DNA, deoxyribonucleic acid, is a nucleic acid that is the chief constituent of the chromosomes carrying genetic information, in the form of genes, necessary for the organization and functioning of living cells. The molecular structure of DNA was first proposed by JD Watson and FH Crick in 1953.
The DNA consists of a double helix of two strands coiled around each other. Each strand is made up of alternating pentose sugar (deoxyribose) and phosphate groups, with an organic base attached to each pentose group. There are four bases: adenine (A); guanine (G); cytocine (C); and thymine (T). The bases on each strand are joined by hydrogen bonds which are always bound in the same way; A always binds with T; and G with C. During replication, the strands of the helix separate and each provides a template for the synthesis of a new complementary strand; thus producing two identical copies of the original helix. This special property for accurate self-replication enables DNA to duplicate the genes of an organism during the cell divisions of growth and production of cells for the next generation.
If it’s evolution and self-replication of cells then how is it that each man has a different DNA?
Chromosomes are thread like structures that carry the genetic information of living organisms, and are found in the nuclei of their cells. Chromosomes consist of a central axis of DNA with associated RNA and Proteins. Before cell division, the long filamentous threads contract and thicken, and each chromosome can be seen as two identical threads (which are known as chromatids.) joined at the centromere. The chromatids later separate to become the daughter chromosomes.
Chromosome number is characteristic of a species. For example, a human cell has 46 chromosomes comprising 22 matched pairs (which are called autosomes.) and two sex chromosomes. A human sperm or egg cell has half this number of chromosomes. Abnormal numbers or parts of chromosomes often lead to abnormalities in the individual concerned. Down syndrome, called mongolism, is caused by the presence of an extra copy of genetic material on 21st chromosome, either in full or in part.
Genes is a unit of the hereditary material of an organism that provides the genetic information necessary to fulfill a single function. The term gene was first coined by WL Johansen in 1909.
Genes were initially conceived as a string of beads comprising of the chromosome; they were later defined as lengths of chromosome that were physically indivisible during the exchange of chromosomal material that occurs during meiosis. Alternatively, a gene was defined as the shortest lenth of the chromosome that could undergo mutation.
However, with the discovery of the structure of DNA and the molecular basis of heredity, a gene was re-defined as “being a functional unit (cictron) corresponding to a specific sequence of the genetic code.” Structural genes code for individual polypeptides while regular genes control the activities of structural genes. Genes are means by which information for the organization and function of living cells is carried by DNA and RNA molecules. Evidence for the nature of genetic code was provided in the 1960s by the work of Crick, Nirenbeg, Khorana and many others.
They found that the basic symbol of the code was a sequence of three consecutive bases mentioned earlier as DNA or RNA. The different triplet codons or sequences couls specify the 20 or so amino acids commonly used by cells synthesis; and give start and stop signal for the process. Investigations in many species have shown that the code seems to apply universally.
RNA, ribonucleic acid, is a nucleic acid that is important in the synthesis of proteins in living organisms. In some viruses RNA is the genetic material. Structurally, it’s similsr to DNA but usually occurs as a single stranded molecule with the sugar ribose and thymine of DNA. There are three main types of RNA, most of which occur in the cell cytoplasm: ribosomal (r) RNA; messenger (m) RNA; and transfer (t) RNA.
The genetic code in DNA contains information necessary for protein synthesis. This code is transcribed into a strand of mRNA and carried to the risomes which are small cytoplasmic particles. There it is translated into a particular polypeptide chain. The amino acids making up the protein are brought to correct positions in chain of tRNA.
Peptide is a chemical compound comprising of a chain of two or more amino acids linked by peptide bonds (-NH-CO-) formed between the carboxyl and amino groups of adjacent amino acids.
Polypeptides, containing between three and several hundreds amino acids, are the constituents of proteins. Some peptides are important as hormones e.g. ACTH and as antibiotics e.g. bactracin, grad mecidin.
Mitosis plays its own part. It’s the process by which the nucleus of a somatic cell (i.e. any cell that is not a germ cell) duplicates itself exactly, producing two daughter nuclei with chromosomes that are identical to those of the parent nucleus. This nuclear division involves the separation of the two chromatids of each chromosome, which move apart to form two groups at opposite ends of the cell. In the final phase each group becomes enclosed in a new nuclear membrane. After this the cytoplasm usually divides to form two new cells. Mitosis occurs in most animals and plants during the normal growth and repair of tissues.
In this way meiosis, which is the process by that the nucleus of a germ cell divides prior to the formation of gamets, such as sperm, pollen or eggs; plays its own part to influence the whole life. Through meiosis the formation of four eggs or sperm cells from one parent cells take place in two divisions, each of which is divided into several phases.
