Download DNA Composition and Structure

DNA Composition and
Structure
Ariana Figueroa, Danielle Nicholson, Dominique Tran, Juliet
Pepe, Kira Mathiesen
The Discovery of DNA Composition
•
DNA structure consists of a backbone, which is a
repeating chain of phosphate-sugar-phosphatesugar, and bases (adenine, thymine, cytosine and
guanine) sticking out on the sides, akin to side chains
of amino acids.
•
Chargaff discovered two very interesting things about
DNA: 1) its base composition was not in fact a ratio of
1:1:1:1 for A, G, C, and T, but the ratios of adenine to
thymine and guanine to cytosine remained constant
at approximately 1:1 between each two; and 2) The
base composition was not the same in all organisms.
Questions
• What tests did Chargaff do exactly to come about his theories?
• How do certain amounts of different bases affect the makeup of an
organism? e.g. what characteristics of an organism would be affected if
the ratio of A:G were 3 to 1?
Replication of DNA
•
Polymerization always goes in the 5' to 3' direction, and new
sequences can be added onto the 3' end only.
•
Base-paired strands (i.e. C-G and A-T or A-U, as in RNA) must be
anti-parallel.
•
Strands are held together with covalent bonds, while base-pairing
between strands can be attributed to hydrogen bonds.
•
The "leading strand" in DNA is the continuous strand that can add
on to its 3' end without a break, while the "lagging strand" is
discontinuous and must jump ahead and restart in order to keep
going, which takes longer (thus- it lags).
Questions
• Why must DNA strands be anti-parallel?
• The 5' end of a DNA strand is hydrophobic, while the 3' end (an oxygen
atom) is hydrophilic. Is this connected to the reason that a strand can only
continue from its 3' end?