DNA. History of DNA Early scientists thought protein was the cell’s hereditary material because it was more complex than DNA Proteins were composed of.

DNA

History of DNA

• Early scientists thought protein was the cell’s hereditary material because it was more complex than DNA

• Proteins were composed of 20 different amino acids in long polypeptide chains

TransformationTransformation• In 1928, Fred Griffith worked with virulent S and nonvirulent R strain Pneumoccocus bacteria

• He found that R strain could become virulent when it took in DNA from heat-killed S strain

• Study suggested that DNA was probably the genetic material

Griffith Experiment

History of DNA• Chromosomes are made of both DNA and protein

• Experiments on bacteriophage viruses by Hershey & Chase in 1952 proved that DNA was the cell’s genetic material

Radioactive 32P was injected into bacteria!

DNA StructureDNA Structure• In 1950’s, Rosalind Franklin took x-ray diffraction photographs of DNA crystals

showed: 1) helicalstructure2) moleculesspaced at regularintervals

• In 1953, Watson & Crick built the first model of DNA using Franklin’s

work as a foundation

DNA STRUCTUREDNA STRUCTURE•Two strands coiled called a double helix

•Sides made of a 5 carbon sugar deoxyribose, bonded to phosphate (PO4) groups

•Center made of nitrogen bases bonded together by weak hydrogen bonds

DNA Double HelixDNA 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 DNA • Stands for Deoxyribonucleic acid

• Made up of subunits called nucleotidesnucleotides

• Each nucleotide is made Each nucleotide is made of:of:

1. Phosphate groupPhosphate group2. 5-carbon sugar5-carbon sugar3. Nitrogenous baseNitrogenous base

DNA NucleotideDNA 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)

Nitrogenous BasesNitrogenous Bases

• 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%

Chargaff’s Rule:Chargaff’s Rule:• AdenineAdenine must pair with ThymineThymine

• GuanineGuanine must pair with CytosineCytosine

• The bases form weak hydrogen bonds

G CT A

DNADNA

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

Base-PairingsBase-PairingsThree hydrogen bonds required to bond Guanine & Cytosine

CG

3 H-bonds

T A

•Two hydrogen bonds are required to bond Adenine & Thymine

Question:Question:

•If there is 30% AdenineAdenine, how much CytosineCytosine is present?

Answer:Answer:•There would be 20% CytosineCytosine

• Adenine (30%) = Adenine (30%) = Thymine (30%)Thymine (30%)

• Guanine (20%) = Guanine (20%) = Cytosine (20%)Cytosine (20%)

• Therefore, Therefore, 60% A-T 60% A-T and 40% C-Gand 40% C-G

Significance of DNA

• DNA is often compared to a set of blueprints it contains the genetic instructions used in the development and functioning of all known living organisms

i.e. it contains the instructions to construct components of cells, such as proteins and RNA molecules.

• DNA segments that carry genetic information are called genes

• Genes code for proteins

• proteins, which include enzymes, do specialized jobs, & control the activities of each cell

Different cells have different activities. By controlling protein synthesis within each cell, the genes that make up DNA control the life of the entire organism.

Thus, sequence of DNA bases ultimately determines how the information in genes is read

Interesting Facts• Every person has two copies of each gene, one inherited from each parent. Most genes are the same in all people, but a small number of genes (less than 1 percent of the total) are slightly different between people. Alleles are forms of the same gene with small differences in their sequence of DNA bases. These small differences contribute to each person’s unique physical features.

• Biology 11 stop here – the remainder of the powerpoint is not part of our study

DNA DNA ReplicatReplicat

ionion

Replication FactsReplication Facts

• DNA has to be copied DNA has to be copied before a cell dividesbefore a cell divides

• DNA is copied during DNA is copied during the the SS or synthesis or synthesis phase of phase of interphaseinterphase

• New cells will need New cells will need identical identical DNA strandsDNA strands

Synthesis Phase (S Synthesis Phase (S phase)phase)

• S phase during 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 ReplicationDNA Replication

• Two strands open forming Two strands open forming Replication Forks (Y-shaped Replication Forks (Y-shaped region)region)

• New strands grow at the forksNew strands grow at the forks

ReplicationReplicationForkFork

Parental DNA MoleculeParental DNA Molecule

3’

5’

3’

5’

DNA ReplicationDNA Replication• As the 2 DNA strands open As the 2 DNA strands open at the origin, at the origin, Replication Replication BubblesBubbles form form

• Eukaryotic chromosomes have MANY bubbles

• Prokaryotes (bacteria) have a single bubble

Bubbles Bubbles

DNA ReplicationDNA Replication

• Enzyme Enzyme HelicaseHelicase unwinds unwinds and separates the 2 DNA and separates the 2 DNA strands by breaking the strands by breaking the weak hydrogen bondsweak hydrogen bonds

• Single-Strand Binding Single-Strand Binding ProteinsProteins attach and keep the 2 DNA strands separated and untwisted

DNA ReplicationDNA Replication• BeforeBefore new DNA strands can

form, there must be RNA RNA primersprimers present to start the addition of new nucleotides

• PrimasePrimase is the enzyme that synthesizes the RNA Primer

• DNA polymerase can then add the new nucleotides

Proofreading New Proofreading New DNADNA

• DNA polymerase initially makes DNA polymerase initially makes about about 1 in 10,0001 in 10,000 base pairing base pairing errorserrors

• EnzymesEnzymes proofread and correct proofread and correct these mistakesthese mistakes

• The new error rate for DNA The new error rate for DNA that has been proofread is that has been proofread is 1 1 in 1 billionin 1 billion base pairing base pairing errorserrors

Semiconservative Model Semiconservative Model of Replicationof Replication

• Idea presented by Idea presented by Watson & CrickWatson & Crick• TheThe two strands of the parental molecule separate, and each acts as a template for a new complementary strand

• New DNA consists of 1 PARENTAL (original) and 1 NEW strand of DNAParental DNA

DNA Template

New DNA

DNA Damage & DNA Damage & RepairRepair

• Chemicals & ultraviolet radiation damage the DNA in our body cells

• Cells must continuously repair DAMAGED DNA

• Excision repair occurs when any of over 50 repair enzymes remove damaged parts of DNA

• DNA polymerase and DNA ligase replace and bond the new nucleotides together

Question:Question:

•What would be the complementary DNA strand for the following DNA sequence?

DNA 5’-CGTATG-3’DNA 5’-CGTATG-3’

Answer:Answer:

DNA 5’-GCGTATG-3’DNA 5’-GCGTATG-3’

DNA 3’-CGCATAC-5’DNA 3’-CGCATAC-5’