dna&rna Flashcards
what does dna stand for
deoxyribonucleic acid
what are the components of a dna nucleotide
deoxyribose (pentose sugar)
phosphate group
nitrogenous base
structure of dna
double helix with two polynucleotide chains held together by hydrogen bonds between complementary base pairs
complementary base pairs
adenine & thymine
cytosine & guanine
how many hydrogen bonds between c & g
3
how many hydrogen bonds between a & t
2
which bases are purines
a & g
which bases are pyrimidine
t & c
what does rna stand for
ribonucleic acid
what are the components of rna
ribose (pentose sugar)
phosphate group
nitrogenous base
semi-conservative replication
- dna helicase breaks the hydrogen bonds between the two polynucleotide dna strands. the helix unzips to form two single strands
- each original single strand acts as a template for a new strand, free floating nucleotides join to the exposed bases on each original template strand by complementary base pairing.
- the nucleotides of the new strand are joined together via condensation reactions catalysed by dna polymerase, hydrogen bonds form between the bases on the original and new strand. the strands twist to make a double helix.
- each new dna molecule contains one strand from the original dna strand and one new strand.
anti parallel dna strands
the active site of dna polymerase is only complementary to the 3’ end of the newly forming dna strand so the enzyme can only add nucleotides to the new strand at the 3’ end.
this means that the new strand is made in 5’ to 3’ direction and that dna polymerase moves down the template strand in a 3’ to 5’ direction. since the strands in the double helix are anti parallel, the dna polymerase working on one of the template strands moves in the opposite direction to the dna polymerase working on the other template strand
who provided evidence for semi conservative replication
meselson and stahl
evidence for semi conservative replication
- two samples of bacteria were grown – one in a nutrient broth containing light nitrogen and the other in heavy nitrogen. as the bacteria reproduced, they took up the nitrogen from the broth to make new nucleotides for new dna, the nitrogen gradually became part of the bacteria’s dna.
- a sample of dna was taken from each batch of bacteria and was spun in a centrifuge. the dna from the heavy nitrogen bacteria settled lower than the lighter nitrogen bacteria bc its heavier.
- then the bacteria grown in the heavy nitrogen was taken out and put inyo a broth containing only light nitrogen. it was then left for one round of dna replication then taken out and spun in a centrifuge.
- if replication was conservative, the original heavy dna would settle at the top and the new light dna would be at the bottom.
- if replication was semi conservative, the new dna molecule would contain one strand of the old heavy nitrogen and one strand of the new light nitrogen. it would settle in the middle of the heavy nitrogen dna and light nitrogen dna.
