3.1.5.2 DNA replication Flashcards
why is the semi-conservative replication of DNA important?
it ensures genetic continuity between generations of cells
why is DNA replication referred to as semi-conservative?
- each DNA strand acts as a template
- therefore the daughter DNA has one new strand and one original (it has been conserved)
what is the first step of DNA replication (unwinding)?
the enzyme DNA helicase unwinds the double helix by breaking the hydrogen bonds between the bases of the two DNA strands
what is the second step of DNA replication (nucleotides attaching through…)?
- each original strand acts as a template to build a new strand
- free DNA nucleotides are attracted to exposed bases on the template strands
- free DNA nucleotides attach to the correct bases through complementary base pairing (A—T and C—G)
what is the third step of DNA replication (two DNA molecules formed)?
- adjacent nucleotides are joined together (to form the phosphodiester bonds) by a condensation reaction catalysed by DNA polymerase
- this allows hydrogen bonds between bases to form (DNA polymerase does NOT form hydrogen bonds)
which three features of DNA allow efficient semi-conservative replication according to the Watson–Crick model and why?
- hydrogen bonds between bases are easily broken so the double helix can easily be unwound to separate into two strands
- bases in DNA are paired through complementary base pairing allowing accurate replication
- there are two DNA strands in a double helix to both can act as a template
what is the role of DNA helicase in the semi-conservative replication of DNA?
unwinding DNA and breaking hydrogen bonds
what is the role of DNA polymerase in the semi-conservative replication of DNA?
- joins adjacent DNA nucleotides
- catalyses condensation reaction
- catalyses the formation of phosphodiester bonds between adjacent nucleotides
what does it mean when DNA has antiparallel strands?
- each DNA strand has a 5’ and a 3’ end
- 3 and 5 primes indicated by location of carbon of deoxyribose
why can nucleotides only be added in a 5’ to 3’ direction during DNA replication?
- DNA polymerase
- which is specific
- is only complementary to the 3’ end of the stand
- shapes of 5’ end and 3’ end are different
DNA polymerase is only complementary to…
- 3’ end of the template strand so can only move along the template strand and add nucleotides in the 3’ to 5’ direction
- therefore, the new strand is built 5’ to 3’ in vivo
which two scientists proposed the chemical structure of DNA and DNA replication?
Watson and Crick
why did Meselson and Stahl’s experiment prove semi-conservative replication?
- grew bacteria mediums containing two different isotopes of nitrogen: 14N which is lighter and 15N which is heavier
- two mediums were then mixed
- DNA produced from each of the three samples was separated according to density using a centrifuge
- found that DNA with a mixture of heavy and light nitrogen settled halfway between heavy and light samples
