DNA
Frederick Griffith (1928) – did an experiment with bacteria and
determined there was something in cells that transferred
information and it could be a gene.
Oswald Avery (1944) – repeated Griffith’s experiments and
determind that the nucleic acid DNA stores and transmits the
genetic information from one generation of an organism to the
next.
Alfred Hershey & Martha Chase (1952) – did an experiment
with viruses that determined the genetic material of a
bacteriophage, a virus that infects bacteria, is DNA not protein.
DNA – long molecule made up of units called nucleotides.
 Nucleotide – consists of a 5-carbon sugar called
deoxyribose, a phosphate group, and a nitrogenous base.
 There are 4 different nitrogenous bases:
o Purines – adenine (A) and guanine (G)
 2 rings in their structure
o Pyrimidines – cytosine (C) and thymine (T)
 1 ring in their structure
 DNA backbone is formed by the sugar and phosphate
groups, the nitrogenous bases stick out to the side.
 Can be joined in any order, so any sequence of bases is
possible.
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).
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.
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 two 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:
o Adenine (A) & Thymine (T)
o Cytosine (C) & Guanine (G)
 This was called base pairing.
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.
RNA and Protein Synthesis
RNA (ribonucleic acid) – very similar to DNA except:
 The sugar in RNA is ribose
 RNA is single stranded
 In place of thymine, the nitrogen base in RNA is uracil (U)
There are three main types of RNA:
 Messenger RNA (mRNA) – carry copies of instructions
for assembling amino acids into proteins
 Ribosomeal RNA (rRNA) – make up ribosomes
 Transfer RNA (tRNA) – transfers each amino acid to a
ribosome during the construction of a protein according to
the instructions on mRNA.
Transcription – the process of copying part of the nucleotide
sequence of DNA to produce a complementary sequence of RNA.
1. RNA polymerase, an enzyme, binds to DNA found in
the nucleus and separates the DNA strands.
2. RNA polymerase then uses one strand of DNA as a
template for assembling a strand of mRNA that is then
released into the cytoplasm.
Codon – three consecutive nucleotides that specify a single amino
acid that is to be added to the polypeptide.
Translation – the decoding of an mRNA message into a
polypeptide chain (protein.)
1. mRNA attaches to a ribosome.
2. each codon allows the proper amino acid to be brought
into the ribosome by tRNA
3. the amino acids are attached together along an
“assembly line”
4. this continues until a “stop” codon is reached.