10 Chemical nature of a gene

What characteristics must genetic material posses?

• Must contain complex information

• must replicate

• must encode the phenotype

• capacity to vary

Established that the nuclear material contained a novel substance that was slightly acidic and high in phosphorus known as the nucleic acid

Miescher

discovered that DNA consists of a large number of linked, repeating units, called nucleotides; each nucleotide contains a sugar, a phosphate, and a base

• believed in tetranucleotide hypothesis

Levene

Contributed to the idea that protein is the genetic material because the structure of protein, with its 20 different amino acids, could be highly variable

Tetranucleotide Hypothesis

Within each species, there is some regularity in the ratios of the bases: the amount of adenine is always equal to the amount of thymine (A = T), and the amount of guanine is always equal to the amount of cytosine (G = C)

• disproved tetranucleotide hypothesis

Chargaff

Succeeded in isolating several different strains of S. pneumoniae and studied on mice

• found that a mixture of the IIIS heat killed bacteria mixed with the IIR bacteria, the mice were dead

• concluded that the type IIR bacteria had somehow been transformed, acquiring the genetic virulence of the dead type IIIS bacteria producing a permanent genetic change in the bacteria

• theorized that a substance (transforming principle) must be the cause of this

Griffith

set out to understand the nature of the transforming substance

• Enzymes capable of destroying DNA, however, eliminated the biological activity of the transforming substance

• Protease and RNase still resulted in IIR being transformed into IIIS

• DNase did not result in this transformation and only had IIR indicating that DNA is the transforming substance/principle/genetic material

Avery, MacLeod, and McCarty

Study of the T2 bacteriophage and e.coli; a series of experiments to determine whether the phage protein or the phage DNA is transmitted in phage reproduction; confirmed that DNA was the genetic material of phages

• DNA was tagged with Phosphorous and protein with sulfur

• Bacteria labeled with Sulfur: no radioactivity in the cells and no sulfur; indicated that protein does not enter the cell and transmitted to progeny

• Bacteria with phosphorous: bacteria was radioactive; many progeny contained phosphorous indicating that DNA is passed on

Hershey Chase Experiment

used x-ray diffraction results from Franklin and Astbury

• recognized that A goes with T and G with C

• showed that DNA consists of two strands of nucleotides that run in opposite directions (are antiparallel) and wind around each other to form a right-handed helix, with the sugars and phosphates on the outside and the bases in the interior

Watson and Crick

True or False: A few viruses, however, use RNA, not DNA, as their genetic material

True an example would be the TMV virus

hydroxyl group (OH) at the 2’ carbon atom

RNA sugar (ribose)

True or False: RNA is less stable because of its hydroxyl group on the 2’ carbon

True

• two membered ring

• adenine and guanine

Purine

• 6 membered ring

• cytosine (in both)

• thymine (DNA only)

• Uracil (RNA only)

Pyrimidine

What type of bond and where does the nitrogenous base form on the carbon atom of the sugar?

Covalent bond on the 1’ position

deoxyribose/ribose and base

Nucleoside

Where is the phosphate group bonded to on the sugar?

5’ carbon atom

Bonds that connect nucleotides

Phosphodiester linkages

What bonds link the bases on opposite strands and how strong are they?

Hydrogen bonds; typically weak but is necessary for transcription

How many hydrogen bonds with A and T?

2

How many hydrogen bonds with G and C?

3; stronger bond

What forces hold the secondary structure of DNA?

hydrogen bonds and stacked base pairs

Consists of a region of paired bases (the stem) and intervening unpaired bases (a loop)

Hairpin

• typically show low levels of transcription

• can also affect the three-dimensional structure of the DNA molecule and is responsible for some epigenetic effects

• adenine and cytosine are common to undergo this in bacteria

DNA Methylation