3.1.5 Nucleic Acids

nucleotide structure

1 phosphate group, 1 pentose sugar (deoxyribose for DNA + ribose for RNA and ATP) and 1 base (nitrogenous)

2 nucleotides

-a condensation reaction between 2 nucleotides forms a phosphodiester bond

differences between DNA + RNA

-DNA = double stranded, double helix, deoxyribose sugar, thymine base, holds genetic info, long

-RNA = single strand, ribose sugar, uracil base, transfers genetic info from DNA to ribosomes, short

who were the scientists to discover DNA structure?

Watson and Crick

DNA structure

-2 strands run antiparallel to form the double helix held by hydrogen bonds

-4 bases adenine + thymine, cytosine + guanine with 2 complementary base pairs A=T C≡G where C≡G is more stable as there are more hydrogen bonds

-polymer of nucleotides joined together by phosphodiester bonds

DNA adaptations

-large = can carry lots of information

-stable = can pass on information without changes (mutation)

-strong + stable phosphodiester bond surrounding bases = genetic info is protected from being corrupted

-weak hydrogen bonds between bases = can separate easily for DNA replication

semi-conservative replication

-when the daughter DNA molecules have 1 original strand + 1 new strand

-Meselon-Stahl = bacteria of ¹⁵N added to medium of ¹⁴N + centrifuged

STEPS of DNA replication

1) DNA helicase breaks hydrogen bonds between complementary base pairs

2) so double helix unwinds and the 2 strands separate and act as templates to determine order of nucleotides

3) free DNA nucleotides are attracted to the exposed bases on strands and attach by complementary base pairing

4) DNA polymerase joins adjacent nucleotides by catalysing condensation reactions to form phosphodiester bonds between phosphate and deoxyribose

DNA polymerase

is an enzyme so has specific active shape and is only complementary to specific substrate/end (either 5’ or 3’) so move in opposite directions of each strand as ends are different shapes