gen bio - lecture 14: DNA

chromosomes have… (2 components)

1. DNA (deoxyribonucleic acid) → built up by only 4 nucleotides

2. proteins → built up by 20 amino acids

** early hypothesis: genes are made up of proteins bc proteins have 20 building blocks vs DNA’s four

griffith’s experiment (overview + result)

• injected 2 strains of streptococcus pneumoniae into mice

  • rough strain (nonvirulent) = mice lived

  • smooth strain (virulent) = mice died

  • heat killed smooth strain = mice lived

  • rough + heat killed smooth strain = mice died

• principle of transformation: dead bacteria transfer genetic material to living bacteria = changes their traits

avery, mcleod, mccarty experiment (overview + result)

built upon griffith to determine what component undergoes transformation

1. lysed smooth strain

2. separated into: lipids, proteins, polysaccharides, nucleic acid

3. tested each for transforming ability

result? only DNA can transform!

hershey-chase experiment (overview + result)

• bacteriophages: viruses that infect bacteria

  • made up DNA + protein

  • radioactivated the DNA → conclusion

result? only DNA of the phage was injected into bacteria → DNA = genetic info

nucleotides are made of… (3 components)

1. phosphate group

2. sugar (deoxyribose)

• make sugar-phosphate backbone

3. nitrogenous bases

pyramidines vs. purines

pyrimidines = one ring

• cytosine, thymine, urasil

purines = two rings

• adenine, guanine

rosalind franklin contribution

determined 3D structure of molecules!!!

• DNA is 2 stranded, double helix

• consistent width

• consistent distance between turns

• nucleotides = stacked like rungs of a ladder

chargaff contribution

chargaff’s rule: total purine (A+G) = total pyrimidine (C+T)

• amount of A = amount of T (A/T ~ 1)

• amount of C = amount of G (C/G ~ 1)

A + T DOES NOT EQUAL C + G

watson and crick model

determined how nitrogenous bases fit in DNA!!

• helps explain how DNA carries genetic info and how it can be replicated

• sugar-phosphate backbone: outside of helix

  • phosphodiester linkages

  • 3’ end has hydroxyl group, 5’ has a phosphate group

  • NO VARIABILITY

• nitrogenous bases

  • attached to backbone by covalent bonds

  • LOTS OF VARIABILITY

dna strands are…

• held together by hydrogen bonds

  • 2 bonds between A + T

  • 3 bonds between C + G

• antiparallel

  • 3’ to 5’ and 5’ to 3’

• complementary — shows mechanism for DNA replication

semi conservative replication definition

each new molecule of DNA has…

• one parental strand (old)

• one newly synthesized strand

two parts: initiation + elongation

initiation (5 steps)

starts @ origin of replication where a replication bubble is formed + replication proceeds along replication forks

1. DNA = unwound by proteins

• single strand binding proteins (ssbp): keep strands apart

2. helicase: unwinds helix by breaking hydrogen bonds

3. topoisomerase: relaxes helix

4. RNA primase: builds RNA primer (short piece of RNA) @ origin

• later removed + replaced w DNA

5. DNA polymerase: builds DNA polymer by latching on to RNA primer

DNA IS READ FROM THE…

3’ to 5’ direction

MONOMERS ARE BUILT IN THE

5’ to 3’ direction

elongation (nucleoside triphosphate, leading strand, lagging strand, ending)

• nucleoside triphosphate are added

  • 5’ phosphate attaches to previous 3’ hydroxyl

  • forms new phosphodiester bond (releases a phosphate)

• leading strand: 5’ to 3’ TOWARDS replication fork

  • elongates smoothly and continuously

• lagging strand: 5’ to 3’ AWAY from replication fork

  • elongates in short, discontinuous, okazaki fragments

  • have many RNA primers

  • start building on 3’ side!!

@ end, RNA primers are removed and DNA is glued together with ligase

• end with 2 identical sister strands of DNA!