DNA replication Flashcards
What are purines
Nitrogenous bases adenine and guanine are purines as they have a double ring structure
What are pyrimidines
Nitrogenous bases cytosine, thymine and uracil are pyrimidines as they have a single ring structure
How many hydrogen bonds between adenine and thymine
Two hydrogen bonds
How many hydrogen bonds between guanine and cytosine
Three hydrogen bonds
When does DNA replication occur
When a cell divides by mitosis an exact copy of the DNA must be produced so that each of the new daughter cells receives a copy
Describe the process of DNA replication
-helicase causes entire double helix unwinds from one end and the two single strands split apart as the hydrogen bonds between bases break
-free nucleotides line up alongside each DNA strand and hydrogen bonds form between the complementary bases
-enzyme DNA polymerase links adjacent nucleotides with phosphodiester bonds in condensation reactions to form new complementary strands
-overall builds two complete DNA molecules identical to each other and to the original DNA molecule
What are the three models for DNA replication and which was accepted
-conservative replication
-fragmentary replication
-semi-conservative replication (accepted)
What is conservative replication
When one DNA molecule has two original parent DNA strands while the other has two new strands
What is fragmentary replication
When both DNA strands are made up of a mixture of original parent DNA nucleotides and new nucleotides
What is semi-conservative replication
When each DNA molecule contains one original parent strand and one new strand
Explain the method that Meselson and Stahl used to investigate the method of DNA replication
-used DNA from bacteria that had been grown in a medium containing only heavy nitrogen isotopes (N15) which all the nucleotides contained at the start
-then moved the bacteria into a medium containing only normal N14 meaning all new nucleotides incorporated into the replicated DNA were light but original nucleotides were heavy
-allowed the bacteria to replicate and their DNA to replicate once and extracted this then centrifuged
Results of the experiment after one round of division
Single band of medium density DNA present (heavy DNA no longer existed so rejected conservative model as half would be light half heavy)
Results of experiment after two rounds of replication
Two bands with one being medium and one light
-confirmed semi-conservative model as with fragmentary would produce one type of DNA with a mixture of heavy and light
