Rosalind Franklin
Woman who used X-ray diffraction to research DNA structure. She improved the resolution to obtain precise pattern measurements, finding the double helix structure of DNA
Watson and Crick
Men who used calculations and a model of DNA structure using Rosalind Franklin's information
Nucleosomes
A core of eight histones with DNA coiled around them. It has 2 copies of 4 types of histones. An H1 protein binds the DNA to the octamer
Linker DNA strand
A short strand of DNA connecting nucleosomes
Supercoiling
The twisting of DNA to make it pack into a tiny nucleus
Complementary base pairing
Structure of DNA where each base is bonded to another complementary base to form a complementary strand
Semiconservative model of replication
Model of DNA replication where each strand of DNA is a template strand for a new DNA helix, and new strands are synthesized based on that strand
Origins of replication
Where DNA replication begins (there are many of these in eukaryotes, but only one in prokaryotes)
Direction in which replication occurs
It occurs in both directions away from the origin, appearing as a replication bubble. Nucleotides are added to the 3' carbon at the end of the chain, so replication occurs in a 5' to 3' direction
Deoxyribose phosphate
The ribose sugar-phosphate backbone of DNA
Nucleoside
A base connected to a sugar
Nucleotide
A base and sugar connected to a phosphate group
Structure of the DNA helix
Double helix arranged in an anti-parallel way. This means synthesis must occur differently on each strand
Replication fork
Region where the DNA is split into two separate strands by helicase
Leading strand
Strand of DNA during replication which is made continuously following the fork as it opens
Lagging strand
Strand of DNA during replication which is made in fragments, moving away from the replication fork
Okazaki fragments
Fragments of DNA replicated on the lagging strand (which do not have phosphodiester bonds)
Phosphodiester bonds
Bonds between sugars in the sugar-phosphate backbone, using a phosphate group and O's on either side
Covalent bonds in DNA structure
Occur between C5 and the phosphate group
Helicase
Unwinds DNA at the replication fork
DNA gyrase
(AKA topoisomerase II) Releases the strain developing ahead of the helicase (isolating breaking so it is happening solely at the helicase)
SSB's
Single-stranded binding proteins keep strands apart until after being separated long enough to allow the template to be replicated
DNA primase
Adds one RNA primer on the leading strand at initiation and many primers on the lagging strand
DNA polymerase III
Covalently links deoxyribonucleotide monophosphates (nucleotides) to the 3' end of the growing strand. It begins off the RNA primer
DNA polymerase I
Removes RNA primers on replicated strands, replacing them with DNA
DNA ligase
Connects gaps between Okazaki fragments by creating phosphodiester bonds
Coding sequences
DNA that codes for polypeptides
Non-coding sequences
Sequences of DNA which do not make polypeptides. They can: (1) Allow tRNA production (2) Regulate gene expression with silencers and enhancers (3) Be introns
Satellite DNA
Highly repetitive sequences of non-coding DNA
Telomeres
Ends of chromosomes with repetitive non-coding DNA which protect it during interphase because in DNA replication, replication does not occur to the end of the chromosome. This prevents genes from being lost, instead losing the telomeres
How X-ray diffraction works
(1) X-rays are directed at a material, and some is scattered through diffraction (2) X-rays' wavelengths are sensitive to diffraction by DNA (3) DNA is arranged in an orderly way (as if it were crystallized, but as DNA cannot be crystallized, this was done) for diffraction patterns to be obtained (4) X-ray detector placed close to the sample to collect scattered rays (5) Sample is rotated to see pattern of scattering (6) Diffraction recorded with X-ray film
What Franklin deduced about DNA
(1) Cross in centre showed the molecule was helical (2) Angle of cross shape showed steepness of helix (3) Distance between horizontal bars showed turns in the helix were 3.4nm apart (4) Distance between middle of the pattern and the top showed there were repeating structures with 0.34nm between repeats
How base sequence is typically determined
(1) Many copies of unknown DNA are placed into test tubes with deoxyribonucleotides and enzymes for replication (2) Small amounts of "dide"oxyribonucleotides are added, labelled with fluorescent markers (3) They are incorporated into some of the new DNA, stopping replication where they are added (4) Fragments synthesized are separated by electrophoresis (5) Base sequence is analyzed by comparing colour of fluorescence with fragment length
Variable number tandem repeat
Short sequence which is repeated a different number of times in different people, inherited as an allele. This allows DNA profiling.
Typical DNA profiling techniques
Finding father: analyzing short tandem repeats from the Y-chromosome Finding mother: analyzing mitochondrial DNA variations in single nucleotides in hyper-variable regions
Hershey and CHase
Scientists who worked with T2 phages, some with sulfur-35 and others with phosphorus-32. The ones with radioactive 32P did not have radioactive material, but 35S did. This indicated that nucleic acids were responsible for genetic material