Question

3. Genetic Information in E. coli DNA The genetic information contained in DNA consists of a linear sequence of coding units, known as codons. Each codon is a specific sequence of three deoxyribonucleotides (three deoxyribonucleotide pairs in double-stranded DNA), and each codon codes for a single amino acid unit in a protein. The molecular weight of an E. coli DNA molecule is about 3.1 3 109 g/mol. The average molecular weight of a nucleotide pair is 660 g/mol, and each nucleotide pair contributes 0.34 nm to the length of DNA. (a) Calculate the length of an E. coli DNA molecule. Compare the length of the DNA molecule with the cell dimensions (see Problem 2). How does the DNA molecule fit into the cell? (b) Assume that the average protein in E. coli consists of a chain of 400 amino acids. What is the maximum number of proteins that can be coded by an E. coli DNA molecule?

Answer 1

Part a)

Number of nucleotides = 3.1 x 109 /660 = 4700000

Length of DNA = 4700000 x 0.34 nm = 1600000 nm = 1.6 mm

However, the approximate length of an e.coli cell is around 0.002 mm. The DNA fit into the cell because it is packed with the basic proteins present in the cell.

Part b)

Since an amino acid is coded by three nucleotides, therefore, for a protein with 400 amino acids, 1200 base pairs are occupied.

So, 4700000 nucleotides will code for 4700000/1200 = 3917 proteins