• Frederick Griffith (1928)
– Did an experiment with bacteria and determined
there was something in cells that transferred
– Could be a gene.
• Oswald Avery (1944)
– Repeated Griffith’s experiment and determined
that DNA (nucleic acid) transmitted genetic
information from one generation to the next.
• Alfred Hershey & Martha Chase (1952)
– Did an experiment with viruses and
determined that viruses contain DNA, also.
• DNA (Deoxyribonucleic Acid)
– Long molecule made up of units called nucleotides.
• Nucleotides –made up of a 5-carbon sugar (deoxyribose),
phosphate group, & a nitrogenous base.
• 4 different nitrogenous bases
– Purines – Adenine & Guanine (A & G)
– Pyrimidines – Thymine & Cytosine (T & C)
• Backbone made up of Sugar & Phosphates
• Can be joined in any order, so any sequence of bases is
Composition of DNA
• Long molecule made up of units called
• Nucleotides are made of 3 parts:
1. *5-carbon sugar (deoxyribose)
3.*nitrogenous base (4 different bases.)
T- thymime A always binds T
G- guanine G always binds C
• Erwin Chargaff (1930s)
– Discovered that percentages of Cytosine (C) &
Guanine (G) were almost equal in all samples
– Same was true for Thymine (T) & Adenine (A)
• Rosalind Franklin (1950)
– DNA is a helix – shaped like a coil
– Might be two strands with the nitrogen bases
pointing towards the center.
• James Watson & Francis Crick (1970s)
– Double helix – 2 strands of DNA wound
around each other like a spiral staircase.
– Hydrogen bonds form between nitrogenous
bases that point toward the center.
• Can only form between certain bases
– Adenine & Thymine (A-T)
– Guanine & Cytosine (G-C)
– Called BASE PAIRING
The DNA in a single human cell is more than
1 meter in length and is found coiled up
inside the nucleus.
• The process where a DNA molecule separates
into two strands, then produces two new
complementary strands following the rules of
• Each strand of the double helix of DNA serves as
a template, or model, for the new strand.
• Carried out by the enzyme DNA polymerase
– Also proofreads the new strands to make sure each is a
perfect copy of the original DNA.