Compare and contrast the packaging and composition of DNA in prokaryotes and eukaryotes
Does the large amount of DNA in eukaryotic cells directly reflect the number of genes in eukaryotic cells compared to prokaryotic cells or are there other factors involved?
What will be an ideal response?
Answer: Prokaryotic cells generally have one circular chromosome (although there are exceptions) within a nucleoid, supplemented by very small circular plasmids outside of the nucleoid that can be exchanged between cells. Eukaryotic cells have multiple linear chromosomes within the nucleus and small circular chromosomes within mitochondria and chloroplasts. Prokaryotic circular chromosomes are negatively supercoiled and packaged into loops by basic histone-like proteins and structural RNAs. Eukaryotic chromosomes are densely packed around histones in nucleosomes, which are further compacted into loops and fibers by scaffold proteins. Prokaryotic genomes contain a very low percentage of repetitive DNA and a relatively high percentage of nonrepetitive DNA and coding DNA compared to eukaryotic genomes. Eukaryotic genomes contain 25%–60% repetitive DNA and less than 10% exons or coding DNA. Some roles for repetitive DNA are structural, such as in centromeres and telomeres, or may play a role in evolutionary adaptability. The large amount of DNA in eukaryotic cells does NOT directly reflect a proportional increase in the number of genes contained in eukaryotic genomes compared to prokaryotic genomes because a large amount of DNA in eukaryotic genomes plays a structural and regulatory role instead of coding for proteins or RNAs.
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