2. Learning Outcomes
• Summarized the evidence that led to the acceptance that genes are
made of DNA
At the end of the lesson, students must have;
• Described how Watson and Crick built their model of a DNA molecule
• Discussed Chargaff’s rule
• Identified the parts of a chromosome and its function.
3. What is D N A ?
DNA stands for deoxyribose nucleic acid.
It’s a group of molecules that is responsible for carrying
and transmitting the hereditary materials or genetic
instructions from parents to offsprings.
4. It stores genetic information necessary for
organisms to develop, survive, and reproduce.
Damage to DNA may cause cells to stop
dividing, cell death, or become malignant.
What is D N A ?
5. DNA is a very large molecule made up of a long chain of sub-
units;
The sub-units are called nucleotides
Each nucleotide is made up of:
a sugar called deoxyribose
a phosphate group PO4 and
an organic base
D N A M o l e c u l e
6. Ribose & Deoxyribose
Ribose is a sugar, like glucose, but with only five
carbon atoms in its molecule
Deoxyribose is almost the same but lacks one
oxygen atom
Both molecules may be represented by the symbol
9. A molecule of DNA
is formed by millions
of nucleotides joined
together in a long
chain
PO4
PO4
PO4
PO4
sugar-phosphate
backbone
+ bases
Joined nucleotides
11. The bases always pair up in the same way
Adenine forms a bond with Thymine
and Cytosine bonds with Guanine
Bonding 1
Adenine Thymine
Cytosine Guanine
15. THE DISCOVERY
James Watson and
Francis Crick
In 1
953, they concluded that the
DNA molecule appears as a
three-dimensional double helix.
Rosalind Franklin
and Maurice Wilkins
They used X-ray crystallography
to study DNA's structure, which
helped Watson and Crick with
their discovery.
16. In the early 1950s, American biologist James Watson
and British physicist Francis Crick came up with their
famous model of the DNA double helix. They were the
first to cross the finish line in this scientific "race," with
others such as Linus Pauling (who discovered protein
secondary structure) also trying to find the correct
model.
Rather than carrying out new experiments in
the lab, Watson and Crick mostly collected and analyzed
existing pieces of data, putting them together in new
and insightful ways. Some of their most crucial clues to
DNA's structure came from Rosalind Franklin, a chemist
working in the lab of physicist Maurice Wilkins.
THE DISCOVERY
17. Franklin was an expert in a powerful technique for
determining the structure of molecules, known as X-ray
crystallography. When the crystallized form of a
molecule such as DNA is exposed to X-rays, some of
the rays are deflected by the atoms in the crystal,
forming a diffraction pattern that gives clues about the
molecule's structure.
THE DISCOVERY
18. Franklin’s crystallography gave Watson and Crick
important clues to the structure of DNA. Some of these
came from the famous “image 51,” a remarkably clear and
striking X-ray diffraction image of DNA produced by
Franklin and her graduate student. To Watson, the X-
shaped diffraction pattern of Franklin's image immediately
suggested a helical, two-stranded structure for DNA.
Watson and Crick brought together data from a
number of researchers (including Franklin, Wilkins,
Chargaff, and others) to assemble their celebrated model
of the 3D structure of DNA. In 1962, James Watson,
Francis Crick, and Maurice Wilkins were awarded the
Nobel Prize in Medicine. Unfortunately, by then Franklin
had died, and Nobel prizes are not awarded
posthumously.
THE DISCOVERY
19. S U M M A R Y
The Discovery
The double-helical
model came from
the combined work
of Watson, Crick,
Franklin, and Wilkins.
DNA Characteristics
DNA is made of nucleotide monomers.
The four bases are adenosine, thymine,
cytosine, and guanine.
DNA Function
DNA stores an organism's genetic information.
21. Who is Erwin Chargaff?
- He was an Austro-Hungarian biochemist, who colonized
in the U.S. during the Nazi era. Moreover, he was a
professor of biochemistry at Columbia University
Medical School.
- (11 August 1905 – 20 June 2002)
- During his career he discovered two major rules by
analyzing the base composition of the DNA of
various species. This led him to propose two main rules
that have been appropriately named Chargaff's rules.
Erwin Chargaff
22. What is Chargaff rule?
Rule 1
Chargaff determined that in DNA, the amount of one base, a purine, always
approximately equals the amount of a particular second base, a pyrimidine.
Specifically, that in any double-stranded DNA the number of guanine units equals
approximately the number of cytosine units and the number of adenine units
equals approximately the number of thymine units.
In the Chargaff’s rules of base pairing are:
• Relation of A with T: The Pyrimidine Thymine (T) always pairs with the Purine
Adenine (A)
• Relation of C with G: The Purine Guanine (G) always pair with the Pyrimidine
Cytosine (C)
23. What is Chargaff rule?
Rule 2
In 1947 Chargaff showed that the composition of DNA, in terms of the relative
amounts of the A, C, G and T bases, varied from one species to another.
24. What is Chargaff rule?
Chargaff's rules state that DNA from any cell of all organisms should have a 1:1
ratio (base Pair Rule) of pyrimidine and purine bases and, more specifically, that
the amount of guanine is equal to cytosine and the amount of adenine is equal to
thymine.
According to his first rule, the DNA is the number of cytosine unit is equal to
the guanine unit. In addition, the number of thymine unit is equal to the
adenine units. Besides, it hinted that the base pair makeup of DNA.
The second rule states that the amount of cytosine, guanine, adenine, and
thymine vary from species to species.
Summary
26. Frederick Griffith: Bacterial transformation
- (born October 3, 1877, England—died 1941, London),
- British bacteriologist whose 1928 experiment with
bacterium was the first to reveal the “transforming
principle,” which led to the discovery that DNA acts as the
carrier of genetic information.
In the 1920s, Frederick Griffith made an important discovery. He was studying two different
strains of a bacterium, called R (rough) strain and S (smooth) strain.
• R strain - The R bacteria were non-virulent, meaning they are not capable of causing
disease.
• S strain – The S bacteria were virulent (capable of causing disease).
27. Frederick Griffith: Bacterial transformation
He injected the two strains into mice. The S strain killed (virulent) the mice, but
the R strain did not (non-virulent). Griffith also injected mice with S-strain
bacteria that had been killed by heat. As expected, the killed bacteria did not harm
the mice. However, when the dead S-strain bacteria were mixed with live R-strain
bacteria and injected, the mice died.
28. Frederick Griffith: Bacterial transformation
Griffith’s Experimental Results - showed that a substance could be transferred
to harmless bacteria and make them deadly.
Based on his observations, Griffith deduced that something in the killed S strain
was transferred to the previously harmless R strain, making the R strain deadly.
He called this process transformation, as something was "transforming" the
bacteria from one strain into another strain. What was that something? What
type of substance could change the characteristics of the organism that received
it?
29. Avery – MacLeod – McCarty experiment
Oswald Avery Colin MacLeod Maclyn McCarty
In the early 1940s, a team of scientists led by Oswald Avery tried to answer the question raised by
Griffith’s results. They inactivated various substances in the S-strain bacteria. They then killed the
S-strain bacteria and mixed the remains with live R-strain bacteria. (Keep in mind, the R-strain
bacteria usually did not harm the mice.) When they inactivated proteins, the R-strain was deadly to
the injected mice. This ruled out proteins as the genetic material. Why? Even without the S-strain
proteins, the R strain was changed, or transformed, into the deadly strain. However, when the
researchers inactivated DNA in the S strain, the R strain remained harmless. This led to the
conclusion that DNA is the substance that controls the characteristics of organisms. In other words,
DNA is the genetic material.
30. Hershey-Chase experiments
- The Hershey-Chase experiment was a series of
experiments conducted in 1952 by Alfred
Hershey and Martha Chase.
- Alfred Hershey and Martha Chase showed that only
the DNA of a virus needs to enter a bacterium to
infect it.
The most well-known Hershey-Chase experiment was the final experiment, also
called the Waring Blender experiment, through which Hershey and Chase
showed that phages only injected their DNA into host bacteria, and that the DNA
served as the replicating genetic element of phages.
31. Hershey-Chase experiments
In this experiment they used a bacteriophage and they name it as bacteriophage T4.
bacteriophage T4 was made of protein and DNA. They also knew that proteins contain
sulfur atoms but no phosphorus, while DNA contains a great deal of phosphorus and no
sulfur. They used radioactive sulfur and phosphorus to label and, so, distinguish viral
proteins from viral DNA. After allowing labeled viruses to infect bacteria, they observed
that the radioactive phosphorus enters the bacteria while the radioactive sulfur always
remains outside.
33. What is Chromosome?
- Chromosomes are present in the nucleus of all
the cells and contain the basic genetic material
DNA, which passes from one generation to
another”.
- Chromosomes were first discovered by
Strasburger in 1815 and the term ‘chromosome’
was first used by Waldeyer in 1888.
- Human beings have 46 chromosomes in their
body. These are arranged into 23 pairs.
A chromosome has generally 7 parts; Centromere or primary constriction or
kinetochore, chromatids, chromatin, telomere, chromomere, chromonema, and
matrix.
34. Parts of Chromosome
Centromere or Kinetochore: It is the primary constriction at the
center to which the chromatids or spindle fibers are attached. Its
function is to enable movement of the chromosome during the
anaphase stage of cell division.
35. Parts of Chromosome
Chromatid: During cell division, a chromosome is divided into 2
identical half strands joined by a centromere. A chromatid is each
half of the chromosome joined. Each chromatid contains DNA and
separates at Anaphase to form a separate chromosome. Both
chromatids are attached to each other by the centromere.
36. Parts of Chromosome
Chromatin: It is a complex of DNA and proteins that forms
chromosomes within the nucleus of eukaryotic cells. Nuclear DNA
is highly condensed and wrapped around nuclear proteins in order
to fit inside the nucleus. In other words, it is not present as free
linear strands. The chromatin consists of DNA, RNA, and protein
37. Parts of Chromosome
Telomere: These are little protective caps at the ends of the DNA
molecules that make up our chromosomes. Their job is to stop the
ends of chromosomes from fraying or sticking to each other
38. Parts of Chromosome
Chromonema: It is a threadlike coiled filamentous
structure along which chromomeres are arranged.
Chromonema controls the size of the chromosome and it
acts as a site of gene bearing.
Chromomeres: These are the bead-like structures present
on threads or chromonema. These are arranged in a row
along the length of chromonema. The number of
chromosomes is constant and it is responsible for carrying
the genes during cell division to the next generation.
Matrix: Pellicle is the membrane surrounding each of the
chromosomes. Matrix is the jelly-like substance present
inside pellicle. It is formed of non-genetic materials.
