4.1 DNA Structure Flashcards
What is DNA?
the genetic material of life
When researchers began their work, they knew that the genetic material must be… (2)
- able to store information that pertains to the development, structure, and metabolic activities of the cell or organism
- stable so that it can be replicated with high accuracy during cell division and be transmitted from generation to generation
By the 1940s, what did scientists recognize?
that genes are on chromosomes and that chromosomes contain both proteins and nucleic acid
Why was there debate about whether protein or DNA was the genetic material?
many thought that the protein component of chromosomes must be the genetic material because proteins contain up to 20 different amino acids that can be sequences in particular ways, while nucleic acids (DNA, RNA) contain only four types of nucleotides as building blocks
Why did some argue that DNA was not the genetic material?
some argued that DNA did not have enough variability to be able to store information and be the genetic material
What did Oswald Avery, Colin MacLeod and Maclyn McCarty demonstrate?
that the transforming substance that allows S. pneumoniae to produce a capsule and be virulent is DNA, this meant that DNA is the genetic material
What did Oswald Avery, Colin MacLeod and Maclyn McCarty also find out? (4)
- DNA from S strain bacteria causes R strain bacteria to be transformed so that they can produce a capsule and be virulent
- the addition of DNase (enzyme that digests DNA) prevents transformation from occurring. this supports the hypothesis that DNA is the genetic material
- the molecular weight of the transforming substance is large. this suggests the possibility of genetic variability
- the addition of enzymes that degrade proteins has no effect on the transforming substance nor does RNase (enzyme that digests RNA). this shows that neither protein nor RNA is the genetic material
What did Oswald Avery, Colin MacLeod and Maclyn McCarty’s experiments show?
that DNA is the transforming substance, and therefore, the genetic material
What did an experiment by Alfred Hershey and Martha Chase help with?
to firmly establish DNA as the genetic material
What was done in Hershey and Chase’s experiment?
- used a virus called a T phage (composed of radioactively labelled DNA and capsid coat proteins) to infect E. coli bacteria
- discovered that the radioactive tracers for DNA, but not protein, ended up inside the bacterial cells, causing them to become transformed
- only the genetic material could have caused this transformation so Hershey and Chase determined that DNA must be the genetic material
Who was the structure of DNA determined by?
James Watson and Francis Crick
How did Watson and Crick deduce that DNA has a twisted, ladder-like structure?
using the following data:
- DNA is a polymer of nucleotides. There are 4 types because there are 4 different nitrogenous bases.
- Erwin Chargaff (chemist) had determined in the late 1940s that regardless of the species under consideration, the number of purines in DNA is always equal to the number of pyrimidines. A=T, G=C, became known as Chargaff’s rules
- Rosalind Franklin and Maurice Wilkins had just prepared an x-ray diffraction photo of DNA that showed that DNA is a double helix of constant diameter and that the bases are regularly stacked on top of one another
What is DNA a chain of?
nucleotides
What is each nucleotide made up of?
three subunits: phosphoric acid (phosphate), pentose sugar (deoxyribose), nitrogen-containing base
What are purines?
nucleotides with a double ring structure, adenine and guanine
