dna replication

how did Watson, Crick and Franklin’s discovery of DNA’s molecular structure serve to explain how DNA replicates?

the complementary bases allow for DNA strands to be predicted as complementary pairs of bases are always the same

describe semi conservative replication:

• DNA helix unwinds

• each strand acts as a template for synthesis of a new complementary strand

• results in 2 DNA molecules w/ 1 original and 1 new strand each

  

name the enzymes involved in semi conservative DNA replication:

• DNA helicase

• DNA polymerase

• DNA ligase

what is the function of DNA helicase?

• unzips DNA helix by breaking H bonds

• DNA unwinds and separates into 2 strands

what is the function of DNA polymerase?

• free nucleotides that have been activated are attracted to their complementary bases and joined together

• this forms a phosphodiester bond

what is the function of DNA ligase?

joins 2 fragments on the lagging strand

compare the synthesis occurring in the leading strand to the synthesis occurring in the lagging strand:

• leading strand - catalysed by DNA polymerase and is constantly synthesising

• lagging strand - must synthesise DNA in short sections

which end of a polynucleotide chain is the 3’ end?

the end having a free hydroxyl group at the 3rd C of the deoxyribose sugar

which end of a polynucleotide chain is the 3’ end?

the end containing the free Pi group attached to the 5th C of the deoxyribose sugar

why does DNA polymerase only work in the 5’ - 3’ direction?

• DNA strands antiparallel

• DNA polymerase active site is specific and only complementary to 5’ endshape of 5’ end different to shape of 3’ end

describe the (incorrect) conservative model of DNA replication:

• original DNA molecule remains intact

• separate daughter DNA copy built up from new molecules of deoxyribose, phosphate and organic bases

• → one molecule made up of entirely new material, another entirely original

describe and explain an experiment that shows evidence for semi-conservative replication:

• grew bacteria on a medium containing ¹⁴N for multiple generations before transferring bacteria to a medium containing ¹⁵N for a single generation

• extracted and centrifuged DNA

• if conservative: original DNA would only contain ¹⁵N and settle at bottom of tube, new DNA would only contain ¹⁴N and settle at top of tube

• if semi-conservative: all DNA would contain both ¹⁵N and ¹⁴N and so all would settle in middle of tube

• one band formed in middle of tube → each DNA molecule contained a mixture of the heavier and lighter N isotopes

• (if left to continue, ratio of ¹⁵N:¹⁴N would go from 1:1 to 3:1 to 7:1 etc.)

who conducted the experiment which provided evidence for semi-conservative replication?

Meselsohn and Stahl