DNA Part II Principles of Life Science Rainier Jr/Sr High School Mr. Taylor

DNA (deoxyribonucleic acid) ► A nucleotide is  a phosphate group  a sugar (in this case = deoxyribose)  a nitrogen base (adenine, thymine, cytosine, or guanine) ► DNA is a polymer … it is a bunch of nucleotides linked together to form a HUGE molecule.

Nitrogen bases ►T►T►T►The two rails of DNA are held together by weak bonds between hydrogen atoms on the nitrogen bases ►B►B►B►Because of structure and number of bonding sites, for DNA AAAAdenine always pairs with Thymine (A with T) TTTThymine always pairs with Adenine (T with A) CCCCytosine always pairs with Guanine (C with G) GGGGuanine always pairs with Cytosine (G with C) ►T►T►T►This is the BASE PAIRING RULE

Complementary Pairing ► The Base Pairing Rule (A with T and C with G) is called complementary pairing. Leading Strand: A G C T A C C G T Lagging Strand: T C G A T G G C A

DNA Replication 1. DNA helicases helicases “unzip” the two complementary strands by breaking the hydrogen bonds between nitrogen bases. The separating area is called the “replication fork”. 2. DNA polymerases polymerases add nucleotides according to the base-pairing rule (A with T, C with G)  The  The DNA polymerase also holds the two strands apart so that they don’t “stick” back together again.

DNA Replication 3. DNA polymerases also check for errors: tttthe polymerase doesn’t move to the next nucleotide in the sequence unless the current one is correct IIIIf an error is detected, the DNA polymerase removes the wrong nucleotide and inserts the correct one. ►T►T►T►The error rate for this process is <1:1,000,000,000 ►A►A►A►An error is called a mutation WWWWe know that some substances can cause mutations and are called mutagens.

This process takes about 6 hours in humans. In some simple bacteria it can be as quick as minutes.