DNA Structure and Function

Chapter 13

Rosalind Franklin:

A Female Scientist Ahead of her

Time

• When we talk about Watson and Crick, Rosalind Franklin provided crucial X-ray diffraction photos that allowed Watson and Crick to develop the DNA model.

DNA X-Ray Diffraction Photo by

Rosalind Franklin

Rosalind Franklin’s Work

• Was an expert in x-ray crystallography

• Used this technique to examine DNA

fibers

• Concluded that DNA was some sort of

helix

DNA Structure

Mystery of the

Hereditary Material

• Originally believed to be an unknown

class of proteins

• Thinking was

– Heritable traits are diverse

– Molecules encoding traits must be diverse

– Proteins are made of 20 amino acids and

are structurally diverse

Structure of the

Hereditary Material

• Experiments in the 1950s showed that DNA is the hereditary material

• Scientists raced to determine the structure of DNA

• 1953 - Watson and Crick proposed that DNA is a double helix

Watson and Crick

Structure of Nucleotides

in DNA

• Each nucleotide consists of

– Deoxyribose sugar (5-carbon sugar)

– Phosphate group

– A nitrogen-containing base

• Four bases

– Adenine, Guanine, Thymine, Cytosine

Watson-Crick Model

• DNA consists of two nucleotide strands

• Strands run in opposite directions

• Strands are held together by hydrogen bonds

between bases

• A binds with T and C with G

• Molecule is a double helix

Watson-Crick

Model

The DNA Molecule

Double-stranded DNA

Nucleotide

Single strand

Nucleotide Bases

sugar

(deoxyribose)

adenine

A

base w ith a

double-ring

structure

guanine

(G )

base w ith a

double-ring

structure

cytosine

(C )

base w ith a

s ingle-ring

structure

thym ine

(T )

base w ith a

s ingle-ring

structure

The DNA Code

• The information in DNA is encoded in the

sequence of bases

• A-T (double hydrogen bonded)

• G-C (triple hydrogen bonded)

A (adenine) T (thymine)

C (cytosine) G (guanine)

• The DNA sequence in a gene specifies which

amino acids go into making a particular

protein, and in what order

Base Pairing

• The bases on opposite strands pair up with

each other in very specific ways

• A large, two-ring base must always pair with a

smaller, one-ring base

• For this reason alone, A could only pair with

either T

• Hydrogen-bond compatability restricts pairing

to A with T, and C with G, always

DNA Strands are Complementary

• If one strand has an “A” at one spot, the other

strand must have a “T” at the same spot

• Likewise, “C” is matched with “G”

• This means that if we have one complete

strand, we can predict the sequence of the

other strand

Semiconservative DNA Copying

• When DNA is

copied, each old

strand functions as

a template to

construct a new,

complementary

strand

• Each resulting

DNA double-helix

is half old and half

new material

Strands Run in Opposite

Directions (5’ to 3’ direction)

Strands Run in Opposite Directions

Direction of a strand is defined by orientation of the

5’ and 3’ carbon atoms in the sugars in its backbone

Strands Run in Opposite Directions

• The two strands of DNA run past each other

in opposite directions

• DNA polymerase always assembles strand

in a particular direction, from the 5’ carbon

toward the 3’ carbon on its nucleotides

Enzymes in Replication

• Enzymes unwind the two strands

• DNA polymerase attaches

complementary nucleotides

• DNA ligase fills in gaps

• Enzymes wind two strands together

http://www.steve.gb.com/images/science/okazaki_fragments.png

Okazaki Fragments

• The leading strand is

continuously assembled

• The lagging strand must

be assembled backward

(relative to movement of

the replication fork)

• DNA Polymerases move

backwards along

lagging strand, making

many short segments,

later attached together

Genes and DNA

• DNA gives directions for making proteins

• Differences in the DNA sequence are the basis

of different alleles for genes

• Sequences may be longer or shorter between

different alleles, or may otherwise differ

• DNA can be chemically chopped into fragments,

that are then separated by size

• Different alleles may be cut into different size

fragments, which are then sorted by size

Electrophoresis Separates DNA Fragments

• DNA found at a crime

scene, as well as DNA

from seven suspects,

is cut into fragments

with special enzymes

• Electrophoresis

separates DNA

fragments by size,

producing unique

pattern

• Who did it?

Composition of DNA

• Chargaff showed:

– Amount of adenine relative to guanine

differs among species

– Amount of adenine always equals amount

of thymine and amount of guanine always

equals amount of cytosine

A=T and G=C

Patterns of Base Pairing

DNA Structure Helps

Explain How It Duplicates

• DNA is two nucleotide strands held

together by hydrogen bonds

• Hydrogen bonds between two strands

are easily broken

• Each single strand then serves as

template for new strand

DNA

Replication

newnew old old

• Each parent

strand remains

intact

• Every DNA

molecule is half

“old” and half

“new”

Base Pairing

during

Replication

Each old strand

serves as the template

for complementary

new strand

Continuous and Discontinuous

Assembly

Strands can

only be

assembled in

the 5’ to 3’

direction

DNA Repair

• Mistakes can occur during replication

• DNA polymerase can read correct

sequence from complementary strand

and, together with DNA ligase, can

repair mistakes in incorrect strand

What about Cloning?

Moral Dilemma or Technological

Revolution

• Making a genetically identical copy of

an individual

• Researchers have been creating clones

for decades

• These clones were created by embryo

splitting

• Showed that differentiated cells could

be used to create clones

• Sheep udder cell was combined with

enucleated egg cell

• Dolly is genetically identical to the

sheep that donated the udder cell

Dolly:

Cloned from an Adult Cell

More Clones

• Mice

• Cows

• Pigs

• Goats

• Guar (endangered species)