Q.Extra nuclear inheritance is a consequence of presence of genes in

(a) Chloroplast (cytoplasmic) inheritance is illustrated by maternally-inherited leaf variegation in Mirabilis jalapa; (b) single cell protein serves as a protein-rich food/feed supplement.

(a) Inheritance of the chloroplast gene

Chloroplasts, like mitochondria, carry their own small DNA genome and are inherited independently of the nuclear (Mendelian) genes, through a phenomenon called cytoplasmic (extranuclear/organelle) inheritance. The classic example, first studied by Carl Correns in 1908, is leaf variegation in the four o'clock plant, Mirabilis jalapa. This plant naturally produces branches bearing leaves of three types: fully green (normal chloroplasts), fully white/pale (defective, non-pigmented plastids), and variegated (a patchy mixture of green and white sectors).

When reciprocal crosses were made -- using flowers on branches of each leaf-colour type as either the female (egg-bearing) or male (pollen-bearing) parent -- the colour of the offspring's leaves was found to depend entirely on which branch supplied the female (ovule/egg) parent, regardless of which branch supplied the pollen:

• Female parent from a green branch -> all offspring green, whatever the pollen source.
• Female parent from a white branch -> all offspring white (often unable to survive long, lacking chlorophyll), whatever the pollen source.
• Female parent from a variegated branch -> offspring show a mixture of green, white, and variegated types, in varying proportions.

This result showed that the trait is transmitted only through the maternal (egg) parent, never through the pollen -- because during fertilisation, the pollen (male gamete) contributes essentially only its nucleus, while chloroplasts are carried in the cytoplasm of the much larger egg cell. Since chloroplasts (containing their own DNA and hence the genes for pigmentation) are passed on almost exclusively via the egg's cytoplasm, this is a clear example of maternal, non-Mendelian (cytoplasmic) inheritance, distinct from the biparental inheritance pattern of nuclear genes.

(b) Applications of single cell protein (SCP)

Single Cell Protein (SCP) refers to the dried, protein-rich cellular biomass obtained by cultivating microorganisms -- algae (e.g. Spirulina, Chlorella), yeasts (e.g. Candida utilis, Saccharomyces), bacteria (e.g. Methylophilus methylotrophus), or fungi -- on an industrial scale, often using cheap, renewable substrates such as molasses, whey, agricultural or food-processing waste.

Applications include:

1. Protein supplement in animal feed: SCP is widely added to poultry, livestock, and aquaculture (fish/shrimp) feed to boost the protein content economically.
2. Human food supplement: SCP can help address protein-calorie malnutrition, particularly in developing countries, since it is a rich, relatively fast-to-produce source of protein, vitamins, and minerals, requiring far less land and time than conventional livestock or crop protein.
3. Sustainable/economic production: because SCP organisms grow rapidly on cheap, often waste-derived substrates, SCP production also helps utilise industrial and agricultural waste, reducing both cost and environmental pollution.
4. Source of vitamins and other nutrients: besides protein, SCP biomass can also supply useful vitamins and other nutritional compounds.

(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.

In plants, RNA editing occurs prominently in the organelle genomes -- mitochondria and chloroplasts. The most common form is C-to-U editing, in which a cytidine (C) nucleotide in the pre-mRNA is chemically converted (deaminated) to a uridine (U) at specific sites. This single-base change can alter a codon so that it specifies a different amino acid, or can even create a new start or stop codon, thereby changing the length or sequence of the resulting protein compared to what the unedited transcript would produce.