DNA Discovery and Structure Interest Grabber Order! Genes are made of DNA, a large, complex molecule. DNA is composed of individual units called nucleotides

DNA Discovery and Structure

Interest Grabber• Order! Order!• Genes are made of DNA, a large, complex molecule. DNA is

composed of individual units called nucleotides. Three of these units form a code. The order, or sequence, of a code and the type of code determine the meaning of the message.

1. On a sheet of paper, write the word cats. List the letters or units that make up the word cats.

2. Try rearranging the units to form other words. Remember that each new word can have only three units. Write each word on your paper, and then add a definition for each word.

3. Did any of the codes you formed have the same meaning?4. How do you think changing the order of the nucleotides in

the DNA codon changes the codon’s message?Go to Section:


Section Outline

• 12–1 DNAA. Griffith and Transformation

1. Griffith’s Experiments

2. Transformation

B. Avery and DNA

C. The Hershey-Chase Experiment

1. Bacteriophages

2. Radioactive Markers

D. The Structure of DNA

1. Chargaff’s Rules

2. X-Ray Evidence

3. The Double Helix

Section 12-1

Go to Section:


Discovery of Genetic Material

• Mendel’s work actually created more questions in the minds of scientists.

• What are genes made of?• Chromosomes consist of two materials DNA

wrapped around a protein.• Which of the two materials carried the genetic

code?• Transformation• A process where one strain of bacteria is

changed to another


KEY CONCEPT DNA was identified as the genetic material through a series of experiments.


DNA Discovery and StructureDNA

I.Deoxyribonucleic Acid

A. Chromatin/chromosomes

B. In nucleus

C. Functions:

i. Stores , copies & transmits genetic information (genes); genes code for proteins determining cell functions & traits

13.1 Ecologists Study Relationships

•Is DNA the genetic material?•3 experiments = evidence

1.1928 Frederick Griffith – discovers that a heritable factor in bacteria can transform harmless bacteria into deadly bacteria in mice

II. A Genetic MysteryII. A Genetic Mystery


1. Griffith’s Transformation

•An army medical officer•attempting to develop a vaccine against pneumonia, but unexpectedly advances molecular biology instead•isolated and cultured two different strains of the pneumonia bacteria

R = rough surface, not deadly

S = smooth surface (protein coat), deadly

Turn to p 339 in your book.


TWO types of Streptococcus pneumonia

a. Smooth edged colony (S type) - have a polysaccharide (sugar) capsule around the bacteria's cell membrane:capsule which protects the bacteria from the cell's defenses and causes PNEUMONIA

b. Rough edged colony (R type) - lack the sugar capsule and does not cause pneumonia


Griffith’s Transformation

CLUE: Live S cells found in mouse body injected with heat-killed S cells!!!!


What Happened?

Maybe the heat-killed “S” cells in the mixture weren’t really killed. If this was true, what would have happened to the mice in Experiment 3?

Maybe the harmless “R” cells in the mixture can mutate into a killer form. If this was true, what would have happened to the mice in Experiment #1?


DISCOVERY:

1) The heat killed = harmless R bacteria was changed (transformed) into the deadly S form of bacteria

2) Genetic material transferred between dead

bacteria and living bacteria.

WHY did transformation take place? What was transferred to the R bacteria?

We know now that some bacteria can take up DNA released from other cells and use it.


The Importance of Griffith’s Work

Griffith called this “transformation.”

He had shown that something had been transferred from the heat-killed disease-causing bacteria to the live harmless bacteria that could be inherited.

.

Investigators began searching for this unknown material.


2.1944 Oswald Avery – shows that DNA is the factor in Griffith’s experiment by selectively destroying different types molecules & checking whether transformation still occurred



Oswald Avery's Experiment (1944)

Project: Repeat Griffith's experiment with a TWIST

R bacteria + "Heat killed" S bacteria mixture: Used an

ENZYME that destroyed RNA and sugars

(polysaccharides)

RESULT: Transformation still took place and the mice died.

R bacteria + "Heat killed" S bacteria mixture: Used an

ENZYME that destroyed DNA.

RESULT: Transformation did not take place, and mice

lived!!!


Avery’s Experiments

CLUE: Only culture not transformed is treated with DNAse (enzyme cuts up DNA).


DISCOVERY:

1) DNA-destroying enzymes prevented harmless bacteria from transforming into deadly bacteria

2) DNA was the genetic material NOT protein.


3.1952 Alfred Hershey & Martha Chase - use radioactive markers to trace where DNA and proteins are located during a bacteriophage infection; supports Avery.

DNA is the genetic material!



Alfred Hershey and Martha Chase Experiment (1952)

Background of Experiment:

Studied the effect of a bacteriophage on bacteria

BACTERIOPHAGE = a virus that infects bacteria only

Bacteriophage attaches to bacteria and injects its DNA

into the bacteria - protein capsule remains outside the

Bacteria.

DNA becomes part of the bacteria's DNA - instructs the

bacteria to make hundreds of new phages.

Bacteria becomes full of new virus and finally bursts -

releasing new bacteriophages


Hershey and Chase confirm that DNA is the genetic material.

• Hershey and Chase studied viruses that infect bacteria, or bacteriophages.

• Tagged DNA was found inside the bacteria; tagged proteins were not.

– They tagged viral DNA with radioactive phosphorus.– They tagged viral proteins with radioactive sulfur.


Hershey & Chase


Hershey & Chase

Clue: Radioactivity inside cell only when attached to DNA


Mystery Solved: Mystery Solved:

All evidence points to DNA as All evidence points to DNA as the genetic material. the genetic material.

No evidence points to protein.No evidence points to protein.



Checkpoint

1. What is the function of DNA?

2. What is the molecular function of genes?

3. Describe the three experiments that demonstrated that DNA must be the genetic material?

24ANY QUESTIONS?


III. A Race to DNA Structure

Once known that DNA is the molecule of heredity – new question:

HOW DOES IT WORK?

Like all biological molecules and their chemistry, the key to function lies in their structure!

Probably the most important biological discovery of the 20th century.


• Competition, personality clashes and secret collaborations describe the race to be the first to find the actual molecular structure of DNA.

• What did we know in 1920?• – DNA is a nucleic acid.

III. A Race to DNA Structure III. A Race to DNA Structure


KEY CONCEPT DNA structure is the same in all organisms.


A. DNA is a Nucleic Acid

1. nucleic acid - chains of nucleotides linked by covalent bonds.

2. DNA Nucleotide

a) phosphate,b) deoxyribose

(sugar) c) Nitrogen base


2. DNA Nucleotide 2. DNA Nucleotide

O

O=P-O

O

PhosphateGroup

NNitrogenous baseNitrogenous base

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

CH2

O

C1C4

C3 C2

5

Deoxyribose Deoxyribose (sugar)(sugar)

Draw & label in your notes


• The nitrogen containing bases are the only difference in the four nucleotides.


Purines PyrimidinesAdenine Guanine Cytosine Thymine

Phosphate groupDeoxyribose

Figure 12–5 DNA NucleotidesSection 12-1

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TAC

G

Nucleotides always pair in the same way.

• The base-pairing rules show how nucleotides always pair up in DNA.

• Because a pyrimidine (single ring) pairs with a purine (double ring), the helix has a uniform width.

– A pairs with T– C pairs with G


Percentage of Bases in Four Organisms

Section 12-1

Source of DNA A T G CSource of DNA A T G C

Streptococcus 29.8 31.6 20.5 18.0

Yeast 31.3 32.9 18.7 17.1

Herring 27.8 27.5 22.2 22.6

Human 30.9 29.4 19.9 19.8

Streptococcus 29.8 31.6 20.5 18.0

Yeast 31.3 32.9 18.7 17.1

Herring 27.8 27.5 22.2 22.6

Human 30.9 29.4 19.9 19.8

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B. Chargaff’s Rules – 1949 Erwin Chargaff - amount of Adenine always equal to amount of Thymine, Cytosine always equal to Guanine and amount of Purines = pyrimidines in a DNA Purines = pyrimidines in a DNA sample sample


Due to Due to Base Pair Rules: Base Pair Rules: AA bonds with bonds with TT

& & GG bonds with bonds with CC


Checkpoint Quiz

1. What kind of macromolecule is DNA?

2. What are the three parts of a nucleotide?

3. How do you tell purines and pyrimidines apart?

4. Which bases are paired together?

5. According to Chargaff’s rules, if 30% of DNA sample is Adenine, how much is Thymine?

6. How much is Cytosine?

35 3535

ANY QUESTIONS?


1950 - How do these 4 building blocks fit together to form the molecule responsible for all of the similarity and diversity of LIFE??

C. 1952 Linus Pauling – Nobel American chemist uses modeling (toys!) to propose first structure.

– he suggests a

triple helix DNA



D. 1952 Rosalind Franklin & Maurice Wilkins capture X-ray pattern of DNA – key clue to structure; not a triple helix

Hated each other but worked in same lab




• Watson and Crick’s discovery built on the work of Rosalind Franklin and Erwin Chargaff.

– Franklin’s x-ray images suggested that DNA was a double helix of even width.

– Chargaff’s rules stated that A=T and C=G.


At the college down the road, two students, using Pauling’s models, see Franklin’s photo from Wilkins and publish their own idea – WITHOUT crediting Franklin!



E. 1953 Watson & Crick reveal 3-D model of DNA molecule: DOUBLE HELIX with 2 complementary antiparallel strands joined by hydrogen bonds between base pairs


- used the information from the above scientists and established the structure of DNA (with the unknowing help of Rosalind Franklin)


Watson and Crick determined the three-dimensional structure of DNA by building models.

• They realized that DNA is a double helix that is made up of a sugar-phosphate backbone on the outside with bases on the inside.


Hydrogen bonds

Nucleotide

Sugar-phosphate backbone

Key

Adenine (A)

Thymine (T)

Cytosine (C)

Guanine (G)

Figure 12–7 Structure of DNASection 12-1

Go to Section:


III. A Race to DNA Structure III. A Race to DNA Structure 1.Double Helix – 1.Double Helix – 2 strands in a twisted ladder 2 strands in a twisted ladder

with covalent bonded sugar-phosphate rails with covalent bonded sugar-phosphate rails and H-bonded base pair rungsand H-bonded base pair rungs

2.Complementary – 2.Complementary – strands strands match according to base match according to base pairing rules (A-T, C-G)pairing rules (A-T, C-G)

• If know one strand, then can If know one strand, then can code for the other.code for the other.

• Important for copying!Important for copying!


DNA Model3.Antiparallel = 2 strands of DNA run in opposite directions (5’ carbon v 3’carbon at end)

Draw & label in your notes

Sugar-

phosphate rail

3’

5’


E.1958- Franklin dies of cancer at 37.

F. 1962 Watson & Crick (& Wilkins)

Win Nobel Prize for double helix DNA



Checkpoint

1. Who is credited with the double helix model?

2. What is a double helix?

3. Which nucleotide parts make up the rails of a DNA molecule?

4. Which nucleotide parts make up the rungs?

5. If a DNA strand reads ATCGTAG, what does the complementary strand read?

47ANY QUESTIONS?

Documents

DNA Discovery and Structure Interest Grabber Order! Genes are made of DNA, a large, complex molecule. DNA is composed of individual units called nucleotides