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Tertiary structure, DNA supercoiling
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Tertiary structure, DNA supercoiling notes
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SUBJECT 2: TERTIARY STRUCTURE. DNA SUPERCOILINGStructure 1aria → nucleotide sequence
Structure 2aria → double helix, triple helix, hairpin, cruciform ...
3aria structure → supercoiling
Unlike proteins, DNA has no 4aria structure.
DNA supercoiling:
- The axis of the double helix coils on itself and forms a superhelix →structure
Tertiaryof DNA that involves thermodynamic stress.
- Compacts, takes up less space → can enter smaller places.
- TheDNA molecule must not have free ends (neither beginning nor end), if this condition is
not given, the supercoil will never form:
- Circular DNA → Plasmidis: small molecules of circular, closed and
double-stranded DNA, which are in the cytoplasm of many bacteria.
- Linear DNA: theDNA loop in the middle of a chain → in a linear chain loops can be
formed (as if it had no free ends), which have stabilizing proteins that maintain this
shape.
, FunctionBiology: supercoiling
- Augmenta the degree of DNA compaction.
- It allows good DNA replication and transcription.
Compaction ofDNAEscherichia Coli:
- Its genome is a single circular DNA molecule (4.7 million bp; 1.6 mm), highly compacted
and condensed in a central area of the cell cytoplasm; E. coli, 2 mm). → It is a double
circular helix attached to proteins forming 40 loops or domains of about 100 Kb
with negative supercoiling.
INTRODUCTION
In general, the supercoil can be:
- Negative: if the superhelix rotates in the same direction as the double helix
(clockwise).
- Positive: if the superhelix rotates in the opposite direction to the double
helix (lever).
Inliving things, permanently, there is only supercoiling negative.
The positive is only found in a transient way, as it dissolves quickly.
To study all this, we observe the
supercoiling of DNA by agarose gel
electrophoresis:
- Relaxed DNA band: it migrates
more slowly, since its linear shape
presents difficulties in crossing the pores
of the ice.
- Rolled DNA Band: Migrates
faster, as the compact shape allows it to
easily penetrate the pores of the ice.
The positive pole is at the bottom.
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