Q.(a) Bring out the inheritance of chloroplast gene with example. OR

(a) Chloroplast genes are inherited maternally, independent of nuclear Mendelian genes, as shown by Mirabilis jalapa variegation; (b) RNA editing changes an mRNA's sequence post-transcriptionally, notably by C-to-U conversion in plant organelles.

(a) Inheritance of Chloroplast Genes

Chloroplasts, like mitochondria, possess their own small circular DNA genome (the plastid genome) distinct from the nuclear genome, and this genome is inherited independently of the Mendelian, nuclear pattern of inheritance -- a phenomenon called cytoplasmic (extranuclear or extrachromosomal) inheritance.

The classic example is leaf variegation in the four-o'clock plant, Mirabilis jalapa, first described by the German botanist Carl Correns in 1908. In this plant, branches can bear green leaves, white (pale) leaves, or variegated (green-and-white) leaves, depending on whether their cells' plastids are normal (chlorophyll-containing), defective (lacking chlorophyll), or a mixture of both.

When flowers from branches of different leaf colour are crossed, the phenotype of the offspring is determined almost entirely by the female (seed) parent, not the male (pollen) parent -- because the egg cell contributes essentially all of the cytoplasm, and hence all the plastids, to the zygote, whereas pollen contributes mainly just its nucleus, with little or no cytoplasm/plastids. Thus, a cross using a variegated-leaf plant as the mother produces progeny reflecting the mother's own mixture of normal and defective plastids, irrespective of the leaf colour of the father plant, and reciprocal crosses give different results -- unlike nuclear genes, which give the same Mendelian ratios regardless of which parent contributes which allele.

(b) RNA Editing in Plants

RNA editing is a post-transcriptional process in which the nucleotide sequence of an RNA molecule (typically mRNA) is altered after it has been transcribed from DNA, so that the final RNA sequence differs from what is literally specified by the DNA template. This means the amino acid sequence of the resulting protein cannot always be reliably predicted just by reading the DNA sequence.

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