How to generate a random string and compute the percent of a specific string?

I'm doing my first ever R course. One of the exercises is to create a random DNA strings with 1000 bases and For compute the GC percentage (GC%).

I created the DNA bases vector and tryed to create the sequence, however, the result is nor the correct

DNA <- c("A","G","T","C")
seq <- strrep(DNA, 250)

Any suggestion for this noob?