MOLECULAR GENETICS I

 

Nature of DNA

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    Miescher discovered DNA in the 1880s and coined the term "nuclein"

 

    Levene in the 1920s who worked out the chemistry of nucleic acids

 

        RNA and DNA are two types of nucleic acids

                 -difference lies in the SUGAR not the bases,

 

    Nucleic acids have three different components

 

    1. a pentose sugar    ribose in RNA ; deoxyribose in DNA

 

    2. a nitrogenous base   

        purines: adenine (A) and guanine (G)

        pyrimidines: thymine (T), cytosine (C), and uracil (U)

 

    3. phosphate     one, two, or three phosphates.

 

    sugar + base = nucleoside

    sugar + base + phosphates = nucleotide

    polymer of nucleotides = nucleic acid

 

 

    Naming the nucleotides is easy. Follow the rules! And remember the abbreviations!

 

    Ribonucleotides

 

    the name of the nucleoside comes first, then the phosphate with a prefix denoting the

    number of phosphates

 

    e.g. adenosine monophosphate (AMP) 

           adenosine diphosphate (ADP)

           adenosine triphosphate (ATP)

 

    Let's stick with abbreviations!

 

    Deoxyribonucleotides

 

    With deoxyribose, the nucleotide has deoxy added to the name

    e.g. deoxyadenosine monophosphate (dAMP), et cetera

 

    It's easier to name the DNA nucleotides with their bases alone (A,C,G,T).

 

    Numbering conventions

 

        The carbons of the nitrogenous base are given cardinal numbers (1,2,3...)

        The numbers  on the pentose are given prime numbers

 

    The 1' carbon (read: one-prime) is the carbon of the sugar that's attached to the base.

 

    The 2' carbon has an OH group in RNA but an H only in DNA. It's an important

    difference!

 

    The 3' carbon has an OH group in both acids and participates in building the

    sugar-phosphate backbone (condensation synthesis - remember?)

 

    The 4' carbon sits there ...

 

    The 5' carbon is attached to the phosphate group(s)

 

     Making polymers of nucleic acids

 

        Nucleic acids have a polarity.

        The nucleotides are synthesized, and strung, in a 5' to  3' direction.

        RNA is single-stranded but may contain regional loops due to H-bonding

    (all that chemistry is coming back!) 2° RNA structure

DNA forms a double helix, with two strands that lie anti-parallel to each other.

In 1953, James Watson and Francis Crick unravelled the secret of the structure of DNA and ushered in the era of molecular genetics.

It  was a monumental step in science. Everything started to make sense due to the

simplistic yet amazing features of the helix. By the way, most DNA is right-handed!

Watson, Crick, and Wilkins were awarded the Nobel Prize in 1962.

 

    The double helix

 

    DNA is a molecule consisting of nucleotides strung in a 5' to 3' direction held

    together by phosphodiester bonds between the sugar and the phosphate. Two strands

    form a double helix that is stabilized by hydrogen bonds between adjacent bases on

    the inside of the helix. It is helpful to think of the sugar-phosphate backbone of DNA

    as the sides of a ladder and the base pairs as the rungs of the ladder.

 

 

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