DNA Structure and Replication

I.  General Characteristic of Nucleic Acids

Nucleic Acids:


A.  Types:

        1. DNA :

                        Adenine   (A), Cytosine (C), Guanine (G), and Thymine (T).

        2.  RNA:

                        Adenine   (A), Cytosine (C), Guanine (G), and Uracil (U).

B.  Nucleotides-----  Building blocks of DNA

        1.  Nitrogenous base

            a)  Purines:  Two carbon rings:  Adenine (A), Guanine (G)

            b)  Pyrimidines:  One carbon ring:  Thymine  (T), Cytosine (C), and Uracil (U) for RNA.

        2.  Five - carbon sugar

            a)  deoxyribose for DNA:  missing an oxygen atom at its #2 carbon.

            b)  ribose for RNA.

        3.  Phosphate group:  Connecting the 5-Carbon sugars by covalent bond to form the back bone for DNA and RNA.

C.  Structure of  DNA

    1.  Cast of Characters:

        a)  Erwin Chargaff:  A== T; G== C, always come in pairs.

        b)  Maurice Wilkin and Rosalid Franklin  published the first X-ray diffraction photograph.

        c)  Linus Pauling:  Three -stranded DNA model.

        d)  James Watson and Francis Crick

            Double Helix Structure, and base pairing rule:  G---> C, and  A---> T, three hydrogen bonds between G and C,

            two hydrogen bonds between A and T.

    2.  The double helix structure:

D.  DNA Replication:

        Semi conservative model:  something old:  parental strand; something new:  newly synthesized strand

        DNA Polymerase and DNA ligase together are responsible for DNA replication, plus some other enzymatic factors.

        Editing out mistakes:  nuclease.

        Heritable mutations:  germ-line cell mutations are heritable.

        Value of mutations (mistakes):  Evolutionary adaptation.