Sunday, January 29, 2017

David Allen writes


The crime lab tech said
there was no doubt that
the DNA from the defendant’s
saliva matched DNA from sperm
samples collected at two rape scenes.
“There’s just one chance
in 4.8 billion it could have
been someone else,” he said.
The accused hung his head.
It sucks when your own body
rats you out. 
 Image result for thierry ehrmann dna

DNA -- Thierry Ehrmann


  1. Deoxyribonucleic acid (DNA) is a molecule that stores and carries the genetic instructions used in the growth, development, functioning and reproduction of all known living organisms and many viruses. Most DNA molecules consist of two antiparallel (running in opposite directions to each other) biopolymer strands coiled around each other to form a double helix; these are called polynucleotides since they are composed of simpler monomer units called nucleotides. Each nucleotide is composed of a sugar (deoxyribose), a phosphate group, and one of four nitrogen-containing nucleobases (either cytosine, guanine, adenine, or thymine). The nucleotides are joined to one another in a chain by covalent bonds between the sugar of one nucleotide and the phosphate of the next, resulting in an alternating sugar-phosphate backbone. A gene, a unit of heredity, is a sequence of DNA that contains genetic information that influences a particular characteristic in an organism. The genetic information in a genome is held within genes, and the complete set of this information in an organism is called its genotype. Within a gene, the sequence of bases along a DNA strand defines a messenger RNA sequence, which then defines one or more protein sequences. (RNA, ribonucleic acid, a polymeric molecule also assembled as a chain of nucleotides, but often a single-strand folded onto itself). A DNA sequence is called "sense" if its sequence is the same as that of a messenger RNA copy that is translated into protein; the sequence on the opposite strand is called the "antisense" sequence. The relationship between the nucleotide sequences of genes and the amino-acid sequences of proteins is determined by the rules of translation, known collectively as the genetic code, which consists of 64 three-letter 'words' called codons formed from a sequence of three nucleotides. In transcription, the codons are copied into messenger RNA by an enzyme called RNA polymerase; this RNA copy is then decoded by a ribosome that reads the RNA sequence by base-pairing the messenger RNA to transfer RNA which carries amino acids. Cell division is essential for an organism to grow, but when a cell divides it must replicate the DNA in its genome so that the two daughter cells have the same genetic information as their parent; this is accomplished by DNA-dependent DNA polymerases making copies of DNA polynucleotide chains. Nucleases are enzymes that cut DNA strands by catalyzing the hydrolysis of the phosphodiester bonds. The most frequently used nucleases in molecular biology are the restriction endonucleases, which cut DNA at specific sequences within strands. (Nucleases that hydrolyse nucleotides from the ends of DNA strands are called exonucleases.) These enzymes digest the phage DNA when it enters the bacterial cell, thus protecting bacteria against phage infection, and these sequence-specific nucleases are used in molecular cloning and genetic fingerprinting (DNA profiling). Forensic scientists can use DNA in blood, semen, skin, saliva, or hair to identify a matching DNA of an individual; the lengths of variable sections of repetitive DNA, such as short tandem repeats and minisatellites, are compared between people. DNA profiling was developed in 1984 by British geneticist Sir Alec Jeffreys.

  2. In 1869 Swiss physician Friedrich Miescher discovered various microscopic phosphate-rich chemicals in the pus of discarded surgical bandages; since they resided in the nuclei of cells, he called the substance "nuclein," but now it is known as DNA; later he suggested that nucleic acids could be involved in heredity. Albrecht Kossel made the initial inquiries into its chemical structure in 1878, isolating nucleic acid, its non-protein component, and later he isolated its five primary nucleobases; he received the Nobel Prize for Physiology or Medicine in 1910 for his work. In 1919, Phoebus Levene identified the base, sugar, and phosphate nucleotide units and suggested that DNA consisted of a string of nucleotide units linked together through the phosphate groups.
    In 1927, Nikolai Konstantinovich Koltsov proposed that traits were inherited via a "giant hereditary molecule" made up of "two mirror strands that would replicate in a semi-conservative fashion using each strand as a template." (In 1920 he had been denounced as a member of a non-existent "anti-Soviet Tactical Center," but author Maxim Gorky 's direct appel to V. I. Lenin restored him to his position as the head of the Koltsov Institute of Experimental Biology; he was targeted again in 1937 and 1939 by supporters of Trofim Lysenko; in 1940 he was poisoned by the NKVD, and his wife committed suicide the same day.) Frederick Griffith's 1928 experiment was the first widely accepted demonstration of bacterial transformation, whereby a bacterium distinctly changes its form and function. In 1937 William Astbury produced the first X-ray diffraction patterns that showed that DNA had a regular structure. In 1943 Oswald Avery, Colin MacLeod, and Maclyn McCarty built upon Griffith's work by identifying DNA as the transforming principle, rather than proteins (the word "protein" had been coined in 1838 by Jöns Jacob Berzelius from the Greek "proteios," meaning "primary," when his colleague Gerardus Johannes Mulder first described them). In 1952, Alfred Hershey and Martha Chase conducted a series of experiments showing that when bacteriophages, which are composed of DNA and protein, infect bacteria, their DNA enters the host bacterial cell but most of their protein does not; Hershey won the 1969 Nobel Prize. Based on an X-ray diffraction image taken by Rosalind Franklin and Raymond Gosling in 1952, James Watson and Francis Crick suggested the double-helix model of DNA structure in 1953; after Franklin's death, Watson, Crick, and Wilkins jointly received the 1962 Nobel Prize. In 1957 Crick laid out the central dogma of molecular biology, which foretold the relationship between DNA, RNA, and proteins, and articulated the "adaptor hypothesis." Final confirmation of the replication mechanism that was implied by the double-helical structure followed in 1958 when Matthew Meselson and Franklin Stahl supported the hypothesis that DNA replication was semiconservative (meaning that when the DNA helix is replicated, each of the two new double-stranded DNA helices consists of one strand from the original helix and one that is newly synthesized.) Further work by Crick showed that the genetic code was based on non-overlapping triplets of bases (codons), allowing Har Gobind Khorana, Robert W. Holley, and Marshall W. Nirenberg to decipher the genetic code by demonstrating that the order of nucleotides control the cell’s synthesis of proteins n 1964, leading to their joint Nobel Prize in 1968.


Join the conversation! What is your reaction to the post?