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Learning objectives

  • Explain the meaning of semiconservative DNA replication
  • Explain why DNA replication is bidirectional and includes both a leading and lagging strand
  • Explain why Okazaki fragments are formed
  • Describe the process of DNA replication and the functions of the enzymes involved
  • Identify the differences between DNA replication in bacteria and eukaryotes
  • Explain the process of rolling circle replication

The elucidation of the structure of the double helix by James Watson and Francis Crick in 1953 provided a hint as to how DNA is copied during the process of replication . Separating the strands of the double helix would provide two templates for the synthesis of new complementary strands, but exactly how new DNA molecules were constructed was still unclear. In one model, semiconservative replication , the two strands of the double helix separate during DNA replication, and each strand serves as a template from which the new complementary strand is copied; after replication, each double-stranded DNA includes one parental or “old” strand and one “new” strand. There were two competing models also suggested: conservative and dispersive, which are shown in [link] .

Diagram showing 3 models of DNA replication. In the conservative model the original double helix produces two double helices; one of which has two of the parent strands and one of which has two of the new strands. Another round produces 4 helices; one of which has two of the parent strands and three of which have all new strands. In semiconservative replication the first round leads to two double helices each with one old strand and one new strand. The next round leads to four double helices; two of these have an old and a new strand and two have all new strands. In dispersive replication each new round of replication results in strands with random bits from the parent strand and random bits of new strands.
There were three models suggested for DNA replication. In the conservative model, parental DNA strands (blue) remained associated in one DNA molecule while new daughter strands (red) remained associated in newly formed DNA molecules. In the semiconservative model, parental strands separated and directed the synthesis of a daughter strand, with each resulting DNA molecule being a hybrid of a parental strand and a daughter strand. In the dispersive model, all resulting DNA strands have regions of double-stranded parental DNA and regions of double-stranded daughter DNA.

Matthew Meselson (1930–) and Franklin Stahl (1929–) devised an experiment in 1958 to test which of these models correctly represents DNA replication ( [link] ). They grew E. coli for several generations in a medium containing a “heavy” isotope of nitrogen ( 15 N) that was incorporated into nitrogenous bases and, eventually, into the DNA. This labeled the parental DNA. The E. coli culture was then shifted into a medium containing 14 N and allowed to grow for one generation. The cells were harvested and the DNA was isolated. The DNA was separated by ultracentrifugation, during which the DNA formed bands according to its density. DNA grown in 15 N would be expected to form a band at a higher density position than that grown in 14 N. Meselson and Stahl noted that after one generation of growth in 14 N, the single band observed was intermediate in position in between DNA of cells grown exclusively in 15 N or 14 N. This suggested either a semiconservative or dispersive mode of replication. Some cells were allowed to grow for one more generation in 14 N and spun again. The DNA harvested from cells grown for two generations in 14 N formed two bands: one DNA band was at the intermediate position between 15 N and 14 N, and the other corresponded to the band of 14 N DNA. These results could only be explained if DNA replicates in a semiconservative manner. Therefore, the other two models were ruled out. As a result of this experiment, we now know that during DNA replication, each of the two strands that make up the double helix serves as a template from which new strands are copied. The new strand will be complementary to the parental or “old” strand. The resulting DNA molecules have the same sequence and are divided equally into the two daughter cells.

Questions & Answers

characteristic of Gram negative bacteria
jane Reply
Characteristics of Gram Negative Bacteria As with Gram positive bacteria, Gram negative bacteria also contain the peptidoglycan polymer in their cell wall. While this polymer is thin (2 to 4 nanometers in thickness with just about 3 layers of peptidoglycan) in Gram negative bacteria, it's also com
it's also composed of long glycan strands that are cross-linked by peptide molecules. This composition serves a number of functions including protecting the bacterial cell from lysis
Good shot
Thanks 😊
what was Hans Christian Gram's supported in the modern Microbiology?
Wilson Reply
what is microbial growth
Chisa Reply
The organism responsible for vulva ulcers
nyiter Reply
Why are vascular pathogen poorly communicable from person to person?
Aj Reply
Most vascular pathogens are poorly communicable from person to person because they need a medium to be communicated i,e a vector that would carry them from one person to other
what's the habit of protista
Afieahngwi Reply
They show both autotrophic and heterotrophic mechanisms...
let me mention some water. Air .Food and so on
Gattiek Reply
causes of infectious diseases
Afieahngwi Reply
infectious disease are caused by pathogenic micro organisms like bacteria ,fungi..
What is pasteurization?
are fungi prokaryote or eukaryotes?
Afieahngwi Reply
fungi are eukaryotes.
All fungi are eukaryotes. Even micro fungi.
have..complex cellular organization and membrane bound nucleus ...and..also... having loops of DNA( like plasmids) as.bacteria
what enzyme replaces rna nucleotides with dna nucleotides during replication?
Remi Reply
an enzyme called DNA ligase.
describe the acid fast staining procedure used in the diagnosis of tuberculosis
Salma Reply
bacterial morphology
lf_ Reply
what is the difference between biogenesis & abiogenesis
Mayuri Reply
biogenesis is when living comes out from other living things as a result of reproduction while a biogenesis is the process where living things comes out from non living things
living things come form other form living things is biogenesis. ....right?
what is mean by pasturation method?
Mayuri Reply
Pasteurization is a process that kills harmful bacteria and creates an extended shelf life for your milk. ... It's pretty simple—we take the milk from the cows, we rapidly heat it to a high enough temperature to kill the bacteria, and then we cool it back down before packaging and shipping it to you
tell me about abiogenessis &biogenesis
discribe aristol spontaneous generation theory in brif
Mayuri Reply
The Greek philosopher Aristotle (384–322 BC) was one of the earliest recorded scholars to articulate the theory of spontaneous generation, the notion that life can arise from nonliving matter. Aristotle proposed that life arose from nonliving material if the material contained pneuma (“vital heat”).
thank you 😊
No mention dear 😊

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Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
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