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.

Learn about how bacteria and viruses differ, how they each can make you sick, and how they can or cannot be treated. Discover more resources from NIGMS’ Pathways collaboration with Scholastic. View the video on YouTube for closed captioning.

Pigment cells are cells that give skin its color. In fishes and amphibians, like frogs and salamanders, pigment cells are responsible for the characteristic skin patterns that help these organisms to blend into their surroundings or attract mates. The pigment cells are derived from neural crest cells, which are cells originating from the neural tube in the early embryo. Investigating pigment cell formation and migration in animals helps answer important fundamental questions about the factors that control pigmentation in the skin of animals, including humans. This image shows a pigment cell from zebrafish at high resolution. Related to images 5755, 5756, 5757 and 5758.

Sensory organs have cells equipped for detecting signals from the environment, such as odors. Receptors in the membranes of nerve cells in the nose bind to odor molecules, triggering a cascade of chemical reactions tranferred by G proteins into the cytoplasm.

This painting shows a cross section of a small respiratory droplet, like the ones that are thought to transmit SARS-CoV-2, the virus that causes COVID-19. The virus is shown in pink, and the droplet is also filled with molecules that are present in the respiratory tract, including mucins (green), pulmonary surfactant proteins and lipids (blue), and antibodies (tan).

A replica of a human retina grown from stem cells. It shows rod photoreceptors (nerve cells responsible for dark vision) in green and red/green cones (nerve cells responsible for red and green color vision) in red. The cell nuclei are stained blue. This image was captured using a confocal microscope.

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 image visualizes a part of the yeast molecular interaction network. The lines in the network represent connections among genes (shown as little dots) and different-colored networks indicate subnetworks, for instance, those in specific locations or pathways in the cell. Researchers use gene or protein expression data to build these networks; the network shown here was visualized with a program called Cytoscape. By following changes in the architectures of these networks in response to altered environmental conditions, scientists can home in on those genes that become central "hubs" (highly connected genes), for example, when a cell encounters stress. They can then further investigate the precise role of these genes to uncover how a cell's molecular machinery deals with stress or other factors. Related to images 3732 and 3733.

This image shows the structure of a synapse, or junction between two nerve cells in three dimensions. From the brain of a mouse.

Hydra magnipapillata is an invertebrate animal used as a model organism to study developmental questions, for example the formation of the body axis.

The receptor is shown bound to an inverse agonist, ZM241385.

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.

Endocytosis is the process by which cells are able to take up membrane and extracellular materials through the formation of a small intracellular bubble, called a vesicle. This process, called membrane budding, is generally by a coating of proteins. This protein coat helps both to deform the membrane and to concentrate specific proteins inside the newly forming vesicle. Clathrin is a coat protein that functions in receptor-mediated endocytosis events at the plasma membrane. This animation shows the process of clathrin-mediated endocytosis. An iron-transport protein called transferrin (blue) is bound to its receptor (purple) on the exterior cell membrane.  Inside the cell, a clathrin cage (shown in white/beige) assembles through interactions with membrane-bound adaptor proteins (green), causing the cell membrane to begin bending. The adaptor proteins also bind to receptors for transferrin, capturing them in the growing vesicle. Molecules of a protein called dynamin (purple) are then recruited to the neck of the vesicle and are involved in separating the membranes of the cell and the vesicle. Soon after the vesicle has budded off the membrane, the clathrin cage is disassembled. This disassembly is mediated by another protein called HSC70 (yellow), and its cofactor protein auxilin (orange).

This 2-hour-old fly embryo already has a blueprint for its formation, and the process for following it is so precise that the difference of just a few key molecules can change the plans. Here, blue marks a high concentration of Bicoid, a key signaling protein that directs the formation of the fly's head. It also regulates another important protein, Hunchback (green), that further maps the head and thorax structures and partitions the embryo in half (red is DNA). The yellow dots overlaying the embryo plot the concentration of Bicoid versus Hunchback proteins within each nucleus. The image illustrates the precision with which an embryo interprets and locates its halfway boundary, approaching limits set by simple physical principles. This image was a finalist in the 2008 Drosophila Image Award.

A 3D model of the human endoplasmic reticulum membrane protein complex (EMC) that identifies its nine essential subunits. The EMC plays an important role in making membrane proteins, which are essential for all cellular processes. This is the first atomic-level depiction of the EMC. Its structure was obtained using single-particle cryo-electron microscopy.

Speckle microscopy analysis of actin cytoskeleton force. This is an example of NIH-supported research on single-cell analysis. Images in related series; Related to 2799, 2800, 2801, 2802 and 2803.

Automated crystal screening systems such as the one shown here are becoming a common feature at synchrotron and other facilities where high-throughput crystal structure determination is being carried out. These systems rapidly screen samples to identify the best candidates for further study.

Colonies of bacteria growing despite high concentrations of antibiotics. These colonies are visible both by eye, as seen on the left, and by bioluminescence imaging, as seen on the right. The bioluminescent color indicates the metabolic activity of these bacteria, with their red centers indicating high metabolism.

More information about the research that produced this image can be found in the Antimicrobial Agents and Chemotherapy paper “Novel aminoglycoside-tolerant phoenix colony variants of Pseudomonas aeruginosa” by Sindeldecker et al.

Human embryonic stem cells were differentiated into cells like those found in the pancreas (blue), which give rise to insulin-producing cells (red). When implanted in mice, the stem cell-derived pancreatic cells can replace the insulin that isn't produced in type 1 diabetes. Image and caption information courtesy of the California Institute for Regenerative Medicine.

A protein called tubulin (green) accumulates in the center of a nucleus (outlined in pink) from an aging cell. Normally, this protein is kept out of the nucleus with the help of gatekeepers known as nuclear pore complexes. But NIGMS-funded researchers found that wear and tear to long-lived components of the complexes eventually lowers the gatekeepers' guard. As a result, cytoplasmic proteins like tubulin gain entry to the nucleus while proteins normally confined to the nucleus seep out. The work suggests that finding ways to stop the leakage could slow the cellular aging process and possibly lead to new therapies for age-related diseases.

Proteins are long chains of amino acids. Each protein has a unique amino acid sequence. It is still a mystery how a protein folds into the proper shape based on its sequence. Scientists hope that one day they can "watch" this folding process for any given protein. The dream has been realized, at least partially, through the use of computer simulation.

Histone proteins loop together with double-stranded DNA to form a structure that resembles beads on a string. See image 2560 for an unlabeled version of this illustration. Featured in The New Genetics.

This image shows an abnormal, tetrapolar mitosis. Chromosomes are highlighted pink. The cells shown are S3 tissue cultured cells from Xenopus laevis, African clawed frog.

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 CCD-1 molecule and a molecule of the antibiotic cefotaxime bound together. The structure revealed that CCD-1 provides extensive hydrogen bonding and stabilization of the antibiotic in the active site, leading to efficient degradation of the antibiotic.

Related to images 6764, 6765, and 6767.

These neurons are derived from mouse embryonic stem cells. Red shows cells making a protein called TH that is characteristic of the neurons that degenerate in Parkinson's disease. Green indicates a protein that's found in all neurons. Blue indicates the nuclei of all cells. Studying dopaminergic neurons can help researchers understand the origins of Parkinson's disease and could be used to screen potential new drugs. Image and caption information courtesy of the California Institute for Regenerative Medicine. Related to images 3270 and 3285.

This pig cell is in the process of dividing. The chromosomes (purple) have already replicated and the duplicates are being pulled apart by fibers of the cell skeleton known as microtubules (green). Studies of cell division yield knowledge that is critical to advancing understanding of many human diseases, including cancer and birth defects.

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

The molecules that glow blue in these cultured cells prevent the expression of the mutant proteins that cause Huntington's disease. Biochemist David Corey and others at UT Southwestern Medical Center designed the molecules to specifically target the genetic repeats that code for harmful proteins in people with Huntington's disese. People with Huntington's disease and similar neurodegenerative disorders often have extra copies of a gene segment. Moving from cell cultures to animals will help researchers further explore the potential of their specially crafted molecule to treat brain disorders. In addition to NIGMS, NIH's National Institute of Neurological Disorders and Stroke and National Institute of Biomedical Imaging and Bioengineering also funded this work.

A model of thymidylate synthase complementing protein from Thermotoga maritime.

At the root tips of the mustard plant Arabidopsis thaliana (red), two proteins work together to control the uptake of water and nutrients. When the cell division-promoting protein called Short-root moves from the center of the tip outward, it triggers the production of another protein (green) that confines Short-root to the nutrient-filtering endodermis. The mechanism sheds light on how genes and proteins interact in a model organism and also could inform the engineering of plants.

Those of us who get sneezy and itchy-eyed every spring or fall may have pollen grains, like those shown here, to blame. Pollen grains are the male germ cells of plants, released to fertilize the corresponding female plant parts. When they are instead inhaled into human nasal passages, they can trigger allergies.

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

The arrangement of identical molecular components can make a dramatic difference. For example, carbon atoms can be arranged into dull graphite (left) or sparkly diamonds (right). See image 2507 for an illustration with examples.

A cell in metaphase during mitosis: The copied chromosomes align in the middle of the spindle. 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 light organ (~0.5 mm across) of a Hawaiian bobtail squid, Euprymna scolopes, with different tissues are stained various colors. The two pairs of ciliated appendages, or “arms,” on the sides of the organ move Vibrio fischeri bacterial cells closer to the two sets of three pores (two seen in this image) at the base of the arms that each lead to an interior crypt. This image was taken using a confocal fluorescence microscope.

Related to images 7016, 7018, 7019, and 7020.

Living primary mouse embryonic fibroblasts. Mitochondria (green) stained with the mitochondrial membrane potential indicator, rhodamine 123. Nuclei (blue) are stained with DAPI. Caption from a November 26, 2012 news release from U Penn (Penn Medicine).

These images show frog cells in interphase. The cells are Xenopus XL177 cells, which are derived from tadpole epithelial cells. The microtubules are green and the chromosomes are blue. Related to 3442.

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.

Floral pattern emerging as two bacterial species, motile Acinetobacter baylyi (red) and non-motile Escherichia coli (green), are grown together for 48 hours on 1% agar surface from a small inoculum in the center of a Petri dish.

See 6557 for a photo of this process at 24 hours on 0.75% agar surface.
See 6555 for another photo of this process at 48 hours on 1% agar surface.
See 6556 for a photo of this process at 72 hours on 0.5% agar surface.
See 6550 for a video of this process.

The green in this image highlights a protein called TonB, which is produced by many gram-negative bacteria, including those that cause typhoid fever, meningitis and dysentery. TonB lets bacteria take up iron from the host's body, which they need to survive. More information about the research behind this image can be found in a Biomedical Beat Blog posting from August 2013.

A biofilm is a highly organized community of microorganisms that develops naturally on certain surfaces. These communities are common in natural environments and generally do not pose any danger to humans. Many microbes in biofilms have a positive impact on the planet and our societies. Biofilms can be helpful in treatment of wastewater, for example. This dime-sized biofilm, however, was formed by the opportunistic pathogen Pseudomonas aeruginosa. Under some conditions, this bacterium can infect wounds that are caused by severe burns. The bacterial cells release a variety of materials to form an extracellular matrix, which is stained red in this photograph. The matrix holds the biofilm together and protects the bacteria from antibiotics and the immune system.

Arranging exons in different patterns, called alternative splicing, enables cells to make different proteins from a single gene. Featured in The New Genetics.

See image 2552 for an unlabeled version of this illustration.

Scientists in Scotland were the first to clone an animal, this sheep named Dolly. She later gave birth to Bonnie, the lamb next to her.

The receptor is shown bound to an antagonist, JDTic.

To become products, reactants must overcome an energy hill. See image 2526 for a labeled version of this illustration. Featured in The Chemistry of Health.

A cell in anaphase during mitosis: Chromosomes separate into two genetically identical groups and move to opposite ends of the spindle. 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.

Using techniques that took 4 years to design, a team of developmental biologists showed that certain proteins can direct the subdivision of fruit fly and chicken nervous system tissue into the regions depicted here in blue, green, and red. Molecules called bone morphogenetic proteins (BMPs) helped form this fruit fly embryo. While scientists knew that BMPs play a major role earlier in embryonic development, they didn't know how the proteins help organize nervous tissue. The findings suggest that BMPs are part of an evolutionarily conserved mechanism for organizing the nervous system. The National Institute of Neurological Disorders and Stroke also supported this work.

Vibrio fischeri (2 mm in length) is the exclusive symbiotic partner of the Hawaiian bobtail squid, Euprymna scolopes. After this bacterium uses its flagella to swim from the seawater into the light organ of a newly hatched juvenile, it colonizes the host and loses the appendages. This image was taken using a scanning electron microscope.

Proteins are made of amino acids hooked end-to-end like beads on a necklace. To become active, proteins must twist and fold into their final, or "native," conformation. A protein's final shape enables it to accomplish its function. Featured in The Structures of Life.

Like a watch wrapped around a wrist, a special enzyme encircles the double helix to repair a broken strand of DNA. Without molecules that can mend such breaks, cells can malfunction, die, or become cancerous. Related to image 3493.

In the worm C. elegans, double-stranded RNA made in neurons can silence matching genes in a variety of cell types through the transport of RNA between cells. The head region of three worms that were genetically modified to express a fluorescent protein were imaged and the images were color-coded based on depth. The worm on the left lacks neuronal double-stranded RNA and thus every cell is fluorescent. In the middle worm, the expression of the fluorescent protein is silenced by neuronal double-stranded RNA and thus most cells are not fluorescent. The worm on the right lacks an enzyme that amplifies RNA for silencing. Surprisingly, the identities of the cells that depend on this enzyme for gene silencing are unpredictable. As a result, worms of identical genotype are nevertheless random mosaics for how the function of gene silencing is carried out. For more, see journal article and press release. Related to image 6532.

Amid a network of blood vessels and star-shaped support cells, neurons in the brain signal each other. The mists of color show the flow of important molecules like glucose and oxygen. This image is a snapshot from a 52-second simulation created by an animation artist. Such visualizations make biological processes more accessible and easier to understand.