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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.
Dejene
Thanks
kelvin
explain 3 points why gender is relevant to health.
kelvin
wat causes sor throat
Ezeama Reply
Streptococcus
Rajat
Streptococcus
Panda
Gram positive
shamim
streptococcus pyrogenes
Sherma
group A streptococcus pyogenes. But not the only one.
Carlos
does the bac of s.aures gives choronic diseas in bone?
Elyas
causes virus bac group A strep allergies smoking
Elyas
truth also virus
Shania
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.
Ridwan
thanks
micah
Best scope in microbiology
Rahul
wat abut skin
Elyas
whatis scope
Amare
what is scope
Funmi
what happened when someone died and what happened to the blood
Franca Reply
nice question
Attari
when you die you have no memory and the blood dry off
Falere
what microorganisms is all about?
Ridwan
And how many does the microorganisms is classified?
Ridwan
How are microbs organized
Saskia
what is endoparasites and exo parasite and their example
OGEDE Reply
Endo : inside the body Exo/ecto : outside or on the body
Paul
examples please
OGEDE
Louse for exo/ecto Helminths (worms) for endo
Paul
what's prokaryotic
mohaiminul Reply
A prokaryotic cell is a cell with no membrane bounds organelles
Paul
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
Amare
D N A ar purno rup ki
Ruba Reply
বাংলায় দেখবো DNA এর পূর্রন্যরুপ কি
Ruba
what's microbiology
micah
micro biology is the study of micro organisms,those being unicellular multicellular or a cellular
aniisha
deoxyribonucleic acid is a long molecule that contains our unique genetic code.
aniisha
Does the parasite/parasitology are under microbiology or not
yunusa
Define bacteria
Kainat
Bacteria are a type of biological cell & they constitute a large domain of prokaryotic microorganisms.
raisa
what kind of microbiology
Mira
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)
Janet
what is hepatitis B
Sunday
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...
Dejene
What are fundamental experiment of microbiology?
Lucky Reply
study of microscopic organisms, unicellular and multicellular
Falere
what are the characteristics of bacteria, viruses, and eukaryotes
Ami
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
Dejene
virus ribosomes absence. bacteria living organism. virus b/n living and non-living things
Dejene
eukaryotic cell has a true membrane bond nucleus and has other membranous organelles that allow for compartmenalization of function
Dejene
This answer is wrong
Jeffrey Reply
who discovered the growth curve
Joy Reply
Example of coccus bacteria
Nweke Reply
staphylococcus aureus
Elton
MRSA any bew information
Satish
pl. read any new information about MRSA
Satish
Any stru tural finding about novel corona virus
Satish

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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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