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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

what is pathogenic
kelvin Reply
biological pathogen is anything that can produce disease. a pathogen may also be referred to as an infectious agent, or simple a germ.
explain 3 points why gender is relevant to health.
wat causes sor throat
Ezeama Reply
Gram positive
streptococcus pyrogenes
group A streptococcus pyogenes. But not the only one.
does the bac of s.aures gives choronic diseas in bone?
causes virus bac group A strep allergies smoking
truth also virus
what z microbiology
Mwelwa Reply
microbiology is all about the scientific study of microorganisms, their life changes, their life span, structural punctuality and their phenomenon.
Best scope in microbiology
wat abut skin
whatis scope
what is scope
what happened when someone died and what happened to the blood
Franca Reply
nice question
when you die you have no memory and the blood dry off
what microorganisms is all about?
And how many does the microorganisms is classified?
How are microbs organized
what is endoparasites and exo parasite and their example
Endo : inside the body Exo/ecto : outside or on the body
examples please
Louse for exo/ecto Helminths (worms) for endo
what's prokaryotic
mohaiminul Reply
A prokaryotic cell is a cell with no membrane bounds organelles
what are the main effect of parasite?
yunusa Reply
nutritional group of micro organism
Mana Reply
what are the charateristic bacteria, eukaryotes, and viruses have in commom
Ami Reply
what type of bacteria ismor serious
D N A ar purno rup ki
Ruba Reply
বাংলায় দেখবো DNA এর পূর্রন্যরুপ কি
what's microbiology
micro biology is the study of micro organisms,those being unicellular multicellular or a cellular
deoxyribonucleic acid is a long molecule that contains our unique genetic code.
Does the parasite/parasitology are under microbiology or not
Define bacteria
Bacteria are a type of biological cell & they constitute a large domain of prokaryotic microorganisms.
what kind of microbiology
hello what is main function of T and B cells
Ishaq Reply
T cells are involved in cell-mediated immunity while B cells are primarily responsible for humoral immunity (relating to antibodies)
what is hepatitis B
it's viral infection that attacks the liver and can cause both acute and chronic diseases. the virus is most commonly transmitted from mother to child during birth and delivery, as well as through contact with blood or other body fluids...
What are fundamental experiment of microbiology?
Lucky Reply
study of microscopic organisms, unicellular and multicellular
what are the characteristics of bacteria, viruses, and eukaryotes
bacteria-are living organism. contain a peptidoglycan/lipopolysaccharide cell wall. VIRUS-are considered as organic structure which interact with living organism don't have acell wall. they have many differents like bacteria size -large(1000nm) virus size-smaller(20-400nm). bacteria ribosomes presen
virus ribosomes absence. bacteria living organism. virus b/n living and non-living things
eukaryotic cell has a true membrane bond nucleus and has other membranous organelles that allow for compartmenalization of function
This answer is wrong
Jeffrey Reply
who discovered the growth curve
Joy Reply
Example of coccus bacteria
Nweke Reply
staphylococcus aureus
MRSA any bew information
pl. read any new information about MRSA
Any stru tural finding about novel corona virus

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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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