From the 2011 Holiday Lectures —Bones, Stones, and Genes: The Origin of Modern Humans

Lactose Digestion in Infants

The lactase enzyme is produced in the small intestine of infants. It digests lactose by breaking it into glucose and galactose.

52 seconds

Natural Selection of Lactose Tolerance

Environmental and cultural factors can affect whether a new human mutation becomes common in a population.

46 seconds

Regulation of Eukaryotic DNA Transcription

General transcription factors, activators, and repressors interact to regulate the transcription of eukaryotic DNA into RNA.

2 minutes 6 seconds

From the 2005 Holiday Lectures — Evolution: Constant Change and Common Threads

Fossil Record of Stickleback Evolution

A quarry site in Nevada carries the evolutionary history of a population of stickleback fish that resided there when it was a freshwater lake. In a short time span in evolutionary terms—about 10,000 years—the fish population can be seen to dramatically reduce the size of their pelvic spines. This particular fossil record is remarkably complete with nearly year-by-year detail which includes documentation of intermediate forms.

1 minute 26 seconds

Gene Switch

Regulatory "switches" are found upstream from a gene. Regulatory molecules bind to the switches and recruit RNA polymerase to bind to the gene's promoter region, increasing the transcription of the gene into messenger RNA.

1 minute 14 seconds

Pitx1 Expression

In the stickleback fish, pelvic-fin reduction resulted from changes in the regulatory switch elements of the Pitx1 gene. In the marine ancestor, the Pitx1 gene is activated in the pelvic-fin region during development to generate the fin. In the pelvic-reduced stickleback, the regulatory switch that normally turns on the Pitx1 gene is either missing or non-functional.

54 seconds

Paintbrush Gene

In two related Drosophila species, a so-called paintbrush gene is activated to "paint" the pigment on the body. In one species, an extra switch activates the gene, resulting in spotted wings.

49 seconds

Pocket Mouse and Predation

The rock pocket mouse is found in two color variants, or morphs: light and dark. In different environments, their visibility to predators such as owls varies. The dark morph is more vulnerable on light sandy desert, and the light morph on dark lava rock.

20 seconds

Pocket Mouse Evolution

This simulation shows the spread of a favorable mutation through a population of pocket mice. Even a small selective advantage can lead to a rapid evolution of the population.

1 minute 4 seconds

Stickleback CT Scan

This animation shows a rotating 3-D image of a stickleback skeleton. The pelvic region, including the pelvic spines, is highlighted in red. Armored plating covers the flanks of the fish. The three prominent dorsal spines give the fish its name.

37 seconds

Wing Morph

This "morph" animation demonstrates how the expression of a particular toolkit gene in a butterfly larva corresponds to the location of the wing eyespots in an adult butterfly.

28 seconds

From the 2010 Holiday Lectures — Viral Outbreak: The Science of Emerging Disease

Dengue Fever Re-Emergence in the Americas

Since the 1960s dengue fever has spread to many countries and total case numbers have exploded.

24 seconds

Viral Geometry and Structural Diversity

The geometric structures of viruses are beautiful and can be used, along with genomic information, to identify them.

3 minutes 22 seconds

Structure of Dengue Virus

The dengue virus's outer envelope proteins form symmetrical units and overlay the lipid envelope, capsid, and the RNA genome.

58 seconds

The Chemical Structure of DNA

DNA's chemical properties can be harnessed for a variety of biotechnology applications.

2 minutes 45 seconds

The Polymerase Chain Reaction (PCR)

PCR is a standard laboratory technique that allows amplification of specific segments of DNA based on complementarity.

55 seconds

Running a Virochip Experiment

A sample is put on a Virochip microarray, and results are compared to databases of all known viral sequences.

2 minutes 9 seconds

Dengue Virus Enters a Cell

Infection begins when the dengue virus uses receptors on an immune cell's surface to gain entry and release its genome.

1 minute 24 seconds

Dengue Virus Life Cycle

Dengue virus has sophisticated mechanisms for entering a cell, for replicating its RNA genome, and for transcribing proteins.

4 minutes 12 seconds

Life cycle of malaria

Part 1: Human host

When a malaria-carrying mosquito bites a human host, the malaria parasite enters the bloodstream, multiplies in the liver cells, and is then released back into the bloodstream, where it infects and destroys red blood cells.

4 minutes 17 seconds

Part 2: Mosquito host

A mosquito becomes infected with malaria when it sucks the blood from an infected human. Once inside the mosquito, the parasites reproduce in the gut and accumulate in the salivary glands, ready to infect another human host with the next bite.

3 minutes 59 seconds

From the 2007 Holiday Lectures — AIDS: Evolution of an Epidemic

Antigen presentation and CTL

How a cell infected by a virus signals cytotoxic T lymphocytes to kill the cell before the virus replicates and spreads.

2 minutes 34 seconds

AZT blocks reverse transcriptase

HIV's reverse transcriptase mistakes AZT for thymidine. Once incorporated, AZT stops reverse transcription.

1 minute 46 seconds

HIV life cycle

How HIV infects a cell and replicates itself using reverse transcriptase and the host's cellular machinery.

4 minutes 52 seconds

Protease inhibitors

Protease inhibitors prevent maturation of viral proteins inside HIV particles.

1 minute 6 seconds

U.S. AIDS epidemic

A visual representation of the U.S. AIDS epidemic from 1981 to 1997. Each dot represents 30 cases.

31 seconds

From the 1999 Holiday Lectures — 2000 and Beyond: Confronting the Microbe Menace

Bacterial Conjugation

Bacteria can transfer genetic material, and thus drug resistance, to other bacteria via conjugation.

23 seconds

E. coli Infection Strategy

Watch this animation to see the molecular tricks that an infectious strain of Escherichia coli uses to infect your gut.

2 minutes 52 seconds

Intracellular Infection by Salmonella

In this animation, you can see how one S. typhimurium invades an epithelial cell of the intestinal tract, survives the intracellular defense mechanisms of the host cell, and multiplies.

1 minute 18 seconds

Recombination of Viral Genome

When two different strains of influenza infect a single cell, their genetic material can mix freely, resulting in a new third strain of influenza.

3 minutes 5 seconds

Viral Lifecycle

Delivering a single virus to a cell allows the virus to infect the cell, replicate, and give rise to many progeny viruses. These viruses can then infect many neighboring cells.

1 minute 8 seconds

From the 2009 Holiday Lectures — Exploring Biodiversity: The Search for New Medicines

Prialt blocks motor synapse in fish

Prialt, a drug derived from cone snail venom, paralyzes fish by blocking calcium channels at a motor synapse.

2 minutes 31 seconds

Prialt blocks pain signaling in mice

Prialt does not block the mammalian motor synapse, but blocks the pain pathway in the spinal cord.

2 minutes 58 seconds

The LUX operon controls light production

A single transcription factor controls this operon, which contains five genes necessary to produce bioluminescence.

2 minutes 26 seconds

Motor cabal toxins block motor neuron synapses

Multiple cone snail toxins attack different molecules of the nervous system and cause paralysis.

3 minutes 28 seconds

Lightning-strike cabal acts like a Taser

Some cone snail toxins chemically hyperactivate neurons and immobilize prey, much like a Taser.

2 minutes 9 seconds

The molecular cascade in bacterial quorum sensing

Quorum sensing regulates gene expression by a protein phosphorylation cascade that controls transcription.

3 minutes 20 seconds

From the 2008 Holiday Lectures — Making Your Mind: Molecules, Motion, and Memory

Development of the human embryonic brain

The fetal brain grows enormously during pregnancy, both in terms of its size and the number of neurons it has.

1 minute 40 seconds

Molecular activity in Aplysia long-term memory

Long-term memory requires the activation of CREB, turning on specific genes that support new synaptic growth.

1 minute 39 seconds

Molecular activity in Aplysia short-term memory

Short-term memory relies on serotonin activating a protein kinase to modify existing synaptic strength.

2 minutes 30 seconds

Molecular basis of early LTP (short-term memory)

Early LTP (short-term memory) depends on a calcium-dependent protein kinase to strengthen an existing synapse.

1 minute 28 seconds

Molecular basis of late LTP (long-term memory)

Late LTP (long-term memory) involves dopamine activation of CREB to support new synaptic growth.

56 seconds

Molecular mechanism of synaptic function

Electrical and chemical signals are used by neurons to communicate with one another at contact points called synapses.

1 minute 9 seconds

Neurons in parietal cortex are active during straddling

Neurons in the cortical area 5 are active when a cat is straddling an obstacle.

1 minute 3 seconds

Repellant ephrin signals guide limb innervations

The growth cone of a neuron avoids repellant molecules and navigates to innervate the appropriate muscle.

1 minute 35 seconds

Signal molecules trigger transcription factors

Varying concentrations of a signaling molecule activate different transcription factors and determine cell fate.

2 minutes 4 seconds

From the 2003 Holiday Lectures — Learning From Patients: The Science of Medicine

Exclusion Mapping

A useful technique for narrowing down the location of a gene involves comparing the chromosomes of affected siblings. Two sisters with Rett syndrome allow researchers an opportunity to map the most likely location of the gene by excluding areas of the chromosome that are not alike. Includes audio narration.

32 seconds

MECP2

This animation shows how the protein MECP2, in conjuction with another protein complex, can act as an "on-off' switch for gene expression. Includes audio narration.

43 seconds

The Proteasome

A 3D animation showing how proteins in the cell are tagged for disposal and degraded by the proteasome. Includes audio narration.

1 minute 43 seconds

SCA1 Pedigree

Illustrates how studying one family's pedigree can reveal an entire history of passing on a genetic disorder such as SCA1.

30 seconds

Tri Nucleotide Repeat

Slippage during DNA replication can lead to expanding sections of repeating nucleotides. Watch this animation to see how this problem occurs. Includes audio narration.

1 minute 7 seconds

X Inactivation

This animation shows how the random deactivation of one of the X chromosomes in a pair can lead to a mozaicism in the expression genes. Includes audio narration.

55 seconds

From the 1997 Holiday Lectures — Senses and Sensitivity: Neuronal Alliances for Sight and Sound

The Cochlea

A dramatic illustration of how hearing happens in the ear. Includes audio narration.

1 minute 30 seconds

From the 2006 Holiday Lectures — Potent Biology: Stem Cells, Cloning, and Regeneration

Human Embryonic Development

Human embryonic development depends on stem cells. During the course of development, cells divide, migrate, and specialize. Early in development, a group of cells called the inner cell mass (ICM) forms. These cells are able to produce all the tissues of the body. Later in development, during gastrulation, the three germ layers form, and most cells become more restricted in the types of cells that they can produce.

2 minutes 18 seconds

Differentiation and the Fate of Cells

As a human embryo develops, its cells become progressively restricted in the types of specialized cells that they can produce. Inner cell mass (ICM) cells of the blastocyst can make any type of body cell. Gastrula-stage cells can give rise to the cells of a given germ layer. Later, cells become even more restricted. For example, the pancreatic bud of the endoderm layer can only make the cells of the pancreas.

1 minute 28 seconds

Creating Embryonic Stem Cell Lines

The inner cell mass (ICM) cells of blastocyst-stage early human embryos can be removed and cultured. These cells can be grown in the lab indefinitely. Various growth factors cause these cells to develop into a variety of differentiated cells, such as muscle or nerve cells.

1 minute 37 seconds

Cytoplasmic Factors

Cytoplasmic factors play a significant part in determining how a cell develops. This segment discusses their importance in turning the appropriate genes on and off for proper development.

56 seconds

Somatic Cell Nuclear Transfer Animation

Somatic cell nuclear transfer (SCNT) is a technique for cloning. The nucleus is removed from a healthy egg. This egg becomes the host for a nucleus that is transplanted from another cell, such as a skin cell. The resulting embryo can be used to generate embryonic stem cells with a genetic match to the nucleus donor (therapeutic cloning), or can be implanted into a surrogate mother to create a cloned individual, such as Dolly the sheep (reproductive cloning).

51 seconds

Zebrafish Heart Regeneration

The zebrafish heart is similar to the human heart in many respects. But unlike the human heart, the fish heart closes wounds rapidly and then regenerates to nearly full function. Fibroblast growth factor (FGF) is an important molecule in the regeneration process.

2 minutes 29 seconds

Newt Limb Regeneration

Urodele amphibians—newts and salamanders—are able to regenerate fully functional limbs in response to amputation. Cells in and near the limb stump dedifferentiate to form a mass of stemlike cells that can produce all the specialized tissues of the limb, such as muscle, nerves, and blood vessels.

1 minute 20 seconds

From the 2004 Holiday Lectures — Science of Fat

Body Mass Index (BMI)

Comparison of the change in BMI for a given height and varying weights.

1 minute 21 seconds

Leptin Feedback Control System

Demonstrates how changes in the amount of fat tissue lead to changes in leptin levels and thus changes in appetite.

1 minute 1 second

Location of the Hypothalamus

A 3-D animation that shows the location of the hypothalamus in a mouse's brain.

33 seconds

Leptin Neuronal Rewiring

Illustrates how providing leptin to an obese mouse rapidly rewires its hypothalamus neurons.

1 minute 18 seconds

The Fate of Fat

An overview of how dietary fat gets digested, packaged, and sent to various tissues for storage or energy.

2 minutes 7 seconds

Obesity-Related Health Problems

A timeline illustrating the gradual effects of obesity on the body, including diabetes, atherosclerosis, and heart attack.

1 minute 43 seconds

How a Heart Attack Occurs

A 3-D animation that shows how plaques form in a blood vessel, leading to blockage and a heart attack.

37 seconds

PPAR-gamma Activation in the Fat Cell

The PPAR-gamma receptor activates certain genes in a fat cell, resulting in the storage of fat and changes in hormone levels.

2 minutes 48 seconds

PPAR-delta Activation in the Muscle Cell

The PPAR-delta receptor activates certain genes in a muscle cell, resulting in the burning of fat.

1 minute 44 seconds

From the 2003 Holiday Lectures — Learning From Patients: The Science of Medicine

Angiogenesis

A cancer tumor forms in a bed of healthy cells. The animation goes on to show how the tumor recruits blood vessels and how metastasis occurs.

1 minute 12 seconds

Gleevec

Gleevec is a drug designed to interfere with the stimulation of growth in leukemia cells. This 3D animation shows how this is achieved.

1 minute 3 seconds

Mismatch Repair

This animation illustrates how mistakes made during DNA replication are repaired.

1 minute 22 seconds

p53

A 3D animation showing the molecule p53 binds to DNA and initiates the transcription of mRNA.

25 seconds

Using p53 to Fight Cancer

This animation demonstrates how cancerous cells could be destroyed using a modified virus.

1 minute 1 second

VEGF

This animation shows how a growing tumor can recruit nearby blood vessels in order to gain a supply of blood.

29 seconds

From the 2002 Holiday Lectures — Scanning Life's Matrix: Genes, Proteins, and Small Molecules

Cellular Screening

One technique for discovering small molecules of biological relevance is to expose cultured cells to a variety of small molecules and look for changes in the cells' appearance, behavior or other measurable qualities.

1 minute 30 seconds

Chemspace

The hypothetical relationship of chemical space and biological space is plotted on a three-dimensional graph, giving a glimpse of the future direction of research at the intersections of various disciplines.

1 minute 34 seconds

Diversity of Small Molecules

A molecular menagerie of small molecules is displayed, with two particular molecules singled out for attention: rapamycin and furrowstatin, which are discussed in the remainder of Dr. Schreiber's lectures on chemical genetics.

1 minute 56 seconds

DOS Matrix

In diversity-oriented synthesis, many combinations of chemical building blocks undergo relatively few reaction steps to form a vast variety of different molecules. In this example, 45 x 45 x 45 combinations yield more than 88,000 novel molecules.

1 minute 20 seconds

Furrowstatin

The small molecule 'furrowstatin' exemplifies the power of using small molecules to investigate life's processes. When applied to dividing cells, the furrowstatin halts cell division.

4 minute 41 seconds

Gene Chip Manufacturing

Gene chips, also called DNA microarrays, have a broad range of applications in current research, including enabling researchers to measure the activity of thousands of genes simultaneously. Dr. Eric Lander describes the process used to manufacture gene chips.

1 minute 56 seconds

Molecular Screening

After a chemical biologist has made many novel small molecules by diversity-oriented synthesis, the next step is to find those that are useful. Molecules need to be "screened." Conceptually, screening is like using proteins as a custom filter to catch potentially useful small molecules.

38 seconds

Myosin II Mechanism

Myosin II is one of the molecules involved in furrow formation in dividing cells. This animation shows how the molecule operates, and how furrowstatin blocks the mechanism and halts division of a cell.

57 seconds

Rapamycin

Rapamycin is a small molecule originally isolated from nature. It has antibiotic and immunosuppressive properties. It also allows two proteins which do not normally interact to bind together in the cell, which causes problems in the nutrient-sensing pathway.

1 minute 9 seconds

Small-Molecule Microarrays

To screen many small molecules at once, microarray technology is useful. Automated devices have made it possible for thousands of different small molecules to be printed as an array of spots on a glass slide. A single type of protein which has been tagged with a fluorescent marker can then be washed across the array. Any small molecule that binds to the protein can be detected by scanning for spots that are fluorescent.

59 seconds

Using Small Molecules to Modulate a Protein

This animation illustrates how a small molecule binds to a protein. As a result of the binding, the protein alters its shape and becomes inactivated.

1 minute 10 seconds

From the 2001 Holiday Lectures — The Meaning of Sex: Genes and Gender

Evolution of the Y Chromosome

How did the human Y chromosome become so small relative to its X counterpart? This animation depicts the 300-million-year odyssey of the sex chromosomes that began when the proto X and Y were an identical pair.

5 minutes 38 seconds

Meiosis

Meiosis, the form of cell division unique to egg and sperm production, sets the stage for sex determination by creating sperm that carry either an X or a Y sex chromosome. But what is it about the X or Y that determines sex?

5 minutes 52 seconds

MIX-1

This animation shows how MIX-1 facilitates both chromosome condensation and dosage compensation.

3 minutes 38 seconds

The Y Chromosome

The Y chromosome has been likened to a hall of mirrors because its sequence contains many sections that appear to be palindromes. These palindromes provide a clue to some interesting events that may have occurred during the course of the chromosome's evolution.

2 min 45 seconds (no audio narration)

From the 2000 Holiday Lectures — Clockwork Genes: Discoveries in Biological Time

The Drosophila Molecular Clock Model

Watch these animations display the dynamic orchestration of the molecular events of the Drosophila biological clock.

7 minutes 34 seconds

The Human Suprachiasmatic Nucleus

In mammals, the controlling clock component that generates a 24-hour rhythm is the suprachiasmatic nucleus (SCN), located in a part of the brain called the hypothalamus. The SCN produces a signal that can keep the rest of the body on an approximately 24-hour schedule. This animation illustrates the location of the SCN in the human brain.

1 minute 40 seconds

The Mammalian Molecular Clock Model

This animation shows the molecular interactions involved in the negative feedback loop responsible for circadian rhythms in mammals.

3 minutes 40 seconds

Measuring Circadian Activity in Drosophila

This animation series shows four experiments that compare the activity patterns of a wild-type fly keeping a normal schedule with those of a mutant fly apparently following a 19-hour internal clock.

2 minutes 2 seconds

From the 1998 Holiday Lectures — Of Hearts and Hypertension: Blazing Genetic Trails

The Visible Heart

This animation focuses on the gross anatomy of the human heart. The model of the heart is semitransparent, allowing you to see through the thick cardiac muscle into the four heart chambers.

Heart Function

The job of the human heart—in fact of all vertebrate hearts—is to pump oxygenated blood throughout the cells of the body and to return deoxygenated blood to lungs or gills for replenishment

33 seconds

Diffusion Across Membranes

This animation describes two different ways by which chemicals migrate through membranes: passive diffusion and active transport.

Cloning an Army of T Cells for Immune Defense

View the animation to see how one type of immune cell—the helper T cell—interprets a message presented at the surface of the cell membrane. The message is an antigen, a protein fragment taken from an invading microbe. A series of events unfolds that results in the production of many clones of the helper T cell. These identical T cells can serve as a brigade forming an essential communication network to activate B cells, which make antibodies that will specifically attack the activating antigen.

4 min 20 sec

RNA Folding

Since RNA is single-stranded, it can fold upon itself and form structures that are protein-like in both appearance and functionality.

32 seconds

DNA replication (schematic)

The structure of DNA, discovered by James Watson and Francis Crick, suggests a mechanism of replication. The double helix unwinds, and each strand acts as a template for the construction of the new DNA molecule.

50 seconds

DNA replication (basic detail)

Using information from molecular research, this 3-D animation shows how DNA is replicated at the molecular level. It involves an enzyme that unwinds the DNA, and other enzymes that copy the two resulting strands.

1 minute 6 seconds

DNA replication (advanced detail)

Both strands of the DNA double helix act as templates for the new DNA strands. Incoming DNA is unraveled by the enzyme helicase, resulting in the 3′ strand and the 5′ strand. The 3′ strands and the 5′ strands are replicated by a DNA polymerase enzyme but in different ways.

2 minutes 32 seconds

DNA transcription (basic detail)

The first phase of the process of reading DNA information to make proteins starts with a molecule unzipping the DNA. The molecule then copies one of the strands of DNA into a strand of RNA, a close cousin of DNA. This process is called transcription.

1 minute 54 seconds

DNA transcription (advanced detail)

The process of copying DNA into messenger RNA (mRNA) is called transcription. Transcription factors assemble at the promoter region of a gene, bringing an RNA polymerase enzyme to form the transcription initiation complex. Activator proteins at the enhancer region of DNA then activate the transcription initiation complex. RNA polymerase unzips a small portion of the DNA and copies one strand into an mRNA molecule.

1 minute 55 seconds

Translation (basic detail)

The ribosome is a molecular factory that translates the genetic information in RNA into a string of amino acids that becomes a protein. Inside the ribosome, the genetic code of the RNA is read three letters at a time and compared with the corresponding code on a transfer molecule. When a match occurs between the codes, the amino acid carried by the transfer molecule is added to the growing protein chain.

2 minutes 5 seconds

Translation (advanced detail)

Messenger RNA (mRNA) carries DNA’s genetic information to the ribosome, where it is translated into a sequence of amino acids. mRNA is fed into the ribosome, and it is positioned so that it can be read in groups of three letters, known as codons. Each mRNA codon is matched against the transfer RNA molecule’s anti-codon. If there is a match, the amino acid carried by the transfer RNA is added to the growing protein chain.

3 minutes 4 seconds

DNA packaging

DNA is tightly packed in the nucleus of every cell. DNA wraps around special proteins called histones, which form loops of DNA called nucleosomes. These nucleosomes coil and stack together to form fibers called chromatin. Chromatin in turn forms larger loops and coils to form chromosomes.

1 minute 43 seconds

Building blocks of DNA

Adenine (A), cytosine (C), guanine (G), and thymine (T) are the components of nucleic acid that make up DNA.

26 seconds

Chargaff's Ratio

In 1950, Erwin Chargaff published a paper stating that in the DNA of any given species, the ratio of adenine to thymine is equal, as is the ratio of cytosine to guanine. This became known as Chargaff’s ratio, and it was an important clue for solving the structure of DNA.

48 seconds

CML and Gleevec

Chronic myeloid leukemia (CML) is caused by a mutation that leads to an abnormal protein that is always active. The drug Gleevec has a shape that fits into the active site of the abnormal protein and stops its harmful effects.

41 seconds

Coding sequences in DNA

Of the 3 billion letters in the human genome, only 1% directly code for proteins. Of the rest, about 25% make up genes and their regulatory elements. The functions of the remaining letters are still unclear.

1 minute 4 seconds

Damage to DNA leads to mutation

Reactive molecules, such as free radicals, and solar ultraviolet radiation can lead to mutations in DNA. Most mutations are corrected, but in rare cases mutations can accumulate and cause diseases such as cancer.

1 minute 6 seconds

Genetic engineering

A new gene can be inserted into a loop of bacterial DNA called a plasmid. This is done by cutting the plasmid DNA with a restriction enzyme, which allows a new piece of DNA to be inserted. The ends of the new piece of DNA are stitched together by an enzyme called DNA ligase. The genetically engineered bacteria will now manufacture any protein coded by genes on the newly inserted DNA

1 minute 12 seconds

Human chromosomes

The human genome is organized into structures called chromosomes, consisting of 22 matching pairs and one pair of sex chromosomes.

47 seconds

Human genome sequencing

The public Human Genome Project started by identifying unique marker sequences distributed throughout the genome. Then, many copies of a small section of DNA were randomly cleaved into smaller fragments, and each small fragment was sequenced. Because there were originally many copies of the DNA in question, many fragments represented the same part of the genome. These were aligned by identifying overlapping regions of the sequence, and then they were assembled into the original DNA.

1 minute 48 seconds

mRNA splicing

Once a gene has been transcribed into messenger RNA (mRNA), it is edited in a process called splicing. Noncoding regions called introns are removed, leaving protein-coding regions called exons

39 seconds

Paired DNA strands

DNA has a double helix structure. If untwisted, DNA looks like two parallel strands. Each strand has a linear sequence of A, C, G, and T. The precise order of the letters carries the coded instructions. One strand is a complementary image of the other: A always pairs with T, and C always pairs with G.

1 minute 18 seconds

Pauling triple helix model

One of the failed hypothetical models of DNA is Linus Pauling’s triple helix model. This structure would be unstable under normal cellular conditions.

29 seconds

Polymerase chain reaction

Polymerase chain reaction, or PCR, is a technique for making many copies of a specific DNA sequence. DNA is repeatedly heated and cooled in the presence of primers that bracket the desired sequence and of the enzyme Tac polymerase. In as few as 30 cycles, a billion copies of the target sequence can be made.

1 minute 27 seconds

Sanger method of DNA sequencing

Fred Sanger developed the first technique for sequencing DNA. DNA is replicated in the presence of chemically altered versions of the A, C, G, and T bases. These bases stop the replication process when they are incorporated into the growing strand of DNA, resulting in varying lengths of short DNA. These short DNA strands are ordered by size, and by reading the end letters from the shortest to the longest piece, the whole sequence of the original DNA is revealed.

51 seconds

Shotgun sequencing

In shotgun sequencing many copies of the entire genome are “blown up” into millions of small fragments. Each small fragment is sequenced. Powerful computers then assemble the individual fragments into the original configuration. Repeat sequences pose a problem for this approach because their sizes can be much larger than the small fragments.

59 seconds

Sickle cell anemia

Sickle cell anemia is a genetic disease that affects hemoglobin. A single nucleotide change in the hemoglobin gene causes an amino acid substitution in the hemoglobin protein from glutamic acid to valine. The resulting proteins stick together to form long fibers and distort the shape of the red blood cells.

59 seconds

Triplet code

Once the structure of DNA was discovered, the next challenge was determining how the sequence of letters coded for the 20 amino acids. In theory, one or two letters can only code for 4 or 16 amino acids, respectively. A scheme using three letters, a triplet code, is the minimum necessary to encode for all the amino acids.

1 minute 8 seconds

Watson constructing base pair models

During the process of trying to elucidate the structure of DNA, Jim Watson made some cardboard models to try to understand how DNA nucleotides are paired. It helped him visualize how hydrogen atoms of paired nucleotides interact with each other to form a symmetrical structure that fits the double helix model

1 minute 42 seconds