History of DNA ~Review Discovery of the DNA double helix.... A.Dates back to the mid 1800’s B.DNA images become clear during 1950’s C. Rosalind Franklin – took X-ray photos of DNA. D.Watson and Crick - described the DNA molecule from Franklin’s X-ray.

Question: What is DNA?

Deoxyribonucleic Acid (DNA) Genetic material made up of nucleotides (DNA molecule) in a DNA double helix. Nucleotide: 1.Phosphate group 2.5-carbon sugar 3.Nitrogenous bases

DNA Nucleotide O O=P-O OPhosphate Group Group N Nitrogenous base (A, G, C, or T) (A, G, C, or T) CH2 O C1C1 C4C4 C3C3 C2C2 5 Sugar Sugar(deoxyribose)

DNA Double Helix Nitrogenous Base (A,T,G or C) “Rungs of ladder” “Legs of ladder” Phosphate & Sugar Backbone

DNA Double Helix P P P O O O P P P O O O G C TA

Nitrogenous Bases PURINES 1.Adenine (A) 2.Guanine (G) PYRIMIDINES 3.Thymine (T) 4.Cytosine (C) T or C A or G

Base # of Purines PyrimidinesPairs H-Bonds Adenine (A) Thymine (T)A = T 2 Guanine (G) Cytosine (C)C = G 3

BASE-PAIRINGS CG H-bonds T A

Chargaff’s Rule Adenine must pair with Thymine Guanine must pair with Cytosine Their amounts in a given DNA molecule will be about the same. G C TA

Discovery of DNA Structure Erwin Chargraff showed the amounts of the four bases on DNA ( A,T,C,G) In a body or somatic cell: A = 30.3% T = 30.3% G = 19.5% C = 19.9%

Question: If there is 30% Adenine, how much Cytosine is present?

Answer: There would be 20% Cytosine. Adenine (30%) = Thymine (30%) Guanine (20%) = Cytosine (20%) (50%) = (50%)

Question If 15% of the nucleotides in a DNA molecule contain Guanine, what percentage of the nucleotides contain each of the other 3 bases? Explain your reasoning.

Answer If Guanine has 15% then Cytosine also has 15% Adenine and Thymine will have 35%.

Question: When and where does DNA Replication take place?

Synthesis Phase (S phase) S phase in interphase of the cell cycle. Nucleus of eukaryotes Mitosis -prophase -metaphase -anaphase -telophase G1G1 G2G2 S phase interphase DNA replication takes place in the S phase.

**DNA Replication** How DNA makes a copy of itself Before a cell divides, its DNA is replicated. Because the two strands of a DNA molecule have complementary base pairs, the nucleotide sequence of each strand automatically supplies the information needed to produce its partner. If the two strands of a DNA molecule are separated, each can be used as a pattern or template to produce a complementary strand. Each template and it’s new complement together form a new DNA double helix, identical to the original.

The Process  Step 1 ~The double-helix unwinds separating the two strands (like the two sides of a zipper). With the help of DNA helicase, the hydrogen bonds are broken. The areas where it breaks is known as the replication fork, b/c of the Y shape.ReplicationFork Parental DNA Molecule 3’ 5’ 3’ 5’

The Process Cont..  Step 2 ~ The enzyme DNA polymerase & ligase move along the exposed DNA strand, adding nucleotides to a new DNA strand that’s complementary to the template. Polymerase also “PROOFREADS” to make sure there are no errors and each copy is identical to the original.  Step 3 ~ Two DNA molecules are formed & are identical to the original. Each one contains a new and original strand of DNA

Question: What would be the complementary DNA strand for the following DNA sequence? DNA 5’-GCGTATG-3 ’

Answer: DNA 5’-GCGTATG-3’ DNA 3’-CGCATAC-5’

Animation of DNA Replication DNA Replication - long