chapter 13: dna. i. mystery of dna structure a.griffith (1920s): injected mice with deadly bacteria...

Chapter 13: DNA

I. Mystery of DNA Structure

A. Griffith (1920s): Injected mice with deadly bacteria (rough colony) and they die. Inject mice with smooth bacteria and they live. Inject mice with heat-killed rough and they live. Mix heat-killed rough with healthy smooth, mice

die. Conclusion: Transforming factor smaller than

cell.

I. Mystery of DNA Structure

B. Hershey/Chase (1950s)

Radiolabeled proteins and DNA.

Found viruses injected radiolabeled DNA,

but no proteins.

Conclusion: DNA is transforming factor.

Figure 16.2a The Hershey-Chase experiment: phages

Fig. 13.5, p. 217

virus particle labeled with 35S

virus particle labeled with 32P

bacterial cell (cutaway view)

label outside cell

label inside cell

I. Mystery of DNA Structure

C. Chargaff (1950s): Worked with DNA and found that the amount of adenine (A) equaled the amount of thymine (T) while the amount of cytosine (C ) equaled the amount of guanine (G). Conclusion: Chargaff’s base-pairing

rule.

I. Mystery of DNA Structure

D. Franklin/Wilkins (1953): X-ray

crystalography

They froze DNA and ran x-rays through

resulting crystal.

Conclusion: DNA tightly-wound helix

(at least they provided the evidence

for this conclusion).

Figure 16.4 Rosalind Franklin and her X-ray diffraction photo of DNA

I. Mystery of DNA Structure

E. Watson & Crick (1953):

Assemble all evidence and correctly

interpret data to create double helix.

Conclusion: Current model of DNA

(and Noble prize and fame and

glory and chicks)

II. Model of DNA

A. Nucleotide: Basic building block of DNA.

1 phosphate group, 1 deoxyribose

sugar, 1 nitrogenous base.

II. Model of DNA

A. Nucleotide:

1. Sugar (Deoxyribose): Each carbon is

numbered sequentially.

Fig. 13.7, p. 219

2-nanometer diameter, overall

distance between each pairof bases = 0.34 nanometer

each full twist of the DNA double helix = 3.4 nanometers

A. Nucleotide

2. Phosphate: Negative yet next to each other along DNA backbone, resulting in twisting.

II. Model of DNA

A. Nucleotide:

3. Nitrogenous Bases:

Adenine and Guanine are Purines

(double ring structures)

Cytosine and Thymine are

Pyrimadines (sing ring structures)

Fig. 13.6, p. 218

phosphate group

sugar (ribose)

ADENINE (A)

base with a double-ring

structure

THYMINE (T)

base with a single-ring structure

CYTOSINE (C)

base with a single-ring structure

GUANINE (G)

base with a double-ring

structure

II. Model of DNA

B. Bonds within DNA Molecule

1. Phosphate to Sugar: Covalent (note which carbon in sugar is attached to which phosphate).

II. Model of DNA

B. Bonds within DNA Molecule

2. Sugar to Nitrogenous Base: Covalent.

3. N-base to N-base:

Hydrogen Bonds.

II. Model of DNA

C. Complete molecule is

‘antiparallel’ (one side

runs 3’ to 5’, other side

runs upside down,

or 5’ to 3’)

wx C

in-text, p. 219

For each, 1. How many base pairs?

2. How many nucleotides?

3. How many sugars?

4. Circle a hydrogen bond.

III. DNA Replication

A. Replication: Occurs in the nucleus; process of DNA creating an exact replica of original strand.

III. DNA Replication

B. Semi-Conservative Nature of Replication

The two daughter strands are half new and half original (order and molecules of parent strand are conserved ).

Each half of original strand becomes a template for each new DNA molecule. The new strand is half old, half new.

III. DNA Replication

C. Steps of Replication: 1. DNA uncoils and unzips (done by enzyme DNA helicase). 2. New nucleotides are added to exposed strand and added by DNA polymerase enzyme. 3. DNA ligase fills in gaps in new DNA strands.

III. DNA Replication

D. Okazawi Segements: DNA is read from 3’ to 5’.

1. Leading strand reads 3’ to 5’ as DNA is

unzipped.

2. Lag strand would have to wait for entire

strand of DNA to unzip to begin; rather

it builds short segments, known as

Okazawi segments (later ‘stitched’

together with DNA ligase).

Note 3’ to 5’ direction:

Figure 16.13 Synthesis of leading and lagging strands during DNA replication

Label Middle image 3’ to 5’

Okazaki Segment Illustrated

III. DNA Replication

E. Replication Fork/Bubble:

As DNA unzips, it replication begins

immediately. Replication simultanelously

occurs at many sites along a strand of

DNA.

Figure 16.10 Origins of replication in eukaryotes