DNA Structure and Replication Flashcards
What was the Fredrick Griffith experiment? What were the outcomes?
Griffith grew two strains of Steptrococcus Pneumoniae. A rough strain (non-virulent) and smooth strain (virulent). He then injected mice with the bacteria. As predicted those with the smooth strain died, and those with the rough strain lived. He then injected some mice with the rough strain and heat-killed smooth strain. They died. He concluded the rough strain had taken up a transforming-principle from the heat killed smooth strain which allowed them to be virulent. Oswald Avery, and colleagues, identified this principle as DNA.
What was the Hershey and Chase experiment, and what was its conclusions?
Hershey and Chase confirmed the genetic material was DNA in an experiment in which the protein and DNA components of a bacteriophage were labelled with different radioactive molecules: they used radioactive sulphur and phosphorus. Sulphur is found protein and phosphorus in DNA. Only the phosphorous was detected inside the cells. A bacteriophage is a virus that infects bacteria by injecting its DNA into the bacteria so the bacteria makes many copies of the virus.
What did Erwin Chargaff contribute to the structure of DNA?
He discovered that percentage composition of thymine in DNA was the same as that of Adenine and Cytosine the same as Guanine. Though these percentages varied between species, the pattern was the same.
What was Watson and Crick discover?
The double helix shape of DNA.
What was Rosalind Franklin’s contribution to DNA?
She produced images of DNA using X-ray crystallography.
What is a nucleoside?
Nitrogenous base and a sugar.
What is a nucleotide?
A nitrogenous base linked to a sugar and inorganic phosphate.
What bond links adjacent deoxyribose sugars?
Phosphodiester bonds
What is a chromatin?
DNA wrapped around histone proteins.
What was the Meseleson and Stahl experiment?
They proved DNA replicates using semi-conservative replication, They used E-coli and grew them in N-15. They then added N-14. After each round of DNA replication, they centrifuged the E-coli. The density in the ultracentrifuge showed an intermediate between N-15 and N-14. The N-15 was being gradually diluted out. This showed one strand of the parent DNA was present in the daughter molecule of DNA.
How does DNA replication occur?
DNA synthesis initiates at site called the origin of replication and utilises a dynamic structure called the replication fork
At the replication fork:
helicases unwind the double stranded DNA to allow replication to occur
Single strand binding proteins stabilise the denatured DNA
DNA primase synthesises a short RNA primer to allow replication to commence
DNA polymerase carries out the elongation of the new strand of DNA, which forms by complementary base pairing to the template strand
In which direction does DNA polymerase work in?
5’ to 3’
What does DNA polymerase require to start?
RNA primer
What is different about the leading strand compared to the lagging strand?
A primer is needed at the start of every Okazaki fragment in the lagging strand. A primer is only needed at the start in the leading strand.
How is the lagging stand synthesised?
In Okazaki fragments. as DNA polymerase can only work from the 5’ end to 3’. DNA polymerase I replaces the RNA primers with DNA and DNA ligase seals the gaps between fragments
