.B. the S units do not add up since they represent measures of sedimentation rate, not mass.] While prokaryotes carry out protein synthesis in the cytoplasm, eukaryotes use membrane-bound organelles such as the endoplasmic reticulum, for protein maturation, and the Golgi apparatus for protein sorting and transport.

Genes

Cells are the basic structural and functional units of all life activities. Generally considered:

1. A cell is a protoplasm mass surrounded by a membrane, which communicates material and information with the surrounding environment through the plasma membrane;

2. It is the basic unit that constitutes the organism, has the ability of self-replication, and is the basis for the growth and development of the organism;

3. It is the basic unit of metabolism and function, with a complete set of metabolism and regulation system;

4. It is the basic unit of heredity and has the totipotency of development.

Compared with prokaryotes, eukaryotes have more complex genomes:

1. The eukaryotic genome is much larger than the prokaryotic genome. The Escherichia coli genome is about 4×106bp, and the mammalian genome is on the order of 109bp, which is a thousand times larger than that of bacteria; Escherichia coli has about 4,000 genes, and humans have about 100,000 genes.

2. The main genetic material and histones of eukaryotes constitute chromatin, which is wrapped in the nuclear membrane, and there are genetic components (such as mitochondrial DNA, etc.) outside the nucleus, which increases the level and complexity of gene expression regulation.

3. The genome of prokaryotes is basically haploid, while the genome of eukaryotes is diploid.

4. As mentioned above, most of the genes in bacteria are arranged in clusters related to their functions to form the unit of gene expression regulation of the operator, which is turned on or off together to transcribe polycistron (polycistron) mRNA; eukaryotes are a The structural gene is transcribed to generate an mRNA, that is, the mRNA is monocistronic (monocistron), basically without the structure of an operator, and many active proteins in eukaryotic cells are composed of subunits formed by the same and different polypeptides, which is The coordinated expression of multiple genes is involved, and the coordinated expression of genes in eukaryotes is much more complicated than that in prokaryotes.

5. Most of the sequences in the prokaryotic genome are encoded by genes, but experiments such as nucleic acid hybridization have shown that only about 10% of the sequences in the mammalian genome are encoded by proteins, rRNA, tRNA, etc., and the functions of the remaining 90% of the sequences are still unclear .

6. Most of the prokaryotic genes are continuous for protein-coding sequences, while most of the eukaryotic protein-coding genes are discontinuous, that is, there are exons (exon) and introns (intron) After transcription, the intron needs to be removed by splicing before the complete protein can be translated, which increases the link of gene expression regulation.

7. Except for multiple copies of rRNA and tRNA genes in the prokaryotic genome, there are not many repetitive sequences. There are a large number of repetitive sequences in mammalian genomes. Experiments such as renaturation kinetics have shown that there are three types of repetitive sequences: 1) Highly repetitive sequences (highly repetitive sequences), such sequences are generally short, 10-300 bp long, and repeat about 106 times in the mammalian genome, accounting for the genomic DNA sequence. 10-60% of the total, this type of sequence accounts for about 20% in the human genome, and its function is still unclear. 2) Moderately repetitive sequences, most of these sequences are 100-500bp long, repeat 101-105 times, accounting for 10-40% of the genome. For example, the sequence called Alu, which is the most abundant in mammals, is about 300 bp long, similar among different species of mammals, repeated 3×105 times in the genome, accounting for about 7% of the human genome, and its function is still Not very clear. In the human genome, the 18S/28SrRNA gene repeats 280 times, the 5SrRNA gene repeats 2000 times, the tRNA gene repeats 1300 times, and the genes of five histones are clustered and repeated 30-40 times. These genes can be classified as moderately repeated sequence range. 3) Single copy sequences. Such sequences are basically non-repetitive, accounting for 50-80% of the mammalian genome and about 65% of the human genome. The vast majority of eukaryotic protein-coding genes are not repeated in the haploid genome and are single-copy genes.

It can be seen from the above that the eukaryotic genome is much more complex than the prokaryotic genome, and so far human beings have very limited understanding of the eukaryotic genome, so that the international human genome research plan (human gene project) has now been completed, and the chromosome location of all human genes has been mapped Figure, after measuring the entire 109bp DNA sequence of the human genome, it will take a long and arduous research process to understand the functions of all human genes and their interrelationships, especially to understand all the laws of gene expression regulation.

Eukaryotic cells evolved from prokaryotic cells between 1.6 and 2.7 billion years ago. Today, all complex organisms and most multicellular ones are eukaryotes, making this evolution a major event in the history of life on Earth. There are about 75 separate lineages of eukaryotes, most of which evolved into protists. Eukaryotes are more closely related to archaea, unicellular organisms sometimes found in extreme conditions such as hot springs, than to bacteria.

Eukaryotic cells developed specific organelles, which are structures within the cell that perform a specific task. These organelles include mitochondria, which make energy, chloroplasts, which are found in plants and make food from light and carbon dioxide, and the endoplasmic reticulum, which sorts and packages proteins. Some organelles, such as mitochondria and chloroplasts, may have evolved when free-living bacteria were taken up into cells. Under this theory, bacteria and the cells had a symbiotic relationship, where each benefited from the presence of the other. Over time, these bacteria formed the organelles incorporated within eukaryotic cells that are seen today, and became a necessary part of the eukaryotic cell. Mitochondria have DNA that is separate from the chromosomal DNA found in a cell’s nucleus. However, another theory is that small amounts of DNA already in the cell were simply infolded within the cell membrane and evolved into organelles such as mitochondria. Theories involving mitochondria appearing in a cell that was already almost eukaryotic, with a nucleus, are known as autogenous models. Yet another theory proposes that eukaryotic cells evolved when an archaeon and a bacterium merged to form one cell. This is known as a chimeric model.

Some eukaryotes reproduce asexually, while others reproduce sexually. The development of sexual reproduction is another defining feature in the evolution of eukaryotes. It is believed that the common ancestor of all eukaryotes reproduced sexually, and that asexual eukaryotes (such as some amoebas) evolved asexuality from an ancestor that was sexual. Prokaryotes only reproduce asexually; genes can be exchanged between individuals through horizontal gene transfer, but this is not sexual reproduction.

The direct ancestor of the most primitive eukaryotes is likely to be an unusually large prokaryote, which has an endoplasmic reticulum-like endoplasmic reticulum-like inner membrane system and a primitive microfiber system capable of deformation movement and devour. Later, part of the inner membrane system surrounded the chromatin, thus forming the most primitive nucleus. Other parts of the endomembrane system develop into organelles such as Golgi apparatus and lysosome respectively. According to the "endosymbiosis theory" re-proposed by American scholar L. Margulis and others (see cell origin), mitochondria originated from intracellular symbiotic eubacteria capable of oxidative phosphorylation, while chloroplasts originated from intracellular symbiotic eubacteria. Cyanobacteria that perform photosynthesis.

American scholar R.W. Whitaker divided eukaryotes into five kingdoms in 1969: prokaryotes, protists, fungi, plants and animals. The kingdom Protista includes protozoa, unicellular algae and unicellular fungi. Fungi are saprophytic or parasitic in the kingdom of fungi. Most types of cells have chitin walls, and the bacteria are mostly composed of hyphae. The plant kingdom has chloroplasts, which can carry out photosynthesis, and the cells have cellulose walls. The animal kingdom lives by feeding or predating, most species can move, cells have no cell walls, and have a complicated embryonic development process.

In the form of mitosis, the division of the nucleus is usually coordinated with cell division, a process whereby each daughter nucleus receives a copy of the parent's chromosome. In most eukaryotic cells, there is another sexual reproduction process called meiosis, in which a diploid parent cell divides twice to become haploid, halving the amount of DNA. However, meiosis itself comes in many varieties.

Compared to prokaryotic cells, eukaryotic cells have a smaller surface-to-volume ratio, resulting in slower metabolic rates and longer cell cycles. In some multicellular organisms, specialized cells for metabolism have enlarged surface area, such as intestinal villi.

sexual reproduction

Sexual reproduction is widely adopted by eukaryotes, and there is evidence that this is a primitive, fundamental characteristic of eukaryotes. Based on phylogenetic analysis, the biologists Dax and Roger proposed that a common ancestor of eukaryotic cells behaved randomly. A core set of genes in meiosis is present in Trichomonas vaginalis and Giardia intestinalis, two organisms previously thought to be asexual. On the eukaryotic cell evolution tree, these two species were separated very early, so it can be inferred that the core gene of meiosis exists in the common ancestor of all eukaryotic organisms, and therefore this ancestor is sexual. Other studies of eukaryotic species have also uncovered evidence of reproductive cycles. For example the parasitic protist Leishmania has recently been shown to have a reproductive cycle. Evidence shows that amoebas, previously thought to be asexual, were also sexual in ancient times, and that most asexual organisms today have independently evolved asexuality only recently.