DNA and RNA Structure

discovery of DNA

• in the 1950s, Rosalind Franklin performed X-ray crystallography

  of DNA

• her work showed a pattern and was regular and repetitive

• Edwin Chargaff analyzed DNA samples from different species

• he found that A = T and C = G

• then Watson and Crick combined these 2 findings to create a 3D double helix chain

nucleotides

• purines

  • double ring structure

  • (A, G)

• pyrimidines

  • single ring structures

  • (C, U, T)

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• base pairs are held together by hydrogen bonds

• A & T have 2 hydrogen bonds

• C & G have 3 hydrogen bonds

dna structure

• DNA is a double-stranded helix

  • backbone → sugar-phosphate

  • center → nucleotide pairing

• DNA strands are antiparallel (two strands run in opposite directions)

  • one strand runs from 5’ to 3’, the other strand is upside down from 3’ to 5’

  • 5’ → free phosphate group

  • 3’ → free hydroxyl group

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• 5' end → The strand's "start" has a free phosphate group attached to the 5th carbon of the sugar (deoxyribose).

• 3' end → The strand's "end" has a free hydroxyl group (-OH) attached to the 3rd carbon of the sugar.

function of DNA

• DNA is the primary source for heritable information

• generation information is stored and passed on through DNA

  • exception: RNA is the primary source of heritable information in some viruses

prokaryotic vs eukaryotic DNA

• eukaryotic cells

  • DNA in the nucleus

  • linear chromosomes

• prokaryotic cells

  • DNA is in the nucleoid region

  • chromosomes are circular

• prokaryotes & (some eukaryotes) also contain plasmids

  • small, circular DNA molecules that are separate

    from the chromosomes

plasmids

• they replicate independently from the chromosomal DNA

  • often found in prokaryotes

  • carry genes that are useful to the prokaryote when it is in a certain environment, but are not required for survival

• plasmids can be manipulated in labs

  • they can be removed from bacteria, then a favorable gene can be placed inside to form a recombinant plasmid DNA, to express something new

• bacteria can exchange genes found on plasmids

  with neighboring bacteria

  • once the DNA is exchanged, the bacteria can express the genes acquired

  • assists with the survival of prokaryotes

RNA vs. DNA