Easy way to extract line ranges (from line 200 to line 210) with sed.
sed -n '200,210p' data/Homo_sapiens.GRCh38.dna.chromosome.18.fa
# then pipe it to a new file with
sed -n '200,210p' data/Homo_sapiens.GRCh38.dna.chromosome.18.fa > new.fa
head or tail could be used to extract from begining of the end of the file.