Deoxyribonucleic acid (DNA) is a very important molecule found in all living cells. It contains information used in everyday metabolism and growth and influences most of our characteristics.
DNA is often described as the blueprint of an organism. It enables various cells to develop and work together to form a fully functional body and controls characteristics such as eye colour. How much DNA influences very complex features, such as intelligence, is not yet fully understood.
The information that DNA contains is passed from one generation to the next. There is much debate over how much of what we are like is due to inheritance and defined by our DNA, and how much is defined by the influence of the environment. This is sometimes referred to as the ‘nature/nurture’ debate.
Using gene technology, DNA can be modified or transferred from one organism to another. Genes are made up of short lengths of DNA and modern gene technology is able to make changes at the level of individual genes.
Where is DNA is Located?
You are made up of billions of cells. Current estimates put the figure somewhere between 10 billion and 100 billion cells. Inside nearly every cell is a nucleus containing your own unique set of 46 chromosomes. Each of these chromosomes consists of a compact coil of an incredibly long molecule of deoxyribonucleic acid (DNA).
DNA is so tightly coiled that approximately 1.8 metres of it is able to fit into the nucleus of a human cell.
DNA stores all the coded information needed for everyday growth and metabolism. Its information is passed down generations and influences your appearance and the way your body functions. Almost every cell in your body has a nucleus. Inside the nucleus is where you find the DNA, wrapped up as chromosomes
The Full Set
Half of your DNA comes from your mum and half from your dad. When the sperm and egg combined to make you, 23 chromosomes from the egg combined with 23 chromosomes from the sperm to form a full complement of human DNA – 46 chromosomes.
Chromosomes pair up and copy themselves every time before cells divide. This division happens billions of times in your lifetime as you grow, and to replace old cells (like skin cells or cells in the lining of your mouth).
If a cell is stopped during cell division and stained with Giemsa dye, the 23 pairs of human chromosomes are visible with a light microscope. The dye stains regions of chromosomes that are rich in the base pairs adenine (A) and thymine (T), producing banding patterns in the chromosomes, each one different from the rest.
Chromosomes are best seen during cell division when they bunch up ready to copy themselves.
DNA Structure- What does DNA look like?
Surprisingly, while the DNA molecule is very long, it is stunningly simple. DNA looks like an incredibly long twisted ladder. This shape is called a double helix. The sides of the ladder are a linked chain of alternating sugar and phosphate molecules. The rungs connect to the sugar molecules and are known as bases.
There are four bases
- Adenine (A)
- Thymine (T)
- Guanine (G) and
- Cytosine (C).
Each rung is made up of two bases that link together. Because of their chemical nature, A will only link with T and G will only link with C. DNA from all living organisms is made of the same sugar and phosphate molecules and the same four bases. Whether DNA is in your cells, those of cactus, of a worm or a bacterium, it is made of the same chemicals and has the same structure.
The only difference is the order or the sequence of the bases in the DNA molecule. It is this sequence that is referred to as the genetic code, and why it is sometimes called the code of life. DNA is packaged so tightly together that even the thinnest bands contain over a million base pairs and potentially hundreds of genes.
The chromosomes can be matched in their pairs, arranged and numbered by size from largest to smallest based on the banding patterns that you see and the position of the centromere. The centromere is the central most condensed and constricted region of a chromosome. It is also the part that the spindle fibre attaches to during cell division, allowing the chromosomes to separate.
A human karyotype. The 23 pairs of chromosomes are lined up according to size, from largest to smallest. Can you tell whether this DNA came from a male or female?
Lining up the chromosomes produces an image called a karyotype.
Genetic diseases can result if a person:
- has too many or too few chromosomes
- is missing pieces of chromosomes
- has mixed up pieces of chromosomes.
Karyotyping is one of many techniques that can detect chromosomal abnormalities by looking at the number and structure of chromosomes. Cytogenetics is the study of chromosomes using a microscope.
Chromosome preparations can be taken from different types of tissue including blood, bone marrow, amniotic fluid, and embryonic tissue.
Function of DNA
DNA is an ideal molecule to transfer genetic messages to every cell of your body. When an egg and sperm met to form the first cell that was to become you, you were given the complete genetic code that all of your cells will use for the rest of your life.
In that first cell, half of the chromosomes (half of the DNA molecules) came from your father and the other half came from your mother.
The first cell divided to become two cells, these both divided to become four, then eight then 16 and so on. Some of the cells in your body are still dividing, for example, to produce new skin or blood cells. Most of the time a cell divides perfectly and each of the DNA molecules is copied exactly, with one copy going to each of the new cells. If mistakes are made, they are fixed or the cell is marked for destruction.
If a problem occurs in this process the new cells often die, but on rare occasions, the faulty cells survive and can cause a wide range of problems. However, sometimes these faults (mutations) can be beneficial for the organism: this is the basis for evolution.
In order to make a copy of itself, the DNA molecule unzips lengthwise, leaving unpaired bases along each backbone. Nucleotides, which are made up of a sugar, a phosphate and one of the four bases, float freely in the nucleus. Because A can only pair with T and G can only pair with C, the nucleotides match up with the unpaired bases along the DNA backbone. Like building blocks, they form a new strand that is complementary to (matching) the sequence. This forms strands identical to the original strand before it unzipped.
Several teams of scientists are trying to make a new form of a living being from non-living chemicals. They will need to find this new ‘Los Alamos Bug’ the equivalent of a cell wall and make sure it can metabolise and reproduce itself. The Bug will use a completely different way to hold genetic instructions than DNA. Currently, scientists are looking to use a molecule called peptide nucleic acid (PNA). Like DNA, PNA is made up of two strands containing the nucleotides A, T, G and C which complement each other, but the molecule itself is soluble in fat instead of water.
What is DNA Unknown/junk DNA or non-coding DNA?
In between the well-structured genes are large sections of DNA for which no function has yet been identified. These areas have been called ‘junk DNA’ or ‘non-coding DNA’ and make up a large proportion of the genomes of both plants and animals.
But is it junk at all?
We don’t really know. This DNA appears to act as a filler in between genes and a number of ideas are starting to emerge about what role it plays. This is a mystery to be solved in the next couple of decades.
Some of the ideas are:
- It is where defective genes, or pseudogenes, are dumped
It is the accumulated DNA of viruses that have infected the body and failed to take over the cell
- It acts as a protective buffer against genetic damage and harmful mutations because the area is irrelevant to the metabolic and developmental processes (if a random change occurs in the sequence, there is no effect on the body)
- it acts as a reservoir of sequences from which potentially advantageous new genes can emerge
Researchers believe that this unknown DNA probably plays some role in regulating the ‘coding DNA’ and therefore cellular processes. But there is currently very little knowledge about the relationship between non-coding DNA and the DNA of genes.
Onions contain 12 times more DNA per cell than humans. A pufferfish’s genome is only about one-tenth the size of the human, yet seems to have about the same number of genes. The ratio of functional DNA to ‘in-between filler’ DNA of the unknown function differs widely per species.
Chickens have a similar number of genes to humans: 20,000 to 23,000 for chickens and 25,000 to 30,000 for humans. But their genome is much smaller – they have 1 billion DNA bases, compared to about 3 billion in humans. The chicken genome appears to contain less repetitive non-coding DNA than the human genome.