For instance, people who have B blood group have a certain gene resulting in a particular surface protein on red blood cells. Alleles are different, mostly functional, varieties of every gene. One of the ways in which this happens is through mutations in DNA molecules.Ĭhanges to the nucleotide sequence in genetic material allows for the formation of new allele. The structure of DNA ensures that the information coded within every polynucleotide strand is replicated with astonishing accuracy.Įven though it is important for DNA to be duplicated with a very high degree of accuracy, the overall process of evolution requires the presence of genetic variability within every species. In order to create a new generation, genetic information needs to be accurately duplicated and then transmitted. One of the main functions of any hereditary material is to be replicated and inherited. RNA is transcribed in the 5’ to 3’ direction. In the image, the coding strands and the template strands are depicted in orange and purple respectively. Since transcription is also dependent on complementary base pairing, the RNA sequence is nearly the same as the coding strand. This is called the template strand and the other strand is called the coding strand. Only one of the two strands of DNA is involved in transcription. The single-stranded region then acts as the template for the polymerization of a complementary polynucleotide RNA molecule. Like DNA replication, this begins with the transient formation of a single-stranded region. When a protein or RNA molecule needs to be produced, the first step is transcription. The structure and function of these molecules is determined by nucleotide sequences in DNA. Most catalytic and functional roles in the body are carried out by peptides, proteins and RNA. The second important function of genetic material is to direct the physiological activities of the cell. High-fidelity DNA polymerases and a host of error repair mechanisms ensure that there is only one incorrectly incorporated nucleotide for every 10 billion base pairs. This implies that nearly 3 billion nucleotides are accurately read and copied. Each time a cell divides, nuclear genetic material is duplicated. Even the maintenance of normal functions in an adult requires constant mitosis. After this, the entire dazzling array of cells and tissue types are produced by cell division. For humans, this is the zygote formed by the fertilization of an egg by a sperm. These enzymes continue along each strand creating a new polynucleotide molecule until the entire DNA is replicated. In this transient single-stranded region, a number of enzymes and other proteins, including DNA polymerase work to create the complementary strand, with the correct nucleotide being chosen through hydrogen bond formation. When a double-stranded DNA molecule needs to be replicated, the first thing that happens is that the two strands separate along a short stretch, creating a bubble-like structure. The sugar phosphate backbone, on the other hand, is common to all DNA molecules, across species, whether in bacteria, plants, invertebrates or humans. There are individual differences in these nucleotide sequences, but overall, for every organism, large stretches are conserved. For instance, when the human genome was sequenced, the nucleotides constituting each of the 23 pairs of chromosomes were laid out, like a string of words on a page. That is, the order in which nitrogenous bases appear within the macromolecule identify a DNA molecule. Replication and HeredityĮvery DNA molecule is distinguished by its sequence of nucleotides. A series of experiments demonstrated that among the four types of macromolecules within the cell (carbohydrates, lipids, proteins and nucleic acids), the only chemicals that were consistently transmitted from one generation to the next were nucleic acids.Īs it became clear that DNA was the material that was transferred from one generation to the next, its functions began to be investigated. When Mendel’s experiments on genetics were rediscovered, it became clear that heredity was probably transmitted through discrete particles, and that there was a biochemical basis for inheritance. DNA and its related molecule, ribonucleic acid (RNA), were initially identified simply as acidic molecules that were present in the nucleus. DNA was isolated and discovered chemically before its functions became clear.
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