The DNA within the chromosomes contains coded instructions for the production of protein. The coded area for the production of a specific protein is called a gene.
The structure of DNA
The structure of DNA was discovered through X-ray diffraction back in 1953 by the Nobel Prize-winning scientists James Watson and Francis Crick. DNA is composed of bases, sugars and phosphates that combine together to form a double helix. The double helix shape looks like a twisted ladder. The ‘sides’ of the ladder are made of phosphates and sugars, while the ‘rungs’ of the ladder are made of bases. Only four different types of bases exist within the DNA:
• Adenine (A);
• Guanine (G);
• Cytosine (C);
• Thymine (T).
DNA bases pair up with each other to form the ‘ladder rungs’ (see Figure 1.19). Adenine always pairs with Thymine, and Guanine always pairs with Cytosine. Only these two types of base pairing exist in DNA. The order of the base ‘rungs’ along the DNA ladder varies but the base pairings are always complementary.
Figure 1.19 DNA bases
The sequences of bases on one DNA strand can be deduced from the sequence on the opposite strand, because base pairing is always complementary. Each strand independently carries the information required to form a double helix. Therefore, to describe a DNA sequence, only the sequence of the bases in one strand is needed, for example ATTGCAAT, as the other strand is always complementary, i.e. TAACGTTA. Human DNA consists of about 3 billion bases, of which over 99 per cent of the sequence is identical in all people. These bases, within the DNA, form the code for the production of proteins.
PROTEIN
All the functions of the cell depend on protein. Protein maintains cell structure, acts as both intracellular and extracellular messengers, binds and transports molecules and acts as enzymes.
Some proteins exist in every cell, such as the enzymes involved in glucose metabolism. Other proteins are highly specialised and are only found in specialised cells, such as the protein myosin, found only in muscle cells, or the protein insulin that is only produced in pancreatic islet cells.
What are proteins?
Proteins are made up of long chains of amino acids. There are only 20 different types of amino acids but, by varying the order and amount of amino acids in the chain, thousands of different proteins can be produced.
Links within the chain of amino acids are called peptide bonds, while the chain itself is known as a polypeptide. A protein can contain one or more polypeptides. Both the structure and function of the protein depend on the sequence of the amino acids making up the polypeptide chains.
In order to function, cells need information to produce proteins and the ability to pass this information on to new cells during cell division. This important information is provided by the DNA.
How are proteins made?
Proteins are not made in the cell nucleus but by the ribosomes in the cell’s cytoplasm. The coded information in the DNA has to be transferred out of the nucleus. This is done by the use of ribonucleic acid (RNA).
Step 1: Copying the code
Segments of the DNA within the chromosomes separate at specific points and the DNA code is copied. This copy is called the messenger RNA (mRNA). During this process Guanine pairs with Cytosine and Adenine pairs with Uracil. RNA does not have Thymine but this is replaced with Uracil. Once a copy has been made, the DNA reattaches and the mRNA makes its way out of the nucleus into the cytoplasm (see Figure 1.20).
Figure 1.20 Copying the code (transcription)
Step 2: Reading the code
Once out in the cytoplasm, the mRNA attaches itself to a ribosome. Also present in the cytoplasm are amino acids, which are attached to a different type of RNA called transfer RNA (tRNA). The tRNA is only a short molecule of three bases that is attached to a corresponding amino acid. If the messenger RNA, which is attached to the ribosome, has three codes that correspond to the code on the transfer RNA, then the amino acid will be released by the transfer RNA. The released amino acid will then join other amino acids through the same process to form a protein molecule (see Figure 1.21).
Figure 1.21 Reading the code (translation)
Figure 1.22 Making the protein
Step 3: Making the protein
When a peptide bond has been formed between amino acids they detach from the transfer RNA. The protein will now be constructed (see Figure 1.22).
Table 1.3 Summary of the main processes
| Structure | Process | Function |
| DNA | None | Carries the genetic code |
| Messenger RNA (mRNA) | Transcription | Copies the code for a single protein from the DNA. Carries the copied code to the ribosomes |
| Ribosome | Translation | Reads the mRNA code and assembles the correct amino acid sequence |
| Transfer RNA (tRNA) | None | Brings individual amino acids from the cell cytoplasm to the ribosomes |
The RNA code is written in a trinary code. Three bases code for one amino acid; this is known as a codon. There are four bases in RNA (Adenine, Guanine, Cytosine and Uracil) so a total of 64 possible combinations of codons can be achieved. As there are only 20 different types of amino acids, some amino acids can be coded for by more than one codon. This is referred to as degeneracy in the genetic code.
Some codons do not code for any amino acids but act as a start or stop signal. AUG (Adenine, Uracil, Guanine) has been recognised as a start codon and UAG, UGA and UAA act as stop codons.
The RNA base code for all amino acids has been deciphered since the 1960s and is known as the universal genetic code (see Figure 1.23).
The universal genetic code is based on the codons from the RNA where Uracil has replaced the Thymine base in the DNA. There is only one type of DNA whereas there are three different types of RNA (Table 1.4).
• mRNA: messenger RNA is a direct copy of DNA that codes for specific amino acids.
•
