Studied by 1 personHershey and Chase experiment→ Isotopic mediums used
Radioactive SULFUR→ Proteins
Radioactive PHOSPHORUS→ DNA
Pellet
The insoluable portion after centrifugation
Supernatant
The soluable portion after centrifugation
Result of the Hershey and Chase experiment
-Radioactive phosphorus in the PELLET
-Radioactive sulphur in the SUPERNATANT
Process of X-ray diffraction
-DNA fibres streched in a thin glass tube
-X-ray beam is targetted to the tube, it diffracts when it contacts an atom
-The structure is recorded on film
Oucomes of the X-ray diffraction
-DNA is a double-stranded molecule
-Phosphates form an outer backbone
-DNA twists at regular intervals o form a helix (every 34 Angstrom)
Helicase
-unwinds the double-stranded DNA by breaking the hydrogen bonds between base pairs
-creates the replication fork
Topoisomerase (DNA Gyrase)
-reduces the stain created by the undwinding of DNA
-Keeps the DNA from supercoiling
Single Stranded Binding Proteins
-Bind to the DNA strands after they have been separated and prevent the strands from re-annealing
-Prevent the single stranded DNA from being digested by nucleases
Primase
-Generates a short RNA primer(~10–15 nucleotides) on each of the template strands
-The RNA primer provides an initiation point for DNA polymerase III, which can extend a nucleotide chain but not start one
DNA Polymerase III
-Builds the new strand in a 5’ to 3’ direction
-It moves from 3’ to 5’ in the template strand
-It attaches the free nucleotides which are aligned opposite to their complementary bases
Replication Bubble
Open region of DNA where replication occurs
Okazaki Fragments
Synthesising of the DNA in pieces created Okazaki fragments in the lagging strand
DNA Replication is…
Semi-conservative
DNA Polymerase I
Removes the RNA primers from the lagging strand and replaces them with DNA nucleotides
DNA Ligase
Joins Okazaki fragments together to form a continuous strand by covalently joining the sugar-phosphate backbones together with a phosphodiester bond
Free nucleotides exist as…
-deoxynucleoside triphosphates (dNTPs)
-they have 3 phosphate groups
Energy required for DNA replication comes from…
-DNA polymerase splits the two additional phosphates
-uses the energy released to form a phosphodiester bond with the 3’ end of a nucleotide chain
Dideoxynucleotides- how are they different
Dideoxynucleotides (ddNTPs) lack the 3’-hydroxyl group necessary for forming a phosphodiester bond
Dideoxynucleotides
-ddNTPs prevent further elongation of a nucleotide chain and effectively terminate replication
Purines
-Adenine
-Guanine
(2 rings)
Pyrimidines
-Thymine
-Cytosine
(1 ring)
Coding DNA
EXONS
-Very small percentage (about 1.5%)
-codes for our characteristics
Non-coding DNA
-Large percentage
-Structural function
-“junk” DNA
-doesn’t code for proteins
-includes genes
Introns
-Non-coding sequences WITHIN genes
-are removed by RNA splicing prior to the formation of mRNA
Gene
A section of DNA (sequence of nucleotides) that codes for a protein (a sequence of amino acids)
Functions of Introns
-production of RNA
-gene expression (promotes or inhibits genes)
-telomeres
Telomeres
-located on the ends of eukaryote chromosomes
-protective function: DNA cannot replicate all the way to the ends so telomeres prevent loss of important genes
Short tandem repeats
-Structural component of heterochromatin and centromere
-Satellite DNA
-Non-coding
Uses of STRs
-Cut using restriction enzymes and then separated with gel electrophoresis
-individuals have different numbers of repeats at a given satellite DNA locus, they have unique DNA profiles
Structure of nucleosomes
-Eight histone proteins (an octamer) form a complex called a nucleosome
-Nucleosomes are linked by an additional histone protein (H1 histone) to form a string of chromatosomes
-These coil to form a solenoid structure which condense to form a fibre
-These fibres then form loops, which are compressed and folded around a protein scaffold to form chromatin
Note 
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