Nucleic Acids 2 Flashcards
(12 cards)
Describe some alternative DNA conformations
- B-DNA: right handed helix. Almost all cellular DNA is B-DNA.
- A-DNA: right handed helix. Forms when DNA is dehydrated.
- Z-DNA: left handed helix. Moves in an anticlockwise, fashion towards us.
What occurs if DNA is underwound ?
Negative supercoils
Exposes some sites for processes such as DNA replication to take place
What occurs is DNA is overwound ?
Positive supercoils
Creates a hinderance to DNA replication and transcription
What is supercoiling ?
The DNA molecule twists around itself
What state is DNA usually in when in vivo ?
Negatively supercoiled
Describe the denaturation of DNA
Denaturation is the removing of non-covalent bonds
When DNA is heated at 70-110°, it becomes denatured so the strands separate
What happens when DNA is exposed to alkaline conditions ?
Due to the extrac -OH present, all the bonds are broken- not just the covalent bonds. So you are just left with nucleotides.
How is Denaturation monitored ?
By measuring absorbance of UV light at a wavelength of 260nm via spectrophotometry.
Hydrogen bonds absorb light at this wavelength.
Single stranded DNA absorbs more light than double stranded DNA.
What is the melting temperature (Tm) ?
The temperature needed to denature 50% of the DNA molecules in a sample
When is Tm increased ?
1.In DNA with high content of GC base pairs, since there are more H-bonds between the strands
2. In the presence of cations- they reduce repulsion
How does RNA differ from DNA ?
- RNA has ribose instead of deoxyribose
- RNA contains Uracil instead of Thymine
- RNA is single stranded but DNA is double stranded (usually)
RNA is more reactive than DNA.
What are the properties of plasmids ?
- Circular DNA molecules separate from the chromosome
- Size range 1kb-400kb
- A cell may contain between 1 and 200 copies of a plasmid
- They carry non-essential genes
- Have an origin of replication
