Genomes and Their Evolution

Page
5

Introns and

regulatory

sequences

(24%)

Unique

noncoding

DNA (15%)

Repetitive

DNA

unrelated to

transposable

elements

(15%)

L1

sequences

(17%)

Alu elements

(10%)

Simple sequence

DNA (3%)

Large-segment

duplications (5–6%)

Slide 36

Transposable Elements and Related Sequences

The first evidence for wandering DNA segments came from geneticist Barbara McClintock’s breeding experiments with Indian corn

McClintock identified changes in the color of corn kernels that made sense only by postulating that some genetic elements move from other genome locations into the genes for kernel color

These transposable elements move from one site to another in a cell’s DNA; they are present in both prokaryotes and eukaryotes

Slide 37

Fig. 21-8

Slide 38

Fig. 21-8a

Slide 39

Fig. 21-8b

Slide 40

Movement of Transposons and Retrotransposons

Eukaryotic transposable elements are of two types:

Transposons, which move within a genome by means of a DNA intermediate

Retrotransposons, which move by means of an RNA intermediate

Slide 41

Fig. 21-9

Transposon

New copy of

transposon

Insertion

Transposon

is copied

Mobile transposon

DNA of

genome

(a) Transposon movement (“copy-and-paste” mechanism)

Retrotransposon

New copy of

retrotransposon

Insertion

Reverse

transcriptase

RNA

(b) Retrotransposon movement

Slide 42

Fig. 21-9a

Transposon

New copy of

transposon

DNA of

genome

Transposon

is copied

Insertion

Mobile transposon

(a) Transposon movement (“copy-and-paste” mechanism)

Slide 43

Fig. 21-9b

Retrotransposon

New copy of

retrotransposon

Reverse

transcriptase

Insertion

RNA

(b) Retrotransposon movement

Slide 44

Sequences Related to Transposable Elements

Multiple copies of transposable elements and related sequences are scattered throughout the eukaryotic genome

In primates, a large portion of transposable element–related DNA consists of a family of similar sequences called Alu elements