Biology (Holt)

Change in Chromosome Structure

Changes in an organism’s chromosome structure are called mutations.

Breakage of a chromosome can lead to four types of mutations. In a

deletion mutation, a piece of a chromosome breaks off completely. After

cell division, the new cell will lack a certain set of genes. In many cases

this proves fatal to the zygote. In a duplication mutation, a chromosome

fragment attaches to its homologous chromosome, which will then

carry two copies of a certain set of genes. A third type of mutation is an

inversion mutation, in which the chromosome piece reattaches to the

original chromosome but in a reverse orientation. If the piece reattaches

to a nonhomologous chromosome, a translocation mutation results.

124 CHAPTER 6Chromosomes and Cell Reproduction

Modeling Chromosomal Mutations

You can use paper and a pencil to model the ways

in which chromosome structure can change.

Materials

14 note-card pieces, pencils, tape

Procedure

1.Write the numbers 1–8 on

note-card pieces (one num-

ber per piece). Tape the

pieces together in numerical

order to model a chromo-

some with eight genes.

2.Use the “chromosome” you

made to model the four alter-

ations in chromosome struc-

ture discussed on this page

and illustrated at right. For

example, remove the number

3 and reconnect the remain-

ing chromosome pieces to

represent a deletion.

3.Reconstruct the original

chromosome before

modeling a duplication,

an inversion, and a trans-

location. Use the extra

note-card pieces to make

the additional numbers you

need.

Analysis

Describehow a cell might

be affected by each mutation

if the cell were to receive a

chromosome with that

mutation.

Original chromosome

Deletion

Translocation

1

1

12

2

23

4

4

3

12 45

123 45

5

5

Duplication

Inversion

129 3 45

Summarizehow prokaryotic cells divide by

binary fission. 4A 4B 6E

Identifythe point in a eukaryotic cell cycle

at which DNA coils up to form

chromosomes. 6A 6E

Summarizethe difference between a haploid

cell and a diploid cell. 6E

Critical ThinkingEvaluating Conclusions

Do you agree or disagree that homologous chro-

mosomes are found in gametes. Explain. 6A 6E

How does the karyotype of a

person with Down syndrome differ from a normal

karyotype? 4B 6F

AIt lacks a chromosome.

BIt has two sex chromosomes.

CIt has a damaged chromosome.

DIt has an extra copy of a chromosome.

TAKS Test PrepTAKS Test Prep

Section 1 Review

6C