7.1 DNA Structure and replication Flashcards
What did Rosalind Franklin do?
At Kings College she improved the resolution of a camera so that she could make more detailed measurements of the X ray diffraction patterns than had previously been possible.
She obtained the sharpest X-ray diffraction images of DNA in existence.
She passed X-rays through DNA, most passed through but some were scattered this scattering is called diffraction. They wavelengths of X-rays make them particularly sensitive to diffraction by the particles in biological molecules including DNA.
The X-ray detector collects the scattered rays and the patterns can be recorded.
What do histone proteins do?
Histones are used by the cell to package the DNA into structures call nucleosomes. A nucleosome consists of a central core of eight histone proteins with DNA coiled around the proteins. The association of histones with the DNA contributes to a pattern known as supercoiling. An analogy is if you twist an elastic band repeatedly eventually it forms an additional pattern of coils. Supercoiling allows a great length of DNA to be packed into a much smaller space within the nucleus. The nucleosome is an adaptation that facilitates the packing of the large genomes that eukaryotes possess. The H1 histone binds in such a way to form a structure called the 30 nm fibre that facilitates further packing.
The H1 makes the DNA inside the C shaped histone into a D. It closes off.
What are the leading and the lagging strand?
DNA replication is continuous on the leading strand making the DNA just as the fork opens and discontinuous on the lagging strand.
The lagging strand is made instead with little Okazaki fragments which are then linked up afterwards.
How does DNA replication work?
The enzyme helicase unwinds the DNA at the replication fork and the enzyme topoisomerase releases the strain that develops ahead of the helicase. Single stranded binding proteins keep the strands apart long enough to allow the template strand to be copied.
Starting replication requires an RNA primer. There are lots of primers on the lagging strand but just this one on the leading one. The enzyme DNA primase creates one RNA primer on the leading strand and many RNA primers on the lagging strand. The RNA primer is necessary to initiate the activity of the DNA polymerase.
DNA polymerase is responsible for covalently linking the deoxyribosenucleotide monophosphate to the 3’ end of the growing strand.
DNA ligase then connects the gaps between the Okazaki fragments.
What does topoisomerase do?
It is an enzyme involved in DNA replication. It releases the strain head of the helicase as it moves also the DNA strand. It works before helicase does.
How does do the two DNA strands stay open long enough to replicate?
After the topoisomerase and the helicase have opened up and separated the strands, single stranded binding proteins keep the strands apart long enough to allow the template strand to be copied.
What does DNA primase do?
DNA primase attached one RNA primer to the leading strand and many to the lagging strand. The RNA primer is necessary to initiate the activity of DNA polymerase.
What does DNA ligase do?
Connects the Okazaki fragments.
What direction does DNA Polymerase work in?
DNA polymerase can only add nucleotides to the 3’ end of a primer. This is why so many primers are needed in the lagging strand.
What do non-coding parts of the DNA do?
The number of non-coding regions found in genomes have many functions.
- Some are used as a guide to produce tRNA and rRNA .
-Some non-coding regions play a role in the regulation of gene expression such as enhancers and silencers.
- Telomeres
- Introns - non coding parts within genes
- TANDEMLY REPEATING SEQUENCES OF DNA
Most of the Eukaryotic genome is non-coding. There are loads of repetitive sequences.
Some repetitive sequences serve at telomere’s. During interphase when the chromosomes replicate the enzymes cannot continue replication all the way to the end of the chromosome, sacrificing the repetitive sequences found in telomeres serves a protective function.
What are tandem repeats?
Within the non-coding regions of an individual’s genome there exists satellite DNA - long stretches of DNA made up of repeating elements called short tandem repeats.
They can be removed using restriction enzymes and then separated with gel electrophoresis for comparison.
As individuals will likely have different numbers of repeats at a given satellite DNA locus, they will generate unique DNA profiles.
How is DNA sequenced?
USING DIdeoxyribonucleic acid!
The determination of the sequence of bases in a genome is carried out most commonly using a method that employs fluorescence. Many copies of the unknown DNA that is to be sequenced are placed into test tubes with all of the raw materials including deoxyribonucleotides and the enzymes necessary to carry out replication. In addition very small quanitites of dideoxyribonnucleotides that have been labelled with different fluorescent markers are added. The dideoxyribonucleotides will be incorportated into some of the new DNA but when they are incorporated they will stop replication at exactly the point when they were added.
The fragments are separated by length using electrophoresis. The sequence of bases can be automatically analysed by comparing the colour of the fluorescence with the length of the fragment.
