Chromosome organisation

Abstract

The simplest chromosomes, the structures that carry the genetic information and transmit it from one generation to the next, are those of the viruses, as they consist of a single molecule of either DNA or RNA with no associated chromosomal proteins. The following examples illustrate the range of viral chromosomes:

• (1) Tobacco mosaic virus (TMV) has a linear molecule of single-stranded RNA only 6400 nucleotides long.

• (2) Coliphage T2 (coliphages infect E. coli) is a large phage and has a linear molecule of double-stranded DNA about 165 kb long.

• (3) Coliphage lambda (λ) has a linear molecule of double-stranded DNA 48.6 kb long, but as soon as this molecule enters the infected cell, it circularises; this is possible because the opposite ends of the linear molecule of DNA are single-stranded and base-complementary — thus, these ‘sticky’ ends can associate by H bonding between the complementary bases and DNA ligase can seal the remaining single-strand gap (Question 3.7).

• (4) Coliphage X174 is a very small polyhedral phage and its chromosome is a circular molecule of single-stranded DNA, only 5.4 kb long and encoding 11 proteins. After infection this molecule becomes double-stranded by the synthesis of a complementary strand and then enters a complex replication cycle which results in the production of many single-stranded circular molecules of phage DNA.