The structure of deoxyribonucleic acid, or DNA, was first described in 1953 by a paper in the journal Nature and authored by James Watson and Francis Crick. Based on chemical, x-ray diffraction, and structural studies, DNA was described to be a right-handed double helix, where 10 nucleotides comprise a helical turn (Watson & Crick, 1953). The DNA molecule is made up of a long linear polymeric strand made up of numerous deoxyribonucleotides. The nucleotide bases (pyrimidine and purine) are adenine (A), guanine (G), cytosine (C) and guanine (G).
Two DNA strands run in opposite (antiparallel) directions, with the planes of the nucleotide bases perpendicular to each other. The deoxyribose groups are connected to each other through diester phosphate bonds that lie outside the helix, and are arranged similar to the rungs in a ladder. The two DNA strands are connected by non-covalent hydrogen bonds between the bases adenine (A) and thymine (T), and guanine (G) and cytosine (C). The pairings are highly specific and complementary. A and T are connected by two hydrogen bonds, while G and C are bound by three H bonds.
Although the sequences and number of bases are highly variable, the specific pairing of the bases suggests a mechanism for copying the DNA (Watson & Crick, 1953). This initial observation of the DNA structure has led to the central dogma of molecular biology which describes the two major functions of DNA: to copy itself in the process of replication and to carry the genetic information in the processes of transcription and translation therefore dictating the phenotype of the cell (Kornberg & Baker, 2005).
While replication involves the whole DNA strnd, the process of transcription occurs for segments of DNA called genes. All processes in the central dogma are tightly regulated. DNA replication is semi-conservative where one parent strand pairs with a newly formed daughter strand to produce a new DNA duplex with the same sequence. During the cell division process, whole chromosomes are replicated. Chromosomes are packed structures of DNA molecules which contain the totality of the DNA of the cell or the genome. Different organisms have different number of chromosomes.
The chromosome theory of heredity that explains how paternal and maternal parents contribute equally to the genotype of the progeny, came about after it was observed that sex cells (sperm and egg nuclei) contained half the number of chromosomes compared to the other cells (Alberts, Johnson, Lewis, Raff, Roberts, & Walter, 2002). Germ, sex cells or gametes are formed through the nuclear division process of meiosis, where the chromosome complement is halved to form haploid cells, in contrast to diploid cells before meiosis. Meiosis occurs after the completion of DNA replication when the chromosomes of both parents have already duplicated.
The duplicate chromosomes from a parent tightly bind to each other forming sister chromatids. Then, the chromosomes pair with their respective homologs coming from the other parent, to form bivalents, in a process that can last for long periods of time. During the pairing period (also called the prophase), some recombination can occur when some portions of a chromatid can be exchanged with another fragment on its pair. Subsequently, the bivalents separate and move to opposite poles of the cells followed by cell division. After this stage, the chromosome number still remains the same.
A second cell division is necessary to form the gametes nuclei. This occurs when the sister chromatids separate to form cells that contain half the DNA content. A single cell that undergoes meiosis will produce 4 haploid cells (Alberts, Johnson, Lewis, Raff, Roberts, & Walter, 2002). During the process of fertilization, each parent’s sex cells will contribute equally to the fertilized egg to restore the original number of chromosomes. Through this process, the progeny will inherit the DNA of both parents, therefore producing an individual with genetic contribution from both parents but not exactly the same to only one of its parents.