Important Experiments Griffith’s Experiments In 1928, when trying to prepare a vaccine against a pneumonia-causing bacterium, Griffith discovered TRANSFORMATION. Vocab: Vaccine- substance prepared from killed or weakened microorganisms that is introduced into the body to protect the body against future infections from the microorganisms. Transformation- a change in the phenotype caused when bacterial cells take up foreign genetic material.

Griffith (continued) He studied two strains of pneumonia-causing bacteria: S-bacteria (capsule) R-bacteria (no capsule) Mice injected with S- bacteria always got sick and died, but mice injected with R-bacteria did not. Griffith wanted to know if the capsule around S-bacteria is what caused the mouse to get sick.

Even though Griffith killed the S-bacteria by heating it, when he injected it with the healthy R-bacteria, the S- bacteria DNA transformed the R-bacteria DNA so that it became virulent and killed the mouse.

- Structure of DNA James Watson and Francis Crick discovered that: DNA is a double-helix, a twisted ladder.

Watson and Crick: DNA is composed of linked nucleotides. Nucleotides have 3 parts: 5- carbon Sugar, Phosphate and nitrogen Base. The 5-carbon sugar is called deoxyribose.

Nitrogen bases There are 4 different nitrogen bases: Adenine Thymine Cytosine Guanine Adenine and Guanine are called Purines- composed of two-rings Thymine and Cytosine are called Pyrimidines- composed of one ring.

DNA Bases are held together by weak hydrogen bonds.

Base-Pair Rules The base-pair rules state that each base pair can only bond with it’s complement base pair. For example: Adenine always pairs with Thymine Cytosine always pairs with Guanine Therefore, each strand of DNA is called complementary.

Base-Pair Rules Let’s try it: If you have a strand: ATGCACTTGACGC What is its complement strand? _________________________________ T A GTCGATCAGCG

For the Test:

-Replication You’ve already learned that a cell must copy its DNA before it can divide and reproduce. Do you remember what phase DNA is copied in? ___________ Let’s learn HOW DNA is copied! S-Phase of interphase

Replication-Step 1 DNA helicases unwind the DNA and break the hydrogen bonds holding the strands together. Once separated, additional proteins attach that hold the strand apart and prevent them from twisting up again. This creates Replication forks. Keep in mind that multiple replication forks can speed up replication!

Step 2 DNA polymerases (enzymes) move along each of the strands, adding free floating nucleotides to the exposed nitrogen bases. Remember: Base-pair rules apply! As the polymerases move along, two new double-helixes are formed!

Step 2

Step 3 The helicase enzyme remains attached to the strand until the entire DNA strand has been copied and it is signaled to detach. This process produces two DNA molecules, both composed of an original (template) strand and one new (complement) strand! The new DNA is IDENTICAL to that of the original strand.

Proof-Reading Sometimes, the wrong nucleotide can be added to a base. In this case, the polymerase can backup, remove the wrong base and replace it with the correct one. This prevents most errors from occurring. When an error isn’t fixed, a mutation can occur. This only happens 1 time per one billion nucleotides though!