DNA is an abbreviation for deoxyribonucleic acid, but it is usually known by its initials alone. DNA is found in practically all living organisms, and it is now known to carry genetic information from one cell to the next, and from one generation to the next. The units of inheritance, called genes, are actually sections of the DNA molecule. Nuclei of the cells of higher organisms contain thread-like bodies called chromosomes, which consist of DNA, wrapped around proteins.
So understanding how the DNA molecule behaves inside cells helps explain how genetics works at the simplest level.
In the nucleus of every normal cell of the human body there is over 1 metre of DNA, divided between 46 chromosomes.
DNA is a fascinating substance, because it can split into 2 halves, each of which can be built up to re-form the missing sections. It is, therefore, a molecule which is able to reproduce itself – essentially a characteristic of living organisms. For this reason, DNA is sometimes called the basis of life. It can also pass instructions out to the cytoplasm, in order to control the way the cell operates.
In the cell there are also other forms of a similar substance, RNA , ribonucleic acid, which are used to turn the genetic information into proteins that the cell needs. These proteins are mostly enzymes, used to control chemical activities in the cell, collectively known as its metabolism.
The structure of DNA
DNA is an example of a macromolecule, i.e. a large molecule with a special shape, which is built up from many smaller parts called sub-units . If you could magnify part of a nucleus, you would see the DNA molecule looking like a twisted rope ladder – a double helix. The two strands forming the sides of the ladder give it a strong yet flexible structure, which does not vary along its length. Stretched between these are the “rungs” of the ladder, the parts of the DNA molecule which vary, and so the differences carry genetic information. These parts are made up of sections called bases, which fit together in pairs. The 4 bases (so called because on their own they react with acids) are also usually known by their initials, as shown alongside:
A (adenine), paired with T (thymine) and C (cytosine) paired with G (guanine).
Since T pairs with A, and G with C, there are actually 4 different possibilities at any position on a strand. The sequence or order of these bases in DNA is used to store and pass on the genetic information, in a similar way to computer data on a disc or tape.
If one strand of DNA has the base sequence C A T G A G C G C G A T , what will be the sequence on the other strand? > GTA CTC GCG CTA
How DNA replicates
Understanding this goes a long way to explaining how nuclei divide in the process of mitosis , which results in identical copies of chromosomes being transferred during ordinary cell division.
Before a cell divides, its nucleus must divide. But before that happens, the chromosomes must have become double. So the first stage is that DNA which the chromosomes contain must replicate , i.e. become double, by making copies of itself.
The 2 strands of the DNA double helix can separate, under the influence of special enzymes in the nucleus, but each half remains attached along its length, like the 2 sections of a zip, because the sides of the strands are strongly joined.
In the diagrams below, write in the letters for the various bases (using the first few as a key). This should help you understand the results of the process.
Each strand then acts as a basis for rebuilding the missing other strand from which it has been separated. It is said that each strand forms a template on which it reforms its complementary strand.
Enzymes within the nucleus match the appropriate base, which is already attached to strand side subunits, so that A fits against T, G against C, T against A and C against G, according to shape.
Other possibilities are not allowed, so the copying process is accurate in the vast majority of cases.
The result is that one double strand is converted into two identical double strands.
It is interesting to note that each “new” double strand is in fact half composed of a section of the previous DNA molecule, together with a completely new section built up from individual bases. Because of this, it is called semi-conservative replication.
DNA and NUCLEAR DIVISION
These 2 double strands form the 2 sections of chromosomes (called chromatids) that are easily seen when a cell is about to divide. In mitosis the chromosomes are then evenly distributed to different ends of the cell, ready to be incorporated into 2 new cells when the cell itself divides.
For clarity, only 2 pairs of chromosomes are shown in these diagrams
In mitosis, the nucleus divides once to produce 2 nuclei, which then form into 2 genetically identical “ordinary” cells, containing the same number of chromosomes as the original cell (46 in human cells) Because of the reliability of the replication of DNA and mitosis, offspring resulting from asexual reproduction do not usually vary at all, which is the basis of taking cuttings, etc. Similarly, multicellular organisms consists of a harmonious population of identical cells derived from one initial cell, the fertilised egg or zygote.
However, in some cases (about 1 in a million) there may be an error in the copying process; an incorrect copy of the DNA will be passed on to any (body) cells produced following cell division. This may be the cause of different types of cancer, which are associated with exposure to radiation or chemicals and viruses which damage DNA.
Sexual reproduction and meiosis
DNA also replicates reliably in the process of meiosis, which happens before sex cells ( gametes ) are produced, but only half the normal number of chromosomes (and hence genes, and DNA) are distributed to each gamete. The sharing process in halving the number of chromosomes also includes elements of “scrambling” which introduce variation, so each gamete has a unique DNA content.
In meiosis, the nucleus divides twice to produce 4 nuclei, which then form into 4 genetically different sex cells (gametes), each containing half the number of chromosomes of the original cell (23 in human cells) Therefore every organism produced as a result of sexual reproduction varies. However, the DNA built into the nucleus of a gamete may also be changed due to a random event called a mutation, which may alter or even prevent the normal activity of a gene inside cells. In this way a different form of the gene, called an allele , is produced, and will possibly be passed on to the next generation. Because each chromosome usually has a partner in the nucleus, the effect of a mutant allele may be hidden by the DNA of a normal allele of that gene which produces a normal characteristic. This is the basis of dominance and recessiveness in genetics.