Genetics, revised edition. Karen Vipond
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.
• tRNA: transfer RNA carries amino acids from the cytoplasm to the ribosomes.
• rRNA: ribosomal RNA facilitates interaction between mRNA and tRNA.
Figure 1.23 The universal genetic code
Table 1.4 The main differences between DNA and RNA
| DNA | RNA |
| double stranded | single stranded |
| deoxyribose sugar | ribose sugar |
| includes Thymine | includes Uracil |
| exists in one form | exists in different forms |
| ACTIVITIES 1.5 AND 1.6 |
1.5. The following table shows the sequence of bases on part of an mRNA molecule:
| Base sequence on mRNA | CCU CAA AGU GGU GUU CGA |
| Base sequence on DNA |
a. Complete the table to show the DNA base sequence.
b. By using the universal genetic code table in this chapter, identify which amino acids are coded for.
1.6. A particular strand of mRNA is 60 bases long. How many amino acids would this strand code for?
MITOCHONDRIAL DNA
Not all the DNA in the human cell is contained within the chromosomes in the cell nucleus. Mitochondria, in the cell’s cytoplasm, have their own DNA (the mitochondrial genome). This very small amount of DNA in the mitochondria is only inherited from the mother. Mitochondrial DNA is not inherited directly from the father as the mitochondria are placed in the tail of the sperm, which does not penetrate the ovum. In exceptional circumstances, when the tail of the sperm does manage to enter the ovum, the mitochondria are destroyed in the very early stages of embryo development.
The DNA within the mitochondria encodes for proteins that are essential for mitochondrial structure and function. This is a very small genome and most of the mitochondrial proteins are coded for by the nuclear genome.
THE CLASSIFICATION OF GENETIC MATERIAL
For any cellular structure to be classified as genetic, it must display four characteristics.
1. Replication.
2. Storage of information.
3. Expression of the stored information.
4. Variation.
1. Replication: this is achieved through the cell cycle when chromosomes are replicated in order to produce new cells.
2. Storage of information: chromosomes store all the information needed for the production of proteins. The genetic material within cells does not necessarily express all the stored information in every cell, only what is appropriate for that individual cell. For example, eye colour is not expressed in every cell, only in the cells which make up the iris of the eyes and the protein actin is only expressed in muscle cells and not in any other type of cell.
3. Expression of the stored information: expression is a complex process. Information flow requires DNA, RNA and cellular proteins (see Figure 1.24).
4. Variation: Genetic variation includes rearrangements between and within chromosomes as well as ‘crossing over’ during meiosis. This