The following points highlight the two main types of organellar DNA in cell. The types are: 1. Chloroplast DNA (cp DNA) 2. Mitochondrial DNA (mt DNA).
Organellar DNA: Type # 1. Chloroplast DNA (cp DNA):
Each chloroplast contains 20-200 copies of a circular double stranded DNA molecule, which may also be linear as found in maize, cp DNA is characterized
By the presence of three regions:
(i) Two inverted repeats (IR), each 10-24 kb long and carrying ribosomal genes,
(ii) A short single copy (SSC) sequence, 18-20 kb long,
(iii) A long single copy (LSC) sequence (Fig. 2.57).
There are some open reading frames (ORF) with coding sequences beginning with a met-codon and a stop codon at the end of the cp DNA. Chloroplast DNA is relatively large ranging from 120-210 kb, which is comparable to the size of a large bacteriophage (110 kb). Total cp DNA in a cell may make up to 14% of cellular DNA.
Most chloroplast genomes appear to possess the same set of genes. Each molecule of cp DNA is encoded with 110-120 genes and code for about 125 proteins. In maize the genes on cp DNA contains both small and large rRNA genes and sequence for tRNA. It also synthesizes mRNA for some proteins involved in photosynthesis like enzyme rubisco.
Two and eight genes respectively for the polypeptides of PS I and PS II have been identified. Some herbicide resistant genes have also been located in cp DNA.
A circular genetic map was proposed by Ruth Sager (1972) in Chlamydomonas (Fig. 2.58). Chloroplast genetics in higher plants including crops like pea, maize, rice, etc. has also been studied. Bendich (2004) emphasized that cp DNA in plants is generally found as multi-genomic complex and branched linear DNA molecules and not as mono-genomic circular molecules.
Organellar DNA: Type # 2.Mitochondrial DNA (mt DNA):
Like chloroplasts, mitochondria also contain 5-100 copies DNA molecules which are usually circular, but may essentially be also linear. Remarkable variation exists in the size of mt DNA, ranging from 6- 2500 l<b. mt DNA contributes only about less than 1 % of cellular DNA except yeast (18%) (Fig. 2.59).
Mitochondrial genomes display greater variability in gene content. The cloning and sequencing of the entire mt DNA have now been made in several organisms including human (Fig. 2.60). It contains genes for rRNA, tRNA, ribosome associated protein, mitochondrial proteins and enzymes.
A substantial fraction of mt DNA in yeast represents unidentified reading frames containing introns of split genes and appear to code for proteins required for mRNA splicing, called maturates.
In higher plants, smaller circular (sometimes linear also) DNA molecules present in addition to the main circular mt DNA. The master circle contains repetitive sequences and due to extensive recombination between these repeats, smaller molecules originate.
One interesting feature of mt DNA is its unidirectional and highly asymmetric replication. The daughter L-strand starts synthesis when two-third of H-strand is already synthesized. Another interesting point of plant mt DNA is that it can move between organelles which are called promiscous DNA. mt DNA is associated with the male sterility character of plants as in maize.