Lesson 1 of 5 / Genomes and gene regulation
Compare three kinds of genome
Does every genome consist of linear double-stranded DNA?
In this lesson: Compare viral, bacterial and eukaryotic genome organisation.
About 7 min
The key ideaGenome organisation varies in molecule type, strandedness, size, packaging and gene structure; avoid treating one model as universal.
Explore the idea
Compare organisation before comparing size
A compact circular double-stranded DNA chromosome lies in the nucleoid. Smaller plasmids may also be present. Typical bacterial protein-coding genes usually lack introns; DNA is associated with proteins but not packaged as eukaryotic nucleosomes.
Organisation models, not relative sizes or exact chromosome counts. The viral lines represent eight separate RNA segments, not eight cells. Many viruses have genomes of thousands to hundreds of thousands of nucleotides; typical bacteria have millions of base pairs. Exceptions prevent a universal size ranking.
Check the unit before comparing genome amounts
Five million double-stranded base pairs contain ten million individual nucleotides. Matching those amounts does not establish matching sequence, gene number or organismal complexity.
Explanation
A typical bacterial genome has a relatively compact circular double-stranded DNA chromosome in the nucleoid, sometimes with additional plasmids. Protein-coding regions occupy a large proportion of the DNA and introns are uncommon in the usual bacterial protein-coding gene model. DNA is organised with proteins even though bacteria do not package chromosomes as eukaryotic nucleosomes.
A eukaryotic nuclear genome normally consists of several linear double-stranded DNA chromosomes. DNA wraps around histone proteins to form nucleosomes, which organise into chromatin. Protein-coding genes often contain introns, and much of the genome does not encode protein. Genome size does not directly measure organismal complexity.
Viral genomes may be DNA or RNA, single-stranded or double-stranded, linear or circular, and can be segmented. Influenza has a segmented RNA genome; HIV carries RNA that is reverse-transcribed during infection; lambda phage has a DNA genome. Never infer all virus genomes are RNA because two familiar examples are. Viral gene organisation also varies: some influenza A mRNAs are spliced, so viral coding information is not universally intron-free.
Compare genome size with stated units and examples rather than a single mandatory value: many viruses have thousands to hundreds of thousands of nucleotides, typical bacteria millions of base pairs, and many eukaryotes much larger genomes. Exceptions exist. Count nucleotides or base pairs consistently, because one double-stranded base pair contains two nucleotides.
Step by step
- 1
Identify the molecule
State DNA or RNA and one or two strands.
- 2
Describe arrangement
Use linear/circular, chromosome number, segmentation and whether coding information is interrupted and spliced.
- 3
Compare packaging and genes
Consider histones, non-coding regions and introns.
Worked example
Work through the evidence
Genome A is 5 million base pairs; B is 10 million individual nucleotides in a double-stranded molecule. Are they different in total DNA amount?
One way to explain it
They represent the same total nucleotide count under this comparison: 5 million base pairs contain 10 million nucleotides.
Why this answer works
- Do not compare a base-pair count directly with a nucleotide count.
- The calculation does not establish that the sequences or organisms are identical.
Is this true? "All viruses have one small circular DNA chromosome."
Viral genomes are diverse and can be RNA, segmented or linear as well as DNA and circular.