with open('gc.txt', 'w') as gc:
    with open('порезанный геном.txt', 'r') as fasta:
        c=0
        l=0
        for line in fasta:
            if line[0] == '>':
                if l != 0:
                    print(round(c/l*100, 2), file=gc)
                    c=0
                    l=0
            else:
                line = line.strip()
                for i in line:
                    #print(i)
                    l+=1
                    if i=='G' or i=='C':
                        c+=1
        #GC=c/l
        print(round(c/l*100, 2), file=gc)