Genomes and Their Evolution

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Many Alu elements are transcribed into RNA molecules; however, their function is unknown

Slide 45

The human genome also contains many sequences of a type of retrotransposon called LINE-1 (L1)

L1 sequences have a low rate of transposition and may help regulate gene expression

Slide 46

Other Repetitive DNA, Including Simple Sequence DNA

About 15% of the human genome consists of duplication of long sequences of DNA from one location to another

In contrast, simple sequence DNA contains many copies of tandemly repeated short sequences

Slide 47

A series of repeating units of 2 to 5 nucleotides is called a short tandem repeat (STR)

The repeat number for STRs can vary among sites (within a genome) or individuals

Simple sequence DNA is common in centromeres and telomeres, where it probably plays structural roles in the chromosome

Slide 48

Genes and Multigene Families

Many eukaryotic genes are present in one copy per haploid set of chromosomes

The rest of the genome occurs in multigene families, collections of identical or very similar genes

Some multigene families consist of identical DNA sequences, usually clustered tandemly, such as those that code for RNA products

Slide 49

Fig. 21-10

DNA

RNA transcripts

Nontranscribed

spacer

Transcription unit

18S

28S

5.8S

28S

5.8S

rRNA

18S

DNA

(a) Part of the ribosomal RNA gene family

Heme

Hemoglobin

-Globin

-Globin

-Globin gene family

-Globin gene family

Chromosome 16

Chromosome 11





2

1

2

1





G

A







Adult

Fetus

Embryo

Fetus

and adult

Embryo

(b) The human -globin and -globin gene families

Slide 50

Fig. 21-10a

(a) Part of the ribosomal RNA gene family

18S

28S

28S

18S

5.8S

5.8S

rRNA

DNA

DNA

RNA transcripts

Nontranscribed

spacer

Transcription unit

Slide 51

The classic examples of multigene families of nonidentical genes are two related families of genes that encode globins

α-globins and β-globins are polypeptides of hemoglobin and are coded by genes on different human chromosomes