Biology 12

CONCEPT ORGANIZER The Material of Heredity

Storage and expression

of hereditary information

(Chapter 8, section 8.1)

Accuracy of transmission

(Chapter 7, section 7.3)

Possibility of variation

(Chapter 9, section 9.1)

DNA’s function as the

source of genetic

information

288 MHR • Unit 3 Molecular Genetics

Another factor that can rearrange genetic material

is the activity of transposable elements, also known

as jumping genes or transposons. These are short

strands of DNA capable of moving from one location

to another within a cell’s genetic material. A

transposon is flanked by nucleotide splicing

sequences that are recognized by an enzyme called

transposase. Transposase excises the transposon

out of one location and splices it into another. One

effect of transposons is illustrated in Figure 9.4.

Within these flanking splicing sequences, a

transposon may contain one or more regulatory or

structural genes. Thus, the excision and splicing of

a transposon from one place to another can

interrupt the function of a gene or separate a

promoter sequence from the gene it controls. The

effects can be simultaneous yet different at the

transposon’s original location and at its new

location. Mutations of this nature help to explain

some of the rapid genetic changes that may lead to

kernel

original DNA

transposon

flanking splicing sequences

excision integration

after transposon integration,

the resulting kernel is white

target DNA

kernel

kernel

kernel

When a transposon is located within a

strand of DNA that does not code for

kernel colour (top), the default kernel

colour will be purple.

A When the transposon is excised and

re-inserted into a gene that codes for

kernel colour (top), it can disrupt the

coding sequence and block the action

of that gene.

B

Figure 9.4Transposons were

first discovered by American

researcher Barbara McClintock

in 1957. McClintock found that

the random pattern of colours in

the kernels of Indian corn could

be explained by the movement

of short strands of DNA from

one position within a cell’s

chromosomes to another. In

1983, McClintock was awarded

a Nobel prize for her work.

Figure 9.5Three key factors make it possible for DNA to function as the material

of heredity.

Several factors combine to make it possible for DNA to

function as the material of heredity. The molecular structure

of DNA stores the information needed to synthesize

polypeptides. As this information is expressed, the enzymes

and other proteins that cells need to survive and develop

are produced. The high level of accuracy in DNA replication

ensures that the structure and function of cells and organisms

are maintained through countless generations. At the same

time, the possibility that DNA can be modified through

mutations provides an important source of genetic variation

that contributes to the emergence of new types of organisms.