DNA Structure (Chapter 16)

?Chromosomes are found in the nucleus. ?Chromosomes are made up of protein and nucleic acid. ?Genes are located on chromosomes. ?When Watson and Crick began working on their model, scientists knew that genes were found on chromosomes in the nucleus and that chromosomes were composed of proteins and nucleic acid. Watson and Crick were two of the few to focus on the nucleic acid, DNA, instead of proteins.

DNA is composed of a sequence of subunits, each containing a phosphate, a deoxyribose sugar, and one of four nitrogenous bases: adenine (A), thymine (T), guanine (G), or cytosine (C).

?Jim Watson and Francis Crick built theoretical models, incorporating current knowledge about chemical bonding and X-ray data. ?Different scientists took different approaches to understanding the structure of DNA, based upon their training and the tools available. In the end, the problem was solved using all these different approaches.

The "X" pattern of the X-ray crystallography image matched the shape that Crick predicted for a helix.

Chargaff's observation was an important clue to the A-T and G-C pairings that Watson and Crick eventually proposed.

The structure of DNA suggested that the order of bases contains information. Because A is always paired with T and G with C, the order of bases on one strand determines the order on the other strand. Thus, if a DNA molecule were unwound, each strand could be copied into a complementary strand, producing an exact replica of the original molecule.

A nucleotide consists of three things: A nitrogenous base, which can be either adenine, guanine, cytosine, or thymine (in the case of RNA, thymine is replaced by uracil). A five-carbon sugar, called deoxyribose because it is lacking an oxygen group on one of its carbons. One or more phosphate groups.

?With some exceptions, DNA is a double-stranded molecule and RNA is a single-stranded molecule. ?RNA has an extra O in the sugar (ribose instead of deoxyribose) ?DNA contains thymine, whereas RNA contains uracil.

single-ring structures (nucleic acids)

2 linked ring structures (nucleic acids)

The nitrogenous base is attached to the sugar's 1' carbon and the phosphate group is attached to the sugar's 5' carbon.

? 5' to 3' ?New nucleotides are added to the 3' end of a growing polynucleotide.

thymine ... cytosine

Chargaff's rules states that DNA from any cell of all organisms should have a 1:1 ratio (base Pair Rule) of pyrimidine and purine bases and, more specifically, that the amount of guanine is equal to cytosine and the amount of adenine is equal to thymine

chromosome consists of LOOPS of SUPERCOILS made up of TIGHT HELICAL FIBERS composed of NUCLEOSOMES that consist of DNA wrapped around PROTEIN molecules called HISTONES

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(DNA and Proteins) Long DNA molecules are associated with proteins that allow DNA to coil and pack.

It exists as chromatin and is less condensed than mitotic chromosomes. Chromatin is the term for the loosely coiled DNA observed in cells during interphase. Even during interphase, however, parts of the chromosome may still be rather tightly coiled and packed.

A to T C to G

sequence of bases

Histones

a single linear molecule of double-stranded DNA plus proteins

Histones are positively charged, and DNA is negatively charged.