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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 gram staining
Nankya Reply
microbiology?
Bekka
It's technique to differentiate bacterial species.The method is discovered by Hans Christian Gram. The bacteria is differentiated as gram positive and gram negative on the basis of peptidoglycan layer which is present in cell wall. The gram positive have peptidoglycan layer.
shamali
What is study of morphology ? and it's classification
Maryam
taxonomy
Kamaluddeen
what makes a student to forget what he or she has been taught with easily and how can he or she cope up with it
Samson Reply
to be in touch with subject will help to cope up with the learning life long and also interest matters for subject
shriya
Constant review. And don't worry if you do forget something you always have the option to study it again. Each time you study a certain topic the more it make sense and the more it becomes something natural rather than brute memorization.
Abdi
the cause are many 1.lack of interest . 2. poor attitude to the subject. how can he cope up 1. falling in love with the subject . 2. making it part of you at all times and having determination
Ikilai
understanding the concept and relating whatever course of study with something in his/her day to day activities (i.e in which you are more conversant with in your life).
Kamaluddeen
how many types of cross matching method
umesh
which disease is caused by vicera zoster virus
Timaka Reply
chicken pox
Osuoha
Patients with disseminated herpes zostermay present with severe abdominal pain that results from visceral involvement of varicella-zoster-virus infection. In immunocompetent individuals herpes zoster usually is a localised illness, affecting the skin of one or two adjacent dermatomes.
Kamaluddeen
Actually, it is severe abdominal pain thanks
Osuoha
what causes inflammation of the lung?
Samson
what causes the fungi infection of the vigina
Estar Reply
what causes the fungi infection of the vagina?
Estar
poor hygiene of the area and unprotected sex with infected partner
Kamaluddeen
poor hygiene,some times having sex with infected person or when normal flora enters in the vagina
Ikilai
unprotected sex and poor hygiene are the major but keep in mind that there are some other minor causes also
Samson
what brings about dirt in blood
Sumaiyah Reply
So many factors might be the cause viz. Defects of blood capillaries and certain organs like the liver, the kidneys e.t.c
MOHAMMED
with the above any infection of the body easily enters the bloodstream.
MOHAMMED
there may be fevers which cause septicemia
Ikilai
even taking drug, and eating junk foods .
Ikilai
drinking contaminated water and taking drugs which contains chemical and afterwards they gets absorbed in yo the blood stream
Samson
write a essay about gene library's
anaparthi Reply
Structure of bacteria
Maggy Reply
Outline the classification of viruses according to morphology, nucleic acid, type of diseases, antigenic reaction
chilufya Reply
who is da father of microbiology
Mukomya Reply
Lucie pasture
Felicity
Father of microbiology is Anton von Leuuwenhoek
Anshika
mean Luice pasture the father of microbiology
Felicity
Luice pasture is the father of industrial microbiology
Anshika
who is the father of surgery
Talemwa
father of surgery is Sushruta
Agyekum
sushruta is the father of surgery
Ven.phumie
he Anthony van Lee wen hock
Ikilai
who is the father of medical microbiology
Naveen
he is Anthony van Lee wen hock
Ikilai
Anthony van Lee wen hook
Kamaluddeen
where would we be without microbes
Jessa Reply
nowhere because our body alone also contain billions of microbes.
Edward
in the dreams
Abid
Bangladeshi people's body contain thrice the amount of microbes than regular people.
IamTheMegaExploder
sure?
Kamaluddeen
yah
Kamran
why it was like that
Mمهazn
no where
Mukomya
cocci which are arranged in pairs are called?
umesh
diplococci
vijaya
Cocci are bacterias
Anshika
fimbriae is an organ of ?
umesh
what is the choma like vrius?
PRALHAD
Vibrio cholarea
Kamaluddeen
outline the classification of viruses
chilufya
Fimbriae z an organ of bacteria
Maggy
Antoni van Leeuwenhoek, the father of micro biology
Yang
concept of primary& secondary metabolism
yamini Reply
who is the father of microbiology
imran Reply
Louis pasture is a father of modern microbiology
Bharat
anton van leuwen kook
Kavi
anton van is father of ancient microbiolgy
Bharat
Antonie van Leeuwenhoek
Bharat
g
Red
Louis pasture
Atar
what can cause serious stomachache after treating ulcer and typhoid and symptoms persist to leave.
Ven.phumie
Anthony van
Nankya
what is lumbar puncture
amulya Reply
To get the cerespinalfluid
Alphonsia
what is the role of glutamic acid in consideration to ulcer healin
Hyrin Reply
what is role of glutamic acid in peptidoglycan
Pooja Reply
what's the name of Greek word were nursing came from?
Joyce
Take care.
Alphonsia
thanks
Joyce
glutamic acid helps in synthesis of protein
Ganesh
i want some explantion on gram staining
Nankya
gram (+ve) or gram(-ve)
Atar
gram stain is defined as identification methods for bacteria
vijaya
Gram stain or Gram staining, also called Gram's method, is a method of staining used to distinguish and classify bacterial species into two large groups (gram-positive and gram-negative). The name comes from the Danish bacteriologist Hans Christian Gram, who developed the technique.
Atar

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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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