Glossary Plant Breeding

Nucellus. A parenchymatous tissue that extends from the chalaza at the base to the
micropyle at the apex of the embryo sac.
Nuclear Division. The division of the cell nucleus that may follow different patterns such
as amitosis, mitosis, or meiosis.
Nuclear Instability. Any deviation from normal nuclear behaviour producing a nucleus
of abnormal structure, karyotype or behaviour. It may be attributed to higher DNA
content and large telomeric segments of heterochromatin. It affects seed fertility and
endosperm development. It may lead to bridge formation at anaphase and shrivelled
grains.
Nuclear Membrane. The outer boundary of the nucleus. Also called nuclear envelope.
Nuclear Segregation. The segregation of genetically different nuclei contained in a
heterokaryotic multikaryon. It is one of the important sources of variation in asexual
pathogenic fungi.
Nuclease. An enzyme that can degrade DNA/RNA by breaking its phosphodiester bonds.
Nucleases may attack a polynucleotide either from an end (exonucleases), or at
random (endonucleases).
Nucleic Acid. A polymer of nucleotides (deoxyribonucleotides in the DNA or
ribonucleotides in the RNA). Nucleic acid (DNA, and in some cases RNA) acts as the
genetic material in living organisms. The presence of one free acid group in each
phosphoric acid molecule confers on the polynucleotide its acid properties and
enables the molecule to form ionic bonds with basic proteins (histones). The free acid
group also causes nucleic acids to be highly basophilic.
Nucleoid. A DNA mass within a chloroplast, mitochondrion, bacterium or a prokaryotic
cell. It has variable shape but uniform texture. It lacks a membrane (nuclear envelope)
and does not undergo processes comparable to mitosis and meiosis of eukaryotes.
Nucleolar Organiser. A region of the chromosome physically associated with the
nucleolus and containing rRNA genes.
Nucleolus. An organelle found in the nucleus, containing rRNA and amplified multiple
copies of the genes encoding rRNA.
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