Structural Hierarchy and the Cell Cycle
The foundation of heredity is the DNA molecule. Human somatic cells contain a diploid set consisting of 46 molecules. DNA combines with specialized proteins to form a deoxynucleoprotein strand—the chromatid.
The highest level of organization is the chromosome, whose composition is dynamic and depends on the stage of the cell cycle:
- During the main period of cell life: a chromosome consists of just one chromatid. Before preparation for division begins, the terms "chromosome" and "chromatid" are identical.
- Prior to division: replication occurs—DNA is doubled. The total number of chromosomes remains unchanged (46), but each now includes two sister chromatids. As a result, the nucleus contains 46 chromosomes and 92 DNA molecules. The cell retains a diploid chromosome set, but becomes tetraploid in terms of DNA content.
Morphofunctional States of Chromosomes
Chromosomes alternate between two alternative forms depending on cell activity:
- Interphase chromosomes (in a non-dividing cell). Maximally decondensed and visually indistinguishable. Despite their uncoiled state, their nuclear localization is strictly ordered. Each chromosome is individually anchored to the inner nuclear membrane at both ends and intermediate points.
- Metaphase chromosomes (during division). Undergo sharp condensation, shorten, and become clearly visible under a light microscope. It is precisely at the metaphase stage that chromosomes are described in cytogenetic studies for genetic counseling. Homologous chromosomes are paired, with the 23rd pair determining sex.
Structure and Informational Function of DNA
Nuclear DNA takes the form of a double helix consisting of two polynucleotide chains. They are firmly held together by complementary base pairing: adenine (A) always pairs with thymine (T), and guanine (G) always pairs with cytosine (C). This mutual complementation ensures the tightest possible packing of the chains.
DNA encodes the primary structure of proteins, transport and ribosomal RNAs, as well as the program of ontogenesis. The flow of information for protein synthesis goes through three stages:
- Transcription: synthesis of pre-mRNA on a DNA template in the nucleus.
- Processing and transport: maturation of mRNA and its export to the cytoplasm.
- Translation: assembly of the polypeptide chain on ribosomes according to mRNA triplets.
Chemical nuance: in RNA molecules, thymine (T) is always replaced by uracil (U).
Biological Barriers to Cloning
Since the nucleus of any somatic cell contains all the genetic information of an organism, it is theoretically suitable for creating clones. In practice, however, a problem arises related to the genome aging hypothesis.
Somatic cell lines do not undergo meiosis. It is believed that meiosis (in germ cells) "rejuvenates" the genome through hyper-repair processes of DNA and strict chromosome selection. Somatic cells accumulate mutations and age-related changes. As a result, clones derived from such nuclei (like Dolly the sheep) carry an "age load" from the donor from birth, which reduces their viability.