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A comparison of key characteristics from the three domains of life A comparison of key characteristics from the three domains of life
Comparison of key characteristics from the three domains of life.
A cytotoxic T cell attacking a host cell that is expressing foreign antigens A cytotoxic T cell attacking a host cell that is expressing foreign antigens
Binding by both the antigen and an MHC molecule initiates the secretion of lytic enzymes by the cytotoxic T cell (Tc).
Action potential movement through an axon Action potential movement through an axon
A cross-section of an axon, with an action potential (AP) moving from left to right. The AP has not yet reached point 4; the membrane there is still at rest. At point 3, positive sodium ions are moving in from the adjacent region, depolarizing the region; the sodium channels are about to open. Point 2 is at the peak of the AP; the sodium channels are open and ions are flowing into the axon. The AP has passed by point 1; the sodium channels are inactivated, and the membrane is hyperpolarized.
Active site Active site
The active site of the penicillin-binding protein. The gray stick-like structures represent the secondary and tertiary structure of the penicillin-binding protein. Binding of the antibiotic, the substrate, to the active site blocks the normal action of the protein in the bacterial cell, resulting in death of the cell.
Alternative splicing Alternative splicing
More than one protein can be made from a gene. In this case, three different mRNA molecules are made from one gene. The exons (the numbered boxes) can combine in different configurations to eventually form different proteins.
Ambulocetus, an archaic whale Ambulocetus, an archaic whale
An artistic reconstruction of Ambulocetus, an ancestor to modern whales, based on fossil evidence
AMPA receptor figure AMPA receptor figure
Pictured are many of the molecules that are known to interact with AMPA receptors and play some role in long-term potentiation.
Anthrax phylogeny Anthrax phylogeny
A phylogenetic representation of the various anthrax isolates from laboratories around the world
Anthrax rods Anthrax rods
Scanning electron micrograph of Bacillus anthracis rods in the human spleen
Anthrax spores Anthrax spores
Spores of Bacillus anthracis
Antigenic shift Antigenic shift
A new form of a virus can arise when multiple viruses infect the same animal cell. Segments of nucleic acid can be exchanged resulting in a novel pathogen.
ÀApemen” "Apemen"
An early sketch of the evolution of man from the zoological point of view
Arcuate nucleus neurons Arcuate nucleus neurons
Neurons of the arcuate nucleus
Artiodactyl Artiodactyl
An artist's rendering of an ancient artiodactyl, a group of split-hoofed animals that were the ancestors to camels, hippos, pigs, and probably modern whales.
Ascent of man Ascent of man
An early view of the evolution of man from its ancestors
Asteroid Asteroid
A re-creation by NASA scientists of the impact made by an asteroid 500 miles in diameter at Chicxulub, on the Yucatan Peninsula. This impact is thought to be the cause of the K/T mass extinction 65 million years ago.
Attached bacteria Attached bacteria
Scanning electron micrograph of a young bacterial biofilm that formed on the lower surface of an oil slick on the Athabasca River.
Attached bacteria Attached bacteria
Scanning electron micrograph of a young bacterial biofilm that formed on the lower surface of an oil slick on the Athabasca River.
Bacillus anthracis Bacillus anthracis
Transmission electron micrograph of Bacillus anthracis, the bacterium that causes anthrax
Bacillus anthracis Bacillus anthracis
Photomicrograph of Bacillus anthracis, the bacterium that causes anthrax
Bacillus thuringiensis spore and protein crystals Bacillus thuringiensis spore and protein crystals
Protein crystals made by strains of Bacillus thuringiensis are toxic to certain insects and have been put to use as a biopesticide.
Bacterial conjugation Bacterial conjugation
The F-pilus serves as a point of contact between a bacterium containing an F-plasmid (the "male") and a bacterium lacking the plasmid (the "female"). After the female cell is contacted the pilus retracts, pulling the cells together. The exact mechanism of DNA transfer from male to female is not known; it may be by a channel in the pilus or by a temporary fusion of the mating cells.
Bacterial transformation Bacterial transformation
1. Naked DNA fragments from disintegrated cells in the area of a potential recipient cell. This cell must be of the correct genus and be in a state of competence, allowing the entry of the DNA fragments. 2. Entry of naked DNA into competent cell. 3. Recombination.
Biochemical pathway for beta-carotene production in golden rice Biochemical pathway for beta-carotene production in golden rice
The biochemical pathway for beta-carotene synthesis in golden rice.
Biofilms on catheter tips Biofilms on catheter tips
Biofilms can be a problem in medical settings where artificial materials are introduced into the human body, allowing bacteria to colonize and cause infection.
Biomass vs. species number Biomass vs. species number
Tilman's experimental data showing that productivity rose with the number of species grown in a plot
Black Mamo Black Mamo
Another animal that has gone extinct in the "sixth mass extinction"
BLAST search results BLAST search results
The results of a BLAST search using the delta chain of hemoglobin as the query.
Blastula Blastula
A photograph of an early stage blastula from the Xenopus laevis frog.
Breast cancer cells Breast cancer cells
A micrograph of breast cancer cells
Budding HIV viruses Budding HIV viruses
HIV-1 by transmission electron micrograph, showing mature virus particles budding from a lymphocyte
Carl Woese¡s data Carl Woese's data
An autoradiogram of a gel electrophoresis separation and visualization of ribosomal RNA, part of Carl Woese's research that led him to reorganize the Tree of Life into three domains rather than five kingdoms.
Cedar Creek aerial photo Cedar Creek aerial photo
Aerial photo of Cedar Creek research fields at the University of Minnesota
Cedar creek aerial photo Cedar creek aerial photo
Aerial photo of Cedar Creek research fields at the University of Minnesota
Chemical structures of dopamine-like drugs Chemical structures of dopamine-like drugs
The chemical structures of dopamine, Ritalin, and cocaine are structurally similar: they all bind at the dopamine transporter, affecting reuptake of dopamine.
Chick embryo Chick embryo
A microphotograph of a chick embryo
Chimpanzee Chimpanzee
The chimpanzee, humans' closest living relative
Chloroplasts Chloroplasts
A photomicrograph of chloroplasts, key cellular structures found in plants, which originally existed as free-living bacteria
Cloning animals by nuclear transfer Cloning animals by nuclear transfer
A donor cell is fused with enucleated egg cell by subjecting the two cells to pulses of electricity. The cell replicates in culture, generating an embryo, which is then introduced into the uterus of a female for development.
CLUSTAL data CLUSTAL data
A CLUSTAL alignment of a segment of a gene from four species. The red letters show the amino acid sequence (R=arginine, P=proline, G=glycine, etc.). The nucleotides that are conserved in all four species are shown in the columns with an asterisk at the bottom.
Cocaine PET scan Cocaine PET scan
PET scans of a cocaine user's brain
Colinearity Colinearity
Genes that are expressed at the anterior end of an animal are located at the more anterior region of the chromosome. Likewise, posteriorly expressed genes reside on the posterior end of the chromosome. This is referred to as spatial colinearity.
Confocal neuron Confocal neuron
A reconstruction of a neuron in a brain slice using confocal microscopy
Coordinate, gap, pair-rule, and segment polarity genes Coordinate, gap, pair-rule, and segment polarity genes
The cascade of developmental genes in segmentation in Drosophila. Maternal effect/coordinate genes set the anterior-posterior axes. The embryo is subdivided into progressively smaller regions by the actions of each class of segmentation genes.
Corn lilly Corn lilly
A substance called cyclopamine, found in the corn lilly plant, Californicum veratrum, is the cause of birth defects in lambs.
Cortex neurons Cortex neurons
Cortical neurons stained in a slice
Cyclopic lamb Cyclopic lamb
This lamb's mother ate a poisonous plant, the corn lilly, which contains a compound called cyclopamine. This compound inhibits the action of the Sonic Hedgehog protein, which is involved in the formation of the neural system.
David Reimer and his brother David Reimer and his brother
This is the first family gathering after David Reimer announced his intention to live as a male instead of a female. (David is on the left.)
DeCode asthma pedigree figure DeCode asthma pedigree figure
Pedigree of one family and its history of asthma through the generations
Deer tick Deer tick
The tick that causes Lyme disease
Deformed zebrafish embryo Deformed zebrafish embryo
This embryo was treated with retinoic acid, which causes neurological damage.
Deinococcus radiodurans Deinococcus radiodurans
Deinococcus radiodurans is a bacterium that scientists claim is the most radiation-resistant organism on earth.
Deinococcus radiodurans Deinococcus radiodurans
Deinococcus radiodurans is a bacterium that scientists claim is the most radiation-resistant organism on earth.
Diagram of human and chimp chromosome Diagram of human and chimp chromosome
A diagram of human and chimp chromosome similarity
Distribution of Aedes aegypti and dengue hemorrhagic fever Distribution of Aedes aegypti and dengue hemorrhagic fever
Distribution of the mosquito Aedes aegypti, the vector for dengue/dengue hemorrhagic fever. A mosquito eradication program administered by the Pan American Health Organization ended in 1970.
Docked 1 vesicle Docked vesicle
Docked vesicles at axons, showing "post-synaptic density"
Dopamine transporter Dopamine transporter
Left: Dopamine in the synaptic space binds to dopamine receptors on the post-synaptic cell. Dopamine transporters in the presynaptic membrane take up the dopamine molecules from the synaptic cleft and return them to the presynaptic cell. Right: Cocaine blocks the reuptake of dopamine, leading to molecular changes that contribute to addiction.
Dorsal ganglia cells Dorsal ganglia cells
Sensory neurons of the dorsal root ganglia
Drosophila embryo with bicoid protein stain Drosophila embryo with bicoid protein stain
This is a two-hour-old Drosophila embryo that shows the expression of the bicoid protein. The bicoid protein forms a gradient with the highest expression at the anterior end (left side in this photo) of the embryo.
Drosophila eye Drosophila eye
Electron microscope image of a fly eye (Drosophila melanogaster)
Drosophila with antennapedia mutation Drosophila with antennapedia mutation
When Drosophila has a mutation in the antennapedia gene, legs will grow out of its head.
Drug binding to active site of protein Drug binding to active site of protein
In virtual ligand screening, the three-dimensional image of the protein is fed into a computer, which attempts to fit millions of small molecules to a targeted active site. Small molecules that bind well to the protein become good leads for potential new drugs.
Euprymna scolopes Euprymna scolopes
The luminous bacterium Vibrio fischeri colonizes the light organ of the squid Euprymna scolopes, providing camouflage.
Extracellular matrix Extracellular matrix
Bacterial cells enmeshed in extracellular matrix material, creating a biofilm
Fate map Fate map
A representation of a fate map.
Five-kingdom vs. three-domain tree of life Five-kingdom vs. three-domain tree of life
The older five-kingdom tree of life, which has been replaced by Woese's three-domain tree.
Fruit fly With extra eyes Fruit fly with extra eyes
The head of a fruit fly, Drosophila melanogaster, viewed by scanning electron microscope (380x magnification). Targeted expression of the eyeless gene induced the formation of the eye facets on the antenna (to the lower-right of the eye), which are very similar to the facets of the normal eye. This identifies eyeless as the master control gene of eye morphogenesis.
Gene disruption by homologous recombination Gene disruption by homologous recombination
The plasmid contains a gene interrupted by a marker gene (XR). Recombination involving two crossovers between the plasmid and wild type chromosomal DNA with the interrupted gene and the selectable marker.
Gene gun Gene gun
A "gene gun"
Genealogical tree of Icelandic family Genealogical tree of Icelandic family
A genealogical tree of an Icelandic family
Genes involved in human sex determination Genes involved in human sex determination
A chart of the genes that are involved in human sex determination.
Genome of Bacillus anthracis Genome of Bacillus anthracis
A graphical representation of the DNA sequence of Bacillus anthracis
Genome of Deinococcus radiodurans Genome of Deinococcus radiodurans
The genetic sequence of Deinococcus radiodurans
Gingerich¡s revised picture of whale evolution Gingerich's revised picture of whale evolution
In the revised picture of whale evolution, cetaceans evolved from the artiodactyls, and not the mesonychids, as previously thought. The closest living relative to whales is now thought to be the hippo.
Gorilla Gorilla
The gorilla, another living relative to humans
Greater Koa finch Greater Koa finch
Painting of Greater Koa finch.
Group A Streptococci Group A streptococci
Electron micrograph of an ultra-thin section of a chain of group A streptococci. The cell surface fibrils, consisting primarily of M protein, are clearly evident. The bacterial cell wall, to which the fibrils are attached, is also clearly seen as the light-staining region between the fibrils and the dark-staining cell interior. Incipient cell division is also indicated by the nascent septum formation (seen as an indentation of the cell wall) near the cell equator. The streptococcal cell diameter is equal to approximately one micron. (20,000x magnification)
Haeckel¡s evolutionary tree Haeckel's evolutionary tree
A tree-like representation by Ernst Haeckel, a nineteenth-century evolutionary biologist
Harold Bishop and Harold Varmus Michael Bishop and Harold Varmus
Michael Bishop, PhD, and Harold Varmus, PhD. They received the Nobel Prize in 1989 for their discovery that normal cells contain genes capable of becoming cancer genes.
Head of Drosophila melanogaster Head of Drosophila melanogaster
Drosophila, the fruit fly, is a common model organism used in genetic research. It was one of the organisms sequenced in the Human Genome Project.
Heap leach pad Heap leach pad
Heap leach operations, like this one operated by Kennecott, can contain a million tons of ore, piled a leak-proof pad. Adding water on top promotes the growth of microbes and starts the bioleach.
Helper T cells regulate both humoral and cellular immunity Helper T cells regulate both humoral and cellular immunity
A specialized macrophage ingests foreign antigens and displays antigen fragments along with MHC (self) molecules on its surface. A helper T cell (TH) with the appropriate receptor binds and responds by producing cytokines that stimulate antigen-specific B cells, as well as specific cytotoxic T cells.
Hemmagglutinin protein-pH 5 Hemagglutinin protein-pH 5
When the influenza virus enters a cell it is engulfed into an endosome. The pH inside the endosome lowers from 7 to 5, causing the hemagglutinin protein to change shape, open the endosome membrane, and let the viral material into the cell.
Hemmagglutinin protein-pH 7 Hemagglutinin protein-pH 7
When the influenza virus attaches and enters a cell, the hemagglutinin surface protein is at pH 7. A shape change caused by a lower pH is responsible for the hemmagglutinin to open the endosome and release viral material into the cell.
HEP-2 cells in culture HEP-2 cells in culture
Hep-2 cells in culture (200x magnification, crystal violet stain, high density)
Hippocampal neuron Hippocampal neuron
Double-labeled hippocampal neuron stained with yellow and red fluorescence
Hippocampal neurons Hippocampal neurons
Two hippocampal neurons labeled with green fluorescent protein, viewed with confocal microscopy. Such neurons release and sense glutamate and engage in long-term potentiation (LTP). Note the synaptic connections between the lateral processes of the two neurons.
HIV viruses HIV viruses
HIV-1 by transmission electron microscopy, showing the mature virus particles that have budded off the nearby lymphocyte
Hominid skull Hominid skull
Example of a Hominid skull.
Hominoid tree Hominoid tree
A tree showing the evolution of the hominoids, including the great apes and humans.
Homo habilis skull specimen Homo habilis skull specimen
Example of Homo habilis skull.
Human and chimp allele clustering Human and chimp allele clustering
For nearly all genes, human alleles cluster together and chimp alleles cluster together (left). In the case of the major histocompatibility complex (MHC), human alleles are often more closely related to chimp alleles and vice-versa. This occurs due to balancing selection maintaining variation at the MHC (right).
Human chromosomes Human chromosomes
Photo of human chromosomes.
Human chromosomes Human chromosomes
Photo of human chromosomes.
Human chromosomes Human chromosomes
Close-up photo of human chromosomes.
Human fossil Àbush” Human fossil "bush"
The human "bush," as postulated from fossil finds of hominid species.
Human skull specimen Human skull specimen
An example of human skull.
Hypotheses of human migration Hypotheses of human migration
Left: The "Out of Africa," or "Replacement," hypothesis suggests all living humans evolved from a group that originated in Africa. Right: The "Multiregional" hypothesis suggests several groups evolved in parallel to form today's population of humans.
Iceland family genealogy text Icelandic family genealogy text
Icelandic people have kept careful records of their family histories for hundreds of years, aiding efforts to track disease patterns in their genes.
Icelandic family photo Icelandic family photo
A photograph of an Icelandic family.
Influenza epidemic of 1918 Influenza epidemic of 1918
Photo of the precautions taken by officials during the influenza epidemic of 1918.
Janet Reimer holding her twins, Bruce and Brian Janet Reimer holding her twins, Bruce and Brian
Brenda and Brian are identical twins. Brenda was born a male and named Bruce. After a botched circumcision, scientists believed it was best to change him into a girl. This case, termed the "Joan/John case" in medical literature, has been used often in the argument for nurture's role in the gender identity. At 14, Brenda shed her upbringing as a female and began to live as a male. He took the name David, and has said that he never felt right as a female, disproving what the scientists believed: that nurture could fully override his genetic makeup.
John Kendrew John Kendrew
John Kendrew determined the structure of the myoglobin protein in 1957. He shared the 1962 Nobel Prize in Chemistry with Max Perutz, who determined the structure of hemoglobin.
Krings figure Krings figure
Figure from Cell paper on the study of Neanderthal DNA.
Kutchicetus, an archaic whale Kutchicetus, an archaic whale
An artistic reconstruction of Kutchicetus, an ancestor to modern whales, based on fossil evidence
Leroy Hood Leroy Hood
Leroy Hood, MD, PhD, developed the automated genetic sequencer in 1986 and was a strong proponent of the Human Genome Project. He is the founder and president of the Institute for Systems Biology in Seattle, Washington.
Light field of neurons and under fluorescence Light field of neurons and under fluorescence
Neurons under light field and fluorescence microscopy
Light organ of Euprymna scolopes Light organ of Euprymna scolopes
Close-up of the light organ from the squid Euprymna scolopes.
Locu Ceruleus neuron Locus ceruleus neuron
Green fluorescent protein labeled neurons in a brain slice of the locus ceruleus
Map of chromosome 3 showing location of PPARG gene Map of chromosome 3 showing location of PPARG gene
All of the genes that have been identified on chromosome 3. A single nucleotide polymorphism mutation in the PPARG gene has been implicated in Type 2 diabetes.
MAP2NF neuron photo MAP2NF neuron photo
Light microscopy image of branching neurons with many processes
Mass extinctions Mass extinctions
The graph shows an approximate time line of loss of families of species from the earth during the five so-called "mass extinctions."
Maternal inheritance Maternal inheritance
Reciprocal F1 crosses involving maternal effect genes can produce different phenotypes.
Max Perutz Max Perutz
Max Perutz determined the structure of the hemoglobin protein in 1959. The work took him 22 years. He shared the 1962 Nobel Prize in Chemistry with John Kendrew, who discovered the structure of myoglobin.
Megabat Megabat
Photograph of an example of a megabat, the African fruit bat.
Methamphetamine PET scan Methamphetamine PET scan
PET brain scans of a methamphetamine user and a control subject
Mexican freetail bat Mexican freetail bat
Photograph of an example of a microbat, the Mexican freetail bat.
Microarray experiment Microarray experiment
A. RNA is isolated from cells from two samples (in this illustration, infected and uninfected plant cells). B. The mRNA from both samples is copied to a more stable form, called cDNA, using reverse transcriptase. C. At the same time, the cDNA is labeled with fluorescent tags (a different color tag for each sample). D. The tagged cDNA is placed on the microarray chip, where it binds to the corresponding DNA that makes up the genes that have been previously spotted on the chip. E. The chip is placed in a laser scanner, which identifies the genes that hybridize to each sample (uninfected=green; infected=red; and both samples=yellow). F. The data are displayed on a computer screen where expression of the individual genes can be identified.
Microinjection Microinjection
Photo of microininjection.
Mitochondria Mitochondria
Photomicrograph of mitochondria inside a cell
Mitochondrial DNA phylogeny Mitochondrial DNA phylogeny
A phylogeny of mitochondrial DNA sequences from modern humans around the globe
ÀMitochondrial Eve” "Mitochondrial Eve"
The diagram illustrates how one line of mitochondrial DNA came to be carried by all living humans, passed down to us through the "Mitochondrial Eve."
Molecules of LTP 2 Molecules of LTP 2
A schematic figure of the many molecules thought to be involved in long-term potentiation (LTP)
Monkey kidney cells in culture Monkey kidney cells in culture
Monkey kidney cells in culture, showing a typical patch of monolayer cells (100x magnification)
Monophyletic, paraphyletic, and polyphyletic trees Monophyletic, paraphyletic, and polyphyletic trees
Examples of monophyletic (top), paraphyletic (middle), and polyphyletic (bottom) trees.
Monophyly and diphyly of bat evolution Monophyly and diphyly of bat evolution
Alternative possibilities of bat phylogeny. Top: Bats form a monophyletic clade, in which flight evolved once in mamals. Alternately, bottom, bats are diphyletic, and flight evolved twice in mammals.
Mouse Mouse
Mouse with ticks on ear
Mouse ear with engorged ticks Mouse ear with engorged ticks
Mouse ear with engorged ticks attached to it
MRI of brain MRI of brain
A colored magnetic resonance image (MRI) of a human adult brain
MRI of brain, side profile MRI of brain, side profile
Side profile of brain MRI.
Multiple labeled neurons Multiple labeled neurons
Multiple labeled neurons in confocal microscopy
Human and Neanderthal skeletons Neanderthal and human skeletons
A Neanderthal and an adult human skeleton side-by-side
Neanderthal group Neanderthal group
Depiction of Neanderthal group.
Neanderthal painting Neanderthal painting
An artist's rendering of a Neanderthal community
Neanderthal painting Neanderthal painting
An artist's rendering of a Neanderthal man
Neanderthal skeleton upright Neanderthal skeleton upright
Photo of Neanderthal skeleton in the upright position.
Neanderthal skull Neanderthal skull
Skull of a Neanderthal whose DNA was used to determine that "Neanderthals were not our ancestors."
Neuron Neuron
Green fluorescent protein stains a hippocampal neuron.
Neuron Neuron
A neuron branching to many processes
Neuron Neuron
The parts of the neuron: information is received by dendrites, and action potentials are sent out from the cell body down the axon to the synaptic terminals.
Neuron AMPA receptor Neuron AMPA receptor
AMPA receptors stained with green fluorescent protein
Neuron CamKII Neuron CamKII
Calcium-calmodulin kinase II (CamKII), an enzyme involved in long-term potentiation, is stained with green fluorescent protein on this hippocampal neuron.
Neuron labeled in vivo Neuron labeled in vivo
Confocal microscopy of a multiple fluorescent-labeled neuron
Neurons Neurons
Neurons with multiple labeling
Neurons in culture Neurons in culture
Many neurons growing on a grid surface in culture
New adult neuron formed in brain New adult neuron formed in brain
Confocal microscopy reconstruction shows a newly formed adult neuron.
Old Icelandic family photo Old Icelandic family photo
Photo of an Icelandic family
Open reading frames Open reading frames
To find an open reading frame (ORF), a computer program identifies start codons (red arrows) and stop codons (green lines) in all three reading frames (represented by the three stacked rows). The black box is the largest ORF found in this sequence.
Pachyaena Pachyaena
An artist's rendering of Pachyaena, an example of the extinct group of animals called mesonychids. Once thought to be the ancestors of modern whales, it now appears that this is not the case.
Palaeolyngbya Palaeolyngbya
Ancient fossilized bacterium, Palaeolyngbya, from the Bitter Springs Chert
Pancreatic islets Pancreatic islets
The endocrine portion of the pancreas takes the form of many small clusters of cells, called islets of Langerhans or islets. Humans have roughly one million islets. In standard histological sections of the pancreas, islets are seen as relatively pale-staining groups of cells embedded in a sea of darker-staining exocrine tissue. This image shows three islets in the pancreas of a horse.
Parsimony Parsimony
Three possible unrooted trees are shown. The top tree assumes nine changes in character state occurred (each change is represented by a mark), the middle tree assumes ten changes, and the bottom tree assumes eleven. Because the top tree assumes the fewest changes, it is the most parsimonious tree.
Passenger pigeon Passenger pigeon
Once a common bird of eastern North America, the last passenger pigeon died in a zoo in 1914.
Paternal and maternal lineages Paternal and maternal lineages
Maternal lineages can be traced through mitochondrial genes, which are inherited by males and females only from the mother. Paternal lineages can be traced through the Y chromosome, which is inherited only by males and only from the father. (M=male and F=female)
Pathways leading to sex development in mammals Pathways leading to sex development in mammals
The genital ridge in an embryo is converted to a bipotential gonad by the products of the LHX9, SF1, and WT1 genes. This gonad develops into an ovary under the influence of the WNT4 and DAX1 gene products; it develops into a testis under the influence of the products of the SRY and SOX9 genes. The ovary produces cells that make estrogen, which causes the Müllerian duct to differentiate into the female genitalia. The testis makes two hormones, anti-Müllerian duct factor (AMH), which causes the Müllerian duct to regress, and testosterone, which causes the Wolffian duct to differentiate into male internal organs. Testosterone is also converted into dihydrotestosterone (DHT), which is required for development of male external genitalia.
Phagocytes on a biofilm Phagocytes on a biofilm
Scanning electron micrograph of the outer surface of a Tenckhoff catheter recovered from the peritoneum of a chronic ambulatory peritoneal dialysis (CAPD) patient. It shows the extensive microbial biofilm that had developed and a large number of phagocytes that were attracted to this surface by the presence of sessile bacteria.
Phillip Gingerich Phillip Gingerich
Archaeologist Phillip Gingerich working in the field in 1991
Phillip Gingerich Phillip Gingerich
Archaeologist Phillip Gingerich working in the field in 1991
Phylogeny of Bacillus anthracis Phylogeny of Bacillus anthracis
A phylogenetic representation of Bacillus anthracis and related bacteria
Pie chart of species Pie chart of species
A pie chart of the hypothesized distribution of species living on earth today
Pipe corrosion caused by a biofilm Pipe corrosion caused by a biofilm
Example of corrosion on a pipe caused by biofilm.
Pithecanthropus painting Pithecanthropus painting
An artist's rendering of Pithecanthropus man
Pithecanthropus painting Pithecanthropus painting
Hominid scene with tiger
Plasmid DNA Plasmid DNA
A photomicrograph of plasmid DNA, a small circular form of DNA found in bacteria
Polymerase chain reaction (PCR) Polymerase chain reaction (PCR)
1) Double-stranded DNA in the sample is heated to generate single strands. 2) Sequence specific primers are added, which anneal to desired sites on the DNA. 3) Nucleotides and heat-tolerant DNA polymerase allow for primer extension at elevated temperature. 4) The result is two new copies of double-stranded DNA. The process is repeated to generate multiple specific dsDNA molecules.
Pons neurons Pons neurons
Neurons of the pons
Protein crystal Protein crystal
To determine the structure of a protein, scientists use X-ray crystallography, a process in which they crystallize the protein and use an X-ray to determine defraction patterns.
Protein interaction Protein interaction
A network of protein-protein interactions in a yeast cell
Protein structure Protein structure
Primary Structure: The specific sequence of amino acids in a polypeptide chain. Secondary Structure: The folding of the polypeptide chain into specific shapes, such as the alpha helix and beta pleated sheet. Other regions of secondary structure may include turns and random coils. Tertiary Structure: The unique three-dimensional shape that is the result of chemical interactions between amino acids that fold the regions of secondary structure. Quaternary Structure: The specific interaction of two or more polypeptide subunits.
Pseudomonas aeruginosa Pseudomonas aeruginosa
Colonization of P. aeruginosa on a human lung. This bacterium can cause chronic lung infections in people with cystic fibrosis.
Ras protein Ras protein
The model structure of the proto-oncogene ras. Ras plays an important role in the signal transduction pathway that is involved with cell growth. Mutations in Ras have been implicated in many cancers.
RNA processing RNA processing
A gene consists of coding regions, called exons, that are interrupted with intervening noncoding regions, called introns. During transcription, the whole segment of DNA that corresponds to a gene is copied to make RNA. During RNA processing, the introns are removed and the exons are joined. A poly(A) tail is added to the mRNA.
Scanning electron micrograph of Pseudomonas aeruginosa Scanning electron micrograph of Pseudomonas aeruginosa
Colonies of P. aeruginosa are often the cause of chronic lung infections in individuals suffering from cystic fibrosis.
Selective breeding of maize Selective breeding of maize
The ancestor of modern corn had tiny kernels, each protected by a tough husk. Domestication of maize, which began thousands of years ago, selected for large sheathed cobs containing large kernels without husks.
Sequencing Sequencing
Strategies for cloning whole genomes
Signal transduction pathway Signal transduction pathway
A signal (in this example, a growth factor) binds to a tyrosine kinase receptor on the outside of the cell. This activates the membrane protein (through the addition of phosphate groups), which in turn activates proteins in the cytoplasm such as kinases. Several other proteins may be involved in the cascade, ultimately activating one or more transcription factors. The activated transcription factors enter the nucleus, where they stimulate the expression of the genes that are under the control of that factor. This is an example of the RAS pathway, which results in cell division.
16S ribosomal RNA 16S ribosomal RNA
Secondary structure of T. thermophilus 16S rRNA, with its 5', central, 3'-major, and 3'-minor domains shaded in blue, magenta, red, and yellow, respectively.
Skull Homo erectus Skull Homo erectus
Example of Homo erectus skull.
Southern blotting Southern blotting
1) DNA fragments are generated using restriction enzymes. 2) The fragments are separated in a gel by the application of an electric charge. 3) The fragments are then blotted onto a piece of nitrocellulose paper, where they retain their same pattern of separation, but are denatured to become single-stranded DNA. 4) A unique single-stranded portion of the gene of interest (the probe) is radioactively labeled and allowed to anneal with the blotted paper. 5) When exposed to a sheet of photographic film, any DNA fragments that annealed with the labeled probe are identified.
Spiro Spirochetes
Spriochetes of Borelia burgdorferei, the bacterium that causes Lyme disease
Sporulating Bacillus thuringiensis cell Sporulating Bacillus thuringiensis cell
Protein crystals made by strains of Bacillus thuringiensis are toxic to certain insects and have been put into use as a biopesticide.
Stained HIV viruses Stained HIV viruses
HIV-1 by transmission electron microscopy, stained with ruthenium red to show the surface glycoprotein knobs
Synapse Synapse
Synaptic vesicles fuse with the presynaptic membrane to release neurotransmitter into the synaptic space. Here, they bind with neurotransmitter receptors in the postsynaptic membrane.
T. rex T. rex
Illustration of Tyrannosaurus rex, a casualty of the mass extinction at the end of the Cretaceous period
Telomeres Telomeres
Human chromosomes with the telomere tips stained. The telomeres are protective ends of the chromosomes that get shorter each time a chromosome is replicated.
The cell cycle The cell cycle
The cell cycle is an ordered process of events that occurs in four stages. During the two gap phases, G1 and G2, the cell is actively metabolizing but not dividing. In S (synthesis) phase, the chromosomes duplicate as a result of DNA replication. During the M (mitosis) phase the chromosomes separate in the nucleus and the division of the cytoplasm (cytokinesis) occurs. There are checkpoints in the cycle at the end of G1 and G2 that can prevent the cell form entering the S or M phases of the cycle. Cells that are not in the process of dividing are in the G0 stage, which includes most adult cells.
The evolution of the Y chromosome The evolution of the Y chromosome
The degeneration of the Y occurred in four discrete episodes, beginning about 300 million years ago when a reptile-like ancestor acquired the SRY gene on one of its autosomal chromosomes. Each of the four episodes involved a failure of recombination to occur between the X and the Y chromosomes, resulting in subsequent decay of some genes in the non-recombining region.
The life cycle of the malaria parasite The life cycle of the malaria parasite
Sporozoites are delivered to the human bloodstream from the salivary gland of the Anopheles mosquito when the insect bites. In the liver, the sporozoites multiply and become merozoites. The merozoites enter red blood cells and become trophozoites. Red blood cells rupture and new merozoites, which have developed from the trophozoites, are released. Gametocytes (the sexual stage) are eventually produced. Gametocytes taken up by the mosquito in a blood meal fuse to form zygotes, which give rise to sporozoites.
The methane cycle The methane cycle
Methanogens are intolerant to oxygen so they thrive in anoxic sediments. The methane they produce is a carbon and an energy source for methane oxidizers in overlaying water.
The nitrogen cycle The nitrogen cycle
Bacteria are key to the cycling of nitrogen in ecosystems. Different species are involved in decomposition and ammonification, nitrification, denitrification, and nitrogen fixation.
The replication cycle of HIV The replication cycle of HIV
1) Membranes of the virus and the host cell fuse, and viral RNA and reverse transcriptase enter the host¡s cytoplasm. 2) Reverse transcriptase allows viral RNA to be copied to DNA. 3) Viral DNA is incorporated into the host chromosome as provirus. 4) Transcription and translation of viral proteins: viral RNA becomes incorporated into viral particles and is transcribed as well. 5) Viral particles bud out of the host cell, acquiring an envelope in the process.
The ÀShrub of Life” The "Shrub of Life"
Proposed by W. Ford Doolittle, this view of early evolution suggests multiple primitive cells as ancestors to the three domains, and illustrates lateral gene transfer among early organisms.
The structure of HIV The structure of HIV
Shows the binding of HIV to a host cell. gp120 on the virus binds CD4 receptors on the host. A second coreceptor molecule on the host is also required for binding.
Tick Tick
Close-up of tick body
Tick Tick
Tick on the end of a piece of grass, waving pincers, waiting to attach for a bloodmeal
Tick photo Tick photo
The tick that causes Lyme disease
Ticks Ticks
Adult, nymph, and larval stages of the tick
Ticks Ticks
Adult, nymph, and larval stages of the tick
Toilet bowl biofilm Toilet bowl biofilm
Experimental "reactor system" set up to study the formation of biofilm in toilets as part of an industrial-sponsored project at the CBE
Transcription factor Transcription factor
The yellow sphere represents a transcription factor binding to DNA in the nucleus to affect transcription and translation of new proteins.
Transduction by bacteriophage Transduction by bacteriophage
Flowchart of transduction by bacteriophage.
Transposons Transposons
Transposons facilitate the movement of genes throughout bacterial populations. These jumping genes can move from a chromosomal location to a plasmid. Conjugation allows the resultant plasmid to move readily within the bacterial population.
Tree phylogeny Tree phylogeny
Simple phylogeny of plants.
Trilobite Trilobite
Trilobite, a casualty of the mass extinction at the end of the Permian period.
2D gel of proteins 2D gel of proteins
Haemophilus influenzae cell proteins separated by 2D gel electrophoresis. The basic proteins are to the right of the gel and the acidic proteins to the left. High molecular weight proteins are to the top of the gel.
Typical progression of HIV infection and AIDS Typical progression of HIV infection and AIDS
Graph of the typical progression of HIV infection and AIDS.
Undifferentiated embryonic stem cells Undifferentiated embryonic stem cells
Microscopic (5x magnification) view of a colony of undifferentiated human embryonic stems cells. The embryonic stem cell colonies are the rounded, dense masses of cells. The flat, elongated cells in between the embryonic stem cell colonies are fibroblasts that are used as a "feeder layer" on which the embryonic stem cells are grown.
Unrooted tree and possible rooting points Unrooted tree and possible rooting points
Panel A shows an unrooted tree. Panels B, C, D, and E should be the resulting rooted trees, when root is placed in each of the corresponding positions.
Vibrio cholerae Vibrio cholerae
Scanning electron microscope image of Vibrio cholerae bacteria, which can infect the digestive system.
Vikings arrive in Iceland Vikings arrive in Iceland
A painting depicting the arrival of the Vikings to Iceland.
Whorls Whorls
The tissues that will become floral organs are arranged in concentric whorls of a developing flower.
X and Y chromosomes X and Y chromosomes
The Y chromosome is the smallest chromosome in humans. While most chromosomes have about 1,500 genes, the Y has less than 200.
Y chromosome Y chromosome
The Y chromosome is very small compared to the X chromosome. The pseudoautosomal regions at the tips contain the genetic material on the Y that shows similarity to the X chromosome. The SRY gene is located on the p arm of the Y.
Zebrafish embryo Zebrafish embryo
This 18-hour-old zebrafish embryo is labeled with a probe that shows the expression of sonic hedgehog mRNA.
Zebrafish embryo with Sonic Hedgehog protein stain Zebrafish embryo with Sonic Hedgehog protein stain
Zebrafish embryo with a stain that shows where the hedgehog protein is acting during development.


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