Properties and Organization

Nicotiana ct-DNA is a double-stranded structure having a molecular.weight of（95—10）×106 daltons［7，8］.It has an average buoyant density of 1.698—1.700 g/cm3，corresponding to a GC content of 38%—41%.In most cases the buoyant density of ct-DNA is indistinguishable from that of the corresponding nuclear DNA.Ct-DNA，however，possesses some unique features.To date，no detectable 5-methyl cytosine has been found in the ct-DNA from Nicotiana or any plants，whereas the corresponding nuclear DNA invariably contains 4%—5%.Another unique property of ct-DNA is the ease with which it can be renatured.This is a reflection of the degree of simplicity and homogeneity of ct-DNA，which is a much less complicated molecule than nuclear DNA both in structural complexity and in genetic content.

Chloroplast DNA is circular in form and has a contour length of 50 μm.There are multiple copies per organelle ranging from a few to over 100，existing in different forms：circles，supercoiled circles and circular dimers［8］.Each circle can be divided into two inverted repeat regions separated by a small and a large single-copy sequence［9］.The inverted repeats are usually 20—25 kilobases（kb）in length and contain the genes for ribosomal and transfer RNAs［8］.The single-copy regions contains the genes for many chloroplast proteins and transfer RNAs.The coding capacity of ct-DNA may well exceed 100 polypeptides of 4×104 daltons each.However，only a few polypeptides have been identified as the product of chloroplast genes incuding the large subunit（LS）of RuBPCase［7］.(https://www.daowen.com)