celluar control

• A gene mutation is a change in the sequence of base pairs in a DNA molecule that may result in an altered polypeptide.
• Mutations occur continuously.
• Mutations can occur spontaneously, for no reason, during DNA replication.
• The probability of a mutation occurring can increase with the presence of certain factors known as mutagens, such as ionising radiation or deaminating chemicals.
• Ionising radiation, such as X-rays, can break the DNA strands which can then be altered during the repair process.
• Methyl or ethyl groups can be added to bases, leading to incorrect base pairing.
• Viruses can insert sections of viral DNA into the DNA of cells.
• Mutations usually have no effect on us as most mutations do not alter the polypeptide or only alter it slightly so that its structure or function is not changed, as the genetic code is degenerate.
• Triplets of nucleotides often code for the same amino acid.
• Many mutations occur in non-coding sections of DNA and have no effect on the amino acid sequence at all.
• A mutation in a gene can sometimes lead to a change in the polypeptide that the gene codes for, as the DNA base sequence determines the sequence of amino acids that make up a protein.
• There are three main ways that a mutation in the DNA base sequence can occur: insertion of one or more nucleotides, deletion of one or more nucleotides, and substitution of one or more nucleotides.
• An insertion mutation occurs when a nucleotide (with a new base) is randomly inserted into the DNA sequence, changing the amino acid that would have been coded for by the original base triplet.
• An insertion mutation also has a knock-on effect by changing the triplets (groups of three bases) further on in the DNA sequence, sometimes known as a frameshift mutation.
• A deletion mutation occurs when a nucleotide (and therefore its base) is randomly deleted from the DNA sequence, changing the amino acid that would have been coded for by the original base triplet.
• A deletion mutation also has a knock-on effect by changing the triplets (groups of three bases) further on in the DNA sequence, sometimes known as a frameshift mutation.
• A substitution mutation occurs when a base in the DNA sequence is randomly swapped for a different base, changing the amino acid for the triplet (a group of three bases) in which the mutation occurs.
• Substitution mutations can take three forms: silent mutations, which do not alter the amino acid sequence of the polypeptide, missense mutations, which alter a single amino acid in the polypeptide chain, and nonsense mutations, which create a premature stop codon (signal for the cell to stop translation of the mRNA molecule into an amino acid sequence), causing the polypeptide chain produced to be incomplete and therefore affecting the final protein structure and function.
• Based on the effect they have on an organism, gene mutations can be placed into one of three categories: beneficial mutations, harmful mutations, and neutral mutations.
• A small number of mutations result in a significantly altered polypeptide with a different shape, which may alter the ability of the protein to perform its function.
• Cell structure is a fundamental aspect of biology, and understanding cells requires the use of various tools and techniques.
• Microscope use is a crucial part of cell study, and drawing cells is a fundamental skill in biology.
• The cytoskeleton is a crucial component of cell structure, and understanding the role of enzymes is central to understanding biological processes.
• Prokaryotic and eukaryotic cells have distinct structures and functions, and understanding the differences between them is crucial in biological studies.
• If the shape of the active site on an enzyme changes, the substrate may no longer be able to bind to the active site.
• A structural protein (like collagen) may lose its strength if its shape changes.
• In some cases, this alteration to a polypeptide may actually result in an altered characteristic in an organism that causes beneficial effects for the organism.
• An example of a beneficial mutation that occurred in humans involves the production of the pigment melanin.
• Early humans living in Africa had dark skin as they produced high concentrations of the pigment melanin, providing protection from harmful UV radiation from the Sun, whilst still allowing vitamin D to be synthesised due to the high sunlight intensity.
• However, at lower sunlight intensities, pale skin synthesises vitamin D more easily than dark skin.
• As humans moved into cooler temperate climates, certain mutations occurred that led to a decrease in the production of melanin, resulting in paler-skinned individuals having a selective advantage as they could synthesise more vitamin D.
• The mutations that led to a decrease in the production of melanin are referred to as beneficial mutations.
• By altering a polypeptide, some mutations can lead to an altered characteristic in an organism that causes harmful effects for the organism, these mutations are referred to as harmful mutations.
• Many genetic diseases are caused by these harmful mutations, such as haemophilia and sickle cell anaemia.
• An example of a harmful mutation that occurs in humans is that which causes cystic fibrosis.
• In around 70% of cystic fibrosis sufferers, the mutation that causes this disease is a deletion mutation of three nucleotides in the gene coding for the protein CFTR, resulting in the loss of function of the CFTR protein and causing symptoms such as lung and pancreatic problems due to extremely thickened mucus.
• Neutral mutations offer no selective advantage or disadvantage to the individual organism, this can occur either because a mutation does not alter the polypeptide, a mutation only alters the polypeptide slightly so that its structure or function is not changed, or a mutation alters the structure or function of the polypeptide but the resulting difference in the characteristic of the organism provides no particular advantage or disadvantage.
• An example of a neutral mutation that occurs in humans involves the ability to taste a bitter-tasting chemical that is found in Brussel sprouts, this chemical is not toxic so it is not advantageous for us to be able to taste it.
• The ability to taste this chemical is caused by a mutated allele of the TAS2R38 gene, which allows us to taste bitter things by coding for receptor proteins that can detect bitter-tasting chemicals.
• Although this is now seen as a neutral mutation, it may have been advantageous in the past for humans to be able to detect these bitter-tasting chemicals, as large quantities of bitter substances can be harmful and many poisons have a bitter taste.