I. DNA: “deoxyribonucleic acid”

We know traits are inherited but how are

they inherited?

A. X-ray EvidenceA. X-ray Evidence

• 1952 Rosalind Franklin took an x-ray diffraction to take picture of DNA shape

• Picture gave clues to structure of DNA

Watson & CrickWatson & Crick

• Using Rosalind Using Rosalind FranklinFranklin’’s work, s work, Watson & Crick made Watson & Crick made a model of DNAa model of DNA

• Created with cut-out Created with cut-out pieces held together pieces held together with wirewith wire

• Won Nobel PrizeWon Nobel Prize

Enzo di Fabrizio (University of Genoa, Italy) 2012..

Developed a technique that pulls strands of DNA between two miniscule silicone pillars, then photographs them via an electron microscope.

1. Watson and Crick's model of DNA was a double helix- two strands were wound around each other.

B. Why is DNA important?B. Why is DNA important?

Contains instructions for genes, which Contains instructions for genes, which determine traits of an organismdetermine traits of an organism

C. Location of DNA? C. Location of DNA?

NucleusNucleus

The Components & Structure The Components & Structure of DNAof DNA

NucleotideNucleotide1.1. 5-carbon sugar 5-carbon sugar

DEOXYRIBOSEDEOXYRIBOSE

2. Phosphate 2. Phosphate

groupgroup

3. Nitrogenous 3. Nitrogenous

base (4)base (4)

NucleotideNucleotide1.1. 5-carbon sugar 5-carbon sugar

DEOXYRIBOSEDEOXYRIBOSE

2. Phosphate 2. Phosphate

groupgroup

3. Nitrogenous 3. Nitrogenous

base (4)base (4)

i. Adeninei. Adenine ii. Thymineii. Thymine iii. Guanineiii. Guanine iv. Cytosineiv. Cytosine

i. Adeninei. Adenine ii. Thymineii. Thymine iii. Guanineiii. Guanine iv. Cytosineiv. Cytosine

ChargaffChargaff’’s Rules Rule

• Something Something interesting about interesting about nucleotidesnucleotides

• % A = % T% A = % T• % C = % G% C = % G

Erwin Chargaff

Photo credit: http://history.nih.gov/exhibits/nirenberg/popup_htm/03_chargoff.htm

- SUGAR & PHOSPHATE form backbone/sides

- The BASES form the “steps” of ladder, held together by Hydrogen bonds

II. DNA Replication

• Think about DNA’s structure• To know one side, you know the

opposite!• Perfect for use of replication!• Each side of DNA provides a

template to reproduce the other ½

II. DNA Replication

A. When in cell cycle? S phase of Interphase!

B. Purpose?: To provide identical copy of DNA to the 2

new cells that form from mitosis

C.C. How?How? 1. DNA unzips (uses DNA helicase)1. DNA unzips (uses DNA helicase)

2. Complementary sides are built 2. Complementary sides are built (uses (uses

DNA polymerase)DNA polymerase)

D.D. Practice (next slide)Practice (next slide)

E.E. Results? Results? 2 new, identical strands of DNA!!2 new, identical strands of DNA!!

New Strand Original strand

Nitrogen Bases

Replication Fork

Replication ForkDNA Polymerase

Replication Fork

PRACTICE!!• DNA strand is TACGTTWhat is the complementary strand?• ATGCAA

• CAGTAC is the DNA strandWhat is the complementary strand?• GTCATG

• AGCTAGATC is the DNA strandWhat is the complementary strand?• TCGATCTAG

Chromosomes

• Bacteria have single, circular strand of DNA

How is DNA packed in a eukaryotic cell?

DNA ReplicationDNA Replication

• Each side of DNA is called a Each side of DNA is called a ““template”template”

• Parent strandParent strand: original copy of : original copy of DNA, used as templateDNA, used as template

• Daughter strandDaughter strand: newly : newly synthesized complementsynthesized complement

How do you replicate How do you replicate something so long?something so long?

1. Replication occurs at 1. Replication occurs at hundredshundreds of places of places

(“ORIGINS”)(“ORIGINS”)

2. Moves in BOTH directions until each 2. Moves in BOTH directions until each

chromosome is completely copiedchromosome is completely copied

3. Uses ENZYMES to speed up process3. Uses ENZYMES to speed up process

What phase in the cell cycle does replication take What phase in the cell cycle does replication take place? place? S PhaseS Phase!!

Why does DNA replication occur? Why does DNA replication occur? makes sure each makes sure each cell will have a complete set of DNAcell will have a complete set of DNA

Where does this happen? Nucleus!

Where does this happen? Nucleus!

Steps of DNA ReplicationSteps of DNA Replication

1. DNA unwinds1. DNA unwinds

2. 2. DNA HELICASEDNA HELICASE enzyme unzips the weak enzyme unzips the weak hydrogen bonds between base pairshydrogen bonds between base pairs

Steps of DNA ReplicationSteps of DNA Replication

1. DNA unwinds1. DNA unwinds

2. 2. DNA HELICASEDNA HELICASE enzyme unzips the weak enzyme unzips the weak hydrogen bonds between base pairshydrogen bonds between base pairs

3. Complementary 3. Complementary RNA PRIMERSRNA PRIMERS are added to are added to the DNA to the DNA to begin copying.begin copying.

4. 4. DNA POLYMERASEDNA POLYMERASE enzyme matches up enzyme matches up DNA nucleotides to 3’ end continuously DNA nucleotides to 3’ end continuously

(5’ to 3’ directionality) and is called the (5’ to 3’ directionality) and is called the “leading strand”. Later the RNA primers “leading strand”. Later the RNA primers are replaced with DNA nucleotidesare replaced with DNA nucleotides

5. These fragments are “glued” together 5. These fragments are “glued” together using using DNA LIGASEDNA LIGASE

DNA REPLICATIONcan you spot all the enzymes?

New Strand Original strand

““Semi-conservativeSemi-conservative””- why?- why?

PRACTICE!!

DNA strand is TACGTTWhat is the complementary strand?•ATGCAA

CAGTAC is the DNA strandWhat is the complementary strand?•GTCATG

AGCTAGATC is the DNA strandWhat is the complementary strand?•TCGATCTAG