Sequence Length Calculator
Our bioinformatics calculator computes sequence length accurately. Enter measurements for results with formulas and error analysis.
Reviewed by Daniel Agrici, Founder & Lead Developer
Formula
MW(ssDNA) = Sum(nucleotide masses) - (n-1) x 18.02; Physical length = n x 0.34 nm
Molecular weight is the sum of individual nucleotide or amino acid masses minus water molecules lost during polymerization. Physical length for B-DNA is 0.34 nm per base pair, for A-RNA is 0.28 nm per base, and for extended protein chains is approximately 0.35 nm per residue.
Worked Examples
Example 1: Calculating DNA Fragment Properties
Problem:A PCR product is 1500 bp long. Calculate its molecular weight and physical length.
Solution:MW (dsDNA): 1500 bp x 660 Da/bp = 990,000 Da = 990 kDa\nMW (ssDNA): 1500 nt x 330 Da/nt = 495,000 Da = 495 kDa\nPhysical length: 1500 bp x 0.34 nm/bp = 510 nm = 0.51 um\nCoding potential: 1500 / 3 = 500 codons = up to 499 amino acid protein
Result:MW: 990 kDa (dsDNA) | Length: 510 nm | Encodes up to 499 amino acids
Example 2: Protein Size from Gene Length
Problem:A gene is 900 bp in the coding region. What protein size does it encode?
Solution:Codons = 900 / 3 = 300 (including stop codon)\nProtein = 299 amino acids\nEstimated MW = 299 x 110 Da = 32,890 Da = ~33 kDa\nThis would migrate near the 33 kDa marker on SDS-PAGE.
Result:299 amino acids | ~33 kDa protein | Expected at 33 kDa on SDS-PAGE
Frequently Asked Questions
Why is knowing the sequence length important in molecular biology?
Sequence length determines many experimental parameters in molecular biology. For DNA, the length in base pairs affects gel electrophoresis migration, PCR extension time (typically 1 minute per kilobase), ligation efficiency, and transformation frequency. For proteins, the number of amino acids determines the expected molecular weight on SDS-PAGE gels, purification column selection, and structural predictions. Knowing the exact length is essential for calculating molar concentrations from mass measurements, designing expression constructs, and estimating sequencing coverage requirements.
How is molecular weight calculated from sequence length?
For single-stranded DNA, each nucleotide contributes approximately 330 Da (ranging from 289 Da for dCMP to 329 Da for dGMP). The total molecular weight is the sum of individual nucleotide weights minus water molecules released during phosphodiester bond formation (one water per bond). For double-stranded DNA, multiply by 2. A quick estimate is MW = length x 330 Da for ssDNA or length x 660 Da for dsDNA. For proteins, the average amino acid molecular weight is about 110 Da, so MW is approximately length x 110 Da. Precise calculations use the exact mass of each residue in the sequence.
What is the physical length of a DNA molecule?
B-form DNA (the most common conformation under physiological conditions) has a rise of 0.34 nm per base pair between successive base pairs and 10 base pairs per helical turn (3.4 nm pitch). This means a 1 kb DNA fragment is about 340 nm long and a 3 billion bp human chromosome, if stretched out, would be about 1 meter long. The total DNA in one human cell (6 billion bp across 46 chromosomes) extends to approximately 2 meters. RNA in A-form has a slightly shorter rise of 0.28 nm per base pair. These physical dimensions are important for biophysical techniques like atomic force microscopy and DNA nanotechnology.
How does sequence length affect gel electrophoresis migration?
In agarose gel electrophoresis, DNA fragments migrate through the gel matrix at a rate inversely proportional to the log of their molecular weight (and therefore length). Smaller fragments move faster because they navigate through gel pores more easily. Standard agarose gels (0.8-2%) resolve fragments from about 200 bp to 20 kb. For smaller fragments (10-500 bp), polyacrylamide gels provide better resolution. For very large DNA (20 kb to several Mb), pulsed-field gel electrophoresis (PFGE) is required because conventional electrophoresis cannot resolve fragments above approximately 20-25 kb, which all migrate together.
References
Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy