Topic 2 (Chapter 10)

characteristics of genetic material

• contain large amounts of complex information

• must replicate faithfully (accurately to pass on to progeny)

• must encode a phenotype

• genetic material must have the capacity to vary

first discovery of nucleic acid

johann friedrich miescher studies whit blood cells in pus and named the nuclein that is slightly acidic and high in phosphorous. this was later names nucleic acid. this was still not known to be genetic material

discovery of four base pairs

albrecht kossel determined the four nitrogen bases

discovery of dna polymer

phoebus levene discovered the repeating units of nucleotides in the nucleus. proposed the tetranucleotide hypothesis and thought that dna was a repeating invariant, fixed sequence of the four nucleotides.

why were proteins though to be hereditary material

• they are much more structurally diverse with 20 amino acids so it is thought to allow for more complexity which would be required for DNA

• it was very common for scientists to believe this

• proteins were on chromosomes so they were also present in the nucleus

Chargaff’s rules

adenine is always equal to thymine and guanine is always equal to cytosine

• disproves the tetranucleotide hypothesis

discovery of transforming principle

• only virulent strain would kill mice

• only nonvirulent strain did not kill the mice

• heat treated virulent did not kill the mice

• heat treated virulent plus regular nonvirulent did kill the mice

this meant that the nonvirulent was transformed and acquired the virulence from the dead virulent bacteria

identification of nature behind transforming principle

• noticed that enzymes like trypsin and chymotrypsin which break down proteins had no effect on the transforming substance

• Ribonuclease destroys RNA and also had no effect

• DNase which destroys DNA did eliminate the transforming principle

because only cultures treated with proteases or RNase transformed, this means that with no DNA, there is no transformation so DNA is the genetic substance

Hershey-chase experiment

second study that gave evidence of DNA as the genetic component

• Traced radioactivity of sulfur and phosphorous in the genes of viruses with 50:50 dna and protein

• infected bacteria cells with virus and a phosphorous or sulfur isotope

• they found that sulfur and therefore proteins were not found in the cells and was not passed on to progeny, but phosphorous, and therefor DNA was

This showed that DNA from the infecting phages had been passed on to the progeny phages and confirmed that DNA is the genetic material

T2 bacteriophage life cycle

1. phage attaches to bacteria and injects its chromosome

2. bacterial chromosome breaks down and the phage enters and replicates

3. expression of phage genes produces phage structural components

4. progeny phage particles assemble

5. bacterial wall lyses, releasing progeny phages

nucleotides

repeating units of DNA that comprise of a sugar, a phosphate group, and a nitrogen base

nucleotide base pairing

a purine only pairs with a pyrimidine to maintain a specific diameter of the DNA molecule

two features of DNA structure

• double helix of two strands that run antiparallel

• sugar phosphate backbone oriented outside of double helix, bases orient inward towards each other

how are nucleotides connected

the phosphate group on the 5’ carbon of one is covalently bonded to the 3’ carbon with OH

difference between DNA and RNA

DNA lacks the 2’ hydroxyl making its polymers more rigid and stable and RNA polymers more flexible. RNA is also more reactive

• DNA is better for longterm storage of genetic material because of these properties

how are RNA hairpins caused

nucleotides on the same strand are inverted complements

• if the sequences are contiguous, the hairpin has a stem but no loop

ribozymes

RNA molecules capable of specific enzymatic activity

• When RNA molecules fold into complex structures some catalytic activity can occur

RNA world hypothesis

idea that RNA is likely the first genetic material in the first life forms because RNA can be both genetic material and can catalyze enzymatic reactions so theoretically, RNA can replicate itself