DNA
(deoxyribonucleic acid )
**Long molecule made up of units called nucleotides.
*Nucleotide-- consists of:
a.) 5-carbon sugar ( deoxyribose)
b.) phosphate group
c.) nitrogenous base
* There are 4 different nitrogenous bases:
1. Purines—adenine(A) & guanine(G)
*2 rings in their structure
2. Pyrimidines –cytosine(C) & thymine(T)
*1 ring in their structure
***It is the order of the nitrogen bases
that make up the genetic code***
DNA molecules are very long and must be folded into a space
only one one-thousandth of its length. They are found in the
nucleus of the cell.
Example: ** The chromosome of a bacterial cell in the human
colon contains 4,639,221 base pairs.
**A human cell contains almost 1000 times as
many base pairs of DNA as a bacterium. The
DNA of a human cell is more than 1 meter
in length and is found coiled up in the nucleus of
a single cell.
1. Frederick Griffith (1928)
 did an experiment with bacteria and determined there
was something in cells that transferred information and it
could be a gene
2. Oswald Avery (1944)
 repeated Griffith’s experiments and determined that the
nucleic acid DNA stores and transmits the genetic
information from one generation of an organism to the
next.
3. Alfred Hershey & Martha Chase (1952)
 did an experiment with viruses that determined the
genetic material of a bacteriophage, a virus that infects
bacteria.
4. Erwin Chargaff
 discovered that percentages of guanine(G) and
cytosine(C) were almost equal in all samples of DNA and
the same was true for adenine(A) and thymine(T)
5. Rosalind Franklin (1950)
 used x-ray diffraction to get information about the
structure of DNA. She discovered that strands in DNA
are twisted around each other like coils of a spring. This
shape is called a Helix. She also discovered that there
might be two strands and that the nitrogenous bases
might point towards the center. Therefore, making it a
Double Helix.
6. James Watson & Francis Crick
 used the discoveries of all the previous scientists to
develop a model of DNA.
**Double Helix- 2 strands of DNA wound around
each other in the shape of a spiral staircase.
**Hydrogen bonds formed between 2 nitrogenous
bases all the way up and down the strand to hold the
two strands together.
**Hydrogen bonds can only form between certain
base pairs: This is called base pairing
* Adenine(A) & Thymine(T)
* Cytosine(C) & Guanine(G)
DNA REPLICATION
** The process where a DNA molecule separates into 2
strands, then produces 2 new complementary strands
following the rules of base pairing. 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
** This enzyme also “ proofreads” the new
DNA strands to make sure that each
Molecule is a perfect copy of the original
DNA.