Biology (Holt)

242 CHAPTER 11Gene Technology

Problems With Cloning

Since Dolly’s birth in 1996, scientists have successfully cloned ani-

mals. Only a few of the cloned offspring survived for long, however.

Many become fatally oversized. Others encounter problems in devel-

opment. For example, three cloned calves were born healthy in

March, 2001, only to die a month later of immune system failure.

The Importance of Genomic Imprinting

Technical problems in reproductive cloning lie within a developmental

process that conditions eggs and sperm so that the right combination

of genes are turned “on” or “off” during early development. When

cloned offspring become adults, a different combination of genes is

activated. The process of conditioning the DNA during an early stage

of development is called genomic imprinting.

In genomic imprinting, chemical changes

made to DNA prevent a gene’s expression with-

out altering its sequence. Usually, a gene is

locked into the “off ” position by adding methyl

(–CH 3 ) groups to its cytosine nucleotides, as

shown in Figure 13. The bulky methyl groups

prevent polymerase enzymes from reading the

gene, so the gene cannot be transcribed. Later

in development, the methyl groups are removed

and the gene is reactivated.

Why Cloning Fails

Normal vertebrate development depends on precise genomic

imprinting. This process, which takes place in adult reproductive tis-

sue, takes months for sperm and years for eggs. Reproductive

cloning fails because the reconstituted egg begins to divide within

minutes. There is simply not enough time in these few minutes for

the reprogramming to process properly. Key genes fail to become

properly methylated, and this leads to critical errors in development.

Because of these technical problems, and because of ethical prob-

lems, efforts to clone humans are illegal in most countries.

Gene

RNA polymerase

Gene

CH 3

Listthree ways in which food crops have been

improved through genetic engineering. 3C 6D

Comparethe cloning of sheep through the use

of differentiated cells with the cloning of sheep

through the use of embryonic cells. 6A 6D

Critical ThinkingAnalyzing Methods In the

movie Jurassic Park,scientists used DNA to bring

back extinct species. How is that different from the

creation of cloned sheep using differentiated cells?

Critical ThinkingForming Reasoned

OpinionsList reasons you would or would not

be concerned about consuming milk from cows

treated with growth hormone. 11B 11C

Using genetic engineering to

produce rice with high levels of beta-carotene

should help people who suffer from a deficiency in

Avitamin A Cglyphosate

Bgrowth hormone Dcomplex proteins

TAKS Test PrepTAKS Test Prep

Section 3 Review

3C

6A 6D

Figure 13 Methylated
gene. In one model of
genomic imprinting, methyl
groups attached to a gene
prevent the gene from being
expressed.