MEDICAL BIOLOGY, HUMAN ANATOMY, PHYSIOLOGY AND PATHOLOGY - Ya.I. Fedoniuk 2010

BIOLOGY

CHAPTER 1. BIOLOGICAL BASES OF HUMAN VITAL ACTIVITY

1.4. ONTOGENETIC LEVEL OF LIFE ORGANIZATION

1.4.2. Basics of Human Genetics

Non-chromosomal inheritance

T. Morgan's theory of heredity is known as the chromosome theory. This emphasizes the leading role of nuclear Chromosomes in hereditary phenomena. However, heredity is also associated with the Cytoplasm. Unlike chromosomal (nuclear) heredity, this form is called non-chromosomal or CYTOPLASMIC INHERITANCE. It is mediated by genes localized outside the Cell Nucleus—in Mitochondria and Plastids, which possess their own DNA and are capable of autonomous (nucleus-independent) reduplication. The totality of cytoplasmic genes is called the plasmone, while the genes themselves are termed plasmagenes. In their properties, plasmagenes are similar to nuclear genes; they are capable of reduplication and mutation. Cytoplasmic inheritance is transmitted unidirectionally, exclusively through the maternal line via the egg Cell Cytoplasm. Examples of maternal inheritance include streptomycin resistance in Chlamydomonas, the direction of shell coiling in freshwater snails (Lymnaea), variegation in ornamental plants such as four o'clock (Mirabilis jalapa) and snapdragons (Antirrhinum majus), and cytoplasmic male sterility in maize. In Bacteria, cytoplasmic inheritance is associated with Plasmids—small, extrachromosomal circular DNA molecules. Plasmids vary widely in function: F-plasmids confer The ability to undergo bacterial conjugation, R-plasmids determine multiple drug resistance, and Col-plasmids encode colicins (Proteins with lethal activity).

The main forms of cytoplasmic inheritance are plastid and Mitochondrial Inheritance. An example of plastid inheritance is the transmission of leaf variegation in four o'clock and other plants. When the maternal plant is entirely green, regardless of the leaf coloration of the paternal plant, only green offspring appear. Conversely, unpigmented maternal plants yield only white offspring, even if pollen was derived from green plants. When the maternal plant is variegated, offspring of three types—green, white, and variegated—appear regardless of the paternal plant's characteristics. Leaf color is determined by plastids, the most important of which are METABOLISM/14.html">Chloroplasts containing the green pigment chlorophyll. In variegated plants, a proportion of plastids are incapable of synthesizing chlorophyll and appear white. Plastids reproduce autonomously within The Cell and are distributed randomly among daughter Cells. Since the only pathway for plastids to enter the zygote is through the cytoplasm of the egg cell (sperm cells practically lack cytoplasm), variegation is inherited maternally.

Mitochondrial inheritance. Each mitochondrion possesses its own DNA. Compared to nuclear DNA, Mitochondrial DNA (mtDNA) is circular, relatively small in size, contains a correspondingly limited set of genes, and lacks introns. In humans, mtDNA contains 16,569 nucleotide pairs. The Genetic Code of mitochondrial DNA differs from the universal code: certain triplets encode different Amino Acids, distinct nucleotide sequences function as stop codons, and adenine or cytosine is more frequently encountered at the third position. The majority of mitochondrial proteins are encoded by nuclear DNA, synthesized in the cytoplasm, and subsequently imported into the mitochondria. Only about 2% of mitochondrial proteins are encoded by plasmagenes and synthesized on mitochondrial Ribosomes. Mutations in Mitochondrial Genes are associated with various mitochondrial disorders.