Structure of DNA and Replication Discovery of DNA

I. Scientist Frederick Griffith Oswald Avery Alfred Hershey Martha Chase Erwin Chargaff Rosalind Franklin Maurice Wilkins James Watson Francis Crick

I. Frederick Griffith’s Experiment 1928 S (smooth bacteria-pneumonia) bacteria Kills Mouse

b. R (rough bacteria-pneumonia) bacteria DOES NOT KILL MOUSE

c. Heat-Killed S bacteria DOES NOT KILL MOUSE

d. Heat-Killed S bacteria and Live R bacteria KILL MOUSE

e. Results Live, disease causing bacteria (S strain) from dead mouse WHY? Transformation – one type of bacteria (the harmless form) has been changed permanently into another (the disease-causing form) Thus Griffith concluded that the transforming factor had to be a gene

f. Vaccine A substance that is prepared from a KILLED or WEAKENED disease-causing agents, including certain bacteria

II. Oswald Avery Experiment 1944 Repeated Griffith’s Experiment i. Observing bacterial transformation, Avery and his team discovered that the nucleic acid DNA stores and transmits genetic information from one generation of bacteria to the next.

III. Alfred Hershey and Martha Chase’s Experiment 1952 Hershey and Chase’s experiment convinced many scientist that DNA was the genetic material found in genes Not just in viruses and bacteria, in all living cells Bacteriophage – is a virus that infects bacteria i. Remember this is NOT a bacteria it is a VIRUS

IV. Erwin Chargaff Observations 1949 Showed that the amount of Adenine ALWAYS equals the amount of Thymine i. b. Showed that the amount of Guanine ALWAYS equals the amount of Cytosine A = T OR T = A C = G OR G = C

1. So if there is 50 micrograms of adenine in a strand of DNA, there would also be 50 micrograms of ___________________. Thymine

V. Rosalind Franklin’s & Maurice Wilkins Photographs, Early 1950’s Wilkins and Franklin developed high-quality x-ray diffraction of strands of DNA The photograph suggested that the DNA molecule resembled a TIGHTLY COILED HELIX

VI. James Watson and Francis Crick Early 1950’s – published 1953 Discovered the structure of DNA Watson and Crick built a model of DNA with the configuration of a Double Helix (Spiral Staircase) of two strands of nucleotides twisting around a central axis

II. Structure of DNA DNA – Deoxyribonucleic Acid i. Five-carbon sugar found on DNA is called deoxyribose

b. Nucleotides Watson and Crick determined that a DNA molecule is made up of a chain of nitrogen bases called Nucleotides 3 parts of DNA nucleotide 1 - phosphate group 1 - 5-carbon sugar – deoxyribose 1 – nitrogen base (A, T, C, G)

iii. Label the illustration Phosphate Nitrogen base Five-Carbon Sugar

The phosphate and five-carbon sugar group are always the same for each nucleotide The nitrogenous base may be any one of the 4 bases Adenine Guanine Cytosine Thymine

c. Purines Adenine and Guanine are purines Nitrogen bases are made of two rings of carbon and nitrogen

2. How do we remember that? a. Aggies - AGs are pure = purines

d. Pyrimidine Thymine and Cytosine are pyrimidines Nitrogen bases are made up of a single ring of carbon and nitrogen

2. How do we remember that? a. pYrimidine = cYtosine and thYmine i. Hint – for the Y’s

e. Pairing and Structure Watson and Crick, with the help of Chargaff’s work, realized that purines are always paired with pyrimidines Adenine ALWAYS pairs with Thymine and Guanine ALWAYS pairs with Cytosine this is called Complementary Base Pairs Complementary base pair – the sequence of bases on one strand determines the sequence of bases on the other strand

a. Adenine (purine) = Thymine (pyrimidine) AT or TA b. Guanine (purine) = Cytosine (pyrimidine) GC or CG c. Example: T C G T A C ___ ___ ___ ___ ___ ___ A G C A T G

A G C A T G C A A C A T G T C G A G T C A G T A A C C G G T A A C G G T C

ii. Sides of DNA (backbone) 1. Phosphate and five-carbon sugar (deoxyribose) iii. Middle 1.Nitrogen Bases (A, T, C, G) 2. Hydrogen bond holding the helix together

iv. Bonds Adenine forms two hydrogen bonds with Thymine Guanine forms three hydrogen bonds with Cytosine The bonds help keep the DNA strands (2) together

v. Label the DNA illustration Deoxyribose Adenine Guanine Hydrogen Bond Cytosine Thymine Phosphate Adenine – Blue Phosphate – purple Thymine – Yellow 5-Carbon Sugar – Orange Guanine – Pink 2 Hydrogen Bond – red Cytosine – Green 3 Hydrogen Bond - brown

EOC TEST PREP QUESTION: # 1. Which pattern show how bases pair in complementary strands of DNA? a. A-C and T-G c. A-G and T-C d. A-A and C-C b. A–T and C-G

# 2. If the sequence of nucleotides on one strand of DNA molecule is GCCATTG, the sequence of the complementary strand is a. GGGTAAG b. CCCTAAC c. CGGTAAC d. GCCATTC

III.The Replication of DNA a. Roles of enzymes in DNA Replication i. The complementary strands of the double helix serve as a TEMPLATE for building new DNA. ii. The process of making a copy of DNA is called REPLICATION. 1. Remember that DNA replication occurs in the SYNTHESIS (S) phase of the cell cycle, before a cell DIVIDES.

i. Step 1: b. Steps of Replication a. ASE = enzyme 1. The double helix UNWINDS because of enzymes called DNA HELICASES. a. ASE = enzyme 2. DNA helicase breaks open the DOUBLE HELIX that link the complementary bases between 2 strands. 3. PROTEINS keep the strands separated. 4. The areas where the double helix separates are called REPLICATION FORKS because of the Y-SHAPE.

ii. Step 2: At the replication fork, ENZYMES known as DNA POLYMERASES move along each of the DNA strands and add NUCLEOTIDES to the exposed nitrogen bases, according to the BASE-PAIRING rules.

iii. Step 3 : 1. Once all of the DNA has been copied, the POLYMERASES are signaled to DETACH. 2. This process produces TWO DNA molecules (each strand is made up of one OLD and one NEW) that are IDENTICAL to each other.

c. Checking for ERRORS iv. Sometimes ERRORS occur and the wrong NUCLEOTIDE is added to the new strand. v. An important feature of DNA replication is that DNA polymerases have a “PROOFREADING ROLE”

1. DNA Polymerase can only add nucleotides to a growing strand only if the PREVIOUS NUCLEOTIDE is CORRECTLY PAIRED to its complementary base. 2. In the event of a mismatched NUCLEOTIDE, the DNA polymerase can BACKTRACK and remove the incorrect nucleotide and replaces it with the CORRECT one. a. This proofreading reduces error in DNA replication to about ONE error per ONE BILLION nucleotides.

d. The Rate of Replication i. Replication DOES NOT begin at one end and end at the other. Prokaryotic DNA (ex: Bacteria) a. Their circular DNA has TWO replication forks that begins at a single point.

2. Eukaryotic DNA (Ex: animals and plants) a. Their linear DNA has MANY replication forks b. If replication in eukaryotic DNA started at a single point it would take 33 days for a single strand to replicate.

c. Each human chromosome is replicated in about 100 SECTIONS that are 100,000 NUCLEOTIDES long, each section with its own STARTING POINT. d. An entire human chromosome can be replicated in about 8 hours.

EOC PREP QUESTIONS # 3. How many DNA molecules exist after one molecule of DNA has been replicated? A. 1 B. 2 C. 4 D. 8

EOC PREP QUESTIONS # 4. Multiple replication forks along the DNA a. correct replication errors b. reduce DNA replication time c. ensure that the new and old DNA strands are complementary d. signal DNA polymerase to stop