AQA A Level Biology Paper 2 Comprehensive Questions and Answers 2025

What is a gene mutation? A gene mutation is a change in the sequence of base pairs in a DNA molecule that may result in an altered polypeptide How do mutations occur? Mutations occur continuously and spontaneously Why can gene mutations result in altered polypeptides? As the DNA base sequences determines the sequence of amino acids that make up a protein, gene mutations can lead to a change in the polypeptide that the gene codes for . What effect do most gene mutations have and why? Most gene mutations do not alter the polypeptide or only alter it slightly so that its structure or function remains unaltered, as genetic code is degenerate Define degenerate A single amino acid may be coded for by multiple codons What can mutations in the DNA base sequence occur due to? - Insertion, deletion or substitution of a nucleotide - Inversion, duplication or translocation of a section of a gene Describe insertion mutations An insertion mutation is one that occurs when a nucleotide (thus containing a new base) is randomly inserted into the DNA sequence What effect can insertion mutations have and why? AQA A LEVEL BIOLOGY AQA A LEVEL BIOLOGY Insertion mutations change the triplet code that codes for an amino acid and therefore may change the amino acid that would've been coded by the original base triplet as a new triplet of bases is created. As they're frameshift mutations, insertion mutations also have a knock on effect by changing the triplets further on in the DNA sequence. What are the potential consequences of frameshift mutations? Frameshift mutations may dramatically change the amino acid sequence produced from this gene therefore the ability of the polypeptide to function due to the potential knock on effect they may have. Describe deletion mutations and their effects Deletion mutations are mutations that occur when a nucleotide (and therefore its base) is randomly deleted from the DNA sequence and can also potentially change the amino acid that they would've coded for. What type of mutations are deletion mutations? Also frameshift mutations so they may dramatically change the amino acid sequence produced from this gene and therefore the ability of the polypeptide to function. Describe substitution mutations A substitution mutation is one that occurs when aw base in teh DNA sequence is randomly swapped for a different base. What is the effect of a substitution mutation? Unlike insertion or deletion mutations, a substitution mutation will only change the amino acid for the triplet in which the mutation occurs and there will be no knock on effect What are the three types of substitution mutation? silent, missense, nonsense What is a silent substitution mutation? A silent mutation is when the mutation doesn't alter the amino acid sequence of the polypeptide as certain codons may code for the same amino acid as the genetic code is degenerate. What is a missense mutation? the mutation alters a single amino acid in the polypeptide chain AQA A LEVEL BIOLOGY AQA A LEVEL BIOLOGY Give an example of a missense mutation? sickle cell anemia What is a nonsense mutaiton? A nonsense mutation is a mutation that creates a premature stop codon (signal to the cell to stop the translation of the mRNA into an amino acid sequence) causing the polypeptide chain to be incompletely produced therefore affecting the final protein's structure and function Give an example of a nonsense mutation cystic fibrosis (although it can have alternate causes) Where do inversion mutations of a gene section occur? Inversion mutations within a gene section usually occurs during crossing over in meiosis. Describe the process of inversion mutations in gene sections The DNA of a single gene is cut in two places The cut portion is inverted 180 degrees and rejoined to the same place within a gene. As a result, a large section of the gene is backwards causing multiple amino acids to be affected. What do inversion mutations usually result in? Inversion mutations frequently result in non-functional proteins and in some cases an entirely different protein altogether. What is the harm potentially instigated by inversion mutations? This mutation is usually harmful because the original gene can no longer be expressed from that chromosome How can the effect of inversion mutations be reduced? If the other chromosome in the pair carries a working gene then the effect of the mutation may be lessened Describe duplication mutations A duplication mutation is where a whole gene or a section of a gene is duplicated so that two copies of the same gene or section appear on the same chromosome. What are the effects of duplication mutations? AQA A LEVEL BIOLOGY AQA A LEVEL BIOLOGY Original version of the gene remains intact so the mutation is not harmful The second copy of the gene can undergo mutations which can enable it to develop new functions Duplication mutations are an important source of evolutionary change Give an example of a duplication mutation Alpha, beta and gamma Haemoglobin genes evolved due to duplication mutations describe translocation mutations A portion of one non-homologous chromosome is transferred to another non homologous chromosome Translocation mutations involve a gene being cut into two places and that section of the gene cuts off and attaches to a separate gene. What is the result of a translocation mutation? The result is that the cut gene is now non-functional due to having a section missing and the gene that has gained the translocated section is also likely to be non-functional What happens if a section of a proto-oncogene is translocated onto a gene controlling cell diviision? If a section of a proto-oncogene is translocated onto a gene that controls cell division, it could boost expression and lead to tumors What could happen as a result of a section of a tumour suppressor gene being translocated to result in a faulty or non functional tumour suppressor gene? If a section of a tumour suppressor gene is translocated and the result is a faulty tumor suppressor gene, this could lead to the cell continuing to replicate even when it contains faulty DNA. What is the estimation for the rate of mutation occurrence? The rate that mutations occur can be estimated as around on per 100 000 genes per generation What can increase the rate of mutation? Exposure to mutagenic agents can increase the rate of mutation What are examples of mutagenic agents? AQA A LEVEL BIOLOGY AQA A LEVEL BIOLOGY - High energy ionizing radiation such as alpha beta or gamma radiation or UV - Chemicals such as nitrogen dioxide or benzopyrene from tobacco smoke Give examples of how mutations affect the ability of the polypeptide to function If the shape of the active site on an enzyme changes, the substrate is no longer complimentary to the active site so can't bind and form an enzyme substrate complex Structural proteins such as collagen may lose its strength if its shape changes How can gene mutations affect phenotypes? Polypeptides affect the phenotype of organisms by specific cellular mechanisms. If a mutation leads to a major change in a polypeptide, it can affect cellular mechanisms which can impact the phenotype of an organism Give an example of how a mutation can affect a phenotype A mutation in the TYR gene in humans affects the structure of an enzyme required for the production of melanin pigment. The phenotype of the human is affected by the lack of melanin Individuals with the mutation have albinism. How many times can a STEM cell divide? A stem cell can divide by mitosis an unlimited number of times What is a stem cell? An undifferentiated cell that can divide by mitosis an unlimited number of times to either remain a stem cell or develop into a specialized cell What is the ability of stem cells to differentiate into more specialized cell types known as? Potency is the ability of stem cells to differentiate into more specialized cell types What are the types of potency? totipotent, pluripotent, multipotent, unipotent What are totipotent cells? Stem cells in an early embryo that have the potential to become any cell type What is an alternate name for a totipotent stem cell? Embryonic stem cell? Where can totipotent cells be found? AQA A LEVEL BIOLOGY AQA A LEVEL BIOLOGY Totipotent cells can exist for a limited time in early mammalian embryos as well as in extra embryonic cells Give an example of where Extra-Embryonic cells can be found? Placenta What can be said about the potency of the cells found in the zygote? The zygote formed when a sperm cell fertilizes an egg is totipotent as well as the embryonic cells up to the 16-cell stage (the morula) of human embryo development Describe how the unspecialized totipotent cells in the embryo begin to differentiate Initially the totipotent cells in the embryo are unspecialized During development, totipotent cells begin to translate only part of their DNA which results in cell specialization These specialized cells then form tissues and are no longer classified as totipotent There are no totipotent cells present in the later stages of development as cells lose their ability to differentiate into any cell type. What are pluripotent stem cells? Pluripotent stem cells are embryonic stem cells that can differentiate into any cell type found in an embryo but cannot differentiate into extra-embryonic stem cells Describe the ability of pluripotent stem cells to divide Pluripotent stem cells can divide in unlimited numbers and cont