Name: Date: Critical Reading Form for Meselson & Stahl – 10 points Type in answers – Save & Upload to Canvas Purpose:In 1-2 sentences, what was the research question the authors were trying to answer and why? Central Thesis:Using only 1-2 sentences, what was the authors’ thesis, or hypothesis? Key Claims:What are two statements that are used to support the thesis? Conclusions or discussion. 1. 2. Evidence:What evidence do the author’s use to support each of the key claims you identified? These are results of experiments. 1. 2. Figure Analysis (Figure 4 – Meselson & Stahl) What is shown in part a? What is shown in part b? What do the numbers under the generation heading mean? Why is the pattern different between generation 0 and generation 2.5? Why is the pattern different between generation 0 and generation 4.1? Top 10 Human Resources Trends of the Decade Thomas Barwick/ Stone/ Getty Images Human Resources By Susan M. Heathfield Updated August 10, 2016 The top ten trends of the decade for Human Resources staff and the employees served at work were not obvious nor were they easy to pick from my original list. Depending on your company size, your location, and the health and progress of your company and industry, the top ten Human Resources trends may have differed for you. Although the selection was a challenge, and I still go back and forth, these are my top ten Human Resources trends of the decade. These Human Resources trends are presented in no particular order aside from the first trend, which has swamped HR the past couple of years. It's the Economy With US unemployment at 10.2%, as I write this, and extended unemployment benefits and COBRA subsidies keeping many families afloat, this economic downturn has left no one unaffected. Even people still employed have watched as their 401(k)s and savings sunk to new lows. Almost no employees have received a raise without a promotion this past year. Normal bonuses and profit sharing have been replaced with mandatory furloughs and more work to replace that of laid-off coworkers. Mourning the loss of laid-off coworkers with feelings of guilt, anxiety, and fear has also chipped away at the employee’s comfort level at work. Looking over their shoulder and protecting their own job has become commonplace. No one can predict how bad the economy could become or how long the downturn will last. So, business leaders don’t know whether they are managing from an economic perspective that the economy has been reset forever or a down economy that will recover. Business leaders are struggling to manage in times they have never before experienced – and the employees, who may also be experiencing stressful economic trauma outside of work, are watching and concerned. Millennials Are on the March A generation of employees who were pampered and scheduled by their Baby Boomer parents have taken the workplace by storm. They bring pluses and minuses to your workplace, but come on, who e ...

1. Name: Date:
Critical Reading Form for Meselson & Stahl – 10 points
Type in answers – Save & Upload to Canvas
Purpose:In 1-2 sentences, what was the research question the
authors were trying to answer and why?
Central Thesis:Using only 1-2 sentences, what was the authors’
thesis, or hypothesis?
Key Claims:What are two statements that are used to support
the thesis? Conclusions or discussion.
1.
2.
Evidence:What evidence do the author’s use to support each of
the key claims you identified?
These are results of experiments.
1.
2.
Figure Analysis (Figure 4 – Meselson & Stahl)
What is shown in part a?
What is shown in part b?

2. What do the numbers under the generation heading mean?
Why is the pattern different between generation 0
and generation 2.5?
Why is the pattern different between generation 0
and generation 4.1?
Top 10 Human Resources Trends of the Decade
Thomas Barwick/ Stone/ Getty Images Human Resources
By Susan M. Heathfield
Updated August 10, 2016
The top ten trends of the decade for Human Resources staff and
the employees served at work were not obvious nor were they
easy to pick from my original list. Depending on your company
size, your location, and the health and progress of your
company and industry, the top ten Human Resources trends may
have differed for you.
Although the selection was a challenge, and I still go back and
forth, these are my top ten Human Resources trends of the
decade.
These Human Resources trends are presented in no particular
order aside from the first trend, which has swamped HR the past
couple of years.
It's the Economy
With US unemployment at 10.2%, as I write this, and extended
unemployment benefits and COBRA subsidies keeping many
families afloat, this economic downturn has left no one
unaffected. Even people still employed have watched as their
401(k)s and savings sunk to new lows.

3. Almost no employees have received a raise without a promotion
this past year. Normal bonuses and profit sharing have been
replaced with mandatory furloughs and more work to replace
that of laid-off coworkers.
Mourning the loss of laid-off coworkers with feelings of guilt,
anxiety, and fear has also chipped away at the employee’s
comfort level at work. Looking over their shoulder and
protecting their own job has become commonplace. No one can
predict how bad the economy could become or how long the
downturn will last.
So, business leaders don’t know whether they are managing
from an economic perspective that the economy has been reset
forever or a down economy that will recover. Business leaders
are struggling to manage in times they have never before
experienced – and the employees, who may also be experiencing
stressful economic trauma outside of work, are watching and
concerned.
Millennials Are on the March
A generation of employees who were pampered and scheduled
by their Baby Boomer parents have taken the workplace by
storm. They bring pluses and minuses to your workplace, but
come on, who ever heard of a play date before 1990?
So, not only is your workplace trying to absorb these offspring
of the Baby Boomer generation - and millennials bring special
challenges – employers are dealing with helping three
generations of workers happily co-exist to serve customers as a
team.
The economic downturn has made the three generation situation
worst with Boomers who planned retirement, to make way for
up and coming employees, unable to retire - and not happy
about it.
Millennials and Gen X employees are supervising Boomers and
Boomers are mentoring those who wish to learn from the
leaving generation.
For the employer, managing millennials is a skill managers need

4. to develop. The millennial quest for work-life balance and for
having a life outside of work is legend. Employers
accommodate these talented young people and develop their
strengths and ability to contribute, or you'll lose them to an
employer who will.
Many of them have options. They bear no resemblance to the
"company man,” touted as the ideal employee in earlier years.
And, the workplace is changing to accommodate them.
Employee Recruiting and Networking Online
This decade has brought about the transformation of employee
recruiting and social and media interaction and networking.
When I first started writing about recruiting, the big job boards
like Monster, had not been around very long. Employers have
seen a transformation in how people find each other for
networking and jobs this decade.
From large job boards like Indeed to niche job sites, from
networking on discussion lists to sites such as LinkedIn,
Facebook, Twitter, and Ecademy, networking and recruiting will
never be the same again. Human Resources employees have
either kept up with the new ways of interacting and
communicating or they are doing their organizations a
disservice.
Social media networking is the new way to find employees, find
jobs, get answers to questions, build a wide-spread, mutually
supportive network of contacts, and keep track of colleagues
and friends. Social media and online recruiting bring the
employer new challenges.
Developing social media and blogging policies, deciding
whether to monitor employee time online, and checking
candidate backgrounds online, just scratch the surface of new
employer challenges. But, don’t let the power of this online
media pass you by.
Made to Order Employment Relationships
Perhaps it’s the push from the millennials, and definitely, it’s

5. the availability of technology that facilitates the customization,
but the made to order work relationship has become a dominant
force in the past decade. Teleworking or telecommuting, a rare
privilege in the 1990s, has taken workplaces by storm.
One giant computer company reports that over 55% of its
employees not only telecommute, they work from home all of
the time. A New York City publishing company allows
telecommuting two days a week and employees can bargain for
more.
Teleworking is not the only component of the new made to
order work arrangements. Flexible anything has become the new
norm. Flexible work hours, flexible four day work weeks,
flexible time off for appointments, and the most important trend
of all: Paid time off (PTO) allows employees to take time off
when they need the time as it consolidates sick leave, personal
time, and vacation time into a bank of days for employees to
use.
Additionally, trends such as bringing a baby or the family pet to
the office also fall within this workplace flexibility.
Superficially, all of these components of the made to order
Human Resources employment trend offer benefits for
employees. But, they offer benefits for employers, too.
Employers don’t need to police employee time.
They need to make work and communication more transparent
and measurable so the flexibility yields results. Their employees
are more motivated and engaged, and less stressed out about
family and life issues, because they have the time necessary to
address work-life balance issues.
The Big Blur
Online, all the time, and availability via technology, has blurred
the line between work and home. Employees work at home in
the evening on collaborative reports and email. They shop at
work and take brief breaks by playing online games.
Employees do their banking at work and their work accounting
at home. Almost no one goes on vacation without their smart

6. phone, laptop, and kindle-like device.
Employees taking PTO email colleagues with the number of
their cell phone if they won’t have access to email.
No generation has ever been this connected, and for good and
bad, some employees never stop working. This interferes with
down time, relaxing time, and work-life balance, but most
employees just see it as a way of life. Employers need to make
sure this degree of connectivity is not required. They must also
back away from old rules about what an employee was allowed
to do at work.
Employers do need to heed wage and hour laws when dealing
with hourly employees who must be paid for every hour they
work. Indeed, this work – home blurring is a nightmare for
employers who must pay for overtime. So, most employers
forbid hourly employees, for the most part, to work at home.
This emphasizes the differences between exempt and nonexempt
employees, already a distance.
The Rise of Technology
No Human Resources trend list would be complete without an
explicit mention of the impact of technology on all aspects of
the field.
I have mentioned the power of technology all through these
trends, but will still cite technology as a major trend.
Technology has transformed the way in which Human Resources
offices manage and communicate employee information and
communicate with employees, in general.
In a world in which identity theft is prevalent and can cost an
employee countless working hours over several years to correct,
safeguarding employee records is critical.
Identity theft is so serious and rising that every employer needs
a plan to prevent.
Did words like Intranets, wikis, webinars, and blogs exist in
common language ten years ago? I don’t think so; only the early
and earliest adopters used them. Now, employees use them
internally to stockpile information, work collaboratively, and

7. share opinions and project progress.
They can even work virtually and with distant teams
simultaneously. They hold meetings and share visuals with
teams from all over the world.
Employee Training and Development Transformed
This decade has seen the rise of technology-enabled
opportunities for training, employee development, and training
meetings and seminars. Podcasts, teleseminars, online learning,
screen capture and recording software, and webinars provide
employee development opportunities.
Additionally, during this decade, as the technology-enabled
delivery options expanded, so did other training and
development opportunities and definitions, including increased
expectation for learning transfer to the job.
Online learning, earning an online degree or credits, and all
forms of web-enabled education and training provide options
that employees never had when training occurred in a
classroom. Employers are saving millions of dollars in
employee travel expenses, and the employee’s access to the
training does not walk out the door at the seminar’s end.
This is the decade when employers experiment with classroom
training in a virtual world called Second Life. You can expect
even more progress and experimentation in the years to come.
Plus, another Human Resources trend that flourished, though
did not start in this decade, is the concept of alternative forms
of employee learning such as coaching and formal mentoring.
They hit the mainstream in this past decade.
Tension Increases Over Government Intervention in the
Employer-Employee Relationship
A debate exists in the US between people who think the
government is already intervening too much by making laws
that require employers to provide particular benefits for
employees, and those who don’t. People who support the
government intervention believe that the US government has

8. been negligent in not mandating benefits such as paid sick
leave. They consider it the “right” or humanitarian action to
take.
Opponents want benefits for employees but argue that
employers should make benefits choices that their employees
desire and that they can afford. Opponents argue that employer
mandated benefits will cost the country jobs and opportunity.
Small business, the engine of job creation in the US, is
currently sitting on the sidelines because of the uncertain
economy including threatened government mandates and
potential changes to health care.
One of the more significant examples of government
intervention occurred with the passage of the Family and
Medical Leave Act (FMLA) in 1993. The effects of its passage
continued to be an employer’s nightmare this past decade as,
especially, its intermittent leave requirements created
recordkeeping quagmires and made trial lawyers smile. I expect
I will be including this Human Resources trend after the next
decade, too.
The Rising Cost of Health Care
Much as I’d prefer to leave this Human Resources trend off the
table, it’s not going away. The continually rising cost of health
care insurance and health care is affecting what employers can
provide in terms of additional benefits for their employees.
The rise of employee payments for part of insurance coverage,
the practice of seeking insurance first from a spouse’s
employer, increased payments for covered family members, and
higher health care provider co-pay office fees are all highlights
of the rising cost of care.
Americans disagree about what needs to be done in this arena.
(I support capping fees to trial lawyers and limiting payouts in
medical malpractice suits, providing incentives to people
becoming family practice physicians, and making basic
insurance more affordable.)
But, most agree that something does need to happen so that

9. Americans can retain the best healthcare system in the world.
Legislation is currently pending, which as I write this piece, is
not supported by over 56% of Americans, so we shall see.
Health care costs will remain a Human Resources trend in the
next decade.
.
Globalization, Outsourcing, and Offshoring
Increasing government regulation in the US along with
increasing corporate taxation (maybe highest in the world),
higher wages, and less desirable, business friendly policies and
incentives are causing employers to rethink locations for their
operations.
High taxation, high regulation states are seeing an outpouring of
business (and jobs) from their locations.
The US is seeing an overall rise in outsourcing jobs to overseas
locations that are viewed as more friendly to business.
In an era of globalization, this makes sense. Employers seek
global, rather than local markets, so that economic factors in
one location do not hinder progress. Employers see the positive
impacts of locating offices and factories in global markets and
tapping the strengths of local employees who are familiar with
business and employment practices in the new locations.
Whether work is off-shored, outsourced, or the company is
simply expanding globally, the challenges to Human Resources
with a globally-located workforce are serious. If a US company
has five employees in Hong Kong or six in Europe, local Human
Resources offices do not make sense.
In fact, the US HR Director, with assistance from local
employment agencies, probably hired the staff. Managing and
working with these global locations, while obeying the laws and
honoring the customs of the host country, is a challenge for
managers, Human Resources, and coworkers.
I remember hiring my first employee in Hong Kong. I learned
the monetary system, the required holidays, the government
regulations, and more. I also found that, until I had local,

10. trusted employment staffing assistance, the new employee and
subsequent employees took advantage of my limited knowledge.
It's a whole new world of global challenges out there. Get
ready.
Human Resources Trends of the Decade: Honorable Mentions
I considered these Human Resources trends and they are worthy
and deserve a mention. Several of them will see their biggest
impact in the next ten years.
This includes diversity which is already affecting workplaces
and legislation. Discrimination laws have had a profound impact
on recruiting and hiring practices and in all areas of equal
opportunity employment.
The labor union movement in the US is in the process of
undergoing a radical change. Recently, public sector employees
became the majority of union members leaving private sector
union member numbers behind.
Additionally, the Service Employees International Union
(SEIU) has stated publicly that their members include illegal
immigrants. This will produce changes in the next decade about
Political Action Committees (PACs), raise questions about who
is funding union activities, and also impact the illegal
immigration debates in Congress and for employers.
In the wake of the horrific events of 9-11-2001, much of which
most employees watched unfold on their televisions at work, a
feeling of a loss of safety swept the nation. When tragedy struck
the workplace, employers responded with new building
evacuation plans, safety and crisis management plans, and
business continuation strategies.
People who lived closer to the events and who lost family
members and friends were most profoundly affected. But, the
events of 9-11-2001 will never be forgotten in America.
Hopefully, this will never be a trend, but several readers
nominated this event.
The evolution of performance management as an employee
development, goal setting, and performance evaluation strategy

11. is an important Human Resources trend in my book. It allows an
employer to develop an employee from onboarding until they
leave your company.
It moves evaluation and goal setting away from an annual
appraisal administered by the employee’s manager to a mutually
beneficial defined contribution and development plan.
We’ll see considerably more from each of these trends in the
next decade. Hold on to your seat. The next wave of Human
Resources trends for the next decade will soon leave the station.
Are you ready to enhance and take advantage of them in your
workplace?
Source: Heathfield, S.M. (2016). Top 10 human resources
trends of the decade. Retrieved from
http://humanresources.about.com/od/businessmanagement/a/top
_ten_trends.htm. Provided by Saylor.org. License: CC BY.
671
Appearances of regeneration from naked masses of protoplasm
have sometimes been noted, but never in examples in which it
had been established that there were no small, fully formed cells
from which the growth might have arisen.
It is, of course, possible that cultural conditions will be found
under which masses of naked protoplasm will revert to cellular
growth, such as has been reported by Ne&as4 in Saccharomyces
cerevisiae and by Pease"' in Proteus vulgaris.
1 C. Weibull, Symposium Soc. Gen. Microbiol., 6, 111, 1956.
2J. Lederberg, these PROCEEDINGS, 42, 574, 1956; K.
McQuillen, Biochim. et Biophys. Acta, 27,
410,1958.
3 K. McQuillen, Symposium Soc. Gen. Microbiol, 6, 127, 1956;
S. Spiegelman, in The Chemical Basis of Heredity, ed. W. D.
McElroy and B. Glass (Baltimore: Johns Hopkins Press, 1957),
pp. 232-267; J. Spizizen, these PROCEEDINGS, 43, 694, 1957;
D. Fraser, H. R. Mahler, A. L. Shug, and C. A. Thomas, Jr.,

12. these PROCEEDINGS, 43, 939, 1957; E. Chargaff, H. M.
Schulman, and H. S. Shapiro, Nature, 180, 151, 1957.
40. Necas, Biol. Zentr., 75, 268, 1956; A. A. Eddy and D. H.
Williamson, Nature, 179, 1252, 1957.
5 Mary R. Emerson, in manuscript. Our mutant strain, Em
11200, has been shown to be allelic to a number of others of
independent origin: B 135 and M 16 obtained from Dr. D. D.
Perkins, Stanford University; "wooly," from Dr. H. L. K.
Whitehouse, Cambridge University; and "ginger" and an
unnamed mutant from Dr. J. R. S. Fincham, University of
Leicester. From published descriptions it is likely that these are
also allelic to the "cut" mutant of Dr. H. Kuwana
(Cytologia, 18,235, 1953).
6 Mary B. Mitchell and H. K. Mitchell, these PROCEEDINGS,
38, 442, 1952.
Mary B. Mitchell, H. K. Mitchell, and A. Tissieres, these
PROCEEDINGS, 39, 606, 1953. 8 G. W. Beadle and E. L.
Tatum, these PROCEEDINGS, 27, 499, 1941. 9 The
hemicellulase preparation was obtained from the Nutritional
Biochemical Corporation, Cleveland 28, Ohio, who inform us
that it is a concentrate of microbiological origin, containing
cellulase, gumase, and maltase in addition to hemicellulase. A
single test in our laboratory indi cated strong chitinase activity.
1OJ. Lederberg and Jacqueline St. Clair, J. Bacteriol., 75, 143,
1958. 11 Phyllis Pease, J. Gen. Microbiol., 17, 64, 1957.
THE REPLICATION OF DNA IN ESCHERICHIA COLI* BY
MATTHEW MESELSON AND FRANKLIN W. STAHL
GATES AND CRELLIN LABORATORIES OF CHEMISTRY, t
AND NORMAN W. CHURCH LABORATORY OF CHEMICAL
BIOLOGY, CALIFORNIA INSTITUTE OF TECHNOLOGY,
PASADENA, CALIFORNIA
Communicated by Max Delbrick, May 14, 1958
Introduction.-Studies of bacterial transformation and
bacteriaphage infection'-'
strongly indicate that deoxyribonucleic acid (DNA) can carry
and transmit heredi tary information and can direct its own

13. replication.
Hypotheses for the mechanism of DNA replication differ in the
predictions they make concerning the distribution among
progeny molecules of atoms derived from parental molecules.6
Radioisotopic labels have been employed in experiments
bearing on the distribu tion of parental atoms among progeny
molecules in several organisms.6-9 We anticipated that a label
which imparts to the DNA molecule an increased density might
permit an analysis of this distribution by sedimentation
techniques.
To this end, a method was developed for the detection of small
density differences among
6;72
bacterial Distance fuged stages FIG. at 1.-Ultraviolet in lysate
31,410 from the banding containing the rpm axis in absorption a
of of approximately CsCl DNA rotation solution photographs
from increases E. as 108 described coli. lysed showing toward
cells An in was aliquot the successive the centri- right. text. of
after The number reaching beside 31,410 rpm. each photograph
gives the time elapsed
macromolecules.'0 By use of this method, we have observed the
distribution of the heavy nitrogen isotope N15 among molecules
of DNA following the transfer of
a uniformly N"5-labeled, exponentially growing bacterial
population to a growth medium containing the ordinary nitrogen
isotope N'4.
Density-Gradient Centrifugation.- A small amount of DNA in a
concentrated solution of cesium chloride is centrifuged until
equilibrium is closely approached.
BIOLOGY: MESELSON AND STAHL
673
The opposing processes of sedimentation and diffusion have
then produced a stable concentration gradient of the cesium
chloride. The concentration and pressure gradients result in a
continuous increase of density along the direction

14. of centrifugal force.
The macromolecules of DNA present in this density gradient are
driven by the centrifugal field into the region where the solution
density is equal to their own buoyant density.'
This concentrating tendency is opposed by diffusion, with the
result that at equilibrium a single species of DNA is distributed
over a band whose width is inversely related to the molecular
weight of that species
(Fig. 1).
If several different density species of DNA are present, each
will form a band at the position where the density of the CsCl
solution is equal to the buoyant density In this way DNA
labeled with heavy nitrogen (N15) may be of that species.
a
N'14 N5
b
A two at CsCl mixture FIG. 44,770 bands solution 2-a: rpm. of
of N'4 Fig. as The b: gin. described and 2a. A resolution cm.
N"1 microdensitometer The bacterial -- in separation the of text.
N14 lysates, DNA between The tracing each photograph from
containing the showing N"1 peaks DNA was the about
corresponds taken DNA by 10' density-gradtient after
distribution lysed to 24 cells, a hounrs difference was in of the
centrifugation. centrifugation centrifuged region in buoyant of
the in density of 0.014
Figure 2 shows the two bands formed as a result of centrifuging
a mixture of approximately equal amounts of N14 and N1I-
Escherichia coli DNA.
resolved from unlabeled DNA.
In this paper reference will be made to the apparent molecular
weight of DNA
samples determined by means of density-gradient
centrifugation. A discussion has been given"' of the
considerations upon which such determinations are based,
as well as of several possible sources of error.'2

15. Experimental.-Escherichia coli B was grown at 360 C. with
aeration in a glucose salts medium containing ammonium
chloride as the sole nitrogen source." The growth of the
bacteria) population was followed by microscopic cell counts
and by colony assays (Fig. 3).
Bacteria uniformly labeled with N"5 were prepared by growing
washed cells for
S.674 BIOLOGY: MESELSON AND STAHL PROC. N. A.
14 generations (to a titer of 2 X 108/ml) in medium containing
100 ,ug/ml of N'5H4Cl An abrupt change to N14 medium was
then ac of 96.5 per cent isotopic purity. complished by adding
to the growing culture a tenfold excess of N14H4Cl, along with
ribosides of adenine and uracil in experiment 1 and ribosides of
adenine. guanine, uracil, and cytosine in experiment 2, to give a
concentration of 10lg/ml During subsequent growth the
bacterial titer was kept between of each riboside.
-14 -12 -10 -8 -6 -4 -2 0 TIME (hours)
2 4 6 8
medium. withdrawals tion, Thereafter, centrifugation, on FIG.
the the ordinates 3.-Growth generation The during and values
the give additions. of the actual time bacterial the on period the
was actual titer During ordinates 0.81 populations when was
titers hours the kept samples of during period in the first
between Experiment cultures were this in of N sampling later 15
being 1 and and up period 1 then and withdrawn to 2 for the X
in 0.85 density-gradient have N14 108 time hours been medium.
by for of additions in corrected density-gradient addition
Experiment The centrifuga- of of for values fresh N'4. the 2.
1 and 2 X 108/ml by appropriate additions of fresh N 14
medium containing ribosides. Samples containing about 4 X 109
bacteria were withdrawn from the culture just before the
addition of N14 and afterward at intervals for several
generations. Each sample was immediately chilled and
centrifuged in the cold for 5 minutes at After resuspension in
0.40 ml. of a cold solution 0.01 M in NaCl and 1,800 X g.

16. 0.01 M in ethylenediaminetetra-acetate (EDTA) at pH 6, the
cells were lysed by the
addition of 0.10 ml. of 15 per cent sodium dodecyl sulfate and
stored in the cold.
2
2
gradient of cess on CsCl FIG. centrifugation each of solution
Ni4 4-a: centrifugation substrates increases Ultraviolet at
44,770 to to of a growing the lysates absorption rpm under of
Ni"labeled bacteria photographs the conditions sampled culture.
showing at described Each various DNA photograph times in
bands the after text. was resulting the taken The addition
density from after of 20 density- position Ni4 of an hours the
ex- in units the the and DNA species measurements base
unlabeled of bands of the photograph. line DNA generation
shown is DNA directly corresponds of in shown bacterial the
The time. proportional adjacent in time to the right. The growth
the of lowermost photographs. relative generation sampling
Regions to presented the position frame, concentration is of
times measured equal in The which Fig. of for density its
microdensitometer Experiments 3. density serves band When
from of 6 DNA. occupy between Microdensitometer the as
allowance is just a time density 1 the The half-labeled and the
of same pen is degree 2 the bands made reference. were
displacement horizontal addition of estimated of for tracings is
labeling provided fully the A of relative labeled test of above
from of the by of a the conclusion that the DNA in the band of
intermediate the amounts at50 ±[ frame 2 per of showing cent
DNA of in the the the mixture distance three peaks, of between
generations the the peak N" 0 of and and intermediate 1.9. NI,'
peaks. density is found to be centered
BIOLOGY: MESELSON AND STAHL
PROC. NT, A. S.

17. For density-gradient centrifugation, 0.010 mnl. of the dodecyl
sulfate lysate was added to 0.70 ml. of CsCl solution buffered at
pH 8.5 with 0.01 M tris(hydroxy methyl)aminomethane. The
density of the resulting solution was 1.71 gm. cm.-3 This was
centrifuged at 140,000X g. (44,770 rpm) in a Spinco model E
ultracentri fuge at 250 for 20 hours, at which time the DNA had
essentially attained sedimenta tion equilibrium. Bands of DNA
were then found in the region of density 1.71 gm. cm.-3, well
isolated from all other macromolecular components of the
bacterial lysate. Ultraviolet absorption photographs taken
during the course of each cen trifugation were scanned with a
recording microdensitometer (Fig. 4). The buoyant density of a
DNA molecule may be expected to vary directly with The
density gradient is constant in the the fraction of N15 label it
contains. region between fully labeled and unlabeled DNA
bands. Therefore, the degree of labeling of a partially labeled
species of DNA may be determined directly from the relative
position of its band between the band of fully labeled DNA and
the The error in this procedure for the determination of band of
unlabeled DNA. the degree of labeling is estimated to be about
2 per cent. Results.-Figure 4 shows the results of density-
gradient centrifugation of lysates of bacteria sampled at various
times after the addition of an excess of N'4-containing
substrates to a growing N1"-labeled culture. It may be seen in
Figure 4 that, until one generation time has elapsed, half labeled
molecules accumulate, while fully labeled DNA is depleted.
One generation time after the addition of N 14, these half-
labeled or "hybrid" molecules alone are observed. Subsequently,
only half-labeled DNA and completely unlabeled DNA
When two generation times have elapsed after the addition of
N'4, half-labeled and unlabeled DNA are present in equal
amounts.
are found.
Discussion.-These results permit the following conclusions to
be drawn regard ing DNA replication under the conditions of
the present experiment.

18. 1. The nitrogen of a DNA molecule is divided equally between
two subunits which remain intact through many generations.
The observation that parental nitrogen is found only in half-
labeled molecules at all times after the passage of one
generation time demonstrates the existence in each DNA
molecule of two subunits containing equal amounts of nitrogen.
The finding that at the second generation half-labeled and
unlabeled molecules are found in equal amounts shows that the
number of surviving parental subunits is twice the number of
parent molecules initially present. That is, the subunits are
conserved. 2. Followinq replication, each daughter molecule has
received one parental subunit. The finding that all DNA
molecules are half-labeled one generation time after the
addition of N'4 shows that each daughter molecule receives one
parental sub unit.'4 If the parental subunits had segregated in
any other way among the daughter molecules, there would have
been found at the first generation some fully labeled and some
unlabeled DNA molecules, representing those daughters which
received two or no parental subunits, respectively. 3. The
replicative act results in a molecular doubling. This statement is
a corollary of conclusions 1 and 2 above, according to which
each parent molecule passes on two subunits to progeny
molecules and each progeny
BIOLOGY: MESELSON AND STAHL
molecule receives just one parental subunit.
677
It follows that each single molecular reproductive act results in
a doubling of the number of molecules entering into that act.
The above conclusions are represented schematically in Figure
5. The Watson-Crick Model.-A molecular structure for DNA has
been proposed by Watson and Crick.15 It has undergone
preliminary refinement'6 without alteration of its main features
and is supported by physical and chemical studies."7 The
structure consists of two polynucleotide chains wound helically
about a common axis. The nitrogen base (adenine, guanine,
thymine, or cytosine) at each level

19. ORIGINAL 7
PARENT MOLECULE
tions. equally receives the FIG. data 5.-Schematic between one
presented of these. two in subunits. representation Fig. The 4.
subunits The Following nitrogen of are the conserved
duplication, conclusions of each through DNA each drawn
molecule successive daughter in the is text molecule duplica-
divided from
on one chain is hydrogen-bonded to the base at the same level
on the other chain. Structural requirements allow the occurrence
of only the hydrogen-bonded base pairs adenine-thymine and
guanine-cytosine, resulting in a detailed complemen tariness
between the two chains.
This suggested to Watson and Crick"8 a definite and
structurally plausible hypothesis for the duplication of the DNA
molecule. According to this idea, the two chains separate,
exposing the hydrogen-bonding sites of the bases.
Then, in accord with the base-pairing restrictions, each chain
serves as a template for the synthesis of its complement.
Accordingly, each daughter molecule contains one of the
parental chains paired with a newly synthesized chain (Fig. 6).
BIOLOGY: MESELSON AND STAHL
The results of the present experiment are in exact accord with
the expectations of the Watson-Crick model for DNA
duplication. However, it must be emphasized that it has not
been shown that the molecular subunits found in the present ex
periment are single polynucleotide chains or even that the DNA
molecules studied here correspond to single DNA molecules
possessing the structure proposed by Watson and Crick.
However, some information has been obtained about the
molecules and their subunits; it is summarized below.
parent proposed two chain tains FIG. molecules one (white).

20. chains 6.-Illustration by of Watson the remain each Upon
parental with and intact, continued of one Crick. the chains
parental so mechanism that Each duplication, (black) there
chain. daughter paired will of DNA the always with molecule
two duplication be one original found con- new
The DNA molecules derived from E. coli by detergent-induced
lysis have a buoyant density in CsCl of 1.71 gm. cm.-3, in the
region of densities found for T2 and T4 bacteriophage DNA,
and for purified calf-thymus and salmon-sperm DNA. A highly
viscous and elastic solution of N14 DNA was prepared from a
dodecyl sulfate lysate of E. coli by the method of Simmons19
followed by deproteinization with chloroform. Further
purification was accomplished by two cycles of preparative
density-gradient centrifugation in CsCl solution. This purified
bacterial DNA was found to have the same buoyant density and
apparent molecular weight, 7 X 106, as the DNA of the whole
bacterial lysates (Figs. 7, 8).
BIOLOGY: MESELSON AND STAHL
Heat Denaturation.-It has been found that DNA from E. coli
differs importantly from purified salmon-sperm DNA in its
behavior upon heat denaturation. Exposure to elevated
temperatures is known to bring about an abrupt collapse of the
relatively rigid and extended native DNA molecule and to make
available for acid-base titration a large fraction of the
functional groups presumed to be blocked by hydrogen-bond
formation in the native structure. 19, 20, 21, 22
Rice and Doty22 have reported that this collapse is not
accompanied by a reduction in molecu
These findings are corroborated by lar weight as determined
from light-scattering.
density-gradient centrifugation of salmon-sperm DNA.23 When
this material is
relative of FIG. Fig. 8.-The 7 concentration plotted square
against of of the DNA. the width logarithm The of the divisions

21. of band the persity the band. the to along any DNA the absence
point the located weight of Linearity abscissa of the of such at
density banded average the of set a corresponding plot this off
heterogeneity, DNA. molecular intervals plot is directly
indicates The of position weight value proportional 1 the mm.2.
monodis- slope of of in the the the at In
FIG. 7.-Microdensitometer tracing of an ultraviolet optical
density absorption in the region photograph of a band showing
of N14 the E. coli DNA at equilibrium. About 2 fg. of DNA
purified at 31,410 as rpm described at 250 in in 7.75 the molal
text was CsCl centrifuged atpH 8.4. The over the density region
gradient of the maximum band is and essentially is indicates
0.057 gm./cm.4. constant a the The ant optical density position
density of of 1.71 the the above concentration gm. cm.-' the
base In of line this DNA is directly tracing buoy- in the
proportional rotating DNA at the centrifuge maximum to cell. is
about The 50 concentration ,g./ml. of the weight responding
sodium slope for corresponds the to salt. a Cs-DNA molecular
to an salt weight apparent of 9.4 of 7.1 X molecular X 10., 10.
cor- for
kept at 1000 for 30 minutes either under the conditions
employed by Rice and Doty or in the CsCl centrifuging medium,
there results a density increase of 0.014 gm. cm.r3 with no
change in apparent molecular weight. The same results are ob
tained if the salmon-sperm DNA is pre-treated at pH 6 with
EDTA and sodium
Along with the density increase, heating brings about a sharp
dodecyl sulfate.
reduction in the time required for band formation in the CsCl
gradient.
In the absence of an increase in molecular weight, the decrease
in banding time must be ascribed10 to an increase in the
diffusion coefficient, indicating an extensive col lapse of the
native structure.
680
BIOLOGY: MESELSON AND STAHL

22. hours the denaturation. unheated densitometry for density FIG.
position comparison. of 9.-The has centrifugation N1" been
indicated. -3 bacterial of Each and dissociation removed an
Heating a ultraviolet smooth reduction lysates. in Unheated by
CsCi has of subtraction. curve the brought of absorption
solution Heated about lysate subunits connects lysate about half
at was A: photograph 44,770 of points in added A E. alone the a
mixture coli density rpm. apparent obtained to gives DNA this
taken of The one increase experiment heated upon molecular by
after baseline band for micro- heat and one 20 in of weight
generation brid The 0.016 in Fig. density DNA gm. 4. of cm. C:
the contained in difference A N14 DNA. mixture growth in is B:
this of 0.015 medium. Heated heated lysate gm. N14 lysate cm.
forms Before and -' of only heated N16 heat one bacteria
denaturation, N" band, bacterial as grown may lysates. the be
seen hy
The decrease in banding time and a density increase close to
that found upon heating salmon-sperm DNA are observed (Fig.
9, A) when a bacterial lysate containing uniformly labeled N"5
or N14 E. coli DNA is kept at 100° C. for 30 minutes in the
CsCl centrifuging medium; but the apparent molecular weight of
681
the heated bacterial DNA is reduced to approximately half that
of the unheated material. Half-labeled DNA contained in a
detergent lysate of N'5 E. coli cells grown for one generation in
N14 medium was heated at 1000 C. for 30 minutes in the CsCl
centri This treatment results in the loss of the original half-
labeled fuging medium. material and in the appearance in equal
amounts of two new density species, each with approximately
half the initial apparent molecular weight (Fig. 9, B). The
density difference between the two species is 0.015 gm. cm.-,
close to the increment produced by the N'6 labeling of the
unheated DNA. This behavior suggests that heating the hybrid
molecule brings about the dis sociation of the NI5-containing
subunit from the N'4 subunit. This possibility was tested by a

23. density-gradient examination of a mixture of heated N'5 DNA
and heated N14 DNA (Fig. 9, C). The close resemblance
between the products of heating hybrid DNA (Fig. 9 B) and the
mixture of-products obtained from heating N14 and N'5 DNA
separately (Fig. 9, C) leads to the conclusion that the two
molecular subunits have indeed dissociated upon heating. Since
the apparent molecular weight of the subunits so obtained is
found to be close to half that of the intact molecule, it may be
further concluded that the subunits of the DNA molecule which
are conserved at duplication are single, continuous structures.
The scheme for DNA duplication proposed by Delbrfick24 is
thereby ruled out. To recapitulate, both salmon-sperm and E.
coli DNA heated under similar conditions collapse and undergo
a similar density increase, but the salmon DNA retains its initial
molecular weight, while the bacterial DNA dissociates into the
two subunits which are conserved during duplication. These
findings allow two On the one hand, if we assume that salmon
DNA con different interpretations. tains subunits analogous to
those found in E. coli DNA, then we must suppose that the
subunits of salmon DNA are bound together more tightly than
those of the On the other hand, if we assume that the molecules
of salmon DNA bacterial DNA. do not contain these subunits,
then we must concede that the bacterial DNA molecule is a
more complex structure than is the molecule of salmon DNA.
The latter interpretation challenges the sufficiency of the
Watson-Crick DNA model to explain the observed distribution
of parental nitrogen atoms among progeny molecules.
Conclusion.-The structure for DNA proposed by Watson and
Crick brought forth a number of proposals as to how such a
molecule might replicate. proposals6 make specific predictions
concerning the distribution of parental atoms The results
presented here give a detailed answer to among progeny
molecules. the question of this distribution and simultaneously
direct our attention to other problems whose solution must be
the next step in progress toward a complete understanding of
the molecular basis of DNA duplication. What are the molecular

24. structures of the subunits of E. coli DNA which are passed on
intact to each daughter molecule? What is the relationship of
these subunits to each other in a DNA molecule? What is the
mechanism of the synthesis and dissociation of the sub units in
vivo? Summary.-By means of density-gradient centrifugation,
we have observed the distribution of N'5 among molecules of
bacterial DNA following the transfer of a uniformly N'5-
substituted exponentially growing E. coli population to N'4
medium.
These
PROC. N. A. S.
We find that the nitrogen of a DNA molecule is divided equally
between two physi cally continuous subunits; that, following
duplication, each daughter molecule receives one of these; and
that the subunits are conserved through many duplica
tions.
* Aided by grants from the National Foundation for Infantile
Paralysis and the National In stitutes of Health.
t Contribution No. 2344.
R. D. Hotchkiss, in The Nucleic Acids, ed. E. Chargaff and J.
N. Davidson (New York:
· Academic Press, 1955), p. 435; and in Enzymes: Units of
Biological Structure and Function, ed.
· H. Gaebler (New York: Academic Press, 1956), p. 119.
H. Goodgal and R. M. Herriott, in The Chemical Basis of
Heredity, ed. W. D. McElroy and
· Glass (Baltimore: Johns Hopkins Press, 1957), p. 336. 3 S.
Zamenhof, in The Chemical Basis of Heredity, ed. W. D.
McElroy and B. Glass (Baltimore: Johns Hopkins Press, 1957),
p. 351. 4A. D. Hershey and M. Chase, J. Gen. Physiol., 36, 39,
1952. 5 A. M. D. Delbruck Hershey, and Virology, G. S. Stent,
1, 108, in 1955; The Chemical 4, 237, 1957. Basis of Heredity,
ed. W. D. McElroy and
· Glass (Baltimore: Johns Hopkins Press, 1957), p. 699. 7C.
Levinthal, these PROCEEDINGS, 42, 394, 1956.

25. · 2 S.
J. H. Taylor, P. S. Woods, and W. L. Huges, these
PROCEEDINGS, 43, 122, 1957.
9 R. B. Painter, F. Forro,
10 M. S. Meselson, F. W.
Jr., and W. L. Hughes, Nature, 181, 328, 1958.
Stahl, and J. Vinograd, these PROCEEDINGS, 43, 581, 1957.
11 The buoyant density of a molecule is the density of the
solution at the position in the centri fuge cell where the sum of
the forces acting on the molecule is zero. 12 Our attention has
been called by Professor H. K. Schachman to a source of error
in apparent molecular weights determined by density-gradient
centrifugation which was not discussed by In evaluating the
dependence of the free energy of the DNA Meselson, Stahl, and
Vinograd. component upon the concentration of CsCl, the effect
of solvation was neglected. It can be shown that is bound
solvation preferentially. may introduce A method an error for
into estimating the apparent the error molecular due to weight
such selective if either CsCl solvation or water will be
presented elsewhere. 13 In addition to NH4Cl, this medium
consists of 0.049 M Na2HPO4, 0.022 M KH2PO4, 0.05 M NaCl,
0.01 M glucose, 10-3 M MgSO4, and 3 X 10-6 M FeCl3. 14
This result also shows that the generation time is very nearly
the same for all DNA mole This raises the questions of whether
in any one nucleus all DNA mole cules in the population. cules
are controlled by the same clock and, if so, whether this clock
regulates nuclear and cellular division as well.
15 F. H. C. Crick and J. D. Watson, Proc. Roy. Soc. London, A,
223, 80, 1954.
16 R. Langridge, W. E. Seeds, H. R. Wilson, C. W. Hooper, M.
H. F. Wilkins, and L. D. Hamil
ton, J. Biophys. and Biochem. Cytol., 3, 767, 1957.
ed. 18 W. J. D. D. Watson McElroy and and F. B. H. Glass C.
Crick, (Baltimore: Nature, 171, Johns 964, Hopkins 1953. Press,
1957), p. 532. 19 C. E. Hall and M. Litt, J. Biophys. and
Biochem. Cytol., 4, 1, 1958.

26. 17 For reviews see D. 0. Jordan, in The Nucleic Acids, ed. E.
Chargaff and J. D. Davidson (New York: Academic Press,
1955), 1, 447; and F. H. C. Crick, in The Chemical Basis of
Heredity,
20 21 R. P. D. Thomas, Lawley, Biochim. Biochim. et Biophys.
et Biophys. Acta, Acta, 14, 21, 231, 481, 1954. 1956. 22 23 S.
Kindly A. Rice supplied and P. by Doty, Dr. J. Michael Am.
Chem. Litt. Soc., The 79, preparation 3937, 1957. of this DNA
is described by Hall
and 24 M. Litt Delbruck, (J. Biophys. these and
PROCEEDINGS, Biochem. Cytol., 40, 783, 4, 1, 1955. 1958).