Question

A retrotransposon known as Alu1 is about 300 base pairs in length and is present in the human genome in about 1 million copies. Approximately what percentage of the human genome is accounted for by this transposon?

Answer 1

First, it should be known that the human genome has 3 billion (3*10^9) base pairs. 
So, we need to do a simple calculation:
percentage of Alu1 =  $\frac{total-number-of-base-pairs-of-the-Alu1-transposons}{Total-number-of-base-pairs-in-the-human-genome}$ * 100 = $\frac{300 * 1000000}{3000000000}$ *100 = 10%

So, the million copies of Alu1 represent 10% of the human genome.

Answer 2

A retrotransposon is a type of eukaryotic mobile DNA element. The movement if this DNA element in the human genome is mediated by an RNA intermediate and with reverse transcription. An Alu element is a short sequence of DNA in the human genome and the most abundant transposable element.  

Modern Alu elements are short interspersed nuclear elements about 300 base pairs long. They are responsible for the regulation of tissue-specific genes. There are about one million Alu elements interspersed in the human genome contributing to about 10% of the human genome.