double helix

Examples of double helix in the following topics:

• The DNA Double Helix

  • The DNA double helix looks like a twisted staircase, with the sugar and phosphate backbone surrounding complementary nitrogen bases.
  • DNA has a double-helix structure, with sugar and phosphate on the outside of the helix, forming the sugar-phosphate backbone of the DNA.
  • During DNA replication, each strand is copied, resulting in a daughter DNA double helix containing one parental DNA strand and a newly synthesized strand.
  • Native DNA is an antiparallel double helix.
  • In a double stranded DNA molecule, the two strands run antiparallel to one another so that one strand runs 5′ to 3′ and the other 3′ to 5′.

• The Secondary & Tertiary Structures of DNA

  • Before viewing this double helix structure itself, it is instructive to examine the base pairing interactions in greater detail.
  • Coiling these coupled strands then leads to a double helix structure, shown as cross-linked ribbons in part b of the diagram.
  • The double helix is further stabilized by hydrophobic attractions and pi-stacking of the bases.
  • Topoisomerase: This enzyme initiates unwinding of the double helix by cutting one of the strands.
  • Separation of a portion of the double helix takes place at a site called the replication fork.

• Eukaryotic Chromosomal Structure and Compaction

  • In the first level of compaction, short stretches of the DNA double helix wrap around a core of eight histone proteins at regular intervals along the entire length of the chromosome .
  • A DNA molecule in this form is about seven times shorter than the double helix without the histones.
  • The beads are about 10 nm in diameter, in contrast with the 2-nm diameter of a DNA double helix.
  • Double-stranded DNA wraps around histone proteins to form nucleosomes that have the appearance of "beads on a string."

• The Structure and Sequence of DNA

  • DNA is a double helix of two anti-parallel, complementary strands having a phosphate-sugar backbone with nitrogenous bases stacked inside.
  • Watson and Crick proposed that DNA is made up of two polynucleotide strands that are twisted around each other to form a right-handed helix.
  • The diameter of the DNA double helix is 2 nm and is uniform throughout.
  • Therefore, ten base pairs are present per turn of the helix.
  • DNA has (a) a double helix structure and (b) phosphodiester bonds.

• History of DNA Research

  • However, there remained questions of how many strands came together, whether this number was the same for every helix, whether the bases pointed toward the helical axis or away, and ultimately, what were the explicit angles and coordinates of all the bonds and atoms.
  • Using X-ray diffraction, as well as data from Rosalind Franklin and her information about base paris, Watson and Crick arrived at the first accurate model of DNA's molecular structure, a double helix, in 1953 .
  • In 1957, Crick laid out the " Central Dogma, " which explained the relationship between DNA, RNA, and proteins, and articulated the "sequence hypothesis. " A critical confirmation of the replication mechanism that was implied by the double-helical structure followed in 1958 in the form of the Meselson-Stahl experiment.
  • James Watson and Francis Crick are credited with the discovery of the double helix structure of DNA.

• DNA Replication in Prokaryotes

  • Single-strand binding proteins coat the strands of DNA near the replication fork to prevent the single-stranded DNA from winding back into a double helix.
  • As we know, the DNA double helix is anti-parallel; that is, one strand is in the 5' to 3' direction and the other is oriented in the 3' to 5' direction.
  • Topoisomerase prevents the over-winding of the DNA double helix ahead of the replication fork as the DNA is opening up; it does so by causing temporary nicks in the DNA helix and then resealing it.
  • Single-strand binding proteins bind to the single-stranded DNA to prevent the helix from re-forming.

• Biology: DNA Structure and Replication

  • DNA is a large macromolecule that (in three-dimensional space) forms the shape of a double helix, as shown in .
  • The "backbone" of each helix is formed from alternating deoxyribose and phosphate subunits as illustrated in .

• Inhibiting Nucleic Acid Synthesis

  • The process starts when one double-stranded DNA molecule produces two identical copies of the molecule .
  • The double helix is unwound and each strand acts as a template for the next strand.

• Basics of DNA Replication

  • DNA replication uses a semi-conservative method that results in a double-stranded DNA with one parental strand and a new daughter strand.
  • Watson and Crick's discovery that DNA was a two-stranded double helix provided a hint as to how DNA is replicated.
  • The double-stranded structure of DNA suggested that the two strands might separate during replication with each strand serving as a template from which the new complementary strand for each is copied, generating two double-stranded molecules from one.
  • In semi-conservative replication, each of the two parental DNA strands would act as a template for new DNA strands to be synthesized, but after replication, each parental DNA strand would basepair with the complementary newly-synthesized strand just synthesized, and both double-stranded DNAs would include one parental or "old" strand and one daughter or "new" strand.
  • Meselson and Stahl's results established that during DNA replication, each of the two strands that make up the double helix serves as a template from which new strands are synthesized.

• Supercoiling

  • In a "relaxed" double-helical segment of B-DNA, the two strands twist around the helical axis once every 10.4 to 10.5 base pairs of sequence.
  • The two lobes of the figure eight will appear rotated either clockwise or counterclockwise with respect to one another, depending on whether the helix is over or underwound.
  • The twist is the number of helical turns in the DNA and the writhe is the number of times the double helix crosses over on itself (these are the supercoils).
  • A negatively supercoiled DNA molecule will produce either a one-start left-handed helix, the toroid, or a two-start right-handed helix with terminal loops, the plectoneme.