Chromosomes contain genes, regulatory sequences, and non-coding sequences. In prokaryotes, most DNA is made up of genes or regulatory sequences. However, eukaryotes have a large amount of DNA that does not code for proteins and is not part of regulatory sequences. For example, only about 1.5% or less of human DNA is coding DNA. As a result, eukaryotic genomes are often much larger than would be expected from the number of genes.
Which of the following are noncoding DNA that contribute to the large size of eukaryotic genomes? Select all that apply.

a. Alu elements
b. gene duplications
c. multiple variants of a gene
d. Exons
e. repetitive sequences (simple sequence repeats)

The repetitive sequences on DNA are called satellite DNA or short tandem repeats (STR). They are mainly found in the ends of chromosomes to protect the fringes from shortening during replication cycles.

Alu units are small nucleotide segments – averaging 300 bp long- within the human genome that can transpose. They are thought to be remnants of latent phase of viral or bacterial DNA that infected the human genome in the past. They make up the largest part of non-coding DNA.

Exons, on the other hand, are transcribed with introns on mRNA. The nascent mRNA has to be spliced to removing these noncoding regions to create a mature mRNA that can be translated into proteins.

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