Genetics of Living Systems

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  • Created by: LBCW0502
  • Created on: 19-02-17 15:56
What is a mutation?
A change in the sequence of bases in DNA (can disrupt protein synthesis)
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What is a point mutation?
A change in the sequence of bases of DNA which leads to only one nucleotide being affected
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What are the three types of point mutation?
Substitution, insertion and deletion
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What does degenerate mean?
A single amino acid may be coded by more than one codon (redundant)
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How does the change in codon affect the primary structure?
Changes the specific sequence of amino acids in a primary structure of a protein
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What is a frameshift mutation?
A mutation caused by insertion or deletion of base pairs in DNA resulting in the translation of the genetic code in an unnatural reading frame from the position of the mutation to the end of the gene
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What does a triplet code mean?
Sequences of bases are transcribed consecutively in non-overlapping groups of three (reading frame of a sequence of bases)
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How many bases code for one amino acid?
Three
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What happens if the number of nucleotides changes is a multiple of three?
Multiples of three correspond to full codons and therefore, the reading frame will not be changed but the protein formed will still be affected as a new amino acid is added
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Explain why some mutations can have no effect on an organism
There is no effect on the phenotype of an organism because normally functioning proteins are still synthesised
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Explain why some mutations have a damaging effect on an organism
The phenotype is affected in a negative way because proteins are no longer synthesised or proteins synthesised are non-functional. This can interfere with one or more, essential processes
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Explain why some mutations can be beneficial for an organism
Very rarely a protein is synthesised that results in a new and useful characteristic in the phenotype e.g mutation in protein in plasma membrane of human cells means HIV cannot bind/enter cells (immune to infection form HIV)
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What are mutagens?
Chemical, physical or biological agents which cause mutations
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Give an example of a chemical mutagen and describe what they do
Deaminating agents - chemically alter bases in DNA e.g. covert cytosine to uracil, changing the base sequence
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Give an example of a physical mutagen and describe what they do
Ionizing radiations (e.g. X-rays) - break one/both DNA strands. Some breaks can be repaired but mutations can occur in the process
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Describe what alkylating agents do (type of biological agent)
Methyl or ethyl groups are attached to bases resulting in the incorrect pairing of bases during replication
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Describe what base analogs do (type of biological agent)
Incorporated into DNA in the place of the usual base during replication, changing the base sequence
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Describe what viruses do (type of biological agent)
Viral DNA may insert itself into a genome, changing the base sequence
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What is depurination?
The loss of a purine base
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What is depyrimidination?
The loss of a pyrimidine base
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What can the absence of a base lead to?
Insertion of an incorrect base through complementary base paring during DNA replication
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How do oxidising agents affect organisms?
Can affect structure of nucleotides and disrupt base pairing during DNA replication (e.g. free radicals)
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What are anticarcinogens?
Antioxidants such as vitamins A, C and E (found in fruit and vegetables) which have the ability to negate the effects of free radicals
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What is a silent mutation (neutral) ?
The sequence of bases in DNA does not change the proteins or the activity of any proteins synthesised (they have no effect on the phenotype of an organism).
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Where can silent mutations occur?
In non-coding regions of DNA (introns) or code for the same amino acid due to the degenerate nature of the genetic code. They may change the primary structure but not the overall structure/function of the protein
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What is a nonsense mutation?
Result in a codon becoming a stop codon instead of coding for an amino acid - results in the synthesis of a shortened protein (normally non-functional). Mutations can have negative/harmful effects on phenotypes
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What is a missense mutation?
Results in the incorporation of an incorrect amino acid (or amino acids) into the primary structure when the protein is synthesised. Results depends on role amino acid plays in structure/function of protein. Mutation can be silent/beneficial/harmful
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When does a conservative mutation occur?
When the amino acid change leads to an amino acid being coded for which has similar properties to the original, this means the effect of mutation is less severe
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When does a non-conservative mutation occur?
When a new amino acid coded for has different properties to the original, this is more likely to have an effect on protein structure and may cause disease
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Give an example of a recent beneficial mutation
People with a mutation having the ability to digest lactose (found in European populations and some people in East Africa)
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What is sickle-cell anaemia?
A blood disorder where erythrocytes develop abnormally
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What is the cause of sickle-cell anaemia?
A mutation in the gene coding for haemoglobin. Thymine substitutes adenine, making the 6th amino acid valine instead of glutamic acid on the beta Hb chain
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What do gene mutations affect?
Single genes or sections of DNA
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What do chromosome genes affect?
The whole chromosome or number of chromosomes within a cell (can be caused by mutagens and occurs during meiosis)
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Describe how deletion changes the chromosome structure
A section of chromosome breaks off and is lost within the cell
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Describe how duplication changes the chromosome structure
Sections get duplicated on a chromosome
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Describe how translocation changes the chromosome structure
A section of one chromosome breaks off and joins another non-homologous chromosome
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Describe how inversion changes the chromosome structure
A section of chromosome breaks off, is reversed and then joins back onto the chromosome
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State the levels genes are regulated
Transcriptional, post-transcriptional, translational and post-translational
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What is a chromatin?
DNA is a very long molecule and has to be wound around proteins called histones in eukaryotic cells, in order to be packed into a nucleus of a cell. The resulting DNA/protein complex is called a chromatin
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Where are heterochromatin and euchromatin located?
Heterochromatin is tightly wound DNA causing chromosomes to be visible during cell division. Euchromatin is loosely wound DNA present during interphase
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Why is the transcription of genes not possible when DNA is tightly wound?
RNA polymerase cannot access the genes (genes in euchromatin can be freely transcribed)
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Where can protein synthesis not occur?
During cell division. Protein synthesis can occur during interphase between cell divisions. This simple form of regulation ensures proteins necessary for cell division are synthesised in time
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What is the advantage of simple regulation?
Prevents the complex and energy-consuming process of protein synthesis occurring when cells are actually dividing
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Why is DNA able to coil around histones?
Histones are positively charged and DNA is negative charged. Histones can be modified to increase or decrease the degree of packing (or condensation)
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How are histones modified to allow DNA to coil less tightly?
Acetylation and phosphorylation reduces the positive charge on the histones (making them more negative). DNA coils less tightly allowing certain genes to be transcribed
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How are histones modified to allow DNA to coil more tightly?
Methylation makes histones more hydrophobic so they bind more tightly to each other causing DNA to coil more tightly and prevent the transcription of genes
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What is epigenetics?
Term used to describe control of gene expression by modification of DNA. Sometimes used to include all of the different ways in which gene expression is regulated
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What is an operon?
A group of genes that are under the same control of the same regulatory mechanism and are expressed at the same time. They are more common in prokaryotes (simple structure). They have an efficient way of saving resources (genes can turn on/off)
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What is the preferred respiratory substrate for E.coli?
Glucose
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Which respiratory substrate is used if there are low levels of glucose?
Lactose
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What are the three structural genes and what do they code for?
Lac Z (beta-galactosidase), Lac Y (lactose permease) and Lac A (transacetylase)
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What is the function of beta-galactosidase?
The enzyme breaks down lactose into glucose and galactose
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What is the function of lactose permease?
Embedded in the plasma membrane to form channels, making the plasma membrane more permeable to lactose (increase uptake of lactose)
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What is the function of transacetylase?
An enzyme which transfers an acetyl group from acetyl CoA to beta-galactosidase
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What is Lac I and where is it located?
The regulatory gene which is located upstream
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What is the function of the repressor protein?
Prevents the transcription of structural genes in the absence of lactose. Repressor protein binds to operator (blocks promoter). Prevents RNA polymerase binding to DNA for transcription (down regulation)
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What is the operator?
The region which the repressor protein binds to
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What is the promoter?
The region which the RNA polymerase binds to (transcription can take place)
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What happens when lactose is present?
Lactose binds to the repressor protein, causing it to change shape so it no longer binds to operator. RNA polymerase can bind to promoter and structural genes are transcribed (enzymes are synthesised)
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What is the role of cAMP?
A secondary messenger which binds to cAMP receptor protein. This increases the rate of transcription and required quantity of enzymes to metabolise lactose efficiently.
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Why would the levels of cAMP decrease in E.coli?
Transport of glucose is taking place. This reduces transcription of genes responsible for metabolism of lactose
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What would happen if both glucose and lactose were present?
Glucose (preferred respiratory substrate) will be metabolised
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What is the product of transcription?
Pre-mRNA (precursor mRNA)
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How is pre-mRNA modified?
Splicing occurs where the RNA is cut at specific points. Introns are removed and exons are joined together
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Where does the modification of pre-mRNA take place?
In the nucleus of the eukaryotic cell
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What is added to the 5' end and 3' end?
A cap (modified nucleotide) is added to the 5' end and a tail (long chain of adenine nucleotides) is added to the 3' end. These help to stabilise mRNA and delay degradation in the cytoplasm. Cap aids binding of mRNA to ribosomes
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What is the product as a result of the modification of pre-mRNA?
mRNA
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Describe RNA editing
The nucleotide sequence of some mRNA molecules can be changed through base addition, deletion or substitution. These have the same effect as point mutations and results in different proteins/functions. Increases range of proteins from one mRNA
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How does degradation of mRNA regulate the process of protein synthesis?
The more resistant the molecule the longer it will last in the cytoplasm (greater quantity of protein synthesised)
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How does the binding of inhibitory proteins to mRNA regulate the process of protein synthesis?
Binding of inhibitory proteins to mRNA prevents it binding to ribosomes and the synthesis of proteins
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How does the activation of inhibitory factors regulate the process of protein synthesis?
Activation of initiation factors aid the binding of mRNA to ribosomes (eggs of many organisms produce large quantities of mRNA which are not required until after fertilisation, when initiation factors are activated)
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Describe the significance of protein kinases
Enzymes that catalyse phosphorylation (changes tertiary structure of protein). This activates other enzymes. Protein kinases are important regulators of cell activity and are often activated by the secondary messenger cAMP
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Describe the modification of proteins in post-translational control (4)
Addition of non-protein groups (e.g. carbohydrate chains, lipids, phosphates), modifying amino acids/formation of bonds (disulfide bridges), folding/shortening of proteins and modification of cAMP (e.g. Lac Operon)
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What is morphogenesis?
The regulation of the pattern of anatomical development
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What is the advantage of using fruit flies (Drosophila melanogaster) in genetic studies?
They are small, easy to keep and have a short life cycle
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What are homeobox genes?
A group of genes which all contain a homeobox (regulatory genes)
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What is a homeobox?
A section of DNA 180 base pairs long coding for a part of the protein 60 amino acids long that is highly conserved (very similar) in plants, animals and fungi
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What is a homeodomain?
Part of the protein which bonds to DNA and switches genes on or off
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What is Pax 6?
A homeobox gene. When mutated it causes a form of blindness (due to underdevelopment of the retina) in humans. Mice and fruit flies also have this gene. Pax 6 is a gene involved in the development of eyes in all three species
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What are Hox genes?
One group of homeobox genes that are only present in animals. They are responsible for the correct positioning of body parts. In animals, the Hox genes are found in gene clusters (4 clusters on different chromosomes)
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How many Hox genes do humans have?
39 Hox genes. The order in which the genes appear along the chromosomes is the order in which their effects are expressed in the organism
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How are body plans usually represented?
As cross-sections through the organism showing the fundamental arrangement of tissue layers (Diploblastic - 2 tissue layers, triploblastic - 3 tissue layers)
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What is a common feature of animals?
They are segmented. These segments have multiplied over time and are specialised to perform different functions. Hox genes in the head control development of mouthparts and Hox genes in the thorax control the development of wings, limbs or ribs
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What are somites?
The individual vertebrae and associated structures have all developed from segments in the embryo called somites. The somites are directed by Hox genes to develop in a particular way depending on their position in the sequence
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What is radical symmetry?
Seen in diploblastic animals (e.g. jellyfish). They have no left or right sides, only a top and a bottom
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What is bilateral symmetry?
Seen in most animals. Organisms have both left and right sides and a head and a tail rather than just a top and bottom
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What is asymmetry?
Seen in sponges which have no lines of symmetry
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What does mitosis result in?
Cell division and proliferation and apoptosis
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What is apoptosis?
Programmed cell death (essential in shaping organisms)
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What is the role of mitosis?
Increase the number of cells leading to growth
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Describe how apoptosis shapes different body parts
Removes unwanted cell and tissues. Cells undergoing apoptosis can also release chemical signals which stimulate mitosis and cell proliferation leading to the remodelling of tissues
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Which type of genes regulate both mitosis and apoptosis?
Hox genes
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Describe the process of apoptosis (1)
Cell cytoskeleton is broken down by enzymes. Cytoplasm becomes dense with organelles tightly packed. Plasma membrane changes and blebs are formed. Chromatin condenses/nuclear envelope breaks and DNA breaks into fragments.
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Describe the process of apoptosis (2)
Cell breaks into vesicles that are taken up by phagocytosis. Cellular debris is disposed of and does not damage any other cells or tissues. Apoptosis occurs very quickly
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Expression of regulatory genes can be influenced by which factors?
Internal and external factors (factors can have a greater impact during growth and development of an organism)
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What is stress?
The condition produced when the homeostatic balance within an organism is upset. This can be due to external factors such as change in temperature or light intensity, Internal factors can change due to release of hormones or psychological stress
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Which drug affects the activity of regulatory genes?
Thalidomide (given to pregnant women in the '50s/'60s to treat morning sickness
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How does thalidomide affect regulatory genes?
Prevents normal expression of a Hox gene (babies with shortened limbs)
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What is thalidomide used to treat?
Some forms of cancer and prevents the formation of networks of capillaries which are necessary for some tumours to grow and develop
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