DNA structure
- Created by: becky_worth
- Created on: 12-03-19 13:16
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- DNA structure
- Hershey and Chase
- Viruses were grown in radioactive sulfur with radioactively labelled proteins
- Viruses were grown in radioactive phosphorus with radioactively labelled DNA
- The viruses were allowed to infect a bacterium
- The bacteria was found to be infected by the phosphorus (DNA) but not the sulfur
- This showed that DNA was genetic material not proteins
- DNA structure
- Rosalind Franklin used x-ray diffraction to investigate the structure of DNA
- DNA was purified and then stretched into thin glass tubes
- DNA was targeted by an x-ray beam which was diffracted and it's scattering pattern was recorded
- Three findings confirmed
- DNA is a double stranded molecule
- Nitrogenous bases are closely packed inside and phosphates form a backbone
- The DNA strands twist at regular intervals to form a double helix
- There is equal number of purines and pyramidines
- DNA replication
- Helicase - unwinds the double helix at the replication fork by breaking h bonds between base pairs
- Gyrase - reduces torsional strain from helicase by relaxing positive supercoils
- SSB proteins bind to the strands to prevent them reanealing
- DNA primase generates an RNA primer on each strand which provides an initiation point for DNA pol III
- DNA Polymerase III moves towards the replication fork on the leading strand and away from the replication fork on the lagging strand
- DNA Polymerase I - replaces RNA primers with free nucleotides on the lagging strand
- DNA Ligase - joins together Okazaki fragments (short fragments on lagging strand)
- DNA Sequencing
- ddNTPs (Dideoxynucleotides) prevent further elongation of a nucleotide chain and terminate replication
- The Sanger method can be used to determine DNA sequencing
- 4 PCR mixes are set up, fragments are separated using gelelectrophoresis and fragments can be detected by automated sequencing machines
- Non-coding DNA
- Satellite DNA - used for DNA profiling, structural component of centromere
- Telomeres - protects against chromosomal deterioration
- Introns - removed by RNA splicing before mRNA formation
- Non-coding RNA genes - codes for RNA molecules that are not translated into protein
- Gene regulatory sequences - promoters, enhancers, silences. Involved in transcription
- Nucleosomes
- In eukaryotic organisms DNA is packaged with histone proteins
- Nucleosomes help DNA to supercoil for efficient storage
- Nucleosomes are linked by an additional histone protein to form chromatosomes
- Chromatosomes coil to form a solenoid structure which becomes condensed fibre (30nm)
- The fibers form loops to make chromatin
- Chromatin supercoils to form chromosomes
- Hershey and Chase
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