In this lesson we learn that genes are contained in an individual's DNA and are the instructions that make proteins.
The lesson consists of two videos. The first is about the structure of DNA and the second is about its’ function.
Structure of DNA
The structure of DNA, or deoxyribonucleic acid, was published in 1953 by James Watson and Francis Crick, scientists at the University of Cambridge in England. Knowledge of the structure of DNA enabled major scientific advances in the late 20th and early 21st centuries. Scientists have applied this knowledge to health and medical issues, for example by identifying defective genes. This knowledge has also been applied to animal research, and today the dog in particular is an important animal model in human genetics and medicine. Watson and Crick, together with Maurice Wilkins, were awarded the Nobel Prize in 1962 for their discovery (<- see link for more information).
DNA is a double-stranded, ladder-like molecule. The DNA chain is packed into a tight helix and proteins called histones finish the packing so that the DNA fits into the nucleus of the cell. A single cell can hold so much DNA that it would be up to two metres long if straightened out.
The rungs of the ladder hold two strands of nucleotides together. The steps are made up of pairs of four nucleotides, or bases, namely adenine (A), cytosine (C), thymine (T) and guanine (G). Adenine can only be a pair of thymine and cytosine of guanine. A dog, cat, horse, human, in fact all mammals have just under three trillion base pairs in their genome*. The order of the nucleotides is called the sequence. The sequence is unique to each individual. The only exceptions are identical twins.
*(Genome = the entire genetic material of an individual, all DNA)
Genome mapping = base sequence determined The dog genome was first mapped in 2004. For cats and horses, this happened a little later, in 2007. Genome mapping means determining the base sequence of all the DNA, the entire genome. This process is called sequencing. This makes it possible to search for abnormal DNA sites, for example when mapping a disease gene. In these mapping projects, Tasha the Boxer, Cinnamon the Abyssinian cat, and Twilight the English thoroughbred have helped the scientific world. Samples from these animals were used as research material for the projects.
Genes and amino acids
A gene consists of codes made up of three base pairs. These codes are called codons. Each codon corresponds to a particular amino acid. A particular chain of amino acids corresponds to a particular protein.
A gene begins with a start codon, which is recognised by the molecules that read the DNA. This is followed by the codons that code for the amino acids. At the end of the gene is a stop codon.
There are 20 different amino acids. Several codons can code for the same amino acid. For example, serine is encoded by the codons UCU, UCC, UCA and UCG (see table).
Function of DNA
When a cell needs a particular protein to function, the gene that codes for that protein is activated. The recipe formed by the gene is used to start making the protein. The process of making proteins is called protein synthesis.
During protein synthesis, the double helix of DNA splits at just the right place, and the base pairs (ladder steps) diverge. In the space between the start and stop codons of a gene, a molecule called messenger RNA is built to mirror the DNA strand.
Once made, the messenger RNA is released and carries the information about the base sequence of the DNA onward. It acts as a kind of postman. The recipe, transmitted by the messenger RNA, is read and used to build the protein: the transfer RNA reads the information in the messenger RNA and transfers the correct amino acid into the chain.
RNA stands for ribonucleic acid. In RNA, the thymine (T) of DNA is matched by uracil (U). Thousands of messenger RNA molecules are made in each cell every second. The lifespan of a single messenger RNA is a few minutes.
The process of making messenger RNA from a DNA recipe is called transcription. Translation, on the other hand, is the process by which RNA is turned into protein. Together, transcription and translation make up protein synthesis.
The genes that code for proteins cover only a few percent of an individual’s DNA. The rest was once thought to be meaningless filler. It was called junk DNA. We now know that much of this “junk DNA” is involved in gene regulation, which determines in which tissue or organ a gene should work, when, how, and how much. Gene regulation is a very active area of research at the moment, and we are learning more and more about it all the time.
What have we learned?
- DNA is a double-stranded, ladder-like molecule
- The rungs of the ladder are made up of pairs of four nucleotides (bases).
- A gene consists of codons made up of three base pairs.
- Each codon corresponds to a particular amino acid.
- A particular chain of amino acids corresponds to a particular protein.
- When a cell needs a particular protein to function, the gene that codes for that protein is activated. The recipe formed by the gene is used to make the protein.
- The process of making proteins is called protein synthesis.
- Dog genome assembled. MIT News 2004
- Domestic cat genome sequenced. Genome research, CSH Press 2007
- Horse Genome Assembled. National Institutes of Health 2007.
Check these terms in the great Talking Glossary of Genomic and Genetic Terms by National Human Genome Research Institute: