Chapter 6
DNA Structure and Function
Part 2
Deciphering The Structure of DNA
The DNA molecule is a polymer (or chain) of subunits (or
links) called nucleotides.
These nucleotides are composed of only three components:
A five carbon sugar (deoxyribose)
A phosphate group
A nitrogen base (can be adenine (A), guanine (G), cytosine (C), or
thymine (T)) (A and G are called purines. C and T are called
pyrimidines.)
Though there are only these 3 components that make up the
DNA, it is a gigantic molecule.
Chromosomal DNA has a complex structural organization.
Therefore, the puzzle of how the DNA molecule is arranged
took more than 50 years to solve.
A Nucleotide
Deoxyribose
Cytosine
Or
Thymine
Deoxyribose
Adenine
Or
Guanine
Deciphering the Structure of DNA
In 1950, Erwin Chargaff made two discoveries.
1: He determined that the amounts of the nitrogen base
thymine and the nitrogen base adenine are always the same
in all DNA.
He found that the same is true for the amounts of guanine
and cytosine.
Therefore, he deduced that, in DNA, A always binds with and
G always binds with C.
A=T and G=C
We call this Chargaff’s rule.
Deciphering the Structure of DNA
2: Chargaff discovered that the proportion of adenine and
guanine differs among DNA of different species.
This told Chargaff that the composition of DNA varies from
one species to another.
Deciphering the Structure of DNA:
The Race For the Double Helix
Rosalind Franklin, a female British biophysicist , which was rare at
the time, took the first clear x-ray diffraction image of DNA as it
occurs inside of cells.
X-ray crystallography is a technique in which x-rays are directed
though a purified and crystallized substance.
Atoms in the substance’s molecules scatter the x-rays in a pattern
that can be captured as an image.
Researchers can then use this image to calculate the size, shape,
and spacing between any repeating element in the molecules-all of
which are details of molecular structure.
Unbeknownst to her, another British biophysicist, Maurice Wilkins,
worked on the same project just down the hall from Franklin.
Deciphering the Structure of DNA:
The Race For the Double Helix
Using her image of DNA, Franklin was able to decipher the
structure of DNA.
In 1952, Franklin gave a presentation of her work on the
structure of DNA.
In her presentation, she said that DNA has two chains
twisted into a double helix (spiral), with a backbone of
phosphate groups on the outside, and nitrogen bases
arranged in an unknown way on the inside.
She had even calculated the diameter of the DNA molecule,
the distance between its two chains and its bases, the angle
of the helix, and the number of bases in each coil of the
helix.
Deciphering the Structure of DNA:
The Race For the Double Helix
Rosalind Franklin and her famous x-ray diffraction image of DNA
Deciphering the Structure of DNA:
The Race For the Double Helix
Deciphering the Structure of DNA:
The Race For the Double Helix
While Franklin began to write a paper to publish her work so that
she could be recognized for her findings, Wilkins began to review
her image of DNA with American biologist, James Watson.
Watson and a British biophysicist names Francis Crick had been
working on the molecular structure of DNA themselves.
They had already hypothesized that the secondary structure of the
DNA molecule might be a helix.
They had also attempted to build a DNA model with scraps of metal
and wire “bonds”.
They struggled with and argued about the size, shape, and bonding
requirements of the nucleotides that make up DNA.
They had even asked chemists to help them identify possible bonds
that might be present in DNA that they might have overlooked.
Deciphering the Structure of DNA:
The Race For the Double Helix
After Wilkins and Watson reviewed Franklin’s image, Watson
and Crick read a report containing her unpublished data.
Franklin’s DNA image and data provided Watson and Crick
with the information they needed to complete their DNA
model in 1953, revealing the true structure of the DNA
molecule.
Deciphering the Structure of DNA:
The Race For the Double Helix
Crick
Watson
Deciphering the Structure of DNA:
The Race For the Double Helix
Watson and Crick were then the first to publish a theoretical
model of the structure of DNA.
Wilkins research paper on his work with DNA structure was
the second to be published.
Rosalind Franklin’s work was the third paper to be published
in a series of articles about the structure of DNA in Nature.
Rosalind Franklin died at the age of 37 due to ovarian cancer,
probably caused by extensive exposure to x-rays.
Since the Nobel Prize is not given posthumously, Franklin did
not get to share in this honor bestowed upon Watson, Crick,
and Wilkins in 1962 for their work on the discovery of the
structure of DNA.
Deciphering the Structure of DNA:
The Race For the Double Helix
A 1962 photo shows Nobel Prize winners (from left): Maurice Wilkins (medicine),
Max F. Perutz (chemistry), Francis Crick (medicine), John Steinbeck (literature),
James Watson (medicine) and John C. Kendrew (chemistry).
Not pictured are winners Linus Pauling (peace) and Lev Landau (physics).
The Double Helix
Watson and Crick said that DNA is composed of two strands (or
chains) of nucleotides.
They said that these two strands run in opposite directions. They
are said to be antiparallel.
These two strands are coiled into a double helix (or spiral).
Bonds between the sugar of one nucleotide and the phosphate
group of the next nucleotide (called phosphodiester bonds) form
the “backbone” of the DNA molecule.
Hydrogen bonds between the nitrogen bases on the inside of the
DNA molecule hold the two DNA strands together.
Only two kinds of base pairings occur in DNA: A binds to T and G
binds to C, just as Chargaff had said in his first rule.
Due to this complementary base pairing, the two DNA strands are
said to be complementary to one another.
The Double Helix
Patterns of Base Pairing
Just the two kinds of base pairing (A=T and G=C) give rise
to the enormous diversity that we see in all living things on
Earth. How is this possible?
The diversity that we see among living things is due to the
fact that, even though there are only four possible bases in
DNA, the order in which these bases occur varies greatly
from one species to the next.
The DNA sequence, or the order in which one base pair
follows the next in DNA, explains Chargaff’s second rule.
The information encoded by this DNA sequence is
responsible for all traits, seen and unseen, that define
species and distinguish individuals from one another.
DNA Sequence = Genetic Information= Diversity
DNA Replication
Before a cell divides, it must replicate (or make an
identical copy) its DNA so that each new cell that is
produced will receive DNA.
During DNA replication,
1st- an enzyme called DNA helicase breaks the
hydrogen bonds holding the two DNA strands
together, effectively “unzipping” and separating the
two DNA strands.
DNA Replication
2nd- Another enzyme, called DNA polymerase, assembles a
complementary strand of DNA (called a daughter strand) on
each original (parent) strand of DNA.
The DNA polymerase uses each parent as a template (or
blueprint) upon which to construct the new, daughter
strands.
The base sequence of the daughter strand is complementary
to that of the parent strand because DNA polymerase must
follow base-pairing rules.
In this way, two identical copies of DN are produced, ensuring
that each new cell will get identical DNA.
DNA Replication
DNA Replication
3rd- The enzyme, DNA ligase, seals any gaps that remain
in either of the DNA strands in each new DNA molecule,
producing two new continuous DNA strands.
Then the DNA molecules recoil into a double helix.
Because each new DNA molecule consists of one newly
synthesized strand (new) and one original (parent)
strand, DNA replication is said to be semi-conservative,
meaning that part of the original DNA molecule is
conserved in the new DNA molecules.
DNA Replication is
Semi-Conservative
Original
Checking For Mistakes
DNA replication is not always perfectly accurate. Mistakes
can and are made. These mistakes are called mutations if
they are not corrected and become permanent.
Sometimes the wrong nucleotide/base is added to the
growing, newly synthesized strand. Sometimes
nucleotides/bases are lost or extra ones are added.
Errors may also be caused by exposure to radiation or toxic
chemicals because it is difficult for DNA polymerase to add
nucleotides to a damaged parental DNA strand.
However, most of the time, DNA repair mechanisms fix the
mistakes that are made by replacing damaged or
mismatched nucleotides.
Checking for Mistakes
Most errors during replication happen because DNA
polymerase adds nucleotides to the newly
synthesized strand so quickly-up to 1,000 bases per
second.
Luckily, however, DNA polymerase proofreads its
work and correct mismatches immediately.
In the case that DNA polymerase fails to correct a
mistake that it has made, other mechanisms usually
stop the cell from dividing.
Checking for Mistakes
If proofreading and repair mechanisms fail, and the cell is
allowed to divide with the mistake still present, then the
error in the DNA becomes permanent and is called a
mutation- or a permanent change in the DNA.
These mutations can alter the information encoded by the
DNA, possibly resulting in harm to the organism.
Mutations are the original source of variation of traits (for
example, skin color). Therefore, they are the raw material
of evolution.
Mistakes During DNA
Replication