IB biology: topic 2: PCR/gene expression

what is PCR?

polymerase chain reaction

what enzyme is used in PCR?

Taq polymerase

what is Taq polymerase?

a thermostable enzyme found from bacteria in hot springs/volcanoes

how does PCR work?

DNA is heated very strongly so hydrogen bonds between the two strands break,
primers (bases) are added and join up to the exposed complementary base,
Taq polymerase forms a backbone and covalently bonds these bases to form a new strand.
Two new strands are formed

what effects the expression of genes?

various environmental factors that can inhibit or stimulate gene expression. 

how do histones impact gene expression?

how densely the DNA is wrapped around the histone protein will affect the likelihood of transcription.

how can we change how densely DNA is wrapped around a histone?

chemical modification

what chemicals affect histones? how?

Acetyl / Methyl groups add to the tails of the proteins to change how tightly the protein bind to the DNA making it more or less accessible to transcription factors such a RNA polymerase. 

how does Acetylation affect histones?

interferes with the amino acid on the histone tails that normally mean it binds strongly to negative DNA. So there is more gene expression as it is less tightly bound

how does Methylation affect histones?

Methylation of Cytosine in the DNA strand will decrease gene expression by causing increased binding of histone and so less expression.

what are exons?

sections of the mRNA that carry the codons are called exons.

why does removal of introns not take place in prokaryotes?

there is no separation of genetic material from the ribosomes. This means that a gene can be being simultaneously transcribed and translated

what are introns?

sections of RNA produced in transcription that do not code for part of the protein. 

what is a key example of gene expression?

the Laq Operon

what does the regulator gene (I) do in the laq operon?

codes for a repressor protein

what does the control site P do in the laq operon?

promoter DNA which RNA polymerase binds to begin transcription of Z and Y

what does the control site O do in the laq operon?

operator switches structural genes on and off.

what does the structural gene Z do in the laq operon?

catalyses hydrolysis of lactose.

what does the structural gene Y do in the laq operon?

transports lactose into the cell.

what are telomeres?

repetitive sequences of non-coding DNA. 

what is the role of telomeres?

protective function at the ends of chromosomes:

During DNA replication the DNA polymerase can not continue replication all the way to the end of the chromosome. This means that with each replication some DNA is lost. 

what is a nucleosome?

a central core of eight histone proteins (the octomer) with DNA coiled around twice.

what was the first Hershey-Chase experiment?

protein coats of viruses are radiolabelled (made radioactive), they infect bacteria which are then grown in radioactive sulfur,

centrifuge spins it around, the radioactivity is in the supernatant (NO RADIOACITIVTY IN BACTERIA - ONLY IN VIRUSES)

what was the second Hershey-Chase experiment?

DNA of viruses are radiolabelled (made radioactive), they infect bacteria which are then grown in radioactive phosphorous,

centrifuge spins it around, the radioactivity is in the pellet (DNA INJECTED RADIOACTIVITY INTO BACTERIA)

what was the conclusion of the hershey-chase experiments?

DNA is genetic material, not protein

what did Meselson-Stahl provide evidence for?

semi-conservative theory

what was Meselson-Stahl’s experiment for semi-conservative DNA replication?

nitrogen 14 (lighter) and nitrogen 15 (heavier) used.

bacteria are grown in nitrogen 15 (heavier).

bacteria are then placed in nitrogen 14 solution (lighter).

centrifuge spins the solution, as time proceeds n15 band decreases and n14 band increases - it got lighter as n15 was conserved whilst n14 increased.