Tuesday, December 18, 2007

Messenger RNA (mRNA)

The "life cycle" of an mRNA in a eukaryotic cell. RNA is transcribed in the nucleus; once completely processed, it is transported to the cytoplasm and translated by the ribosome.  At the end of its life, the mRNA is degraded.
The "life cycle" of an mRNA in a eukaryotic cell. RNA is transcribed in the nucleus; once completely processed, it is transported to the cytoplasm and translated by the ribosome. At the end of its life, the mRNA is degraded.

Messenger Ribonucleic Acid (mRNA) is a molecule of RNA encoding a chemical "blueprint" for a protein product. mRNA is transcribed from a DNA template, and carries coding information to the sites of protein synthesis: the ribosomes. Here, the nucleic acid polymer is translated into a polymer of amino acids: a protein. In mRNA as in DNA, genetic information is encoded in the sequence of four nucleotides arranged into codons of three bases each. Each codon encodes for a specific amino acid, except the stop codons that terminate protein synthesis. This process requires two other types of RNA: Transfer RNA (tRNA) mediates recognition of the codon and provides the corresponding amino acid, while Ribosomal RNA (rRNA) is the central component of the ribosome's protein manufacturing machinery.

mRNA "life cycle"

The brief life of an mRNA molecule begins with transcription and ultimately ends in degradation. During its life, an mRNA molecule may also be processed, edited, and transported prior to translation. Eukaryotic mRNA molecules often require extensive processing and transport, while prokaryotic molecules do not.


During transcription, RNA polymerase makes a copy of a gene from the DNA to mRNA as needed. This process is similar in eukaryotes and prokaryotes. One notable difference, however, is that eukaryotic RNA polymerase associates with mRNA processing enzymes during transcription so that processing can proceed quickly after the start of transcription. The short-lived, unprocessed or partially processed, product is termed pre-mRNA; once completely processed, it is termed mature mRNA.

Eukaryotic pre-mRNA processing

Processing of mRNA differs greatly among eukaryotes, bacteria and archea. Non-eukaryotic mRNA is essentially mature upon transcription and requires no processing, except in rare cases. Eukaryotic pre-mRNA, however, requires extensive processing.

5' cap addition

A 5' cap (also termed an RNA cap, an RNA 7-methylguanosine cap or an RNA m7G cap) is a modified guanine nucleotide that has been added to the "front" or 5' end of a eukaryotic messenger RNA shortly after the start of transcription. The 5' cap consists of a terminal 7-methylguanosine residue which is linked through a 5'-5'-triphosphate bond to the first transcribed nucleotide. Its presence is critical for recognition by the ribosome and protection from RNases.

Cap addition is coupled to transcription, and occurs co-transcriptionally, such that each influences the other. Shortly after the start of transcription, the 5' end of the mRNA being synthesized is bound by a cap-synthesizing complex associated with RNA polymerase. This enzymatic complex catalyzes the chemical reactions that are required for mRNA capping. Synthesis proceeds as a multi-step biochemical reaction.


Splicing is the process by which pre-mRNA is modified to remove certain stretches of non-coding sequences called introns; the stretches that remain include protein-coding sequences and are called exons. Sometimes pre-mRNA messages may be spliced in several different ways, allowing a single gene to encode multiple proteins. This process is called alternative splicing. Splicing is usually performed by an RNA-protein complex called the spliceosome, but some RNA molecules are also capable of catalyzing their own splicing (see ribozymes).


In some instances, an mRNA will be edited, changing the nucleotide composition of that mRNA. An example in humans is the apolipoprotein B mRNA, which is edited in some tissues, but not others. The editing creates an early stop codon, which upon translation, produces a shorter protein.


Polyadenylation is the covalent linkage of a polyadenylyl moiety to a messenger RNA molecule. In eukaryotic organisms, most messenger RNA (mRNA) molecules are polyadenylated at the 3' end. The poly(A) tail and the protein bound to it aid in protecting mRNA from degradation by exonucleases. Polyadenylation is also important for transcription termination, export of the mRNA from the nucleus, and translation. mRNA can also be polyadenylated in prokaryotic organisms, where poly(A) tails act to facilitate, rather than impede, exonucleolytic degradation.

Polyadenylation occurs during and immediately after transcription of DNA into RNA. After transcription has been terminated, the mRNA chain is cleaved through the action of an endonuclease complex associated with RNA polymerase. The cleavage site is characterized by the presence of the base sequence AAUAAA near the cleavage site. After the mRNA has been cleaved, 80 to 250 adenosine residues are added to the free 3' end at the cleavage site. This reaction is catalyzed by polyadenylate polymerase.


Another difference between eukaryotes and prokaryotes is mRNA transport. Because eukaryotic transcription and translation is compartmentally separated, eukaryotic mRNAs must be exported from the nucleus to the cytoplasm. Mature mRNAs are recognized by their processed modifications and then exported through the nuclear pore.


Because prokaryotic mRNA does not need to be processed or transported, translation by the ribosome can begin immediately after the end of transcription. Therefore, it can be said that prokaryotic translation is coupled to transcription and occurs co-transcriptionally.

Eukaryotic mRNA that has been processed and transported to the cytoplasm (i.e. mature mRNA) can then be translated by the ribosome. Translation may occur at ribosomes free-floating in the cytoplasm, or directed to the endoplasmic reticulum by the signal recognition particle. Therefore, unlike prokaryotes, eukaryotic translation is not directly coupled to transcription.


After a certain amount of time, the message is degraded by RNases into its component nucleotides. The limited longevity of mRNA enables a cell to alter protein synthesis rapidly in response to its changing needs.

Different mRNAs within the same cell have distinct lifetimes. In bacterial cells, individual mRNAs can survive from seconds to more than an hour; in mammalian cells, mRNA lifetimes range from several minutes to days. The greater the stability of an mRNA, the more protein may be produced from that transcript. The presence of AU-rich motifs in some mammalian mRNAs tends to destabilize those transcripts through the action of cellular proteins that bind these motifs. Rapid mRNA degradation via AU-rich motifs is a critical mechanism for preventing the overproduction of potent cytokines such as tumor necrosis factor (TNF) and granulocyte-macrophage colony stimulating factor (GM-CSF).[1][verification needed] Base pairing with a small interfering RNA (siRNA) or microRNA (miRNA) can also accelerate mRNA degradation.

mRNA structure

The structure of a mature eukaryotic mRNA. A fully processed mRNA includes a 5' cap, 5' UTR, coding region, 3' UTR, and poly(A) tail.
The structure of a mature eukaryotic mRNA. A fully processed mRNA includes a 5' cap, 5' UTR, coding region, 3' UTR, and poly(A) tail.

5' cap

The 5' cap is a modified guanine nucleotide added to the "front" (5' end) of the pre-mRNA using a 5',5-Triphosphate linkage. This modification is critical for recognition and proper attachment of mRNA to the ribosome, as well as protection from 5' exonucleases. It may also be important for other essential processes, such as splicing and transport.

Coding regions

Coding regions are composed of codons, which are decoded and translated into one (mostly eukaryotes) or several (mostly prokaryotes) proteins by the ribosome. Coding regions begin with the start codon and end with the one of three possible stop codons. In addition to protein-coding, portions of coding regions may also serve as regulatory sequences in the pre-mRNA as exonic splicing enhancers or exonic splicing silencers. Start codons are indicated by a AUG triplet. Stop codons are indicated by a UAA, UAG, or UGA.

Untranslated regions

Untranslated regions (UTRs) are sections of the RNA before the start codon and after the stop codon that are not translated, termed the five prime untranslated region (5' UTR) and three prime untranslated region (3' UTR), respectively. These regions are transcribed as part of the same transcript as the coding region. Several roles in gene expression have been attributed to the untranslated regions, including mRNA stability, mRNA localization, and translational efficiency. The ability of a UTR to perform these functions depends on the sequence of the UTR and can differ between mRNAs.

The stability of mRNAs may be controlled by the 5' UTR and/or 3' UTR due to varying affinity for RNA degrading enzymes called ribonucleases and for ancillary proteins that can promote or inhibit RNA degradation.

Translational efficiency, including sometimes the complete inhibition of translation, can be controlled by UTRs. Proteins that bind to either the 3' or 5' UTR may affect translation by influencing the ribosome's ability to bind to the mRNA. MicroRNAs bound to the 3' UTR also may affect translational efficiency or mRNA stability.

Cytoplasmic localization of mRNA is thought to be a function of the 3' UTR. Proteins that are needed in a particular region of the cell can actually be translated there; in such a case, the 3' UTR may contain sequences that allow the transcript to be localized to this region for translation.

Some of the elements contained in untranslated regions form a characteristic secondary structure when transcribed into RNA. These structural mRNA elements are involved in regulating the mRNA. Some, such as the SECIS element, are targets for proteins to bind. One class of mRNA element, the riboswitches, directly bind small molecules, changing their fold to modify levels of transcription or translation. In these cases, the mRNA regulates itself.

3' poly(A) tail

The 3' poly(A) tail is a long sequence of adenine nucleotides (often several hundred) added to the "tail" or 3' end of the pre-mRNA through the action of an enzyme, polyadenylate polymerase. In higher eukaryotes, the poly(A) tail is added onto transcripts that contain a specific sequence, the AAUAAA signal. The importance of the AAUAAA signal is demonstrated by a mutation in the human alpha 2-globin gene that changes the original sequence AATAAA into AATAAG, which can lead to hemoglobin deficiencies.[2]

Monocistronic versus polycistronic mRNA

An mRNA molecule is said to be monocistronic when it contains the genetic information to translate only a single protein. This is the case for most of the eukaryotic mRNAs[3]. On the other hand, polycistronic mRNA carries the information of several proteins, which are translated into several proteins. Most of the mRNA found in bacteria and archea are polycistronic[3]. Dicistronic is the term used to describe a mRNA that encodes only two proteins.

Anti-sense mRNA

During transcription, double stranded DNA produces mRNA from the sense strand; the other, complementary, strand of DNA is termed anti-sense. Anti-sense mRNA is an RNA complementary in sequence to one or more mRNAs. In some organisms, the presence of an anti-sense mRNA can inhibit gene expression by base-pairing with the specific mRNAs. In biochemical research, this effect has been used to study gene function, by simply shutting down the studied gene by adding its anti-sense mRNA transcript. Such studies have been done on the worm Caenorhabditis elegans and the bacterium Escherichia coli. This plays a part in RNA interference and RNA transcription.



Types of epithelium
Types of epithelium

In biology and medicine, epithelium is a tissue composed of layers of cells which line the cavities and surfaces of structures throughout the body. It is also the type of tissue of which many glands are formed. Epithelium lines both the outside (skin) and the inside cavities and lumen of bodies. The outermost layer of our skin is composed of dead stratified squamous, keratinized epithelial cells.

Mucous membranes lining the inside of the mouth, the oesophagus, and part of the rectum are lined by nonkeratinized stratified squamous epithelium. Other, open to outside body cavities are lined by simple squamous or columnar epithelial cells.

Other epithelial cells line the insides of the lungs, the gastrointestinal tract, the reproductive and urinary tracts, and make up the exocrine and endocrine glands. The outer surface of the cornea is covered with fast-growing, easily-regenerated epithelial cells.

Functions of epithelial cells include secretion, absorption, protection, transcellular transport, sensation detection, and selective permeability.

Endothelium (the inner lining of blood vessels, the heart, and lymphatic vessels) is a specialized form of epithelium. Another type, Mesothelium, forms the walls of the pericardium, pleurae, and peritoneum.

In humans, epithelium is classified as a primary body tissue, the other ones being connective tissue, muscle tissue and nervous tissue.


Epithelial cells are classified by the following three factors:

  • Shape
  • Stratification
  • Specializations


  • Squamous: All Squamous cells are flat cells with an irregular flattened shape. A one-cell layer of simple squamous epithelium forms the alveoli of the respiratory membrane, and the endothelium of capillaries, and is a minimal barrier to diffusion. squamous cells can be found include the filtration tubules of the kidneys, and the major cavities of the body. These cells are relatively inactive metabolically, and are associated with the diffusion of water, electrolytes, and other substances.
  • Cuboidal: As the name suggests, these cells have a shape similar to a cube, meaning its width is the same size as its height. The nuclei of these cells are usually located in the center.
  • Columnar: These cells are taller than they are wide. Simple columnar epithelium is made up of a single layer of cells that are longer than they are wide. The nucleus is also closer to the base of the cell. The small intestine is a tubular organ lined with this type of tissue. Unicellular glands called goblet cells are scattered throughout the simple columnar epithelial cells and secrete mucus. The free surface of the columnar cell has tiny hairlike projections called microvilli. They increase the surface area for absorption.
  • Transitional: This is a specialized type of epithelium found lining organs that can stretch, such as the urothelium that lines the bladder and ureter of mammals. Since the cells can slide over each other, the appearance of this epithelium depends on whether the organ is distended or contracted: if distended, it appears as if there are only a few layers; when contracted, it appears as if there are several layers.


  • Simple: There is a single layer of cells.
  • Stratified: More than one layer of cells. The superficial layer is used to classify the layer. Only one layer touches the basal lamina. Stratified cells can usually withstand large amounts of stress.
  • Pseudostratified with cilia: This is used mainly in one type of classification (pseudostratified columnar epithelium). There is only a single layer of cells, but the position of the nuclei gives the impression that it is stratified. If a specimen looks stratified, but you can identify cilia, the specimen is pseudostratified ciliated epithelium since stratified epithelium cannot have cilia but may be very rarely found in fetal oesophagus. A cell that contains hairs will be around ten times stronger than a regular cell


  • Keratinized cells contain keratin (a cytoskeletal protein). While keratinized epithelium occurs mainly in the skin, it is also found in the mouth and nose, providing a tough, impermeable barrier.


System ↓ Tissue ↓ Epithelium ↓ Subtype ↓
circulatory blood vessels Simple squamous endothelium
digestive ducts of submandibular glands Stratified columnar -
digestive attached gingiva Stratified squamous, keratinized -
digestive dorsum of tongue Stratified squamous, keratinized -
digestive hard palate Stratified squamous, keratinized -
digestive esophagus Stratified squamous, non-keratinised -
digestive stomach Simple columnar, non-ciliated -
digestive small intestine Simple columnar, non-ciliated -
digestive large intestine Simple columnar, non-ciliated -
digestive rectum Stratified squamous, non-keratinised -
digestive anus Stratified squamous, keratinised -
digestive gallbladder Simple columnar, non-ciliated -
endocrine thyroid follicles Simple cuboidal -
nervous ependyma Simple cuboidal -
lymphatic lymph vessel Simple squamous endothelium
integumentary skin - dead superficial layer Stratified squamous, keratinized -
integumentary sweat gland ducts Stratified cuboidal -
integumentary mesothelium of body cavities Simple squamous -
reproductive - female ovaries Simple cuboidal germinal epithelium (female)
reproductive - female Fallopian tubes Simple columnar, ciliated -
reproductive - female uterus Simple columnar, ciliated -
reproductive - female endometrium Simple columnar -
reproductive - female cervix (endocervix) Simple columnar -
reproductive - female cervix (ectocervix) Stratified squamous, non-keratinised -
reproductive - female vagina Stratified squamous, non-keratinised -
reproductive - female labia majora Stratified squamous, keratinised -
reproductive - male tubuli recti Simple cuboidal germinal epithelium (male)
reproductive - male rete testis Simple cuboidal -
reproductive - male ductuli efferentes Pseudostratified columnar -
reproductive - male epididymis Pseudostratified columnar, with stereocilia -
reproductive - male vas deferens Pseudostratified columnar -
reproductive - male ejaculatory duct Simple columnar -
reproductive - male (gland) bulbourethral glands Simple columnar -
reproductive - male (gland) seminal vesicle Pseudostratified columnar -
respiratory oropharynx Stratified squamous, non-keratinised -
respiratory larynx Pseudostratified columnar, ciliated respiratory epithelium
respiratory trachea Pseudostratified columnar, ciliated respiratory epithelium
respiratory respiratory bronchioles Simple cuboidal -
sensory cornea Stratified squamous, non-keratinised corneal epithelium
sensory nose Pseudostratified columnar olfactory epithelium
urinary kidney - proximal convoluted tubule Simple columnar, ciliated -
urinary kidney - ascending thin limb Simple squamous -
urinary kidney - distal convoluted tubule Simple columnar, non-ciliated -
urinary kidney - collecting duct Simple cuboidal -
urinary renal pelvis Transitional urothelium
urinary ureter Transitional urothelium
urinary urinary bladder Transitional urothelium
urinary prostatic urethra Transitional urothelium
urinary membranous urethra Pseudostratified columnar, non-ciliated -
urinary penile urethra Pseudostratified columnar, non-ciliated -
urinary external urethral orifice Stratified squamous -

Cell junctions

A cell junction is a structure within a tissue of a multicellular organism. Cell junctions are especially abundant in epithelial tissues. They consist of protein complexes and provide contact between neighbouring cells, between a cell and the extracellular matrix, or they built up the paracellular barrier of epithelia and control the paracellular transport.

Secretory epithelia

As stated above, secretion is one major function of epithelial cells. Glands are formed from the invagination / infolding of epithelial cells and subsequent growth in the underlying connective tissue. There are two major classification of glands: endocrine glands and exocrine glands. Endocrine glands are glands that secrete their product directly onto a surface rather than through a duct. This group contains the glands of the Endocrine system



Wöhler observes the synthesis of urea.
Wöhler observes the synthesis of urea.

Biochemistry (from Greek: βίος, bios, "life" and Egyptian kēme, "earth"[1]) is the study of the chemical processes in living organisms. It deals with the structure and function of cellular components, such as proteins, carbohydrates, lipids, nucleic acids, and other biomolecules. Chemical biology aims to answer many questions arising from biochemistry by using tools developed within chemical synthesis.

Although there are a vast number of different biomolecules, many are complex and large molecules (called polymers) that are composed of similar repeating subunits (called monomers). Each class of polymeric biomolecule has a different set of subunit types. For example, a protein is a polymer made up of 20 or more amino acids. Biochemistry studies the chemical properties of important biological molecules, like proteins, in particular the chemistry of enzyme-catalyzed reactions.

The biochemistry of cell metabolism and the endocrine system has been extensively described. Other areas of biochemistry include the genetic code (DNA, RNA), protein synthesis, cell membrane transport, and signal transduction.

This article only discusses terrestrial biochemistry (carbon- and water-based), as all the life forms we know are on Earth. Since life forms alive today are hypothesized by most to have descended from the same common ancestor, they have similar biochemistries, even for matters that seem to be essentially arbitrary, such as handedness of various biomolecules. It is unknown whether alternative biochemistries are possible or practical.

History of biochemistry

Friedrich Wöhler
Friedrich Wöhler

Originally, it was generally believed that life was not subject to the laws of science the way non-life was. It was thought that only living beings could produce the molecules of life (from other, previously existing biomolecules). Then, in 1828, Friedrich Wöhler published a paper on the synthesis of urea, proving that organic compounds can be created artificially.[2][3]

The dawn of biochemistry may have been the discovery of the first enzyme, diastase (today called amylase), in 1833 by Anselme Payen. Eduard Buchner contributed the first demonstration of a complex biochemical process outside of a cell in 1896: alcoholic fermentation in cell extracts of yeast. Although the term “biochemistry” seems to have been first used in 1882, it is generally accepted that the formal coinage of biochemistry occurred in 1903 by Carl Neuberg, a German chemist. Previously, this area would have been referred to as physiological chemistry. Since then, biochemistry has advanced, especially since the mid-20th century, with the development of new techniques such as chromatography, X-ray diffraction, NMR spectroscopy, radioisotopic labeling, electron microscopy and molecular dynamics simulations. These techniques allowed for the discovery and detailed analysis of many molecules and metabolic pathways of the cell, such as glycolysis and the Krebs cycle (citric acid cycle).

Today, the findings of biochemistry are used in many areas, from genetics to molecular biology and from agriculture to medicine.


Molecular biology

Molecular biology is the study of biology at a molecular level. The field overlaps with other areas of biology and chemistry, particularly genetics and biochemistry. Molecular biology chiefly concerns itself with understanding the interactions between the various systems of a cell, including the interactions between DNA, RNA and protein biosynthesis and learning how these interactions are regulated.

Writing in Nature, William Astbury described molecular biology as:

"... not so much a technique as an approach, an approach from the viewpoint of the so-called basic sciences with the leading idea of searching below the large-scale manifestations of classical biology for the corresponding molecular plan. It is concerned particularly with the forms of biological molecules and ..... is predominantly three-dimensional and structural - which does not mean, however, that it is merely a refinement of morphology - it must at the same time inquire into genesis and function." [1]

Relationship to other "molecular-scale" biological sciences

Schematic relationship between biochemistry, genetics and molecular biology
Schematic relationship between biochemistry, genetics and molecular biology

Researchers in molecular biology use specific techniques native to molecular biology (see Techniques section later in article), but increasingly combine these with techniques and ideas from genetics and biochemistry. There is not a defined line between these disciplines. Today the terms molecular biology and biochemistry are nearly interchangeable. The following figure is a schematic that depicts one possible view of the relationship between the fields:

  • Biochemistry is the study of the chemical substances and vital processes occurring in living organisms. Biochemists focus heavily on the role, function, and structure of biomolecules. The study of the chemistry behind biological processes and the synthesis of biologically active molecules are examples of biochemistry.
  • Genetics is the study of the effect of genetic differences on organisms. Often this can be inferred by the absence of a normal component (e.g. one gene). The study of "mutants" – organisms which lack one or more functional components with respect to the so-called "wild type" or normal phenotype. Genetic interactions such as epistasis can often confound simple interpretations of such "knock-out" studies.
  • Molecular biology is the study of molecular underpinnings of the process of replication, transcription and translation of the genetic material. The central dogma of molecular biology where genetic material is transcribed into RNA and then translated into protein, despite being an oversimplified picture of molecular biology, still provides a good starting point for understanding the field. This picture, however, is undergoing revision in light of emerging novel roles for RNA.

Much of the work in molecular biology is quantitative, and recently much work has been done at the interface of molecular biology and computer science in bioinformatics and computational biology. As of the early 2000s, the study of gene structure and function, molecular genetics, has been amongst the most prominent sub-field of molecular biology.

Increasingly many other fields of biology focus on molecules, either directly studying their interactions in their own right such as in cell biology and developmental biology, or indirectly, where the techniques of molecular biology are used to infer historical attributes of populations or species, as in fields in evolutionary biology such as population genetics and phylogenetics. There is also a long tradition of studying biomolecules "from the ground up" in biophysics.



Embryology is the study of the development of an embryo. An embryo is defined as any organism in a stage before birth or hatching, or in plants, before germination occurs.

Morula, 8 cell stage
Morula, 8 cell stage

Embryology refers to the development of the fertilized egg cell (zygote) and its differentiation into tissues and organs. After cleavage, the dividing cells, or morula, becomes a hollow ball, or blastula, which develops a hole or pore at one end.

1 - morula, 2 - blastula
1 - morula, 2 - blastula

In animals, the blastula develops in one of two ways that divides the whole animal kingdom into two halves. If in the blastula the first pore (blastopore) becomes the mouth of the animal, it is a protostome; if the first pore becomes the anus then it is a deuterostome. The protostomes include most invertebrate animals, such as insects, worms and molluscs, while the deuterostomes includes more advanced animals including the vertebrates. In due course, the blastula changes into a more differentiated structure called the gastrula.

1 - blastula, 2 - gastrula with blastopore; orange - ectoderm, red - endoderm.
1 - blastula, 2 - gastrula with blastopore; orange - ectoderm, red - endoderm.

The gastrula with its blastopore soon develops three distinct layers of cells (the germ layers) from which all the bodily organs and tissues then develop:

  • The innermost layer, or endoderm, gives rise to the digestive organs, lungs and bladder.
  • The middle layer, or mesoderm, gives rise to the muscles, skeleton and blood system.
  • The outer layer of cells, or ectoderm, gives rise to the nervous system and skin.

In humans, the term embryo refers to the ball of dividing cells from the moment the zygote implants itself in the uterus wall until the end of the eighth week after conception. Beyond the eighth week, the developing human is then called a fetus.


Developmental biology

"Views of a Fetus in the Womb", Leonardo da Vinci, ca. 1510-1512. The subject of prenatal development is a major subset of developmental biology.
"Views of a Fetus in the Womb", Leonardo da Vinci, ca. 1510-1512. The subject of prenatal development is a major subset of developmental biology.

Developmental biology is the study of the process by which organisms grow and develop. Modern developmental biology studies the genetic control of cell growth, differentiation and "morphogenesis," which is the process that gives rise to tissues, organs and anatomy.



An agar plate streaked with microorganisms
An agar plate streaked with microorganisms

Microbiology is the study of microorganisms, which are unicellular or cell-cluster microscopic organisms.[1] This includes eukaryotes such as fungi and protists, and prokaryotes such as bacteria and certain algae. Viruses, though not strictly classed as living organisms, are also studied.[2] Microbiology is a broad term which includes many branches like virology, mycology, parasitology and others. A person who specializes in the area of microbiology is called a microbiologist.

Although much is now known in the field of microbiology, advances are being made regularly. We have probably only studied about 1% of all of the microbes on Earth.[3] Thus, despite the fact that over three hundred years have passed since the discovery of microbes, the field of microbiology could be said to be in its infancy relative to other biological disciplines such as zoology, botany and entomology.


Cell biology

Cell biology (also called cellular biology or formerly cytology, from the Greek kytos, "container") is an academic discipline that studies cells. This includes their physiological properties, their structure, the organelles they contain, interactions with their environment, their life cycle, division and death. This is done both on a microscopic and molecular level. Cell biology research extends to both the great diversity of single-celled organisms like bacteria and the many specialized cells in multicellular organisms like humans.

Knowing the composition of cells and how cells work is fundamental to all of the biological sciences. Appreciating the similarities and differences between cell types is particularly important to the fields of cell and molecular biology. These fundamental similarities and differences provide a unifying theme, allowing the principles learned from studying one cell type to be extrapolated and generalized to other cell types. Research in cell biology is closely related to genetics, biochemistry, molecular biology and developmental biology.

Understanding cells in terms of their molecular components.
Understanding cells in terms of their molecular components.


Movement of proteins

Proteins (red and green stain) at different locations in a cell.
Proteins (red and green stain) at different locations in a cell.

Each type of protein is usually sent to a particular part of the cell. An important part of cell biology is the investigation of molecular mechanisms by which proteins are moved to different places inside cells or secreted from cells.

Most proteins are synthesized by ribosomes in the cytoplasm. This process is also known as protein biosynthesis or simply protein translation. Some proteins, such as those to be incorporated in membranes membrane proteins, are transported into the ER or endoplasmic reticulum during synthesis and further processed in the Golgi apparatus. From the Golgi, membrane proteins can move to the plasma membrane, to other subcellular compartments or they can be secreted from the cell. The ER and Golgi can be thought of as the "membrane protein synthesis compartment" and the "membrane protein processing compartment", respectively. There is a semi-constant flux of proteins through these compartments. ER and Golgi-resident proteins associate with other proteins but remain in their respective compartments. Other proteins "flow" through the ER and Golgi to the plasma membrane. Motor proteins transport membrane protein-containing vesicles along cytoskeletal tracks to distant parts of cells such as axon terminals.

Some proteins that are made in the cytoplasm contain structural features that target them for transport into mitochondria or the nucleus. Some mitochondrial proteins are made inside mitochondria and are coded for by mitochondrial DNA. In plants, chloroplasts also make some cell proteins.

Extracellular and cell surface proteins destined to be degraded can move back into intracellular compartments upon being incorporated into endocytosed vesicles. Some of these vesicles fuse with lysosomes where the proteins are broken down to their individual amino acids. The degradation of some membrane proteins begins while still at the cell surface when they are cleaved by secretases. Proteins that function in the cytoplasm are often degraded by proteasomes.

Other cellular processes

Internal cellular structures


Techniques used to study cells

Purification of cells and their parts

Purification may be performed using the following methods: