OCR A Level Biology Cellular Control PDF

Head to www.savemyexams.com for more awesome resources OCR A Level Biology Your notes 6.1 Cellular Control Contents 6.1.1 Gene Mutations 6.1.2 Gene Control 6.1.3 Gene Control: Lac Operon 6.1.4 Gene Control...

Head to www.savemyexams.com for more awesome resources OCR A Level Biology Your notes 6.1 Cellular Control Contents 6.1.1 Gene Mutations 6.1.2 Gene Control 6.1.3 Gene Control: Lac Operon 6.1.4 Gene Control: Transcription Factors 6.1.5 Gene Control: Post-Transcriptional Modification 6.1.6 Gene Control: Body Plans 6.1.7 The Importance of Mitosis & Apoptosis Page 1 of 23 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 6.1.1 Gene Mutations Your notes Gene Mutations & Their Effect on Polypeptides 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 (i.e. for no reason) during DNA replication The probability of a mutation occurring can increase with the presence of certain factors known as mutagens, e.g. Ionising radiation such as X-rays can break the DNA strands which can then be altered during the repair process Deaminating chemicals can alter the chemical structure of bases, converting one base into another 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 These 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 i.e. several different triplets often code for the same amino acid) Many mutations occur in non-coding sections of DNA and so have no effect on the amino acid sequence at all However, 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 Substitution of one or more nucleotides Insertion of nucleotides A mutation that occurs when a nucleotide (with a new base) is randomly inserted into the DNA sequence is known as an insertion mutation An insertion mutation changes the amino acid that would have been coded for by the original base triplet, as it creates a new, different triplet of bases Remember – every group of three bases in a DNA sequence codes for an amino acid An insertion mutation also has a knock-on effect by changing the triplets (groups of three bases) further on in the DNA sequence This is sometimes known as a frameshift mutation This may dramatically change the amino acid sequence produced from this gene and therefore the ability of the polypeptide to function Page 2 of 23 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes An example of an insertion mutation Deletion of nucleotides A mutation that occurs when a nucleotide (and therefore its base) is randomly deleted from the DNA sequence Like an insertion mutation, a deletion mutation changes the amino acid that would have been coded for Like an insertion mutation, a deletion mutation also has a knock-on effect by changing the groups of three bases further on in the DNA sequence Like an insertion mutation, this is sometimes known as a frameshift mutation This may dramatically change the amino acid sequence produced from this gene and therefore the ability of the polypeptide to function Substitution of nucleotides A mutation that occurs when a base in the DNA sequence is randomly swapped for a different base Unlike an insertion or deletion mutation, a substitution mutation will only change the amino acid for the triplet (a group of three bases) in which the mutation occurs; it will not have a knock-on effect Substitution mutations can take three forms: Silent mutations – the mutation does not alter the amino acid sequence of the polypeptide (this is because certain codons may code for the same amino acid as the genetic code is degenerate) Missense mutations – the mutation alters a single amino acid in the polypeptide chain (sickle cell anaemia is an example of a disease caused by a single substitution mutation changing a single amino acid in the sequence) Page 3 of 23 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Nonsense mutations – the mutation creates 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 (cystic Your notes fibrosis is an example of a disease caused by a nonsense mutation, although this is not always the only cause) An example of a substitution mutation The effect of gene mutations on polypeptides Based on the effect they have on an organism, gene mutations can be placed into one of three categories: Beneficial mutations Harmful mutations Neutral mutations Beneficial mutations A small number of mutations result in a significantly altered polypeptide with a different shape This may alter the ability of the protein to perform its function. For example: 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 In these cases, the original mutation is referred to as a beneficial mutation Page 4 of 23 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 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 Your notes melanin This provided 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 These paler-skinned individuals would have had a selective advantage, as they could synthesis more vitamin D (a lack of vitamin D causes a range of health problems, including rickets and reduced protection against heart disease and cancers) The mutations that led to a decrease in the production of melanin are therefore referred to as beneficial mutations Harmful mutations By altering a polypeptide, some mutations can lead to an altered characteristic in an organism that causes harmful effects for the organism In these cases, the original mutation is referred to as a harmful mutation Many genetic diseases are caused by these harmful mutations (e.g. 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 The loss of function of the CFTR protein caused by this deletion mutation results in a number of symptoms, including lung and pancreatic problems as a result of extremely thickened mucus Neutral mutations 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 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 to the organism 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 The TAS2R38 gene allows us to taste bitter things by coding for receptor proteins that can detect bitter-tasting chemicals Page 5 of 23 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources However, the mutated allele of this gene causes an increased perception of bitterness, meaning that people with this mutation can taste the bitter-tasting chemical in Brussel sprouts (whereas people without the mutation cannot) Your notes 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 Examiner Tip You may also have read about silent mutations, which is a type of neutral mutation. A silent mutation is a change in the nucleotide sequence that results in the same amino acid sequence.This is possible because some amino acids can be coded for by up to four different triplet codon sequences.Silent mutations are often a change in the 2nd or 3rd base in the codon, rather than the first.For example, the amino acid valine is coded for by four different triplet codon sequences (GUU, GUC, GUA and GUG) – therefore, as long as the first two nucleotides in the codon are guanine and uracil, the amino acid valine will be inserted into the polypeptide. Page 6 of 23 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 6.1.2 Gene Control Your notes Gene Control The nucleus of every cell in the human body contains the same genes However, not every gene is expressed in every cell In addition, not all of these genes are expressed all the time There are several mechanisms that exist within cells to make sure the correct genes are expressed in the correct cell at the correct time These mechanisms are known as regulatory mechanisms They control which genes are expressed at different points in time (e.g. during development) There are three main types of regulatory mechanisms, including: Regulation at the transcriptional level (i.e. regulatory mechanisms that occur during transcription) Regulation at the post-transcriptional level (i.e. regulatory mechanisms that occur after transcription) Regulation at the post-translational level (i.e. regulatory mechanisms that occur after translation) These regulatory mechanisms are controlled by many different regulatory genes Structural and regulatory genes A structural gene codes for a protein that has a function within a cell (e.g. enzymes, membrane carriers, hormones etc.) For example, the F8 gene codes for the protein Factor VIII involved in blood clotting Regulatory genes code for proteins (or various forms of RNA) that control the expression of structural genes Page 7 of 23 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 6.1.3 Gene Control: Lac Operon Your notes Gene Control: Lac Operon Regulatory genes control structural genes and their levels of protein production Regulatory genes sometimes have control over several structural genes at once If the structural genes being controlled are in any way involved in the process of transcription, then gene control is occurring at the transcriptional level The lac operon provides an example of a regulatory mechanism at the transcriptional level (i.e. a regulatory mechanism that occurs during transcription) The lac operon Structural genes in prokaryotes can form an operon: a group or a cluster of genes that are controlled by the same promoter The lac operon found in some bacteria is one of the most well-known of these The lac operon controls the production of the enzyme lactase (also called β-galactosidase) and two other structural proteins Lactase breaks down the substrate lactose so that it can be used as an energy source in the bacterial cell It is known as an inducible enzyme (this means it is only synthesized when lactose is present) This helps prevent the bacteria from wasting energy and materials Structure of the lac operon The components of the lac operon are found in the following order: Promoter for structural genes Operator Structural gene lacZ that codes for lactase Structural gene lacY that codes for permease (allows lactose into the cell) Structural gene lacA that codes for transacetylase Located to the left (upstream) of the lac operon on the bacterium's DNA there is also the: Promoter for regulatory gene Regulatory gene lacI that codes for the lac repressor protein The lac repressor protein has two binding sites that allow it to bind to the operator in the lac operon and also to lactose (the effector molecule) When it binds to the operator it prevents the transcription of the structural genes as RNA polymerase cannot attach to the promoter When it binds to lactose the shape of the repressor protein distorts and it can no longer bind to the operator Page 8 of 23 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The components of the lac operon along with the upstream regulatory gene and its associated promoter When lactose is absent The following processes take place when lactose is absent in the medium that the bacterium is growing in: The regulatory gene is transcribed and translated to produce lac repressor protein The lac repressor protein binds to the operator region upstream of lacZ Due to the presence of the repressor protein RNA polymerase is unable to bind to the promoter region Transcription of the structural genes does not take place No lactase enzyme is synthesized The repressor protein binding to the operator region of the lac operon and preventing transcription of the structural gene Page 9 of 23 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources When lactose is present Your notes The following processes take place when lactose is present in the medium that the bacterium is growing in: There is an uptake of lactose by the bacterium The lactose binds to the second binding site on the repressor protein, distorting its shape so that it cannot bind to the operator site RNA polymerase is then able to bind to the promoter region and transcription takes place The mRNA from all three structural genes is translated Enzyme lactase is produced and lactose can be broken down and used for energy by the bacterium The binding of lactose to the repressor protein frees up the operator region of the lac operon so RNA polymerase can bind and begin transcription of the structural genes Examiner Tip The example above explains how the genetic control of an inducible enzyme works.However, you could get some questions on the genetic control of repressible enzymes.In this mechanism, an effector molecule also binds to a repressor protein produced by a regulatory gene. However this binding actually helps the repressor bind to the operator region and prevent transcription of the structural genes. So it's the opposite of the lac operon: when there is less of the effector molecule, the repressor protein cannot bind to the operator region and transcription of the structural genes goes ahead, meaning the enzyme is produced. Page 10 of 23 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 6.1.4 Gene Control: Transcription Factors Your notes Gene Control: Transcription Factors Eukaryotes can use transcription factors to control gene expression Transcription factors are proteins that bind to specific regions of DNA to control the transcription of genes It is estimated that ~10% of human genes code for transcription factors There are several types of transcription factors that have varying effects on gene expression This is still a relatively young area of research and scientists are working hard to understand how all the different transcription factors function Transcription factors allow organisms to respond to their environment Some hormones achieve their effect via transcription factors How transcription factors work Some transcription factors bind to the promoter region of a gene (i.e. the region of DNA 'upstream' of the gene that controls the expression of the gene) This binding can either allow or prevent the transcription of the gene from taking place The presence of a transcription factor will either increase or decrease the rate of transcription of a gene A transcription factor binding to the promoter region of a gene which allows RNA polymerase to bind and for transcription to occur Gene control: oestrogen In mammals, the hormone oestrogen is involved in controlling the oestrus cycle and also in sperm production Page 11 of 23 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Oestrogen is a lipid-soluble molecule and can therefore diffuse through the plasma membrane of cells It then moves to the nucleus and binds to an oestrogen receptor Your notes These receptors are actually transcription factors that are able to initiate transcription for many different genes by binding to their promoter regions Once bound, oestrogen causes a change in the shape of the receptor As a result, the receptor moves away from the protein complex it is normally attached to and binds to the promoter region of one of its target genes This allows RNA polymerase to bind and to begin transcribing that gene Page 12 of 23 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes A summary of how oestrogen can stimulate the transcription of a gene Gene control: gibberellin Plant cells use transcription factors in a similar way to animal cells Gibberellin is a hormone found in plants (e.g. wheat and barley) that controls seed germination by stimulating the synthesis of the enzyme amylase It does this by influencing transcription of the amylase gene When gibberellin is applied to a germinating seed there is an increased amount of the mRNA for amylase present Mechanism The breakdown of DELLA protein by gibberellin is necessary for the synthesis of amylase The following components are involved: Repressor protein DELLA Transcription factor (the one involved is called PIF) Page 13 of 23 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Promoter of amylase gene Amylase gene Gibberellin Your notes Gibberellin receptor and enzyme The process occurs as follows: DELLA protein is bound to the transcription factor, preventing it from binding to the promoter of the amylase gene so no transcription can occur Gibberellin binds to a gibberellin receptor and enzyme which starts the breakdown of DELLA The transcription factor is no longer bound to DELLA protein and so it binds to the promoter of the amylase gene Transcription of amylase gene begins Amylase is produced The breakdown of DELLA protein by gibberellin allows the transcription factor PIF to bind to the promoter for the amylase gene and for transcription to initiate Page 14 of 23 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Examiner Tip Your notes In your exam you may be asked to explain why RNA analysis is important with regards to gene expression. From the outside most cells look almost identical with the same DNA in their nucleus. However we know that they are most likely expressing different genes.When a cell expresses a gene, RNA is produced by transcription. This RNA present in a cell can be analysed. Scientists can match the RNA present in a cell to specific genes and work out which genes are being expressed in that specific cell. You are not required to recall specific transcription factors for your exam. Page 15 of 23 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 6.1.5 Gene Control: Post-Transcriptional Modification Your notes Post-Transcriptional Modification Within eukaryotic genes, there are both coding and non-coding sequences of DNA The coding sequences are called exons and these are the sequences that will eventually be translated into the amino acids that will form the final polypeptide The non-coding sequences are called introns and are not translated (they do not code for any amino acids) When transcription of a gene occurs, both the exons and introns are transcribed This means the messenger RNA (mRNA) molecule formed also contains exons and introns This RNA molecule is often referred to as primary mRNA or pre-mRNA As the introns are not to be translated, they must be removed from the pre-mRNA molecule The exons are then all fused together to form a continuous mRNA molecule called mature mRNA that is ready to be translated This process is sometimes called ‘splicing’ and is part of the process of post-transcriptional modification (referring to the modification of the RNA molecule after transcription but before translation occurs) Splicing ensures that only the coding sections of mRNA are used to form proteins by translation (if any introns were included in the mature mRNA, the resulting protein would not be formed properly and may not function as it should) Page 16 of 23 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The RNA molecule (known as pre-mRNA) produced from the transcription of a gene contains introns that must be removed (to form mature mRNA) before translation can occur Control at the post-translational level After polypeptides are formed by translation, they undergo modifications in the Golgi apparatus or in the cytosol Some polypeptides may then require activation by cyclic AMP (also known as cAMP) Page 17 of 23 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources cAMP is derived from ATP and is formed by the action of the enzyme adenyl cyclase One important role carried out by cAMP is the activation of protein kinases In eukaryotic cells, cAMP activates protein kinase A (also known as PKA) Your notes PKA is an inactive precursor enzyme Once it is activated, it can activate other proteins (e.g. other enzymes) For example, when muscle cells require energy, an enzyme called glycogen phosphorylase releases glucose from glycogen This enzyme is activated by cAMP, which changes the shape of the enzyme to expose its active site Page 18 of 23 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 6.1.6 Gene Control: Body Plans Your notes Body Plans & Hox Genes Cells in developing organisms need to be able to differentiate and specialise for different roles In order to do this, they must be able to control which genes are functioning at a particular time This is achieved by 'switching on' and 'switching off' genes This must occur in a specific, tightly controlled sequence This sequence is determined by transcription factors (proteins that bind to specific DNA sequences in order to control the rate at which particular genes are transcribed into mRNA) Homeobox genes A homeobox is a DNA sequence that codes for a protein transcription factor The transcription factors (that homeobox sequences code for) attach to DNA at specific locations and regulate the transcription of genes (e.g. genes that control the early development of eukaryotic organisms) by turning various different genes on and off in the correct order A homeobox gene is any gene that contains a homeobox sequence Homeobox gene sequences in plants, animals and fungi are similar and highly conserved (meaning they have been maintained by natural selection i.e. they remain relatively unchanged when travelling back in evolutionary time) The sequences are all similar as they all code for amino acid sequences that will form transcription factors, the DNA-binding regions of which must all have the same shape Mutations that cause changes in these homeobox sequences can lead to organisms that are not viable (not properly developed) so they are not favoured by natural selection. This strong negative selection pressure explains why the sequences are highly conserved Homeobox genes are responsible for the genetic control of the development of body plans in different organisms This means they help to form the basic pattern of the body For example, they control the polarity of the organism (which end will develop into the head and which end will develop into the tail) They also control the segmentation of organisms such as insects and mammals into distinct body parts and they control the development of body parts such as wings and limbs, as well as what organs are present in each section of the body In this way, homeobox genes can be seen as 'master genes' that control which genes function at different stages of development Page 19 of 23 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Eight homeobox genes (specifically, Hox genes) of the fruit fly, Drosophila melanogaster, that control the development of the body plan into specific regions e.g. the head, thorax, and abdomen. The break mark (//) in the chromosome shows that these are two clusters of genes that are separated by a long intervening region of the chromosome that is not shown here Hox genes Hox genes are a very important subset of homeobox genes They determine the identity of embryonic body regions along the anterior-posterior axis (i.e. the head-tail axis) These Hox genes are organised into groups known as Hox clusters Vertebrates have four Hox clusters (each containing 9-11 Hox genes), which are found on different chromosomes There is a linear order to the Hox genes in each Hox cluster and this order is directly related to the order of the regions of the body that they affect Page 20 of 23 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The four Hox clusters containing the Hox genes that control the development of the body plan of vertebrates into specific regions Page 21 of 23 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 6.1.7 The Importance of Mitosis & Apoptosis Your notes The Importance of Mitosis & Apoptosis Mitosis is the type of cell division that produces identical new cells for processes such as growth, cell replacement and tissue repair Apoptosis is programmed cell death (sometimes referred to as natural cell death) In apoptosis, old cells that have already undergone a large number of mitotic cell divisions (approximately 50 divisions, although this depends on the cell type) are systematically taken through various processes leading to cell death These processes include: The DNA of the cell becoming denser and more tightly packed The nuclear envelope of the cell's nucleus breaking down and chromatin condensing Vesicles forming that contain hydrolytic enzymes Phagocytes engulfing and digesting the cell via phagocytosis The importance of mitosis and apoptosis in controlling body plan development By constantly replacing and destroying cells throughout the early development of an organism, mitosis and apoptosis are both key mechanisms controlling the development of body form Apoptosis is important in development as, in some cases, some cells that are produced (by mitosis) earlier on in development may no longer be needed As a result, these cells are destroyed (by apoptosis) as part of the development of the organism For example, structures like fingers and toes first develop as a single combined unit and are then separated later via programmed cell death (apoptosis) of the cells in between the digits The control of mitosis and apoptosis Mitosis is controlled by various different genes that are categorised into two distinct groups: Proto-oncogenes are genes that stimulate cell division Tumour-suppressor genes are genes that reduce cell division Tumour-suppressor genes can also stimulate apoptosis in cells with damaged DNA that cannot be repaired This protects the body as it ensures that any cells that are genetically damaged (and that could, therefore, lead to cancer) are destroyed During the cell cycle (in cells due to undergo mitosis) there are various 'checkpoints' that need to be passed to ensure that damaged cells are not produced These controls ensure that the cell is prepared for the mitosis phase of its cell cycle and that any DNA damage is repaired These controls are regulated by two groups of proteins, known as cyclins and cyclin-dependent kinases (CDKs), that regulate the progress of the cell through the cell cycle Page 22 of 23 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Cyclins act as regulators CDKs act as catalysts (once activated by cyclins) For example, CDKs that have been activated by cyclins will catalyse the phosphorylation of Your notes particular target proteins, which can either activate or inactivate them This ensures the cell cycle progresses from one stage to the next Different cyclins are produced at different stages of the cell cycle in response to internal molecular signals The genes that control the cell cycle and apoptosis are able to respond to: Internal cell stimuli External cell stimuli Examples of internal cell stimuli Internal factors that affect apoptosis and the cell cycle include: Irreparable genetic damage RNA decay Internal biochemical changes that lead to cell changes or cellular injury (e.g. oxidative reactions) Production of cyclin D These factors can all initiate apoptosis in cells Examples of external cell stimuli External factors that affect apoptosis and the cell cycle include: The presence of cell signalling molecules such as cytokines from the immune system, hormones and growth factors Viruses and bacteria, harmful pollutants or ultraviolet light can affect the delicate balance of mitosis and apoptosis by damaging or destroying cells faster than they can be repaired or replaced Cells often respond to such stressful stimuli by activating pathways to increase their chance of survival, or by initiating apoptosis Page 23 of 23 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers