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Summary Gene expression AQA A-Level Biology detailed revision notes, topic 20, unit 3.8.1 to 3.8.3. Section 8- The control of gene expression£6.99
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Detailed, colourful, nicely displayed revision notes with images on AQA A-Level Biology on section 6, topic 14, Gene expression (The control of gene expression). According to the AQA A-Level specification, these notes are on section 3.8.1 to 3.8.3. My notes are collected from many sources to ensur...
Mutations are changes in the sequence of nucleotides in DNA molecules. Types of
mutations include:
- Substitution – where a nucleotide in a section of DNA is replaced by another
nucleotide. There are three possible outcomes: the formation of a stop codon
meaning the production of a polypeptide ends prematurely, the formation of a
codon for a different amino acid so the structure of the polypeptide will differ, the
formation of a different codon but one that codes for the same amino acid as the
DNA code is degenerate.
- Insertion/deletion mutations - 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 a frameshift.
- Duplication - one or more bases are repeated and therefore produces a frameshift.
- Inversion - a group of bases become separated from the DNA sequence and then
rejoin at the same position but in the reverse order. This therefore affects the amino
acid that is produced.
- Translocation - a group of bases become separated from the DNA sequence on one
chromosome and are inserted into the DNA sequence on another chromosome. This
can often lead to significant effects on the phenotype.
Causes of mutations
Gene mutations can arise spontaneously during DNA replication, and can be caused by
mutagenic agents that affect DNA, causes of gene mutations are:
o Chemical mutagens - these include alcohol, benzene and substances in asbestos and
is tar in tobacco.
o Ionising radiation - alpha and beta, but also UV and X-ray.
o Spontaneous errors in DNA replication
Mutations can either have neutral effects where the mutation causes no change to the
organism, for example in a case where the mutation occurs in a non-coding region of DNA or
is a silent mutation. A mutation can also be neutral when a change in tertiary structure of
the protein has no effect on the organism.
Some mutations are beneficial, for instance, humans developed trichromatic vision through
a mutation. Harmful mutations include a mutation in the CFTR protein which causes cystic
fibrosis.
, Stem cells and totipotency
Although all cells contain all genes, only certain genes are expressed in any one cell at any
one time. Only the genes required for a cell to conduct the activities it is specialised for are
expressed. These are genes that code for example specific proteins and enzymes needed to
perform its function.
Stem cells are 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.
Sources of stem cells:
- Embryonic stem cells - can differentiate into any type of cell in the initial stages of
development
- umbilical cord blood stem cells
- placentas themselves- develop into specific types of cells
- adult stem cells - specific to a particular tissue or organ within which they produce
the cells to maintain and repair tissues throughout an organism’s life
Totipotent stem cells that are able to differentiate into any type of cell found in the body
and into extra embryonic cells such as those in the placenta. These cells are found in the
embryo at an early stage called the blastomere. These stem cells are sometimes called
embryonic stem cells.
The totipotent cells in the 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 selectivity switched on and others
switched off.
There are a variety of different types of stem cells and are named according to their ability
to differentiate. They are summarised below:
o Totipotent - can form any type of cells in the body plus extra embryonic cells.
o Pluripotent - these 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.
o Multipotent - can differentiate into other cell types but are more limited e.g. the
cells in the bone marrow and umbilical cord.
o Unipotent - these cells can only differentiate into one type of cell.
Pluripotent stem cells can also be created from unipotent stem cells and are therefore
known as induced pluripotent stem cells (iPS). These are very similar to embryonic stem
cells in form and function. They are capable of self-renewal which means they can
potentially divide indefinitely to provide a limitless supply. Therefore they could replace
embryonic stem cells in medical research and treatment and so overcome many of the
ethical issues surrounding the use of embryos in stem cell research.
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