2. DNA structure and Genetic Code Flashcards
What are the 4 principles DNA structure should fit?
- provide a means for its own replication
- be able to encode the genetic information
- direct cell function
- accomodate changes caused by mutations
Why did Watson move to Cambridge
learn X-Ray diffration/crystalography
What can we learn from the analysis of difracted x-rays?
yields structural information. Based on the angles they defract, we can trace the structure
What were Erwin Chargaff’s rules?
DNA of any species always had equal concentrations of A&T and G&C bases suggesting a fixed relationship
What did Rosalind E Franklin discover?
DNA could exist in dehydrated ‘A Form’ and a fully hydrated ‘B form’
What are important things Franklin deduced from the first X-ray diffraction image?
- DNA was helical and most likely a double helix consisting of 2 anti parallel strands
- phosphates were on the outside of the helicies with the bases on the inside
- distance between bases, the length of the period and the rise of the helix
Are the base tautomersiin DNA most likely Keto or Enol form?
They are Keto form
What are important characteristics of the two complimentary DNA strands?
- have directionality as they are polarized polymers that run antiparallel to each other
is phosphate bonded to the 5’ or 3’ carbon pf the deoxyribose sugar?
5’ phosphate group
is hydroxyl group bonded to the 5’ or 3’ carbon pf the deoxyribose sugar?
3’ hydroxyl group
What DNA replication theories were there?
Semi-conservative, conservative, dispersi ve
How did Meselson-Stahl test the DNA replication theories?
- mixed heavy isotope 15Nitrogen (radioactive) and normal 14 Nitrogen
- followed the sedimentation characteristics of DNA after each cell division
- shows semiconservative replication
Describe Meselson and Stahl’s experiment
- DNA that contained radioactive (15 N) nitrogen was grown in a medium with 14 N (normal nitrogen)
- samples were spun rapidly to form a concentration gradient
- two separate regions were found (14 N and 15N region)
- After one generation, hybrids were found
- second generation showed, hybrid and pure normal nitrogen DNA
How did they crack the genetic code?
Only when using labelled phenylalanine, did the poly-uracil RNA lead to the production of radioactive protein
Which amino acids only have one codon?
methionine and tryptophan
What are mutations?
variations in the DNA sequence eg single base pair substituition
- influence the functionality of protein
What percentage of the human DNA genome directly encodes amino acids?
only 1.5%
how long is a bacterial DNA? and how large is a bacteria?
DNA: 1 mm while Bacteria is only 1 micrometer
What are Topoisomerases
enzymes that insert or remove supercoils
What does topoisomerase I do?
type I: break only one strand -> relaxing or twisting of the helix
What does topoisomerase II do?
Type II breaks both strands and pass another part of the double helix through the gap
How many giant supercoiled loops in the DNA does a typical bacterial chromosome consist of?
50
What proteins does Eukaryotic DNA have?
Histones
What ordered structures in eukaryotic DNA results in chromosomes
Chromatins and nucleosomes
