Indicate whether each of the following descriptions better applies to the genome of mitochondria (M) or nuclei (N) in vertebrates. Your answer would be a four-letter string composed of letters M and N only, e.g. MNMN.
( ) It has a higher percentage of noncoding DNA.
( ) It has a stricter codon usage.
( ) Its genes are present at higher copy numbers per cell.
( ) Its evolutionary clock ticks much faster.