Biology (Holt)

Origin of Heredity

Although scientists disagree about the details of the origin of hered-

ity, many agree that double-stranded DNA evolved after RNA and that

RNA “enzymes” catalyzed the assembly of the earliest proteins. Many

scientists also tentatively accept the hypothesis that some micro-

spheres or similar structures that contained RNA developed a means

of transferring their characteristics to offspring. But researchers do

not yet understand how DNA, RNA and hereditary mechanisms first

developed. Therefore, the subject of how life might have originated

naturally and spontaneously remains a subject of intense interest,

research, and discussion.

Modeling Coacervates

By using simple chemistry, you will see that some properties

of coacervates resemble the properties of cells.

Materials

safety goggles and lab apron, graduated cylinder,

1 percent gelatin solution, 1 percent gum arabic solution,

test tube, 0.1 M HCl, pipet, microscope slide and

coverslip, microscope

Procedure

1.

CAUTION: Hydrochloric

acid is corrosive. Put on

safety goggles, gloves, and

apron. Avoid contact with

skin and eyes. Avoid

breathing vapors. If any of

this solution should spill on

you, immediately flush the

area with water, and notify

your teacher.

2.Mix 5 mL of a 1 percent

gelatin solution with 3 mL

of a 1 percent gum arabic

solution in a test tube.

3. Add 0.1 M HCl to the
   gelatin–gum arabic
   solution one drop at a time
   until the solution turns cloudy.
   4.Prepare a wet mount of the
   cloudy solution, and examine
   it under a microscope at
   high power.
   5.Prepare a drawing of the
   structures that you see.
   They should resemble the
   structures in the micrograph
   above.

Analysis

1. Describewhat happened to
   the solutions after the acid
   was added.


2. Comparethe appearance of
   coacervates with that of cells.


3. Predict what would happen
   to the coacervates if a base
   were added to the solution.
   4.Critical Thinking
   Evaluating Hypotheses
   Based on the evidence
   you obtained, defend the
   hypothesis that coacervates
   could have been the basis
   of life on Earth.

Section 1 Review

Explainhow radioisotopes are used to

determine the age of a rock. 7A

Critiquetwo scientific models that explain

the origin of life. 3A 3E

Describethe first step that may have led toward

cellular organization.

Explainhow heredity may have arisen. 6A

Miller and Urey’s model is

inconsistent with the finding that Earth’s early

atmosphere lacked 3A

Anitrogen. Cammonia.

Bhydrogen. Dozone.

TAKS Test PrepTAKS Test Prep

SECTION 1How Did Life Begin? 257

1A 2B 2C

4A