Genetics Chapter 9: Molecular Structure of DNA and RNA

molecular genetics

study of DNA structure and function at a molecular level

4 crieteria for DNA to fulfill it's role as genetic material

information, transmission, replication, variation

Frederick Griffith

studied streptococcus pneumoniae in mice to suggest that DNA is the genetic material

Avery, McCarty, and MacLeod

used Griffith's observations to prove that DNA is the transforming principal. They created purified type S DNA extract and mixed it with the type R cells

Hershey and Chase

provided evidence that DNA is the genetic material of the T2 phage. They labeled protein with S35 and DNA with P32, and mixed in a blender and centrifuged to separate the 2. There was a much higher percentage of protein outside the cell

Gierer and Schramm

isolated RNA from the tobacco mosaic virus and found that purified RNA still caused infection

4 levels of complexity of DNA and RNA

Nucleotides, strand, double helix, three-demesional structure

components of a nucleotide

at least one phosphate group, a pentose sugar (ribose or deoxyribose), and a nitrogenous base (A,T,C,G, or U)

purines

Guanine and Adenine: contain 2 rings

pyrimidines

Thymine, Uracil, and Cytosine: contain 1 ring

the base of a nucleotide is always attached to which carbon?

1 prime

bonds within DNA

phosphodiester linkages between nucleotides, hydrogen bonds between bases

nucleic acid backbone

Sugar and phosphates

phosphodiester linkage

a phosphate attachment to the 5' carbon in one nucleotide and a 3' carbon in the other

directionality

each strand has directionality based on the orientation of it's sugar molecule

Linus Pauling

proposed that regions of proteins can fold into a secondary structure known as an a-helix. he used ball and stick models.

Rosalind Franklin and Maurice Wilkins

used x-ray defraction to figue out that: DNA is helical, it is too wide to only be a single helix, and a helix contains about 10 base pairs per full turn

Chargaff

A=T, G=C

Watson and Crick

deduced the double helical structure of DNA with a ball and stick model, using Pauling's and Franklin's experiments as a foundation

key features of DNA double helix

right-handed, the bases in opposite strands hydrogen bond according to Chargaff's rule, the two strands are antiparallel with regard to their 5'-3' directionality, and there are about 10 bps in a complete turn.

A DNA

formed in virto, right handed, 11bp per 360 turn, tilted from center axis, occurs under low humidity

B DNA

predominate form of DNA in living cells, right handed, 10bp per 360 turn

Z DNA

formed in vitro, left handed, zig-zag helical backbone, 12bp per 360 turn, tilted from center axis, high salt concentrations-formation is favored by a sequence of bases that alternate between purines and pyrimidines, at low salt concentration-formation is favored by methylation of cytosine bases, favored by negative supercoiling

methylation

a cellular enzyme attaches to a methyl group (-CH3) to the cytosine base

Triplex DNA

tripple helical, formed in vitro by mixing natural double stranded DNA with a synthetic third strand. third strand binds to major groove. has potential to specifically inhibit certain genes, as in cancer cells

possible structures of RNA molecules

bulge loop, internal loop, multibranched junction, stem-loop