3.8 The control of gene expression Flashcards
substitution of bases
A nucleotide in a section of DNA molecule is replaced by another nucleotide that has a different base.
Depending on what new base is substituted, there are 3 possible outcomes:
Formation of 1 of the 3 stop codons. Stop the polypeptide production prematurely.
Final protein will be significantly different and couldn’t perform its normal function
Formation of a codon for a different amino acid, meaning the structure of the polypeptide produced would differ in a single amino acid.
The protein may differ in shape and not function properly
Formation of a different codon but 1 that produces a codon for the same amino acid before.
This is because genetic code is degenerate and so most amino acids have more than 1 codon.
deletion of base
A loss of a nucleotide base from a DNA sequence.
Minor though the loss of a single base might seem, the impact on the phenotype may be large.
The 1 deleted base will cause what is known as a frame shift because the reading frame that contains each 3 letters of the code has been shifted left by 1 base.
The gene is now read in the wrong 3-base groups and the coded information is altered.
1 deleted base at the very start could alter every triplet in the sequence
1 deleted base towards the end could have a smaller impact but can still have consequences.
other types of gene mutation
Addition of bases- an extra base is inserted in the sequence. Usually has a similar effect to base deletion in that there is usually a frame shift and the whole sequence of triplets is altered. Frame shift is to the right.
Duplication of bases- 1 or more bases are repeated. Causes a frame shift to the right.
Inversion of bases- a group of bases are separated from the DNA sequence and re-join at the same position but in the reverse order.
Translocation of bases- group of bases become separated from the DNA sequence on 1 chromosome and become inserted in the DNA sequence of a different chromosome. Translocations often have significant effect on gene expression leading to an abnormal phonotype. Effects include developing cancer and infertility
mutation causes
Can occur spontaneously during DNA replication.
Spontaneous mutations are permanent changes in DNA that occur without any outside influence.
Occur with a predictable frequency
This basic mutation rate can be increased by outside factors known as mutagenic agents or mutagens. These include:
High energy ionising radiation- for example alpha and beta particles as well as short wavelength radiation such as X-rays and ultra violet light. These can disrupt the structure of DNA as it ionises DNA.
Chemicals such as nitrogen dioxide may directly alter the structure of DNA or interfere with transcription. Benzopyrene, a constituent of tobacco is a powerful mutagen that inactivates a tumour-suppressor gene TP53 leading to cancer
cost and benefits of mutations
They produce the genetic diversity necessary for the natural selection and speciation. On the other hand they are almost always harmful and produce an organism that is less well suited to its environment. Mutations that occur in body cells rather than in gametes leading to disruption of normal cellular activities such as cell division
stem cells
an organism develops from a single fertilised egg. Cells such as fertilised specialisation is irreversible in most animal cells.
In mature mammals, only a few cells retain the ability to differentiate into other cells.
Stem cells are undifferentiated dividing cells that occur in adult animal tissues and need to be constantly replaced. They therefore have the ability to divide to form an identical copy of themselves in the process of self-renewal.
where stem cells come from
Embryonic stem cells- come from embryos in the early stage of development. Can differentiate into any type of cell in the initial stages of development
Umbilical chord blood stem cells- derived from the umbilical chord blood and are similar to adult stem cells
Placental stem cells- found in the placenta and develop into specific types of cell
Adult stem cells- found in body tissue of foetus through to adult. Are specific to a particular tissue or organ within which they produce the cells to maintain and repair tissues throughout an organisms life.
totipotency
an organism develops from a single fertilised egg. Cells such as fertilised eggs, which can mature into any type of cell, are known as totipotent cells.
The early cells that are derived from the fertilised egg are totipotent. These later differentiate and become specialised for a particular function. This is because during the process of cell specialisation, only some of genes are expressed, meaning only part of DNA is translated into proteins. The cell therefore only makes those proteins that it requires to carry out its specialised function. These proteins include those required to carry out specialised processes like respiration and membrane synthesis. Although it is still capable of making all the other proteins, these are not needed and so it would be wasteful to produce them. In order to conserve energy and resources, a variety of stimuli ensure the genes for these other proteins aren’t expressed.
The ways in which genes are prevented from expressing themselves include:
Preventing transcription and so preventing the production of mRNA
Preventing translation
totipotent stem cells
found in early embryo and can differentiate into any type of cell.
the zygote is totipotent and as it divides, it cells develop into slightly more specialised cells called pluripotent stem cells
pluripotent stem cells
found in embryos and can differentiate into almost any type of cell
multipotent stem cell
found in adults and can differentiate into a limited number of specialised cells. usually develop into cells of a particular type
unipotent stem cells
can only differentiate into a single type of cell. are derived from multipotent cells and are made into adult tissue
induced pluripotent stem cells
iPS cells
produced from unipotent stem cells
body cells are genetically altered in the lab to make them acquire the characteristics of embryonic stem cells. involves inducing genes and transcriptional factors within the cell to express themselves
are not exact replicates of embryonic stem cells
are capable of self-renewal, so could divide indefinitely to provide a limitless supply
pluripotent in treating human disorders
can be used to regrow tissues that have been damaged in some way i.e. accident ( skin grafts for serious burn damage) or a result of disease (neuro-degenerative disease such as Parkinson’s)
effects of oestrogen on gene transcription
is lipid-soluble and so diffuses easily through the phospholipid portion of membrane
once inside the cytoplasm, oestrogen binds to a site on the receptor molecule of the transcriptional factor. (complementary shape)
by binding, oestrogen changes the shape of DNA binding site on the transcriptional factor, which can now bind to DNA
transcriptional factor can enter the nucleus through a nuclear pore and bind to a specific base sequence on DNA
combination of transcriptional factors with DNA stimulates transcription of the gene that makes up the portion of DNA
general principles in controlling expression of a gene by controlling transcription
for transcription to occur the gene is switched on by specific molecules that move from the cytoplasm into the nucleus. the molecule is called a transcriptional factor
each factor has a site that binds to a specific base sequence of DNA in the nucleus.
when binding, it causes the region of DNA to begin transcription
mRNA is produced and the information it carries is translated into a polypeptide
when gene isn’t expressed, the site on the factor that binds to DNA isnt active
as it is inactive, it cannot cause transcription
epigenetics
now believe environmental factors can cause heritable changes in gene function without chagning the base sequence of DNA
provides explanation as to how environmental influenences such as diet, stress, toxins ect can subtly alter the genetic inheritance of an organisms offspring
epigenome
DNA is wrapped around proteins called histones. both the DNA and histones are covered in chemicals called tags.
determines the shape of the DNA-histone complex
e.g. it keeps genes that are inactive in a tightly-packed arrangement and therefore ensures that it cant be read (epigenetic silencing). it unwraps active genes so DNA is exposed
is flexible as chemical tags respond to environmental changes
the accumulation of the signals it has received during its lifetime and so acts as a cellular memory
DNA- histone complex
association of histones with DNA is weak and so the DNA-histone complex is considerably less condensed. the DNA is accessible by transcriptional factors which can initiate production of mRNA
where assiation is stronger, DNA-histone complex is more condensed and so DNA is not accessible by transcriptional factors which cannot initiate production of mRNA
condensation of the complex therefore inhibits transcription.
can be brought about by decreased acetylation of histones or methylation of DNA
