DNA and RNA

DNA

Discovery of DNA

• 1928 Frederick Griffith

studying bacteria

• Isolated 2 strains of pneumonia

bacteria from mice

•1 caused pneumonia and

produced smooth colonies

•1 didn’t and produced rough

colonies

•Both grew well in culture plates

• Experiments

• Inject w/ disease-causing

bacteria (DCB) = death

• Inject w/ nonDCB = no

death/sick

•Did the DCB produce a

poison?

•Heated the DCB to kill it;

injected into mice = no

death/sick = no poison

•Transformation

•Mixed heat-killed DCB w/

live, harmless bacteria;

injected

•Neither should have made

mice sick individually

•Mice developed pneumonia

and died

•Examined lungs & found

DCB

•Transformation

•One strain of

bacteria is changed

into another

Avery and DNA

• 1944 Avery repeated

Griffith’s work- what caused

transformation

•1st

exper. = enzymes destroyed

carbs, pros, lips, RNA =

transform. still occurred

•2nd

exper. = enzymes

destroyed DNA = no transform.

•Result

•DNA is the nucleic

acid that stores and

transmits genetic

info from one

generation of an

organism to the

next

Hershey-Chase

Experiment

• 1952 – Alfred Hershey &

Martha Chase

• Studied Bacteriophages

•Viruses that infect bacteria

•Protein coat

•DNA/RNA core

•Which part invaded the

bacteria?

Conclusion was

that the genetic

material was DNA,

not protein

Structure of DNA

• Of course, just knowing that

DNA was the source if genetic

info didn’t satisfy scientists,

they wanted to know how.

They knew 3 things:

• Genes had to carry info from one

generation to the next

• Info had to be put to work

• Genes had to be easily copied

•Nucleotides

•3 basic parts

•5-C sugar

•Phosphate group

•Nitrogenous base

−Adenine (A)

−Guanine (G)

−Cytosine (C)

−Thymine (T)

} Purines

} Pyrimidines

Chargaff’s Rules

•G = C

•T = A

•Fig. 12-6

X-Ray Evidence

•1950’s

•Rosalind Franklin used

X-ray diffraction

•Gave clues to the

structure

Double Helix

• 1953 - Francis Crick, James

Watson

• Looked at X-ray and figured

out the double helix

•Two strands wound around

each other

•sugar/phosphate backbone

•Base pairs form hydrogen

bonds

Sugar-phosphate

backbone

Nucleotide

Hydrogen

bonds

Key

Adenine (A)

Thymine (T)

Cytosine (C)

Guanine (G)

DNA and RNA

Chromosomes and

DNA Replication

DNA and C’somes

•Prokaryotes

•DNA located in cytoplasm

•Single, circular DNA

molecule

•Eukaryotes

•1000x more DNA than prok

•Located in nucleus as

c’somes

DNA length

• E. coli – a prokaryote

(bacteria)

•4,639,221 base pairs = 1.6

mm

•Diameter of bacteria = 1.6

um (1 um = 1/1000 of a

mm)

C’some Structure

• Human cell contains

1000x as many base pairs

as a bacteria

• Nucleus contains more

than 1 m of DNA

• How does this work?

Chromosome

Supercoils

Coils

Nucleosome

DNA

double

helixHistones

DNA Replication

• DNA molecule separates

into 2 strands

• 2 new complementary

strands are produced

according to base pairing

rules

• Each strand serves as a

template

• Prokaryotes

•Single point and proceeds in

2 directions

• Eukaryotes

•Hundreds of points and

proceeds in both directions

•Replication fork

Replication

fork

New strand Original

strandDNA

polymerase

DNA

polymeraseGrowth

Growth

Replication

fork

Nitrogenous

bases