Revision summary of Topic/unit 8 of AQA A-Level Biology - the control of gene expression.
Comprehensive, pretty set of summary revision notes.
Used to achieve an A* at A-Level.
Mutations
Changes in the sequence of nucleotides in DNA molecules.
Types of mutation:
-> Insertion/deletion - where one or more nucleotide pairs are inserted or deleted from the sequence. This type of mutation alters the sequence of
nucleotides after the insertion/deletion point known as frameshift.
-> Duplication - one or more bases are repeated which produces frameshift.
-> Inversion - group of bases become separated from DNA sequence and then rejoin at the same position but in reverse order. This affects the amino
acid that is produced.
-> Translocation - group of bases become separated from DNA sequence on one chromosome and are inserted into DNA sequence on another
chromosome. This can often lead to significant effects on phenotype.
Causes of mutations:
Gene mutations can arise spontaneously during DNA replication and can be caused by mutagenic agents that affect DNA.
-> Chemical mutagens - including alcohol, benzene and substances in asbestos and tar in tobacco.
-> Ionising radiation - alpha and beta, but also UV and X-ray.
-> Spontaneous errors in DNA replication.
Mutations can either have neutral effects where the mutation causes no change to the organism e.g., in a case where mutation occurs in non-coding region
of DNA or is a silent mutation.
Mutation can be neutral when a change in tertiary structure of the protein has no effect on the organism.
Some mutations are beneficial e.g., humans developed trichromatic vision through a mutation.
Harmful mutations include mutation in CFTR protein which causes cystic fibrosis.
, Topic 8: the control of gene expression
Stem cells
Undifferentiated cells which can keep dividing to give rise to other cell types.
Types of stem cells include pluripotent cells that are able to give rise to many types of specialised cells apart from embryonic cells and totipotent cells which
can give rise to all types of specialised cells including embryonic cells.
Totipotent stem cells that are able to differentiate into any type of cell found in body and into extra embryonic cells such as those in placenta.
These cells are found in embryo at an early stage called blastomere - embryonic stem cells.
Totipotent cells in embryo are initially unspecialised however, when they become specialised they differentiate to form tissues which make up the foetus.
The cause of this is a change in gene expression where some genes are selectively switched on and others switched off.
There are a variety of different types of stem cells and they're named according to their ability to differentiate:
-> Totipotent - can form any type of cells in body plus extra embryonic cells.
-> Pluripotent - cells can form any cell type in the body, however cannot form extra embryonic cells. They are also found in the early stages of an embryo.
These are often used in replacing damaged tissues in human disorders.
-> Multipotent - can differentiate into other cell types but are more limited e.g., cells in bone marrow and umbilical cord.
-> Unipotent - cells can only differentiate into one type of cell.
Pluripotent cells also have number of different uses in repairing damaged tissue (shown in table).
Pluripotent stem cells can also be created from unipotent stem cells and are known as induced pluripotent stem cells (iPS).
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