leading strand

Examples of leading strand in the following topics:

• 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.
  • One strand (the leading strand), complementary to the 3' to 5' parental DNA strand, is synthesized continuously towards the replication fork because the polymerase can add nucleotides in this direction.
  • The leading strand can be extended by one primer alone, whereas the lagging strand needs a new primer for each of the short Okazaki fragments.
  • The overall direction of the lagging strand will be 3' to 5', while that of the leading strand will be 5' to 3'.
  • On the leading strand, DNA is synthesized continuously, whereas on the lagging strand, DNA is synthesized in short stretches called Okazaki fragments.

• DNA Replication in Eukaryotes

  • DNA polymerase cannot initiate new strand synthesis; it only adds new nucleotides at the 3' end of an existing strand.
  • The "leading strand" is synthesized continuously toward the replication fork as helicase unwinds the template double-stranded DNA.
  • Eventually, the leading strand of one replication bubble reaches the lagging strand of another bubble, and the lagging strand will reach the 5' end of the previous Okazaki fragment in the same bubble.
  • The enzymes FEN1 and RNase H remove RNA primers at the start of each leading strand and at the start of each Okazaki fragment, leaving gaps of unreplicated template DNA.
  • On the leading strand, only a single RNA primer is needed, and DNA is synthesized continuously, whereas on the lagging strand, DNA is synthesized in short stretches, each of which must start with its own RNA primer.

• DNA Repair

  • Uncorrected mistakes may sometimes lead to serious consequences, such as cancer.
  • Repair mechanisms can correct the mistakes, but in rare cases mistakes are not corrected, leading to mutations; in other cases, repair enzymes are themselves mutated or defective.
  • If this remains uncorrected, it may lead to more permanent damage.
  • In E. coli, after replication, the nitrogenous base adenine acquires a methyl group; the parental DNA strand will have methyl groups, whereas the newly-synthesized strand lacks them.
  • In this interactive, you can "edit" a DNA strand and cause a mutation.

• The DNA Double Helix

  • The two strands of the helix run in opposite directions, so that the 5′ carbon end of one strand faces the 3′ carbon end of its matching strand.
  • If the sequence of one strand is AATTGGCC, the complementary strand would have the sequence TTAACCGG.
  • During DNA replication, each strand is copied, resulting in a daughter DNA double helix containing one parental DNA strand and a newly synthesized strand.
  • Each base from one strand interacts via hydrogen bonding with a base from the opposing strand.
  • 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′.

• 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.
  • 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.
  • The new strand will be complementary to the parental or "old" strand and the new strand will remain basepaired to the old strand.
  • So each "daughter" DNA actually consists of one "old"  DNA strand and one newly-synthesized strand.

• Telomere Replication

  • After DNA replication, each newly synthesized DNA strand is shorter at its 5' end than at the parental DNA strand's 5' end.
  • Every RNA primer synthesized during replication can be removed and replaced with DNA strands except the RNA primer at the 5' end of the newly synthesized strand.
  • Therefore, both daughter DNA strands have an incomplete 5' strand with 3' overhang.
  • Parental DNA strands are black, newly synthesized DNA strands are blue, and RNA primers are red.
  • This means that each newly-synthesized DNA strand is shorter at its 5' end than the equivalent strand in the parental DNA.

• Gene Duplications and Divergence

  • When the polymerase reattaches to the DNA strand, it aligns the replicating strand to an incorrect position and incidentally copies the same section more than once.
  • Aneuploidy is often harmful and in mammals regularly leads to spontaneous abortions.
  • For example, trisomy 21 in humans leads to Down syndrome, but it is not fatal.
  • Gene duplications are an essential source of genetic novelty that can lead to evolutionary innovation.
  • This leads to a neutral "subfunctionalization" model, in which the functionality of the original gene is distributed among the two copies.

• 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.
  • The two polynucleotide strands are anti-parallel in nature.
  • The twisting of the two strands around each other results in the formation of uniformly-spaced major and minor grooves bordered by the sugar-phosphate backbones of the two strands.
  • The two strands are held together by base pairing between nitrogenous bases of one strand and nitrogenous bases from the other strand.
  • The two anti-parallel polynucleotide strands are colored differently to illustrate how they coil around each other.

• Elongation and Termination in Eukaryotes

  • Consequently, RNA Polymerase II does not need as many accessory proteins to catalyze the synthesis of new RNA strands during transcription elongation as DNA Polymerase does to catalyze the synthesis of new DNA strands during replication elongation.
  • All RNA Polymerases travel along the template DNA strand in the 3' to 5' direction and catalyze the synthesis of new RNA strands in the 5' to 3' direction, adding new nucleotides to the 3' end of the growing RNA strand.
  • RNA Polymerases unwind the double stranded DNA ahead of them and allow the unwound DNA behind them to rewind.
  • As a result, RNA strand synthesis occurs in a transcription bubble of about 25 unwound DNA basebairs.
  • RNA Polymerases use the DNA strand below them as a template to direct which nucleotide to add to the 3' end of the growing RNA strand at each point in the sequence.

• Virus Classification

  • This genetic material may be single- or double-stranded.
  • The type of genetic material (DNA or RNA) and its structure (single- or double-stranded, linear or circular, and segmented or non-segmented) are used to classify the virus core structures .
  • Viruses can contain double-stranded DNA (dsDNA), single-stranded DNA (ssDNA), double-stranded RNA (dsRNA), single-stranded RNA with a positive polarity (ssRNA), ssRNA with a negative polarity, diploid (two copies) ssRNA, and partial dsDNA genomes.
  • (a) Rabies virus has a single-stranded RNA (ssRNA) core and an enveloped helical capsid, whereas (b) variola virus, the causative agent of smallpox, has a double-stranded DNA (dsDNA) core and a complex capsid.
  • Adenovirus (left) is depicted with a double-stranded DNA genome enclosed in an icosahedral capsid that is 90–100 nm across.