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 gives rise to trait variations between individuals and populations.
Chromosomes are the body’s genetic material as they possess all four characteristics.
Figure 1.24 Expression of stored information
| ACTIVITIES 1.7 AND 1.8 |
1.7. Explain and contrast a chromosome and a gene.
1.8. Identify the role of the following cellular components in the storage, expression and transmission of genetic information:
• chromatin;
• nucleus;
• ribosome;
• mitochondrion;
• centromere.
| SUMMARY |
• Cells are made up of organelles and chromosomes.
• Chromosomes are composed of DNA that encodes for proteins.
• There are 23 pairs of chromosomes in the nucleus of every cell (46 in total). There are 44 autosomes (22 pairs) and 2 sex chromosomes (X and Y).
• 23 individual chromosomes are inherited from each parent.
• Cells replicate to produce identical cells by mitosis. To halve the chromosomal number in germ cells, the cells replicate by meiosis.
• There are four different bases included in the DNA structure. A sequence of three bases (a codon) code for one amino acid. Amino acids link together to form protein.
• RNA is needed to copy and carry the genetic code out of the nucleus and to assemble the amino acid chain within the cytoplasm.
• Proteins are assembled following transcription and translation of the genetic code.
• Mitochondria have their own genome, although most mitochondrial proteins are coded for by the nuclear genome.
FURTHER READING
There are many good physiology texts that have a whole chapter dedicated to the biology of the cell.
Marieb, E. and Hoehn, K. (2006) Human anatomy and physiology. Harlow: Pearson International
Martini, F.H. and Nath, J.L. (2008) Fundamentals of anatomy and physiology. Harlow: Pearson International
Stanfield, C.L. and Germann, W.J. (2007) Principles of human physiology. Harlow: Pearson International
There are also some more in-depth texts on cellular biology.
Cooper, G.M. and Hausman, R.E. (2009) The cell: A molecular approach. Basingstoke: Palgrave Macmillan
Alberts, B., Bray, D., Hopkin, K., Johnson, A., Lewis, J., Raff, M., Roberts, K. and Walter, P. (2009) Essential cell biology. Oxford: Garland Science
For interactive web pages on cellular activities, the following websites provide some good animations in cellular activities.
www.cellsalive.com
