Define the term gene mutation.
A gene mutation is any change to the base (nucleotide) sequence of DNA.
State how gene mutations may arise and name factors that can increase the mutation rate.
Gene mutations may arise spontaneously during DNA replication, and the rate of mutation can be increased by mutagenic agents such as ionising radiation, ultraviolet radiation, and certain chemicals.
List the main types of gene mutation.
The main types of gene mutation include substitution, deletion, addition (insertion), duplication, inversion, and translocation of bases
Explain how a mutation can alter the structure and function of a protein.
The sequence of bases in a gene determines the sequence of amino acids in the encoded polypeptide. Therefore, mutations may result in a change to the amino acid sequence of a polypeptide, which may alter the protein’s structure and function.
Substitution Mutations
Describe what is meant by a substitution mutation.
A substitution mutation occurs when one base is replaced by another.
Describe the three possible effects of a substitution mutation.
It may produce a stop codon, causing premature termination of the polypeptide and resulting in a non-functional protein.
It may code for a different amino acid, changing the primary structure of the polypeptide, which may alter the tertiary structure and function of the protein, for example by changing an enzyme’s active site.
It may code for the same amino acid due to the degenerate nature of the genetic code, in which case the mutation has no effect on the polypeptide produced.
Degenerate Genetic Code
Explain what is meant by the degenerate nature of the genetic code.
The genetic code is degenerate, meaning that most amino acids are coded for by more than one DNA triplet.
Deletion, Addition, and Duplication Mutations
Explain how deletion, addition, and duplication mutations affect a gene.
Deletion, addition, and duplication mutations change the number of bases in a gene. These mutations usually cause a frameshift, which alters the reading frame of base triplets downstream of the mutation.
Explain the effect of a frameshift mutation on a polypeptide.
As a result, all subsequent codons are changed, leading to a significantly altered amino acid sequence and usually a non-functional protein.
Explain why adding or deleting three bases may not cause a frameshift.
If three bases or a multiple of three bases are added or deleted, no frameshift occurs, but the polypeptide will still differ from the original.
Inversion Mutations
Describe an inversion mutation and its possible effects.
An inversion mutation occurs when a sequence of bases is reversed. This may or may not alter the amino acid sequence, depending on whether the resulting codons code for different amino acids.
Translocation Mutations
Describe what is meant by a translocation mutation.
A translocation occurs when a sequence of bases is moved to another location in the genome, either within the same chromosome or to a different chromosome.
State the potential effects of translocation mutations.
Translocations often have major effects on gene expression and can lead to abnormal phenotypes, including cancer and reduced fertility.
Outline the possible consequences of mutations.
Mutations may alter the tertiary structure of proteins.
Mutations may prevent enzymes from binding substrates.
Mutations may cause genetic disorders such as cystic fibrosis or sickle-cell anaemia.
Mutations may increase the risk of cancers, such as mutations in the BRCA1 gene.
Mutations may be inherited if they occur in gametes (hereditary mutations).
Mutations may occur in somatic cells, potentially leading to uncontrolled cell division, such as cancer.
State the key skill students must demonstrate when answering questions on mutations.
Students must be able to relate the type of gene mutation to its effect on the encoded polypeptide.
3.8.2.1 Most of a Cell’s DNA Is Not Translated- STEM CELLS
Explain why not all genes are expressed in every cell.
All cells in an organism contain the same genes, but not all genes are expressed in every cell. Cell specialisation occurs because different cells transcribe and translate different genes.
Totipotent Cells
Define totipotent cells.
Totipotent cells can divide and produce any type of body cell, including extra-embryonic tissues such as the placenta.
State when totipotent cells occur in mammals.
In mammals, totipotent cells occur only for a limited time during the early divisions of the embryo.
Pluripotent Cells
Describe pluripotent stem cells.
Pluripotent stem cells are found in embryos and can differentiate into any type of body cell but not placental cells. Pluripotent stem cells can divide in unlimited numbers and have potential uses in treating human disorders.
Describe unipotent stem cells and give an example.
Unipotent stem cells can differentiate into only one type of cell. Cardiomyocytes are an example of unipotent cells involved in heart muscle repair.
Multipotent and Unipotent Cells
Describe multipotent stem cells and give an example.
Multipotent stem cells are found in mature mammals and can differentiate into a limited range of related cell types, such as blood cells from bone marrow stem cells.
Induced Pluripotent Stem Cells (iPS Cells)
Explain how induced pluripotent stem cells are produced.
Induced pluripotent stem cells are produced from adult somatic cells by introducing specific transcription factors that reactivate genes associated with pluripotency.
Explain what iPS cells demonstrate about adult cells.
This demonstrates that adult cells retain the same genetic information as embryonic cells.
Gene Expression and Cell Differentiation
Explain how gene expression leads to cell differentiation.
During development, totipotent and pluripotent cells become specialised because only part of their DNA is transcribed and translated.
Genes that are expressed are transcribed into mRNA and translated into proteins.
These proteins determine the structure and function of the cell and may also control the expression of other genes.
Once cells become specialised, these changes are usually irreversible.
