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  • Watson constructing base pair models

    Watson constructing base pair models


    (1 min 42 sec) 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.

  • Triplet code

    Triplet code


    (1 min 8 sec) 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.

  • Translation (advanced detail)

    Translation (advanced detail)


    (3 min 4 sec) 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. Also available in Spanish.

  • Translation (basic detail)

    Translation (basic detail)


    (2 min 6 sec) 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. Also available in Spanish.

  • DNA transcription (advanced detail)

    DNA transcription (advanced detail)


    (1 min 55 sec) 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. Also available in Spanish.

  • DNA transcription (basic detail)

    DNA transcription (basic detail)


    (1 min 55 sec) 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. Also available in Spanish.

  • Sickle Cell Anemia

    Sickle Cell Anemia


    (1 min) Sickle cell anemia is a genetic disease that affects hemoglobin.

  • Shotgun sequencing

    Shotgun sequencing


    (1 min) 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.

  • Sanger method of DNA sequencing

    Sanger method of DNA sequencing


    (52 sec) 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.

  • DNA replication (advanced detail)

    DNA replication (advanced detail)


    (2 min 20 sec) 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. Also available in Spanish.

  • DNA replication (basic detail)

    DNA replication (basic detail)


    (1 min 7 sec) 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. Also available in Spanish.

  • DNA replication (schematic)

    DNA replication (schematic)


    (50 sec) 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. Also available in Spanish.

  • Polymerase chain reaction

    Polymerase chain reaction


    (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


    (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


    (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


    (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


    (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


    (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


    (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


    (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.

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