7.1 DNA Structure Flashcards
What problem did scientists in the mid-twentieth century relating to genetic material?
In the mid-twentieth century, scientists were still unsure as to whether DNA or protein was the genetic material of the cell
Why did scientists had trouble deciding whether protein or DNA were the genetic material?
It was known that some viruses consisted solely of DNA and a protein coat and could transfer their genetic material into hosts
Who carried out an experiment to determine whether DNA or protein is the genetic material of the cell?
In 1952, Alfred Hershey and Martha Chase conducted a series of experiments to prove that DNA was the genetic material
- What was grown to prove DNA was the genetic material?
Viruses (T2 bacteriophage) were grown in one of two isotopic mediums in order to radioactively label a specific viral component
- What were the 2 conditions in which the viruses were grown?
Viruses grown in radioactive sulfur (35S) had radiolabelled proteins
Viruses grown in radioactive phosphorus (32P) had radiolabeled DNA
Why were some of the viruses grown in sulfur?
h&c
sulfur is present in proteins but not DNA
Why were some of the viruses grown in phosphorus?
h&c
phosphorus is present in DNA but not proteins
- What were the viruses then allowed to do? h & c
The viruses were then allowed to infect a bacterium (E. coli) and then the virus and bacteria were separated via centrifugation
- What happened during the centrifugation? h&c
The larger bacteria formed a solid pellet while the smaller viruses remained in the supernatant
- What were the results? h&c
The bacterial pellet was found to be radioactive when infected by the 32P–viruses (DNA) but not the 35S–viruses (protein)
- What could they conclude? h&c
This demonstrated that DNA, not protein, was the genetic material because DNA was transferred to the bacteria
Who researched the structure of DNA? What did they use?
Rosalind Franklin and Maurice Wilkins used a method of X-ray diffraction to investigate the structure of DNA
- What is the first step of x-ray diffraction?
DNA was purified and then fibres were stretched in a thin glass tube (to make most of the strands parallel)
- What is concentrated on the DNA? x-ray
The DNA was targeted by a X-ray beam, which was diffracted when it contacted an atom
- What does the x-ray diffraction show?
The scattering pattern of the X-ray was recorded on a film and used to elucidate details of molecular structure
What 3 things can be determined from the x-ray diffraction?
composition
orientation
shape
What was concluded about the composition of DNA from the x-ray diffraction?
DNA is a double stranded molecule
What was concluded about the orientation of DNA from the x-ray diffraction?
Nitrogenous bases are closely packed together on the inside and phosphates form an outer backbone
What was concluded about the shape of DNA from the x-ray diffraction?
The DNA molecule twists at regular intervals (every 34 Angstrom) to form a helix (two strands = double helix)
What did Franklin’s research conclude?
Franklin’s x-ray diffraction experiments demonstrated that the DNA helix is both tightly packed and regular in structure
What is the structure of DNA?
Phosphates (and sugars) form an outer backbone and nitrogenous bases are packaged within the interior
What did Chargriff determine?
Chargaff had also demonstrated that DNA is composed of an equal number of purines (A + G) and pyrimidines (C + T)
What can be inferred from Chargraff’s findings?
This indicates that these nitrogenous bases are paired (purine + pyrimidine) within the double helix
How did Chargraff’s findings help in determining the directions of the DNA strands?
In order for this pairing between purines and pyrimidines to occur, the two strands must run in antiparallel directions
What information about the hydrogen bonds was concluded by Watson and Crick?
When Watson & Crick were developing their DNA model, they discovered that an A–T bond was the same length as a G–C bond
How many bonds are formed between A and T, G and C?
Adenine and thymine paired via two hydrogen bonds, whereas guanine and cytosine paired via three hydrogen bonds
What does the DNA structure suggest about the 2 mechanisms for DNA replication?
Replication occurs via complementary base pairing (adenine pairs with thymine, guanine pairs with cytosine)
Replication is bi-directional (proceeds in opposite directions on the two strands) due to the antiparallel nature of the strands
What type of process is DNA replication?
DNA replication is a semi-conservative process that is carried out by a complex system of enzymes
What is helicase’s role in DNA replication?
Helicase unwinds and separates the double-stranded DNA by breaking the hydrogen bonds between base pairs
Where does helicase unwind DNA?
This occurs at specific regions (origins of replication), creating a replication fork of two strands running in antiparallel directions
What is the role of DNA gyrase?
DNA gyrase reduces the torsional strain created by the unwinding of DNA by helicase
How does DNA gyrase reduce the torsional strain?
It does this by relaxing positive supercoils (via negative supercoiling) that would otherwise form during the unwinding of DNA
What 7 proteins are involved in DNA replication?
helicase DNA gyrase SSB Proteins (single-stranded binding proteins) DNA primase DNA polymerase III DNA polymerase I DNA Ligase
What is the role of SSB’s in DNA replication?
SSB proteins bind to the DNA strands after they have been separated and prevent the strands from re-annealing
Apart from separating the DNA strands, what is the role of SSB’s?
These proteins also help to prevent the single stranded DNA from being digested by nucleases
What happens to SSBs once DNA replication is finished?
SSB proteins will be dislodged from the strand when a new complementary strand is synthesised by DNA polymerase III
