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1.6.5Inquiry Process

Comparing Genome Sizes: Skills — IB Biology HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 19 of 160

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Part 1 of 5Reading Genome Data

1.6.5 Comparing Genome Sizes: Skills
Units
  • Genome sizes are given in base pairs, usually megabases (Mb, millions) or gigabases (Gb, billions).
Large ranges
  • Sizes span many orders of magnitude, so ratios or logarithmic axes make comparisons clearer.
Fair comparison
  • Compare haploid genome sizes in the same unit to avoid errors caused by ploidy differences.
Calculations
  • To find how many times larger one genome is, divide the larger value by the smaller value.
OrganismGenome size (Mb)
Bacteriophage T20.17
Escherichia coli4.6
Drosophila melanogaster140
Homo sapiens3100
Paris japonica150 000

Part 2 of 5Interpreting the Pattern

Interpreting the Pattern
Not linked to complexity
  • The plant Paris japonica has about 48 times more DNA than a human, yet is a simpler organism.
Non-coding DNA
  • Large genomes usually contain huge amounts of repeated and non-coding DNA rather than extra genes.
Gene number differences
  • Gene counts vary far less than genome sizes, so size is a poor predictor of gene number.
Prokaryote versus eukaryote
  • Bacteria generally have small genomes, because they carry little non-coding DNA.

Part 3 of 5Skills Checklist

1
Write down the data with units and check all values use the same unit.
2
Choose a suitable graph, such as a bar chart with a logarithmic axis for sizes that vary enormously.
3
Calculate ratios when asked how many times larger, and give the answer to a sensible number of figures.
4
Describe the overall trend before commenting on anomalies.
5
Link the finding to the idea that genome size is not a measure of biological complexity.

Part 4 of 5Worked Example

Question

  • The genome of E. coli is 4.6 Mb, that of Drosophila is 140 Mb, that of Homo sapiens is 3100 Mb and that of the plant Paris japonica is 150 000 Mb.
  • Calculate how many times larger the Paris japonica genome is than the human genome.

  1. Use the same unit for bothboth are in Mb.
  2. Divide150 000 ÷ 3100.
  3. 150 000 ÷ 3100 = 48.4, which is about 48.

Answer The Paris japonica genome is about 48 times larger than the human genome, so genome size does not reflect complexity.

Part 5 of 5Exam Checklist & Tips

1.6.5 – Key Points for the Exam

  • Megabase (Mb) One million base pairs.
  • Gigabase (Gb) One billion base pairs.
  • Non-coding DNA DNA that does not code for a protein.
  • Ratio One quantity divided by another to compare size.

Common Mistakes

  • Mixing Mb and Gb in a calculation without converting.
  • Concluding that the largest genome belongs to the most complex organism.
  • Reading a logarithmic axis as if it were linear.
  • Quoting a ratio without checking which genome is the numerator.

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