Transcription and Translation
- The DNA of a gene is not used to make a polypeptide in the nucleus because this would be too slow a process. Instead, RNA copies of the gene's code are made.
- Many RNA copies of the coded information contained in a stretch of DNA can be made. This enables polypetide products to be produced rapidly.
- One strand of the gene's DNA is used to make the copies of mRNA, which having a matching code. This process is transcription.
- In many eukaryotes, mRNA is transcribed as pro- mRNA that contains both exons and introns.
- In a process called splicing, introns are removed from pro- mRNA and the exons are spliced to form mature mRNA.
- The mature mRNA passes out of the nucleus and attached to ribosomes in the endoplasmic reticulum.
- The endoplasmic reticulum has a plentiful supplt of tRNA molecules that are attached to specific amino acids. The tRNA molecules have anticodons that recognise and bind to the corresponding mRNA codon
Transcription and Translation Continued...
- As the mRNA moves through a ribosome, the amino acids carried by the tRNA are combined in the correct sequence to form the polypeptide. This process is translation.
- The polypeptides formed can then be used to make a specific protein, for example, an enzyme, a membrane protein or a structural protein.
- A gene mutation occurs when there is a change in the sequences in the DNA of a gene. Bases may be added, deleted or substituted. Segments of DNA may be invented or duplicated.
- A mutation produces a change in the DNA codons and is likely to result in a polypeptide with a different amino acid sequence.
- New alleles arise from mutations in existing alleles.
- Mutations in reproductive cells can be passed on to following generations, but mutations in body cells will only affect the tissue in which they occur.
- Mutations occur naturally at random, but the rate of mutation is increased by mutations such as radiationand some organic chemicals.
- The rate of cell division is controlled by proto-oncogenes that stimulate cell division and tumour-supressor genes that slow cell division.
- A mutated proto-oncogene, called an oncogene, stimulates cells to divide too quickly. A mutated tumour-suppressor gene is inactivated, allowing the rate of cell division to increase.
Control of Gene Expression
- Totipotent cells can mature into any body cel
- During development, totipotent cells translate only part of thier DNA, resulting in cell specialisaton.
- In mature plant cells many cless remain totipotent. In micropropagation, these cells can develop into organs or whole plants.
- Only a few totipotent cells, called stem cells, remian totipotent in mammals.
- Stem cells are used to treat some human disorders, but research into, and use of, stem cell raises moral and ethical issues.
Dicer and the RISC
- Transcription of target genes is stimulated only when specific transcriptional factors move from the cytoplasm into the nucleus.
- Transcriptional factors prevent transcriptionand thereby prevent the synthesis of mRNA.
- Cells in some types of breast cancer have oestrogen receptors in thier cell membranes. The binding of oestrogen with these receptors results in transcription switching on genes for cell growth and division. This results in rapid division of the cells forming the tumour.
- Small interfering RNA (siRNA) are short pieces of double-stranded RNA that interfere with the expression of specific genes by degrading mRNA