Box 1
Estimating Lower Limits of Cell Size—Some Assumptions and Results

Assumptions

    1.  

    Molecular masses:

    •  

    One DNA nucleotide = 312 Da

    •  

    One RNA nucleotide = 324 Da

    •  

    One amino acid residue = 110 Da

    2.  

    Both DNA strands are coding.

    3.  

    The cell contains one ribosome, one set of 20 tRNA molecules (average molecular mass 25,000 Da), and one mRNA molecule for each protein species.

    4.  

    Each ribosome consists of 50 protein molecules of average molecular mass 30,000 Da and of an equivalent quantity of RNA.

    5.  

    The cell contains N nonribosomal protein species of average molecular mass 30,000 Da, each present in 10 copies. At least 100 such protein species are deemed indispensable.

    6.  

    Wet weight = 3 × weight of (DNA + RNA + Protein).

    7.  

    Density of naked cell is 1.10.

    8.  

    Cell membrane is 6 nm thick.

    9.  

    Cell wall is 10 nm thick.

Calculations

Ribosome

Genome

rRNAs and tRNAs: 2,000,000 Da = 3.3 × 10-3 fg

Ribosomal proteins: 12,800,000 Da = 21 × 10-3 fg

Other proteins: N × 255,000 Da = N × 0.42 × 10-3 fg

Total: (24.3 + N × 0.42) × 10-3 fg

RNAs Other Than Ribosomal

tRNAs: 500,000 Da = 0.8 × 10-3 fg (× (N + 50))

mRNAs: Ribosomal proteins: 13,260,000 Da = 21.9 × 10-3 fg

Other proteins: N × 265,000 Da = N × 0.44 × 10-3 fg

Total: (61.9 + N × 1.24) × 10-3 fg

Other Proteins

N × 10 × 30,000 Da = N × 300,000 Da = N × 0.05 × 10-3 fg

(DNA + RNA + Protein) = (336.2 + N × 6.71) × 10-3 fg



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