DNA History, Structure, and Replication

QUESTION:

Where is genetic information stored?*

a) in the ribosomes of cells

b) within the proteins of cells

c) within the DNA of cells

d) within the membrane of cells

*But scientists did not always know this!

DNA is a very small molecule and had to

be discovered. Let’s look at the history,

structure, and replication of DNA in today’s

discussion.

The answer is here

somewhere!

Discovering DNA -

Once Mendel understood that “factors” could be

passed to offspring, scientists began to wonder

what these factors were. They were sure of a few

things…

1.The factors needed to be able to store information

2. The factors needed to be replicable

3. And the factors needed to sometimes undergo

changes (mutations)

1869 – Discovering Nucleic Acids

Swiss Physician, Johannes Friedrich

Miescher isolated a new biomolecule

he called “nuclein” from the nuclei of

white blood cells. This later became

called nucleic acids, but to be honest

– we had no idea what it did. We just

knew it was there.

Miescher used pus and bloodstained

bandages from a hospital to study

“nuclein”

Chemistry of NUCLEOTIDES (Nucleic Acid Monomers)

Analysis of the nucleic acid showed that it

contained a sugar, phosphate and one of four

nitrogen bases

Adenine

Guanine

Cytosine

Thymine

- Now we know what’s in it, but

what does it look like? What does it

do?

**THE BIG QUESTIONS**

Were these new nucleic acids the secret to

Mendel’s factors? Or were the ever-

present proteins the secret? Where is the

cell’s genetic code?

a) Proteins contain 20 amino acids that can be

organized in countless ways to determine traits

b) Nucleic acids only contained 4 different nucleotides

Thanks to their 20 amino

acids, scientists were

leaning toward proteins.

As a information storage

option, amino acids would

have a lot of options, since

you could combine these in

different ways

The 4 bases of the NAs

didn’t seem to have as much

versatility.

It’s like have an alphabet of

20 letters versus and

alphabet of 4.

The Experiment that finally gave us an answer…

Frederick Griffith was attempting

to find a vaccine against

pneumococcus bacteria that

caused pneumonia. In this

experiment, he accidentally

determined the true source of

the genetic code by discovering

that one type of bacteria could

actually turn into another. Let’s

take a look at his work…

An overview of the transformation experiment.

Can you summarize this in your own words?

DNA was determined to be the genetic code.

● DNA from the dead S strain bacteria was taken in by the

live R strain, causing them to transform into S strains.

● Denaturing the proteins or using enzymes that stopped S

proteins did not stop the transformation

● Using enzymes that denatured the nucleic acid did stop

the transformation

● We knew the DNA contained the instructions, but we still

didn’t understand how… All that information transferred

using only four “letters”?! How is that possible?!

Quick Recap:

1) What caused scientists to believe that proteins contained the

genetic code?

2) What was Miescher’s contribution to genetic studies?

3) How were the nonvirulent R strain bacteria transformed into a

virulent strain?

4) Griffith’s experiment resulted in which conclusion?

Alfred Hershey and Martha Chase Experiments

The bacteriophage – a virus used for studying DNA

Reminder: Viruses infect by injecting their DNA into a cell and taking

control of the host. Two types– Lytic viruses immediately use the host to

replicate their own DNA, make more viruses, and then lyse the cell to

release offspring. Lysogenic viruses actually insert their DNA into the

genome of the host. They go lytic eventually, but for a while, they “hide”

in the host DNA and become a part of the host organism. The

bacteriophage is a lysogenic virus for bacteria. How could we use them?

Bacteriophages – viruses that infect bacteria

●Using the bacteriophage to prove DNA as

the genetic code

● Bacteriophages have a protein capsid surrounding a piece

of DNA

● Experiments used radioactive sulfur to tag proteins and

radioactive phosphorous to tag the DNA.

●The goal was to see which substance (protein or DNA)

moved into the infected cell

Conclusion: The radioactive tag on the DNA went into the bacteria, not

the tagged proteins

But we still had NO clue what it looked like!

Imagine you had all of the pieces to a puzzle but you didn’t

know how they fit together. Scientists had the pieces of DNA.

Fame and fortune would go to the one who solved this puzzle...

The Race to Establish the Structure of DNA

THE PLAYERS

THE PIECES

adenine

guanine

cytosine

thymine

deoxyribose

phosphate

Examine the data below. Do you notice a pattern?

So did Erwin Chargaff...

Chargaff’s Rule

Amount of A, T, G, C varies by

species, but #A = #T and #G = #C

(#Purines = #Pyrimidines)

all species

had similar

ratios of

A, T, G, C

Purines (A&G) have two rings on the nitrogen base

Pyrimidines (C&T) have one ring on the nitrogen base

Could it be that

these pieces of

the puzzle fit

together…..

But what about

deoxyribose and

phosphates,

where do those

pieces fit? And

what is the

shape of DNA?

ROSALIND FRANKLIN & WILKENS Took pictures of DNA structure with X-RAY DIFFRACTION

These

images

provided

clues to

the shape

of the

DNA

molecule.

Competition in science was a major theme during this period of time.

(1940-1953 ish)

Scientists often wanted to get sole credit for a discovery, and were

reluctant to share results with others. None of the work by Chargaff,

Franklin, and Wilkins were shared, they existed as isolated facts.

For consideration…..

1) Do you think scientists today are more likely to collaborate?

2) How has the internet changed science?

Enter……...WATSON & CRICK

They were criticized for their

methods which included

- hanging out in pubs and

talking about stuff

- playing cricket

- stealing data from other

scientists (Franklin/Wilkins)

- But they used Chargaff’s

rules and F/W’s picture to

mathematically determine

the double helix shape of

DNA. They won the Nobel

Prize for “solving the

puzzle” and mostly- they get

the credit. Fair?

DNA:

DOUBLE HELIX (Twisted Ladder)

Steps of ladder are bases (A, T,

G, C)

Sides of ladder are sugar &

phosphate

Sugar and phosphate are

covalently bonded along the

sides (strong!) while the steps

are hydrogen bonded (weak.)

1'

2' 3'

4'

5'

1'

2' 3'

4'

5' Each side is antiparallel (runs in the

opposite direction). The numbers used to

represent each side refer to the carbons

attached to the deoxyribose and

covalently bonded to the phosphate. If

it’s connected at the 3rd Carbon, it’s the

3’ end. Vice versa for 5’.

5' and 3' ends

5’ and 3’ ENDS

Each Side is ANTIPARALLEL

Nucleotide =

o1 base

oDeoxyribose (sugar)

o1 phosphate

DNA is a polymer made

from nucleotide

monomers

What’s wrong with

this drawing?

DNA REPLICATION -the process by which DNA

makes a copy of itself during S

phase of interphase prior to

ANY cell division

-replication is semi-

conservative, because one half

of the original strand is always

saved, or "conserved” in the

new strands

Looking at the pic, explain

replication in your words.

DNA Replication Steps 1. Protein enzyme(s) DNA helicase unzips the hydrogen bonds and

creates replication forks at several places along the molecule. DNA

binding proteins hold the separated strands apart to prevent

reannealing (reattaching).

2. Protein enzyme Primase creates a small section of nucleotides to

which protein enzymes DNA polymerases can fully attach and run down

the strand adding free nucleotides (following base pair rules) to the

exposed strand, covalently binding the sugars and phosphates on the

new side, and proofreading/correcting mistakes along the way.

***DNA polymerase can only travel down the strand from the 3' to the 5' end. Careful:

it only “reads” the OLD strand from 3-5, meaning it “builds” the NEW strand from 5-3.

What issue does this create?***

5. Since one side of the DNA runs in the 3’ to 5’ direction, it

is copied continuously and called the leading strand. The

other side runs in the 5' to 3' direction and is called the

lagging strand. Since the DNA polymerase can only READ

from 3’ to 5’ and BUILD from 5’ to 3’, this lagging strand

must be done in chunks called OKAZAKI FRAGMENTS.

Primase places a primer periodically, and DNA Polymerase

“jumps” from primer to primer working in reverse. These

fragments are eventually connected by another protein

enzyme called DNA LIGASE

6. Using multiple replication forks (sometimes called

replication bubbles) all down the strand, these enzymes

are able to copy all of the DNA relatively quickly.

DNA Replication

Which side is

leading? Which

side is lagging?

Which side will

be made

continuously?

Which side will

be made in

Okasaki

fragments?

Pg 235

Figure 13Ac