DNA and Genetics

DNA AND GENETICS

DNA is the basic carrier of genetic information in almost all forms of life and in all cellular life. In humans and higher order animals, each cell (except red blood cells) will have the complete DNA message, even though not all genes are expressed in every cell. There is nuclear DNA and mitochondrial DNA. Because no mitochondria enter the ovum from the sperm cell at the time of conception, all mitochondrial DNA comes from the mother.

As mentioned, there are four bases in DNA. These include adenine (A), thymine (T), cytosine (C), and guanine (G). The order of the bases in the DNA molecule is known as the “DNA sequence”. The double helix of the DNA consists of two sequences that match with each other. The base pairs are connected by means of weak hydrogen bonds, with certain bases only matches of A with T and G with C. This means that the complementary base is found on the opposite DNA strand. Figure 30 shows what the DNA strand looks like:

The DNA sequence is made up of nucleotides. A nucleotide is a pentose sugar molecule (deoxyribose sugar) that has a three-prime (hydroxyl side) and a five-prime (phosphate side) segment. The covalent bonds (strong bonds) connect the three-prime side on one sugar molecule to the five-prime side on another sugar molecule. Attached to the sugar molecule is a nitrogenous base (the A, T, G, C bases just mentioned). Figure 31 shows a guanine nucleotide with the pentose sugar and its base:

When nucleotides link together, they are called polynucleotides or, in the case of a long polynucleotide of deoxyribose nucleotides, DNA. There is a phosphodiester bond that connects the sugars, forming what’s called the backbone of the DNA molecule. The DNA molecule is stabilized as a double strand, with nitrogenous bases connecting to one another via hydrogen bonding. This is a weaker bond than the covalent bond seen in most molecules, which means it doesn’t take as much energy to split the two strands as it does to split each strand apart by itself. These strands, when connected, are considered anti-parallel. One strand runs from the five-prime to the three-prime end, while the opposite strand runs from the three-prime end to the five-prime end.