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By the end of this section, you will be able to:
  • Explain transformation of DNA
  • Describe the key experiments that helped identify that DNA is the genetic material
  • State and explain Chargaff’s rules

Modern understandings of DNA have evolved from the discovery of nucleic acid to the development of the double-helix model. In the 1860s, Friedrich Miescher ( [link] ), a physician by profession, was the first person to isolate phosphate-rich chemicals from white blood cells or leukocytes. He named these chemicals (which would eventually be known as RNA and DNA) nuclein because they were isolated from the nuclei of the cells.

Photo of Friedrich Miescher.
Friedrich Miescher (1844–1895) discovered nucleic acids.

To see Miescher conduct an experiment step-by-step, click through this review of how he discovered the key role of DNA and proteins in the nucleus.

A half century later, British bacteriologist Frederick Griffith was perhaps the first person to show that hereditary information could be transferred from one cell to another “horizontally,” rather than by descent. In 1928, he reported the first demonstration of bacterial transformation    , a process in which external DNA is taken up by a cell, thereby changing morphology and physiology. He was working with Streptococcus pneumoniae, the bacterium that causes pneumonia. Griffith worked with two strains, rough (R) and smooth (S). The R strain is non-pathogenic (does not cause disease) and is called rough because its outer surface is a cell wall and lacks a capsule; as a result, the cell surface appears uneven under the microscope. The S strain is pathogenic (disease-causing) and has a capsule outside its cell wall. As a result, it has a smooth appearance under the microscope. Griffith injected the live R strain into mice and they survived. In another experiment, when he injected mice with the heat-killed S strain, they also survived. In a third set of experiments, a mixture of live R strain and heat-killed S strain were injected into mice, and—to his surprise—the mice died. Upon isolating the live bacteria from the dead mouse, only the S strain of bacteria was recovered. When this isolated S strain was injected into fresh mice, the mice died. Griffith concluded that something had passed from the heat-killed S strain into the live R strain and transformed it into the pathogenic S strain, and he called this the transforming principle ( [link] ). These experiments are now famously known as Griffith's transformation experiments.

On the left is a photo of a live mouse, representing a mouse injected with heat-killed, virulent S strain. On the right is a photo of a dead mouse, representing a mouse injected with heat-killed, virulent S strain and live, non-virulent R strain.
Two strains of S. pneumoniae were used in Griffith’s transformation experiments. The R strain is non-pathogenic. The S strain is pathogenic and causes death. When Griffith injected a mouse with the heat-killed S strain and a live R strain, the mouse died. The S strain was recovered from the dead mouse. Thus, Griffith concluded that something had passed from the heat-killed S strain to the R strain, transforming the R strain into S strain in the process. (credit "living mouse": modification of work by NIH; credit "dead mouse": modification of work by Sarah Marriage)

Questions & Answers

why drinking excess alcohol causes thirst and dehydration
uwikuzo Reply
what is reproduction
smart Reply
it is d act of bringing young ones to life
Oyebanji
to ensure survival of a species🚴‍♀️
Michelle
what is a genotype
Collins
what is hazardous
smart
a cell is the smallest unit of a living thing. so we all have cell
smart
It is the formation of a zygote resulting from the fusion of the sperm cell with the ovum.Thus,this results in the production of new species which are genetically dissimilar from their parent cells.
Pallavi
what is size of cell
Mohd Reply
what is size of Hart
Mohd
nanometers=um sign thingie
Michelle
microns=nanometers
Michelle
monomers and polymers of nucleic acids?
Jyrl Reply
dna and rna involvement
Michelle
give me the elements of the soil
Iguma Reply
Air, water, organic matter, inorganic matter
Tshwano
soil water humus air
Veronicah
yap
Beloved
silica, iron
Patience
potassium, sulfur, calcium, carbon
Michelle
what is cell
iyaji Reply
A cell is a smallest fundamental unit of a living organisms.
Kem
the basic structural and functional unit of life
Patience
what is size of cell
Mohd
all things are made up of.....all things cannot exist without pre-exisiting cells...check out the 16th ce tury to learn more about microscope use and cells. i will give a hint: Mr. L.
Michelle
we are all made of cells
Michelle
Nutrition - sensitive intervention
Therowda Reply
what does the sori In fern mean
arhin Reply
biology is the study .exactly what is life?
Jerry
i'm sorry , the study of life
Jerry
how can U identify a person through his blood
Frankyx Reply
through genetic fingerprinting where specific DNA sequences in a person genome can be identified.
Pallavi
by help of genetics and DNA test
Meerbadini
can it also be detected using an RNA test
Frankyx
environmental biology
Ojesola Reply
what is phytoplankton
Ojesola
What is anabolism
Treasure Reply
the break down of substance
Aunty
how many teeth has an adult person
TSEGAY Reply
36
Treasure
32
Favour
32
Kamaluddeen
32 teeth
Meerbadini
32
OLAMIDE
32 (or 36 including the wisdom teeth)
Hailey
32
Sani
26?🤓
Michelle
what is homoestastis
Ayo Reply
hypothalamus negative feedback vs. postive feedback systems.
Michelle
it is the maintenance of a steady internal environment.it is controlled largely by the brain especially the hypothalamus.
Oyebanji
What is a cell
Eric Reply
A cell is the building block of all organisms
Millicent
yes it is , it also helps in functions .
Joezman
cell is structural & functional unit of oraganisms
Meerbadini
it is a building blocks of a living organism
Joezman
it is the basic unit of life in living organisms
Nabwonso
it is the basic unite of life in living organisms
Nabwonso
it is the fundamental unit of life
Adewoye
it is the structural and functional unit of life
Ojesola
Why cells are classify as living and non living things
Millicent Reply
due to such as virus which acts as a living cell inside the cell of an organisms and out side of cell in form of crystals
Meerbadini
because there are living cells which performs complex processes such as reproduction and metabolism then there are dead cells like the top most part of the epidermis or the bark
irish
because there are living cells which performs complex function such as reproduction and metabolism
David
cause otherwise you could call fire alive
Marissa

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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