lagging strand

Examples of lagging strand in the following topics:

• 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 "lagging strand" is synthesized in the direction away from the replication fork and away from the DNA helicase unwinds.
  • This lagging strand is synthesized in pieces because the DNA polymerase can only synthesize in the 5' to 3' direction, and so it constantly encounters the previously-synthesized new strand.
  • 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.
  • 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 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.
  • The other strand (the lagging strand), complementary to the 5' to 3' parental DNA, is extended away from the replication fork in small fragments known as Okazaki fragments, each requiring a primer to start the synthesis.
  • 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.

• 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.
  • Therefore, both daughter DNA strands have an incomplete 5' strand with 3' overhang.
  • Once the 3' end of the lagging strand template is sufficiently elongated, DNA polymerase adds the complementary nucleotides to the ends of the chromosomes; thus, the ends of the chromosomes are replicated.
  • 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.

• 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.

• 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.

• DNA Repair

  • The polymerase checks whether the newly-added base has paired correctly with the base in the template strand.
  • 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.
  • The mismatch-repair proteins detect this base and remove it from the newly-synthesized strand by nuclease action.
  • A special enzyme, DNA ligase (shown here in color), encircles the double helix to repair a broken strand of DNA.
  • In this interactive, you can "edit" a DNA strand and cause a mutation.

• Transcription in Prokaryotes

  • Transcription always proceeds from the same DNA strand for each gene, which is called the template strand.
  • The RNA product is complementary to the template strand and is almost identical to the other (non-template) DNA strand, called the sense or coding strand.
  • The nucleotide on the DNA template strand that corresponds to the site from which the first 5' RNA nucleotide is transcribed is called the +1 nucleotide, or the initiation site.
  • Nucleotides preceding, or 5' to, the template strand initiation site are given negative numbers and are designated upstream.
  • Conversely, nucleotides following, or 3' to, the template strand initiation site are denoted with "+" numbering and are called downstream nucleotides.