GC content
GC content is the share of bases that are G or C: (G + C) ÷ length. ATGCGC has 4 of 6, or 66.7%. G–C pairs have three hydrogen bonds instead of two, so GC-rich DNA melts at a higher temperature, which matters for PCR primers and probes. DNA Statistics gives base counts, GC and AT content and dinucleotide counts.
Molecular weight
A quick estimate for double-stranded DNA is about 650 daltons per base pair, and for proteins about 110 daltons per amino acid. The DNA Molecular Weight and Protein Molecular Weight tools add up the exact masses of each base or residue instead.
Isoelectric point
The isoelectric point (pI) is the pH at which a protein has no net charge. It is estimated from the numbers of acidic and basic residues and the pKa of each. Different sets of pKa values give answers a few tenths apart, so treat the result as a guide, for example when choosing a buffer for ion-exchange chromatography. Protein Isoelectric Point also gives the charge at any pH.
GRAVY
The grand average of hydropathy is the mean Kyte–Doolittle hydropathy value of all residues. Positive values mean hydrophobic overall, negative values hydrophilic; membrane proteins tend to score higher than soluble ones. Protein GRAVY also gives a hydropathy value for each window along the sequence, where stretches of about 20 hydrophobic residues can point to transmembrane helices.
CpG islands
CpG islands are stretches rich in CG dinucleotides, often near gene promoters in vertebrates. A common definition is a window of at least 200 bases with GC content above 50% and an observed/expected CpG ratio above 0.6; the CpG Island Finder uses those settings by default.
Codon usage
Most amino acids have several codons, and organisms favor some over others. A codon usage table shows how often each is used, per thousand codons and as a share of its synonyms, which helps when adapting a gene for expression in another organism. See Codon Usage, and Protein Statistics for amino acid composition.