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  • Polymerase chain reaction

    Polymerase chain reaction

    Animations

    (1 min 28 sec) 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.

  • Paired DNA strands

    Paired DNA strands

    Animations

    (1 min 18 sec) 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.

  • DNA packaging

    DNA packaging

    Animations

    (1 min 44 sec) 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.

  • mRNA splicing

    mRNA splicing

    Animations

    (39 sec) 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.

  • Human genome sequencing

    Human genome sequencing

    Animations

    (1 min 49 sec) 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.

  • Human chromosomes

    Human chromosomes

    Animations

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

  • Genetic engineering

    Genetic engineering

    Animations

    (1 min 13 sec) 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.

  • Damage to DNA leads to mutation

    Damage to DNA leads to mutation

    Animations

    (1 min 7 sec) 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.

  • Coding sequences in DNA

    Coding sequences in DNA

    Animations

    (1 min 5 sec) 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.

  • Chargaff's Ratio

    Chargaff's Ratio

    Animations

    (49 sec) 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.

  • Building blocks of DNA

    Building blocks of DNA

    Animations

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

  • MECP2

    MECP2

    Animations

    (43 sec) This animation shows how the protein MECP2, in conjuction with another protein complex, can act as an "on-off' switch for gene expression.

  • p53

    p53

    Animations

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

  • Mismatch Repair

    Mismatch Repair

    Animations

    (1 min 22 sec) This animation illustrates how mistakes made during DNA replication are repaired.

  • Tri Nucleotide Repeat

    Tri Nucleotide Repeat

    Animations

    (1 min 8 sec) Slippage during DNA replication can lead to expanding sections of repeating nucleotides. Watch this animation to see how this problem occurs. 

  • X Inactivation

    X Inactivation

    Animations

    (55 sec) 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. 

  • Using DNA to Trace Human Migration

    Using DNA to Trace Human Migration

    Click & Learn

    All living humans originated from populations of ancestors who migrated out of Africa less than 100,000 years ago. Learn how scientists have used genetic markers to trace the migration routes and origins of modern human populations.

  • How to Analyze DNA Microarray Data

    How to Analyze DNA Microarray Data

    Click & Learn

    DNA microarrays, or gene chips, are an important new technology for genomic research. Learn how researchers use computing to analyze and interpret the huge datasets generated by microarray experiments.

  • Test Your Knowledge of Sex Determination

    Test Your Knowledge of Sex Determination

    Click & Learn

    Answer the questions in this online quiz to test your knowledge of what determines the sex of a human, a fruit fly, and a snapping turtle.

  • Meiosis

    Meiosis

    Animations

    (5 min 53 sec) 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? Also available in Spanish.

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