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  • AQA
  • A Level Biology
  • Unit 4 - Genetic information, variation and relationships between organisms
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Summary of DNA, genes and chromosomes - AQA A Level Biology

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Unit 4 - Genetic information, variation and relationships between organisms
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DNA, genes and chromosomes – AQA A Level Biology Summary Notes

Eukaryotic cells contain a nucleus whereas prokaryotic and bacterial cells do not contain a nucleus
Eukaryotes store DNA in nucleus whereas prokaryotes store DNA free in cytoplasm

- Due to greater quantity of DNA present in eukaryotes it must be compacted to a greater
extent to fit into nucleus – DNA is supercoiled around proteins called histones (forming
complex called chromatin)
- Prokaryotes also supercoil their DNA, by action of enzymes called topoisomerases

Compare eukaryotic and prokaryotic DNA

Comparisons
1. Nucleotide structure is identical (ie phosphate, pentose sugar and base)
2. Nucleotides joined by phosphodiester bond
3. DNA in mitocondria/chloroplasts has similar strucute to DNA in prokaryotes

Contrasts
4. Eukaryotic DNA is longer
5. Eukaryotic DNA contains introns, prokaryotic does not
6. Eukaryotic DNA is linear, prokaryotic DNA is circular (plasmids)
7. Eukaryotic DNA is associated with histones, prokaryotic DNA is not

Histones = Proteins

Together a DNA molecule and its associated proteins form a chromosome
- Chromosome = DNA molecule and associated proteins

In diploid cells, chromosomes come in homologous pairs: two chromosomes carry the same genes




Chromosome structure:
- Chromatin wound around histones
- Coiled into chromosomes
- Genes position on a chromosome = locus

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