Discovery of DNA
3 Experiments led to the discovery of DNA
1. Fredrick Griffith
Medical officer who studied Streptococcus pneumoniae (bacteria) which causes
pneumonia. He was trying to develope a vaccine for a disease causing (virulent) bacteria.
S-strain=virulent form, causes pneumonia
R-strain=harmless strain
E.1. Mouse-->R cells-->lives
E.2. Mouse-->S cells-->dies
E.3. Mouse-->kills S cells w/heat-->dies
E.4. Mouse-->heat killed S cellss are mixed with R cells-->dies
Conclusion:
The heat killed S cells released a hereditary factor that transferred the ability to
cause the disease to the live, harmless cells.
Transformation - transfer genetic material from one cell to another cell.
2. Averys experiment
-Knew either protein, dna, or rna was the transforming agent.
-Used enzymes to destroy protein, dna, rna in heat killed S cells.
Protease is the enzyme that killed protein (Exp.1)
RNase is the enzyme that killed RNA (Exp.2)
DNase is the enzyme that killed DNA (Exp.3)
Took the heat killed S an live R mixture and mixed seperately each of the results from the
first 3 experiments.
Batch 1=Dead protein, dead S cells an live R cells, with live RNA an live DNA.
Batch 2=Dead RNA, dead S cells and live R cells, DNA an Protein.
Batch 3=Dead DNA, dead S cells and live R cells, protein an RNA.
Batch 1= Mouse-->dead
Batch 2= Mouse-->dead
Batch 3= Mouse-->live
Conclusion:
The experiments that contained live DNA killed the mice. Therefore, DNA is the
transforming agent. DNA transfers genetic material.
Batch 3, (dead DNA), mice lived so there was no transforming agent. (Contained protein
an RNA)
3. Hershey - Chase experiment
Is protein or DNA responsible for carrying genetic material?
Worked with viruses. When a virus lands on bacterium, do they inject protein or DNA into
a cell? (bacteriaphage)

Since radio active phosphorous was found in the bacteria, that ment DNA carried the
genetic material in viruses.
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Hydrogen bonds between the base pairs help hold the DNA structure together.
4 Nitrogen bases:
a- adenine
t- thymine
c- cytosine
g- guanine
Base pairing rules:
Adenine always pair with thymine
Cytosine always pairs with guanine
Complimentary base pairs = a & t, c & g
One strand of DNA serves as the template (pattern) for the complimentary strand (other
side).
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DNA Replication
DNA replication-vthe process by which DNA is copied before the cell divides
(mitosis/meiosis).
Steps of replication:
1. Enzyme called helicase, seperates the DNA strands (splits the double helix) by
breaking the hydrogen bonds connecting the nitrogenous bases. Forms a
replication fork (Y-shaped fork).
2. DNA polymerase (an enzyme) picks up free floating nucleotides (A>T>C>G) & adds
them to the correct pair. Hydrogen bonds form to hold bases.
3. DNA polymerase has reached the DNA strand and falls off. Two new identical strands
of DNA have formed.
Semi-conserved replication - for each new strand of DNA, one side is new the other is
original.

Replication occurs in more than one place an more than one direction. (allows the
replication process to be rapid).
Mutation- a change in nucleotide sequence of a DNA molecule. Mutations are very rare. 1 per
every billion pares. DNA polymerase hasrepair functions which are corrected.
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Protein Synthesis
Gene- a segment of DNA on a chromosome which codes for a hereditary character.
RNA- ribo nucleic acid
involved in protein synthesis
sugar=ribose
single strand (no double helix)
cytosine, guanine, adenine, an uracil (no thymine)
C pairs with G, A pairs with U
Types:
1. mRNA=messanger RNA
carries instructions from a gene to make proteins
2. rRNA=ribosomal RNA
a subunit of ribosome (organelle where proteins are made).
3. tRNA=transfer RNA
transfers amino acids to ribosomes to make proteins.
Transcription
Where genetic instructions are transcribed/rewritten into RNA.
Steps of transcription
1. RNA polymerase (an enzyme) which catalyzes the formation of RNA from DNA
template. RNA polymerase binds promoter. Promoter - a specific nucleotide on DNA
where RNA polymerase can bind an start the process of transcription . RNA polymerase
splits the DNA strand an starts to unwind.
2. RNA polymerase starts adding free RNA nucleotides. Once added the fresul is an
RNA strand.
3. RNA polymerase reaches a termination signal, the RNA strand is released. Can be
mRNA, tRNA, or rRNA.
Comparason of DNA to RNA:
DNA: Double helix, deoxyribose, thymine
RNA: Single strand, ribose, uracil
Translation-the making of proteins.
Proteins are made up of one or more polypeptides. Polypeptides are chains of amino
acids. There are 20 different amino acids found in proteins.
5 steps in translation:
1. Initiation- 2 ribosomal subunits, tRNA, and mRNA join together. tRNA carries
methionine (the start codon). Anticodon on tRNA binds with the codon on mRNA. ALL G
2. Elongation- polypeptide chain is put together.
3. Elongation- tRNA detaches and leaves its amino acid. A new tRNA will bind to the

mRNA and the polypeptide continues to grow.
4. Termination- polypeptide grows one amino acid at a time until a stop codon is
reached. The polypeptide falls off.
5. Disassembly- the ribosome complex falls apart. tRNA, mRNA, and ribosomal sub
units seperate from each other.
Human genome- the entire gene sequence of the human body.
3.2 billion base pairs in the 23 pairs of chromosomes.