SBI4U – Grade 12 AP Biology – DNA Test


Thanks, Bairavi Sripalan!


DNA replication

– Conservative – parental helix remains intact and a second all new copy is made

– Semi Conservative – two strands of the parental helix separates and each functions as a template for synthesis of a new complementary strand

– Dispersive – mixture of old and new synthesized parts

– DNA replicates Semi conservatively

– In prokaryotes since they have circular DNA there is only one origin of replication

– In eukaryotes they have linear DNA which allows there to be many origins of replication

– There are 5 steps to replicate DNA

1. Helicase (an enzyme) unwinds the helix

2. Single stranded binding proteins (S.S.B.P.) help keep DNA open and unwound

3. The leading strand is synthesized continuously; made 5’ è 3’ but is read as 3’ è 5’ by DNA Pol. III

4. The lagging strand is synthesized in fragments called Okazaki fragments which are made in a process similar to 2 steps forward, 1 step back. RNA primes made by RNA primase flag the DNA Pol. III

5. The DNA Pol I replaces the RNA primers with DNA

6. DNA ligase joins Okazaki fragments

7. Gyrase relases tension generated by the uncoiling (in bacteria). Topoisomerase relieves some of the stress of over-wound DNA by creating small cuts in the DNA ( in people)



– Each daughter cell will have a relatively shorter telomere

– Human telomeres lose 100 base pairs from telomeric DNA at each mitosis

– At this rate after 125 mitotic divisions the telomeres would be completely gone and the cell would therefore die

– Exceptions are stem cells, unicellular eukaryotes and cancer cells


Protein Synthesis

– Proteins are the links between genotype and phenotype

– One gene – one protein hypothesis : that each gene dictates production of a specific enzyme

– Gene expression – process by which DNA directs protein synthesis, includes two stages

– Transcription (at the nucleus) : the synthesis of RNA under the direction of DNA, produces mRNA. Initiation: copying of RNA Elongation : removal of introns and some segments are added Termination : adding the poly A tail

– Translation ( at the ribosome) : the synthesis of a polypeptide which occurs under the direction of mRNA


DNA -> RNA -> Protein

Lac Operon (E-Coli & Lactose)