Quiz 6 Flashcards
timeline of discoveries for DNA structure
- Morgan’s lab: genes located on chromosomes, 2 components (DNA and protein)
- Griffith (1928): used bacteria to show DNA is genetic material
- Chargaff (1950): DNA composition varies among species but ratios of purine and pyrimidine bases remains constant
- hereditary info encoded in DNA found in all cells; directs biochemical, anatomical, physiological, and behavioral traits
- Watson and Crick (1953): double helical DNA model, images stolen from Franklin and Wilkins
DNA structure
- nucleotide polymer (nitrogenous base, sugar, phosphate group)
- 3’ and 5’ end
- bases for Chargaff’s rules about equal A/T and G/C not understood until discovery of double helix
Watson and Crick’s discoveries
- deduced DNA is helical, helix width, spacing of nitrogenous bases, and that DNA has 2 strands in double helix
- ***models built to conform to chemistry
- Franklin had already concluded 2 outer sugar-phosphate backbones with nitrogenous bases paired in interior
- Watson-Crick model found that backbones were antiparallel (subunits run in opposite directions)
- model explains Chargaff’s rules: purine/pyramidine pairing resulted in uniform width
purines and pyramidines
purines: adenine and guanine (2 rings)
pyrimidines: thymine and cytosine (1 ring)
semi-conservative model of replication basics and how did Watson and Crick’s work provide evidence for this?
- Watson and Crick’s discovery of specific base paring suggested a possible copying mechanism (since 2 strands are complementary, each strand acts as a template for building a new strand)
- parent molecule unwinds and 2 new daughter strands are built based on base-pairing rules
- “Semi-conservative” because each daughter molecule has one parent strand and one newly made strand (proven using nitrogen isotopes)
where does DNA replication happen?
*fast and accurate process with more than a dozen enzymes
- begins at origins of replication, where 2 DNA strands separate to form a replication bubble (eukaryotic chromosomes may have thousands of these)
- replication proceeds in both directions from the site until the entire molecule is copied
- at the end, the replication bubble is a y-shaped replication fork, where new DNA strands elongate
important enzymes before replication
- helicase: untwist double helix at replication fork by breaking hydrogen bonds
- single-stranded binding proteins: bind to stabilize single-stranded DNA
- topoisomerase: corrects “overwinding” ahead of replication forks by breaking, swiveling, and rejoining DNA strands
how does DNA replication begin?
- primase starts RNA chain at 3’ end using parent DNA as a template (initial nucleotide strand is a short RNA primer 5-10 nucleotides long)
- works around 50 nucleotides/sec in humans
- nucleotides are added to the 3’ end of the growing strand (new strand elongates in 5’ –> 3’ direction due to antiparallel structure)
how does antiparallel structure impact DNA replication? (2 strands)
2 strands are synthesized differently, replication goes in both directions from the origin of replication
leading strand: synthesized continuously moving towards replication fork (where DNA is unzipped)
lagging strand: synthesized as a series of Okazaki fragments moving away from the replication fork and then shifting back towards it; fragments joined by DNA ligase
DNA replication machine
name for the large complex of proteins that participate in DNA replication
how is DNA proofread and repaired? how is it protected?
- DNA polymerases replace incorrect nucleotides
- repair enzymes correct errors in mismatched base pairing
- nucleotide excision repair: nuclease cuts out and replaces damages DNA stretches
*DNA is protected by telomeres, special nucleotide sequences at the end of chromosomes that prevent erosion of genes
what damages DNA?
- exposure to harmful chemicals
- physical agents (x-rays)
- mutations (although some good)
important enzymes during DNA replication
- DNA polymerase: catalyze elongation of new DNA at replication fork
* can’t initiate synthesis (requires a primer and template strand) and can only add nucleotides to 3’ end - DNA primase: starts RNA chain
- DNA ligase: joins together Okazaki fragment
makeup of nucleic acids
- polymer “polynucleotides” made up of nucleotide mononers
- monomers joined by covalent bonds to create backbone
- include 2 types of nitrogenous bases: purines (adenine, guanine) and pyramidines (cytosine, thymine, uracil)
nucleotides without phosphate: nucleosides
difference between DNA and RNA
DNA - 2 polynucleotide chains; antiparallel double helix; complementary base pairing for A/T and G/C
RNA - single polypeptide chains; T replaced by U; complementary base pairing can occur between 2 RNA molecules of parts of the same RNA molecule