history of dna ~review discovery of the dna double helix.... a.dates back to the mid 1800s b.dna...

History of DNA ~ReviewHistory of DNA ~Review• Discovery of the DNA double

helix....

A. Dates back to the mid 1800’sB. DNA images become clear during 1950’sC. Rosalind Franklin – took X-ray photos of DNA.D. Watson and Crick - described the DNA molecule from Franklin’s X-ray.

Question:Question:•What is DNA?

Deoxyribonucleic Acid (DNA)

• Genetic material made up of nucleotides (DNA molecule) in a DNA double helix.

• Nucleotide:1. Phosphate group2. 5-carbon sugar3. Nitrogenous bases

DNA Nucleotide

OO=P-O O

PhosphatePhosphate GroupGroup

NNitrogenous baseNitrogenous base (A, G, C, or T)(A, G, C, or T)

CH2

O

C1C4

C3 C2

5

SugarSugar(deoxyribose)(deoxyribose)

DNA Double Helix

NitrogenousNitrogenousBase (A,T,G or C)Base (A,T,G or C)

““Rungs of ladder”Rungs of ladder”

““Legs of ladder”Legs of ladder”

Phosphate &Phosphate &Sugar BackboneSugar Backbone

DNA Double Helix

P

P

P

O

O

O

1

23

4

5

5

3

3

5

P

P

PO

O

O

1

2 3

4

5

5

3

5

3

G C

T A

Nitrogenous Bases• PURINES

1. Adenine (A)2. Guanine (G)

• PYRIMIDINES3. 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 CT A

Discovery of DNA Discovery of DNA StructureStructure

• 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%)

QuestionQuestion

• 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.

AnswerAnswer• 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

G1 G2

Sphase

interphase

DNA replication takesDNA replication takesplace in the S phase.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 ProcessStep 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.

ReplicationReplicationForkFork

Parental DNA MoleculeParental 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