Meiosis is the process whereby a cell reduces its chromosomes from diploid to haploid in creating eggs or sperm. See image 2545 for an unlabeled version of this illustration. See image 2544 for an unlabeled version of this illustration. Featured in The New Genetics.

A crystal of sheep hemoglobin protein created for X-ray crystallography, which can reveal detailed, three-dimensional protein structures.

This is a fibroblast, a connective tissue cell that plays an important role in wound healing. Normal fibroblasts have smooth edges. In contrast, this spiky cell is missing a protein that is necessary for proper construction of the cell's skeleton. Its jagged shape makes it impossible for the cell to move normally. In addition to compromising wound healing, abnormal cell movement can lead to birth defects, faulty immune function, and other health problems.

This image was part of the Life: Magnified exhibit that ran from June 3, 2014, to January 21, 2015, at Dulles International Airport.

PETase enzyme degrades polyester plastic (polyethylene terephthalate, or PET) into monohydroxyethyl terephthalate (MHET). Then, MHETase enzyme degrades MHET into its constituents ethylene glycol (EG) and terephthalic acid (TPA).

Find these in the RCSB Protein Data Bank: PET hydrolase (PDB entry 5XH3) and MHETase (PDB entry 6QGA).

RNA polymerase (purple) is a complex enzyme at the heart of transcription. During this process, the enzyme unwinds the DNA double helix and uses one strand (darker orange) as a template to create the single-stranded messenger RNA (green), later used by ribosomes for protein synthesis.

From the RNA polymerase II elongation complex of Saccharomyces cerevisiae (PDB entry 1I6H) as seen in PDB-101's What is a Protein? video.

Adult Drosophila abdominal fat tissue showing cell nuclei labelled in magenta. The upper panel is from well-fed flies, and the lower panel is from flies that have been deprived of food for 4 hours. Starvation results in the accumulation of a key adipokine—a fat hormone (blue-green dots).

Related to images 6982, 6983, and 6985.

Cdc42, a member of the Rho family of small guanosine triphosphatase (GTPase) proteins, regulates multiple cell functions, including motility, proliferation, apoptosis, and cell morphology. In order to fulfill these diverse roles, the timing and location of Cdc42 activation must be tightly controlled. Klaus Hahn and his research group use special dyes designed to report protein conformational changes and interactions, here in living neutrophil cells. Warmer colors in this image indicate higher levels of activation. Cdc42 looks to be activated at cell protrusions.

Related to images 2451, 2453, and 2454.

Drugs enter different layers of skin via intramuscular, subcutaneous, or transdermal delivery methods. See image 2532 for a labeled version of this illustration. Featured in Medicines By Design.

Cell-like compartments that spontaneously emerged from scrambled frog eggs, with nuclei (blue) from frog sperm. Endoplasmic reticulum (red) and microtubules (green) are also visible. Image created using epifluorescence microscopy.

For more photos of cell-like compartments from frog eggs view: 6585, 6586, 6591, 6592, and 6593.

For videos of cell-like compartments from frog eggs view: 6587, 6588, 6589, and 6590.

In the top snapshots, the brain of a sleep-deprived fruit fly glows orange, marking high concentrations of a synaptic protein called Bruchpilot (BRP) involved in communication between neurons. The color particularly lights up brain areas associated with learning. By contrast, the bottom images from a well-rested fly show lower levels of the protein. These pictures illustrate the results of an April 2009 study showing that sleep reduces the protein's levels, suggesting that such "downscaling" resets the brain to normal levels of synaptic activity and makes it ready to learn after a restful night.

This short video shows a model of the DNA helicase in yeast. This DNA helicase has 11 proteins that work together to unwind DNA during the process of copying it, called DNA replication. Scientists used a technique called cryo-electron microscopy (cryo-EM), which allowed them to study the helicase structure in solution rather than in static crystals. Cryo-EM in combination with computer modeling therefore allows researchers to see movements and other dynamic changes in the protein. The cryo-EM approach revealed the helicase structure at much greater resolution than could be obtained before. The researchers think that a repeated motion within the protein as shown in the video helps it move along the DNA strand. To read more about DNA helicase and this proposed mechanism, see this news release by Brookhaven National Laboratory.

Arranging exons in different patterns, called alternative splicing, enables cells to make different proteins from a single gene. See image 2553 for a labeled version of this illustration. Featured in The New Genetics.

The molecule on the left is an electrostatic potential map of the van der Waals surface of the transition state for human purine nucleoside phosphorylase. The colors indicate the electron density at any position of the molecule. Red indicates electron-rich regions with negative charge and blue indicates electron-poor regions with positive charge. The molecule on the right is called DADMe-ImmH. It is a chemically stable analogue of the transition state on the left. It binds to the enzyme millions of times tighter than the substrate. This inhibitor is in human clinical trials for treating patients with gout. This image appears in Figure 4, Schramm, V.L. (2011) Annu. Rev. Biochem. 80:703-732.

Centrioles (green) anchor cilia (red), which project on the surface of pharynx cells of the freshwater planarian Schmidtea mediterranea. Centrioles require cellular structures called centrosomes for assembly in other animal species, but this flatworm known for its regenerative ability was unexpectedly found to lack centrosomes. From a Stowers University news release.

To better understand cell movements in developing embryos, researchers isolated cells from early zebrafish embryos and grew them as clusters. Provided with the right signals, the clusters replicated some cell movements seen in intact embryos. Each line in this image depicts the movement of a single cell. The image was created using time-lapse confocal microscopy. Related to video 6776.

DNA consists of two long, twisted chains made up of nucleotides. Each nucleotide contains one base, one phosphate molecule, and the sugar molecule deoxyribose. The bases in DNA nucleotides are adenine, thymine, cytosine, and guanine. See image 2541 for an unlabeled version of this illustration. Featured in The New Genetics.

A crystal of bovine milk alpha-lactalbumin protein created for X-ray crystallography, which can reveal detailed, three-dimensional protein structures.

Three-dimensional image of misfolded proteins (green) within mitochondria (red). Related to image 5878. Learn more in this press release by The American Association for the Advancement of Science.

These yeast cells were exposed to a glucose (sugar) shortage. This caused the cells to compartmentalize HMGCR (green)—an enzyme involved in making cholesterol—to a patch on the nuclear envelope next to the vacuole/lysosome (purple). This process enhanced HMGCR activity and helped the yeast adapt to the glucose shortage. Researchers hope that understanding how yeast regulate cholesterol could ultimately lead to new ways to treat high cholesterol in people. This image was captured using a fluorescence microscope.

A Trigonium diatom imaged by a quantitative orientation-independent differential interference contrast (OI-DIC) microscope. Diatoms are single-celled photosynthetic algae with mineralized cell walls that contain silica and provide protection and support. These organisms form an important part of the plankton at the base of the marine and freshwater food chains. The width of this image is 90 μm.

More information about the microscopy that produced this image can be found in the Journal of Microscopy paper “An Orientation-Independent DIC Microscope Allows High Resolution Imaging of Epithelial Cell Migration and Wound Healing in a Cnidarian Model” by Malamy and Shribak.

Stained kidney tissue. The kidney is an essential organ responsible for disposing wastes from the body and for maintaining healthy ion levels in the blood. It also secretes two hormones, erythropoietin (EPO) and calcitriol (a derivative of vitamin D), into the blood. It works like a purifier by pulling break-down products of metabolism, such as urea and ammonium, from the blood stream for excretion in urine. Related to image 3725.

This video condenses 6.5 minutes into less than a minute to show how the toxin in bee venom, called melittin, destroys an animal or bacterial cell. What looks like a red balloon is an artificial cell filled with red dye. Melittin molecules are colored green and float on the cell's surface like twigs on a pond. As melittin accumulates on the cell's membrane, the membrane expands to accommodate it. In the video, the membrane stretches into a column on the left. When melittin levels reach a critical threshold, countless pinhole leaks burst open in the membrane. The cell's vital fluids (red dye in the video) leak out through these pores. Within minutes, the cell collapses.

Cross-section of skin anatomy shows layers and different tissue types.

DNA (blue) and actin (red) in cultured fibroblast cells.

A cross section of the measles virus in which six proteins (enlarged on the outside of the virus) work together to infect cells. The measles virus is extremely infectious; 9 out of 10 people exposed will contract the disease. Fortunately, an effective vaccine protects against infection. Portions of the proteins that have not been determined are shown with dots.

Learn more about the six proteins on PDB 101’s Molecule of the Month: Measles Virus Proteins. Structures are available for the ordered regions of nucleoprotein and phosphoprotein (PDB entries 5E4V, 3ZDO, 1T6O), but the remaining regions are thought to form a flexible, random tangle. For a larger look at the measles virus, see 6995.

Two adult fruit flies (Drosophila)

On the left, a cross-section slice of a rat pancreas cell captured using cryo-electron tomography (cryo-ET). On the right, a color-coded, 3D version of the image highlighting cell structures. Visible features include insulin vesicles (purple rings), insulin crystals (gray circles), microtubules (green rods), ribosomes (small yellow circles). The black line at the bottom right of the left image represents 200 nm. Related to image 6608.

A special "messy" region of a potassium ion channel is important in its function.

Multiphoton fluorescence image of HeLa cells stained with the actin binding toxin phalloidin (red), microtubules (cyan) and cell nuclei (blue). Nikon RTS2000MP custom laser scanning microscope. See related images 3518, 3519, 3520, 3522.

A cell in interphase, at the start of mitosis: Chromosomes duplicate, and the copies remain attached to each other. Mitosis is responsible for growth and development, as well as for replacing injured or worn out cells throughout the body. For simplicity, mitosis is illustrated here with only six chromosomes.

A tomographic reconstruction of the colon shows the location of large pools of HIV-1 virus particles (in blue) located in the spaces between adjacent cells. The purple objects within each sphere represent the conical cores that are one of the structural hallmarks of the HIV virus.

In SARS-CoV-2, the virus that causes COVID-19, nucleocapsid is a complex molecule with many functional parts. One section folds into an RNA-binding domain, with a groove that grips a short segment of the viral genomic RNA. Another section folds into a dimerization domain that brings two nucleocapsid molecules together. The rest of the protein is intrinsically disordered, forming tails at each end of the protein chain and a flexible linker that connects the two structured domains. These disordered regions assist with RNA binding and orchestrate association of nucleocapsid dimers into larger assemblies that package the RNA in the small space inside virions. Nucleocapsid is in magenta and purple, and short RNA strands are in yellow.

Find these in the RCSB Protein Data Bank: RNA-binding domain (PDB entry 7ACT) and Dimerization domain (PDB entry 6WJI).

A light microscope image of cells from the endosperm of an African globe lily (Scadoxus katherinae). This is one frame of a time-lapse sequence that shows cell division in action. The lily is considered a good organism for studying cell division because its chromosomes are much thicker and easier to see than human ones. Staining shows microtubules in red and chromosomes in blue. Here, condensed chromosomes are clearly visible and have separated into the opposite sides of a dividing cell.

Related to images 1010, 1012, 1013, 1014, 1015, 1016, 1017, 1018, 1019, and 1021.

The structure of a gene-regulating zinc finger protein bound to DNA.

By mixing fluorescent dyes like an artist mixes paints, scientists are able to color code individual chromosomes. The technique, abbreviated multicolor-FISH, allows researchers to visualize genetic abnormalities often linked to disease. In this image, "painted" chromosomes from a person with a hereditary disease called Werner Syndrome show where a piece of one chromosome has fused to another (see the gold-tipped maroon chromosome in the center). As reported by molecular biologist Jan Karlseder of the Salk Institute for Biological Studies, such damage is typical among people with this rare syndrome.

Myotonic dystrophy is thought to be caused by the binding of a protein called Mbnl1 to abnormal RNA repeats. In these two images of the same muscle precursor cell, the top image shows the location of the Mbnl1 splicing factor (green) and the bottom image shows the location of RNA repeats (red) inside the cell nucleus (blue). The white arrows point to two large foci in the cell nucleus where Mbnl1 is sequestered with RNA.

Dynein (green) is a motor protein that “walks” along microtubules (red, part of the cytoskeleton) and carries its cargo along with it. This video was captured through fluorescence microscopy.

Researchers use cluster analysis to study protein shape and function. Each green circle represents one potential shape of the protein mitoNEET. The longer the blue line between two circles, the greater the differences between the shapes. Most shapes are similar; they fall into three clusters that are represented by the three images of the protein. From a Rice University news release. Graduate student Elizabeth Baxter and Patricia Jennings, professor of chemistry and biochemistry at UCSD, collaborated with José Onuchic, a physicist at Rice University, on this work.

The human body keeps time with a master clock called the suprachiasmatic nucleus or SCN. Situated inside the brain, it's a tiny sliver of tissue about the size of a grain of rice, located behind the eyes. It sits quite close to the optic nerve, which controls vision, and this means that the SCN "clock" can keep track of day and night. The SCN helps control sleep by coordinating the actions of billions of miniature "clocks" throughout the body. These aren't actually clocks, but rather are ensembles of genes inside clusters of cells that switch on and off in a regular, 24-hour cycle in our physiological day.

A blue laser beam turns on a protein that helps this human cancer cell move. Responding to the stimulus, the protein, called Rac1, first creates ruffles at the edge of the cell. Then it stretches the cell forward, following the light like a horse trotting after a carrot on a stick. This new light-based approach can turn Rac1 (and potentially many other proteins) on and off at exact times and places in living cells. By manipulating a protein that controls movement, the technique also offers a new tool to study embryonic development, nerve regeneration and cancer.

This image captures Purkinje cells (red), one of the main types of nerve cell found in the brain. These cells have elaborate branching structures called dendrites that receive signals from other nerve cells.

This image was part of the Life: Magnified exhibit that ran from June 3, 2014, to January 21, 2015, at Dulles International Airport.

These neural precursor cells were derived from human embryonic stem cells. The neural cell bodies are stained red, and the nuclei are blue. Image and caption information courtesy of the California Institute for Regenerative Medicine.

A genetic disorder of the nervous system, neurofibromatosis causes tumors to form on nerves throughout the body, including a type of tumor called an optic nerve glioma that can result in childhood blindness. The image was used to demonstrate the unique imaging capabilities of one of our newest (at the time) laser scanning microscopes and is of a wildtype (normal) mouse retina in the optic fiber layer. This layer is responsible for relaying information from the retina to the brain and was fluorescently stained to reveal the distribution of glial cells (green), DNA and RNA in the cell bodies of the retinal ganglion neurons (orange) and their optic nerve fibers (red), and actin in endothelial cells surrounding a prominent branching blood vessel (blue). By studying the microscopic structure of normal and diseased retina and optic nerves, we hope to better understand the altered biology of the tissues in these tumors with the prospects of developing therapeutic interventions.

The three fibers of the cytoskeleton--microtubules in blue, intermediate filaments in red, and actin in green--play countless roles in the cell.

Acetylsalicylate (bottom) is the aspirin of today. Adding a chemical tag called an acetyl group (shaded box, bottom) to a molecule derived from willow bark (salicylate, top) makes the molecule less acidic (and easier on the lining of the digestive tract), but still effective at relieving pain. See image 2530 for a labeled version of this illustration. Featured in Medicines By Design.

Multiphoton fluorescence image of cultured HeLa cells with a fluorescent protein targeted to the Golgi apparatus (orange), microtubules (green) and counterstained for DNA (cyan). Nikon RTS2000MP custom laser scanning microscope. See related images 3518, 3519, 3520, 3521.

Human embryonic stem cells differentiated into dopaminergic neurons, the type that degenerate in Parkinson's disease. Image courtesy of the California Institute for Regenerative Medicine. Related to images 3271 and 3285.

CCD-1 is an enzyme produced by the bacterium Clostridioides difficile that helps it resist antibiotics. Using X-ray crystallography, researchers determined the structure of a complex between CCD-1 and the antibiotic cefotaxime (purple, yellow, and blue molecule). The structure revealed that CCD-1 provides extensive hydrogen bonding (shown as dotted lines) and stabilization of the antibiotic in the active site, leading to efficient degradation of the antibiotic.

Related to images 6764, 6765, and 6766.

A number of environmental factors cause DNA mutations that can lead to cancer: toxins in cigarette smoke, sunlight and other radiation, and some viruses.

A computer-generated sketch of a DNA origami folded into a flower-and-bird structure. See also related image 3690.