virus

Virus adalah parasit berukuran mikroskopik yang menginfeksi sel organisme biologis. Virus bersifat parasit obligat, hal tersebut disebabkan karena virus hanya dapat bereproduksi di dalam material hidup dengan menginvasi dan memanfaatkan sel makhluk hidup karena virus tidak memiliki perlengkapan selular untuk bereproduksi sendiri. Biasanya virus mengandung sejumlah kecil asam nukleat (DNA atau RNA, tetapi tidak kombinasi keduanya) yang diselubungi semacam bahan pelindung yang terdiri atas protein, lipid, glikoprotein, atau kombinasi ketiganya. Genom virus akan diekspresikan menjadi baik protein yang digunakan untuk memuat bahan genetik maupun protein yang dibutuhkan dalam daur hidupnya.

Istilah virus biasanya merujuk pada partikel-partikel yang menginfeksi sel-sel eukariota (organisme multisel dan banyak jenis organisme sel tunggal), sementara istilah bakteriofage atau fage digunakan untuk jenis yang menyerang jenis-jenis sel prokariota (bakteri dan organisme lain yang tidak berinti sel).

Virus sering diperdebatkan statusnya sebagai makhluk hidup karena ia tidak dapat menjalankan fungsi biologisnya secara bebas jika tidak berada dalam sel inang. Karena karakteristik khasnya ini virus selalu terasosiasi dengan penyakit tertentu, baik pada manusia (misalnya virus influenza dan HIV), hewan (misalnya virus flu burung), atau tanaman (misalnya virus mosaik tembakau/TMV).

Struktur Virus - Setelah mengetahui Definisi Virus, kurang lengkap rasanya kalo kita ga lanjut menyimak Struktur Virus. Virus terdiri dari genom asam nukleat (DNA atau RNA) yang berasosiasi dengan protein yang dikode oleh asam nukleat. Protein tersebut merupakan selubung pelindung yang disebut kapsid. Selubung protein dibentuk oleh sejumlah molekul protein individu yang disebut subunit struktural, yang tersusun dalam pola yang tepat dan sangat repetitif di sekeliling asam nukleat. Genom asam nukleat ditambah selubung protein pelindung disebut nukleokapsid. Beberapa virus memiliki membran lipid dua lapis, tetapi membran tersebut diperoleh dari sel inang melalui proses bertunas (budding). Membran virus mengandung satu atau beberapa protein viral yang bertindak sebagai ligan untuk berinteraksi dengan reseptor pada sel inang.

Pada Struktur Virus, ukuran genom yang kecil pada kebanyakan virus membatasi jumlah protein viral yang dihasilkan. Beberapa virus hanya memiliki satu macam protein dalam kapsidnya, tetapi kebanyakan virus memiliki beberapa macam subunit struktural yang berbeda secara kimia dan mereka mengasosiasikan diri mereka sendiri dalam cara khusus untuk membentuk rakitan yang lebih besar yang disebut kapsomer. Informasi untuk agregasi yang tepat pada perakitan struktur subunit menjadi kapsomer terkandung dalam struktur proteinnya.

Nukleokapsid virus dikonstruksi dalam susunan yang sangat simetris. Simetris mengacu pada bagaimana unit morfologi protein disusun dalam cangkang virus.

Lima bentuk struktural dasar virus di alam  yaitu :

• Ikosahedral tidak bermembran, contohnya poliovirus, adenovirus, virus hepatitis A
• Helical tidak bermembran, contohnya TMV
• Ikosahedral bermembran, contohnya virus herpes, virus rubella
• Helical bermembran, contohnya virus rabies, virus influenza
• Kompleks, contohnya poxvirus

Bentuk ikosahedral adalah bentuk segi 20 yang semua sisinya identik. Cangkang kapsid dibuat dari subunit protein virus yang berulang. Asam nukleat dikemas di dalam cangkang kapsid dan dilindungi dari lingkungan oleh kapsid. Protein-protein bergabung menjadi unit struktural yang disebut sebagai kapsomer. Kapsomer bisa mengandung satu atau beberapa rantai polipeptida. Subunit protein dapat berinteraksi dengan asam nukleat maupun subunit protein lain untuk membentuk struktur berpilin (helical). Contohnya adalah TMV. Struktur berpilin pada TMV membentuk struktur kaku seperti batang. Pada virus helical yang bermembran, kapsid lebih fleksibel dan pada pewarnaan negatif tampak seperti kabel telepon. Virus yang memiliki simetris kompleks sebenarnya memiliki bentuk yang teratur, tetapi simetrinya tidak sepenuhnya dimengerti. Contohnya adalah poxvirus.

I think that’s all is enough, thanks for your attenntion…..

FUNNY: The hormonal (endocrine) system is made up of the ...

FUNNY: The hormonal (endocrine) system is made up of the ...: The   hormonal (endocrine) system   is made up of the   endocrine glands   that secrete   hormones . Although there are eight major endocrin...

PLANT HORMONE

 

What is a hormone?

As plants grow their genotype is expressed in the phenotype which is modified by the environmental conditions that they experience. Somehow the rates of growth and differentiation of cells in different parts of the plant are coordinated in response to these inputs.

There has to be communication between these levels: how does the plant receive and respond to environmental inputs or "signals"? What comunication is there inside the plant to adjust growth and development to the environment?

When growing plants commercially we can ask similar questions:

• what environmental input will produce the kind of growth that we want?
• or can we modify the growth by applying a chemical regulator?
• can change the genotype to achieve the kind of growth we want (by traditional breeding or by genetic manipulation)?

The answers to each of these questions depends on an understanding of how plant growth is regulated. Hormones in animals cooordinate body functions by being produced in one place and acting in another. Plants do not have a circulatory system and "action at a distance" may not be a feature of plant hormones. They are molecules that are not directly involved in metabolic or developmental processes but they act at low concentrations to modify those processes.

There are five generally recognized classes of plant hormone, some of the classes are represented by only one compound, others by several different compounds. They are all organic compounds, they may resemble molecules which turn up elsewhere in plant structure or function, but they are not directly involved as nutrients or metabolites.

Auxins

There is only one naturally occurring auxin: indole-3-acetic acid (IAA) and this is chemically related to the amino acid tryptophan.

There are many synthetic auxins - aromatic compounds with carboxylic sidechains often affect plant growth in the same way that IAA does. These are used commercially rather than IAA because they are cheaper and more stable. For example naphthalene acetic acid (NAA) is used to control fruit set and sucker growth on trees after pruning. Indole butyric acid is used to promote rooting in cuttings. Far and away the biggest use of auxin-like compounds is as herbicides (2,4-D and MCPA). Applied at high concentration they promote uncoordinated growth and finally death, particularly in broad-leaved weeds.

Cytokinins

There are a number of naturally occuring cytokinins all related to the nucleotide adenine. They can occur as the free base or as a riboside. Synthetic cytokinins include benzyladenine and kinetin. Cytokinins are used in tissue culture media, and for growth control in fruit.

Ethylene

Ethylene is the only gaseous hormone in the plant world; it is a simple hydrocarbon gas that is derived from the amino acid, methionine, via an unusual cyclic compound which is also an amino acid, ACC (1-aminocyclopropane-1-carboxylic acid).

The gas is used commercially for ripening fruit, particularly bananas. There are also synthetic compounds, such as ethephon (chloro-ethanephosphonic acid) that can be sprayed onto plants in solution; once inside the tissues ethephon breaks down to liberate ethylene. Ethephon is used to promote ripening on the tree, leaf abscision in ornamentals, growth control in seedlings and flowering in pineapples.

Abscisic acid

Abscisic acid (ABA) is one of two related compounds (the other is xanthoxin) that are in the isoprenoid group and related to carotenoids ABA is a very expensive material and so far there are no synthetic analogs or practical uses

Gibberellins

The gibberellins (GAs) are the largest group with over 70 compounds although not all are biologically active. Like ABA they are derived from the isoprenoid pathway. Gibberellins are used commercially to break dormancy of "difficult" seeds, and to promote set of grapes and other fruits.

Many growth retardants used on flowering pot plants, woody plants and turf are "anti- gibberellins". Compounds such as ancymidol and uniconazole block GA synthesis and produce dwarf plants. Genetic dwarfs are often deficient in gibberellin.

Hormone action
At the cell level hormones attach to a protein receptor which sends a signal down a transduction pathway to switch on particular genes. Through transcription and translation this leads to production of an enzyme protein which actually causes the change in plant growth. A good example from the early stages of plant development is the role of GA in cereal seed germination. As the seed imbibes water the embryo produces GA. This induces synthesis of amylase in the aleurone layer which secretes the enzyme to the endosperm. Amylase breaks down starch to glucose which diffuses to the embryo and is used for the early stages of plant growth.

CELLULAR REPRODUCTION

The Basics of Cellular Reproduction

We humans, like other multicellular organisms, begin life as a single cell. In nine short months, however, we become trillions of cells because cellular reproduction has occurred over and over again. Even after we are born, cellular reproduction doesn’t stop –it continues as we grow, and when we are adults, it replaces worn-out or damage tissues. Right now, your body is producing thousands of new red blood cells, skin cells, and cells that line your respiratory and digestive tracts. If you suffer a cut, cellular reproduction helps repair the injury.

                Cellular reproduction is also necessary for the reproduction of certain organisms. When an amoeba splits, two new individual amoebas are produced. The process is called asexual reproduction because it doesn’t require a sperm and an egg. The next chapter concerns the production of egg and sperm, which are needed for sexual reproduction.

                One way to emphasize the importance of cellular reproduction is to say that “all cells come from cells.” You can’t have a new cell without a pre-existing cell. And you can’t have a new organism without a pre-existing organism. Cellular reproduction is necessary for the production of both new cells and a new organism.

Chromosomes

Cellular reproduction always involves two important processes: growth and cell division. During growth, a cell duplicates the contents of its cytoplasm and it DNA. Then, during division, the cytoplasm and the DNA of the parent cell are distributed to the so-called daughter cells. (these terms have nothing to do with gender; they are simply a way to designate the beginning cell and the resulting cells.)

                The passage of DNA to be daughter cells is critical because cells cannot continue to live without a copy of the genetic materials. Especially in eukaryotic cells, passage of DNA to the daughter cells present a problem because of the large quantity of DNA in the nucleus. For example, a human cell contains about 2 meters of DNA and a nucleus is only 5 to 8 micrometers (µm) in diameter. During cellular reproduction, DNA is packaged into chromosomes, which allow DNA to be distributed to the daughter cells. A chromosome contains DNA, and it also contains proteins that help package the DNA and possibly function in utilizing the DNA as well.

Chromatin into chromosomes

When a eukaryotic cell is not undergoing cell division, the DNA and associated proteins have the appearance of thin threads called chromatin. Closer examination reveals that chromatin is periodically wound around a core of eight protein molecules so that it looks like beads on a string. The protein molecules are histones, and each bead is called nucleosome.

                Just before cell division occurs, the chromatin coils tightly into a fiber that has several nucleosomes to a turn. Then the fiber coils again before it loops back and forth and condenses to produce highly compacted chromosomes. Each species has a characteristic number of chromosomes; a human cell has 46 chromosomes. We can easily chromosomes with a light microscope because just before division occurs a chromosome is 10,000 times more compact than is chromatin.

                Another important event, that occurs in preparation for partition of chromosomes is DNA replication, when a DNA make a copy of itself. By the time we can clearly see the chromosomes, they are duplicated.  A duplicated chromosome is composed of two identical halves called sister chromatids held together  at a constricted region called a centromere. Each sister chromatid contains an identical DNA double helix.

The Cell Cycle

We have already indicated that cellular reproduction involves duplication of cell contents followed by cell division. For cellular reproduction to be orderly, you would expect the first event to occur before the second event, and that’s just happens during the so-called cell cycle. The cell cycle is an orderly sequence of stages that takes place between the time a new cell has arisen from division of a parent cell to the point when it has given rise to two daughter cells. Duplication of cells contents occurs during the stage called interphase.

Interphase

Most of the cell cycle is spent in interphase. This is the time when a cell performs its usual functions, depending on its location in the body. The amount of time the cell takes for interphase varies widely. Some cells, such as nerve and muscle cells, typically remain in interphase and cell division is permanently arrested. These cells are said to have entered a G₀ stage. Embryonic cells complete the entire cell cycle in just a few hours. In contrast, interphase alone in a rapidly dividing mammalian cell, such as an adult stem cell, may last for about 20 hours, which is 90% of the cell cycle.

                DNA replication occurs in the middle of interphase and serves as a way to divide interphase into three stages: G₁, S, and G₂. G₁ is the stage before DNA replication, and G₂ is the stage following DNA synthesis. Originally, G stood for “gap,” but now we know how metabolically active the cell is, it is better to think of G as standing for “growth.” Protein synthesis is very much a part of these growth stages.

                During G₁, a cell doubles its organelles (such as mitochondria and ribosomes) and accumulates materials that will be used for DNA replication. Following G₁, the cell enters the S stage. The S stand for synthesis, and certainly DNA synthesis is required for DNA replication. At the beginning f the S stage, each chromosomes has one DNA double helix. At the end of this stage, each chromosomes is composed of two sister chromatids, each one have a double helix. Another way of expressing these events is to say that DNA replication result in duplicated chromosomes.

                Following the S stage, G₂ is the stage that extends from the completion of DNA replication to the onset of mitosis. During this stage, the cell synthesize proteins that will be needed for cell division, such as the protein found in microtubules. The role of microtubules in cell division is described in a later section.

M (Mitotic) Stage

Cell division occurs during the M stage, which encompasses both division of the nucleus and division of cytoplasm. The type of nuclear division associated with the cell cycle is called mitosis, which accounts for why this stage is called the M stage. As a result of mitosis, the daughter nuclei are identical to the parent cell and to each other-they all have the same number and kinds of chromosomes. Division of the cytoplasm, which starts even before mitosis is finished, is called cytokinesis.

Mitosis and Cytokinesis

Each sister chromatid of a duplicated chromosome carries the same genetic information because its DNA double helix has the same sequence of base pairs as did  the original chromosome. Thus, it is proper, once the chromatids have separated, to call them daughter chromosomes. Because each original chromosome goes through the same process of DNA replication followed by separation of the chromatids to form daughter chromosomes, the daughter nuclei produced by mitosis are genitically identical to each other and to the parent nucleus. In the simplest of terms, if the parent nucleus has 4 chromosomes, each daughter nucleus also has 4 chromosomes of exactly the same type. One way to keep track of the number of chromosomes in drawings is to count the number of centromeres. Because every chromosome has a centromere.

                Every animal has an even number of chromosomes because each parent contributed half of the chromosomes to the new individual. In drawing of mitosis, some chromosomes are colored red and some are colored blue to represent that half of the chromosomes are derived from those contributed by one parent and the other half are derived from chromosomes from the other parent.

The Spindle

While it may seem easy to separate the chromatids of only for 4 duplicated chromosomes, imagine the task when there are 46 chromosomes, as in humans, or 78, as in dogs. Certainly it is helpful that chromosomes be highly condensed before the task begins, but clearly some mechanism is needed to complete separation in an organized manner. Most eukaryotic cells rely on a spindle, a cytoskeletal structure, to pull the chromatids apart. A spindle has spindle fiber made of microtubules that are able to assemble and disassemble. First, the microtubules assemble to form the spindle that takes over the center of the cell and separates the chromatids. Later, they disassemble.

                A centrosome is the primary microtubule organizing center of a cell. Centrosome duplication occurs at the start of the S phase of the cell cycle and is completed by G₂.during the first part of the M stage, the centrosome separate and move to opposite side of the nucleus, where they form the poles of the spindle. As the nuclear envelope breaks down, spindle fibers take over the center of the cell. Certain ones overlap at the spindle equator, which is midway between the poles. Others attach to duplicated chromosomes in a way that ensures the separation of the sister chromatids and their proper distribution to the daughter cells. Whereas the chromosomes will be inside the newly formed daughter nuclei, a centrosome will be just outside.

                Traditionally, mitosis is divided into sequence of event, even though it is a continuous process. We will describe mitosis as having four phase: prophase, metaphase, anaphase, and telophase. These phases for a dividing plant nucleus. Plant cell have centrosomes but they are not clearly visible especially because they lack centriols. In animal centriols.