Cell Division 



43 



present time this identification cannot be made with certainty, but 

 there is no doubt that one chromosome contains many genes and 

 also many particles of dna. We might ask how they are assembled 

 in the chromosome. The results of genetics requires that they should 

 be arranged in a Unear order, i.e. in one long line. How the individual 

 particles are joined up is not precisely known, but it is known that 

 the DNA is not the sole constituent of the chromosomes. They also 

 contain some rather specialized kinds of protein called protamines 

 (present in fish sperm) and histones (present in many mammalian 

 cells). These proteins are of a basic character, i.e. they are capable of 

 combining firmly with the acidic nucleic acid. Their precise function 



FIG. 8. Double-threaded structure of deoxyribonucleic acid 

 of Crick and Watson 



is unknown. It is Hkely that, by combining with and neutralising the 

 phosphate groups of the dna, they protect it and perhaps also shield 

 it from other cell chemicals until the time comes for it to function. 

 It is also possible that they are used to effect the junctions between 

 the separate dna particles. Another possibility, which has been sug- 

 gested, is that the junctions are effected by the ions of metals such as 

 calcium. However the junction between the individual dna particles 

 is made, there is no doubt that when the whole chromosome is 

 duplicated, a replica of every dna particle is formed in the new 

 chromosome. 



Experiments have been made using radioactive materials as 

 markers to find in more detail how the replication of the chromo- 

 some occurs. The experiments of Taylor and others seem to show 



