Types of DNA sequences (As per text pages 178-179)

1) Unique sequences; one to a few sequences (64% of genome, not all protein coding)

2) Moderately repetitive sequences; 10³ to 10⁵ sequences (25% of genome)

3) Highly repetitive sequences; 10⁵ to 10⁷ sequences (10% of genome)

The non-unique (repeated) sequences may be classified as:

A) Tandem repeated sequences

1. all the same function; as ribosomal RNA genes; 106 - 200 copies

2. related but not identical in function as the hemoglobin genes

3. function unknown; as centromeric and telomeric heterochromatin

4. simple telomeric sequences

 

B) Dispersed repeated sequences

1. histone genes in birds and mammals

 

2. Transposable Elements

a. Retroposons - move via RNA - use reverse transcriptase

aa. SINEs (short interspersed elements); 10-500 base pairs

the Alu-I family in primates

200-300 base pairs

repeated as many as 9 x 10⁵ times

about 9% of genome, one per ~ 5,000 base pairs (10%*)

bb. LINEs (long interspersed elements); up to 7,000 base pairs

the LINE-1 family in mammals

 14.6% human genome*

b. transposons (move via DNA; 1.6% of human genome*) - use transposase (chapter 19)

 

 Interspersed repeats in the human genome = 35%*

 

* = Kazazian, H. H. J. and J. V. Moran (1998). "The impact of L1 retrotransposons on the human genome." Nature Genetics 19(1): 19-24.