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23 Cards in this Set
- Front
- Back
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1. Explain how the genetic information in cells is organised as chromosomes
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DNA wraps around a histone centre twice and then does the same thing to the next histones to form a beads on a string appearance called a nucleosome. This is the uncondensed form called euchromatin. The nucleosome is tightly packed into solenoid structures, forming 30nm fibres. The 30nm fibres compact into solenoid loops which make up the chromosome. The chromatin here is condensed and called - heterohromatin. In S phase, when DNA is replicated, the chromosome is duplicated to create the X shape appearance of chromosomes then seen in mitosis. The 2 chromatids are joined by a centromere that can be metacentric, submetacentric or acrocentric. Barr bodies are spare X chromosomes in their condensed form ( euchromatin). Usually only seen in females but can occur in males with sex chromosome abnormalities eg klinefelter’s
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2. Describe the chromosomal basis of sex determination
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A healthy individual has 23 pairs of chromosomes, 22 autosomes (chromosomes 1-22) and one pair of sex chromosomes, chromosomes 23. Healthy males have X and Y chromosomes and healthy females have 2 X chromosomes. Chromosome 1 is the largest chromosome and chromosome 22 is the smallest . chromosomes are numbered and grouped according to their size and position of centromere. Centromeres can be placed in the middle, arms are equal length – metacentric, above/below the middle, arms are unequal length – submetacentric, near the top – acrocentric ( very short p arms). Telocentric is not seen in humans.
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3. Describe numerical chromosome abnormalities and their significance
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Numerical abnormalities: a chromosome number different to 46
- Polyploidy – abnormal number of copies of all chromosomes - a multiple of the haploid number eg 4n tetraploidy, 3n triploidy - Aneuploidy – abnormal number of copies of 1 or 2 chromosomes than the usual 2 copies. It is not a multiple of the haplioid number eg monosomy, trisomy, robertsonian translocation – superchromosome, deletion of chromosome, polysomy Aneuploidy occurs by non dysjunction – either anaphase lagging where a sister chromatid gets left behind or where a sister chromatid fails to separate and so X chromosome moves entirely into one gamete. |
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how could 2 cell lines form in an individual?
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Mosaciasm can occur which is the formation of 2 or more different cell lines (karotypes). If the non dysjunction occurred in the first mitosis of the zygote then 2 cells lines would form. If non dysjunction occurred in the 2nd mitotic division of the zygtote then 3 lines would form eg. 46 -> 46 and 46> 46&46, 45&47.
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4. Describe the structural chromosome abnormalities and their significance
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Structural chromosome abnormalities – arises from the transverse breakage in one or more chromosomes usually followed by reunion of broken ends.
Balanced – the exchange or rearrangement of genetic material does not cause any missing or extra genetic information. Therefore no problems are caused Unbalanced – the structural chromosomal changes cause missing or extra genetic material and can cause mental retardation along with other disorders. Structural Chromosomes mutations within one chromosome ( single chromatin) - usually UNBALANCED - Additions – some genetic material is doubled UNBALANCED - Deletions – some genetic material is lost. UNBALANCED - Ring formations - the telomeres on both arms are removed and the arms join together to form ring structure. UNBALANCED - Isochromosome – creation of 2 non-identical chromosomes, one is a combination of 2 p arms and the other 2 y arms. UNBALANCED - Inversion – segment is cut out, turned around and re-inserted to form inversed form. BALANCED Structural chromosome mutations with 2 chromosomes: may be balanced or unbalanced - Inversion – no loss of genetic material by a rearrangement of genetic material to a non homologous chromosome. - Reciprocal translocation – no loss of genetic material but an exchange of material between 2 non-homologous chromosomes - Robertsonian translocation - rearrangement of genetic material between 2 chromosomes: the p arms of 2 acrocentric chromosomes are lost and the q arms combine to form one super chromosome (homologous or non homologous) – eg chromosome 14 and 21. The loss of p arms isn’t usually important as there isn’t usually any vital information on them. |
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Describe some diseases associated with chromosomal abnormalities caused by aneuploidy
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Diseases caused by aneuploidy can be in the autosomal cells ie can’t be inherited : down’s syndrome (21), Edwards syndrome (19), patau’s syndrome (19)
Aneuploidy can also occur in sex chromosomes: klinefelters syndrome ( 47, XXY) and turners disease ( 45, X) sex chromosomes which are 47, XXX or 47 XYY don’t have any clinical problems. Non-dysjunction is more likely to occur with age eg over 35 mothers are more at risk. 1/3 of all down’s syndrome patients come from mothers over the age of 35. Unbalanced Inversions can cause cancer – the inversion may be paracentric (includes centromere) or pericentric ( doesn’t include centromere). Balanced translocations don’t usually cause any problems, however sometimes important gene sequences occur at chromatid ends and so when the break occurs, this may separate a gene to its promoter sequence and so the correct gene won’t be expressed. Cancer may develop if the promoter sequence is very active and so the wrong genes are expressed to a great quantity . |
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6. Be familiar with the concept of karotyping
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A karotype is a picture of the systematic assortment of the complete set of chromosomes of an individual; starting with chromosome 1 and ending with the sex chromosomes. In moascism 2 or more cell lines arise by non- dysjuction.
The chromosomes are grouped and numbered depending on their size, largest first, and position of centromere: metacentric, subcentric, acrocentric. Karotyping is carried out by investigating chromosomes in metaphase state in somatic cells in tissue culture. Cells are from rapidly dividing type eg bone, blood, skin, fetal cells. The cells accumulate in vitro with spindle fibre inhibitors, cell harvest- accumulated metaphases are removed and prepared on a slide |
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7. Recognise, comprehend and apply chromosome nomenclature |
Karotype formula – start with total no. Of chromosome in the cells, X chromosomes and Y chromosomes. + or – sign followed by a number indicate an extra or missing chromosome. When a part of a chromosome is missing the chromosome no is indicated followed by p or q ( which arm its missing from)
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8. Outline the reasons for referral of patients for karotyping
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Patient ascertainment - they want to know
Congenital problems - Prenatal testing – test for down’s syndrome, more like in mother over 35, choice of abortion - Birth defects – want to know the cause, mental retardation, malformation - Recurrent foetal loss – - Infertile - Abnormal sexual development – klinefelter’s syndrome ( 47, XXY) – low testosterone levels, higher female sex hormone levels, breast tissue but male genitalia, infertile Acquired problems - Leukaemia Diagnosis of chromosome abnormalitites - Allows better diagnosis/ prognosis of clinical problems in patient - Allows better management of disorder eg in abnormal sexual development like klinefelter’s male hormones could be given. - Knowledge helps decision to have children, may be infertile or likely to pass on condition - Help to decide whether to terminate pregnancy |
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9. Explain how fluorescent in situ hybridisation(FISH) works and recognise its importance in the detection of chromosomal abnormalities
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FISH involves a fluorescent probe hybridising with specific gene in a chromosome. 2 probes may be used; one to detect the correct chromosome and one to detect the mutant gene, if 2 fluorescent signals from the same chromosome are seen, this indicates the mutant gene of interest.
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10. Show an appreciation of the ethical issues associated with genetic testing
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- Guilt of passing on to children, fears for future, worry from individual and by family, can’t do the things you used to- lifestyle change, might constantly have to be going for check-ups, job might not allow all days off, discrimination by employers or insurance companies if genetic testing results become known.
Define centromere, chromosome, chromatid, telomere p and y arms. Centromere – the region on chromosome which separates the p arm from the y arm Chromosome – condensed DNA – euchromatin. Nucleosomes are stacked into solienoid structures forming 30nm fibres which form tighly packed solenoid loops, forming the structure of the chromosome. Duplicated after S phase to form X shaped form of chromosome comprising of 2 chromatids. Telomere – end sequences on p and y arms which stablise chromosome. Without telomeres cell recognises DNA damage and either telomere is added or cell dies – apoptosis. |
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Explain the relationshi between changes in nucleotide and amino acid sequences
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An amino acid is coded for by a triplet code of nucleotides called a codon.
61 triplet codons represent 20 amino acids and 3 triplet codons represent 3 stop codons: UAA, UAG, UGA. AUG is the start codon recognised by methionyl tRNA Codons in a reading frame are read in 5’->3’ direction, they don’t overlap and are degenerate. Amino acids are coded for by more than one codon because there are more codons than amino acids. The wobble position in the 3rd position of codon allows degeneracy of the genetic code. |
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Describe the different types of mutational changes eg point mutation, insertion, deletion.
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Point mutation is a single base substitution. Transition : purine/pyrimidine is replaced by purine/pyrimidine eg G -> A. Transversion: purine/pyrimidine is replaced by pyrimidine/purine eg G>T. Point mutations in non coding regions or outside genes can also be detrimental as they can change protein binding sites, promoter sequences, splice sites etc.
Insertions/ Deletions– the sequence that is added or removed from the nucleic acid can be a single nucleotide(single base mutation), a few nucleotides ( eg triplet repeats) to millions of nucleotides (tandem duplications). Addition or subtraction of nucleotides other than multiples of 3: frameshift mutation. Addition or subtraction of 3 nucleotides or multiples of 3: no change to reading frame but the amino acid sequence is changed. |
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Describe triplet expanding.
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Expanding repeats is an insertion mutation of many triplet repeats producing extra amino acids if codons are repeated. Some genes are particularly prone to expanding repeats. The increased number of repeats correlates with earlier onset and more severe phenotype as seen in myotonic dystrophy when onset starts at different stages in someone’s life.
Non coding intron repeats can affect phenotype as the m,RNA may be too large to pass through the nuclear pore into the cytoplasm so translation can’t take place. All triplet expanding disorders eg huntington’s disease, myotonic dystrophy and X fragile syndrome are neurological disorders and can be tested for with southern blotting, not PCR – too unstable. X fragile is an X linked disease predominantly in males. Tandem duplication is the addition of off million of nucleotides. |
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Predict and explain the effect that different mutations may have eg silent mutation, missense mutation, nonsense mutation, frameshift mutation
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Silent mutation is a point mutation, usually on the wobble position, that doesn’t change the amino acid coded for and so doesn’t cause a mutant allele. This is due to the degeneracy of the genetic code and the codons are called synonymous codons.
Missense mutation is a point mutation which changes the amino acid coded for, non-synonymous codons. Missense mutations may affect protein function severely, mildily or not at all. Non-synonymous codons may not affect the phenotype if the amino acids have a similar chemical structure. An example where they do affect the phenotype severely is GLU>VAL in beta polypeptide of haemoglobin causing sickle cell anaemia as glutamate is a negatively charged amino acid and valine is hydrophobic, hence hydrophobic interactions occur between haemoglobin. Nonsense mutation is a point mutation that changes a codon for an amino acid into one of the stop codons. Premature stop codons cause truncated proteins which are often non-functional. Eg most common cause of factor XI deficiency – short protein cannot function in blood clotting, reducing cascade amplification for intrinsic pathway. PCR followed by DNA sequencing or hybridisation with allele specific oligonucleotide(southern blotting), 2D PAGE to see change in protein size and charge or western blotting to see change in size. Frameshift mutation caused by insertion/deletion of bases not in a multiple of three as genetic code is read in codon sequence, 5>3, during translation. |
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Describe how spontaneous and induced mutations may occur
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Spontaneous mutations are new mutations not caused by exposure to mutagens. They are caused by errors in DNA replication, mismatching of base pairs causing tautomers – new DNA exists as 2 alternating forms and have slight chemical instability. Spontaneous mutation rates differ in different genes depending on size and sequence of gene and hot spots. On average there is 1 in 100,000 chance of acquiring a spontaneous mutation in a gene each round of replication. Each individual has multiple new mutations, usually in non-coding regions.
Induced mutations are caused by chemicals and radiation. Chemicals which cause mutations – mutagens. Chemicals that cause cancer – carcinogens. Most carcinogens are mutagens. Mutagens can come from a variety of souces eg X rays, UV ray from sunlight, nitrosamines from cigarette smoke and pesticides, 2,4-diaminotoluene. |
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Describe the genetic link between mutation and mutant and explain how some mutations can be inherited.
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A mutant allele /phenotype is one which differentiates from the common or wildtype allele/phenotype.
A mutation is a change in DNA sequence which causes a mutant allele and thus phenotype and are sources of genetic variation. Somatic mutations occur in cells of the body excluding the germline. The mutation only affects the cell itself and those that it produces by mitosis, it cannot be passed down to offspring. Germline mutations occur in germline cells (gonads) and have the possibility of being passed down to offspring. |
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Describe the process and the role of DNA repair
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Most mutations are repaired by the cell and in a healthy cell there is a fine balance between DNA damage and DNA repair.
Mismatch repair involves spontaneous mutations - the newly formed DNA molecule is proofread using enzymes which detect nucleotides that don’t base pair. Incorrect base is exicised and replaced with correct base. Excision repair may involve induced mutations – damaged DNA eg thymine dimer caused by UV radiation. In nucleotide excision repair small single stranded stretches of damaged DNA, up to 30 nucleotides can be exicised. In base excision repair a single nucleotide or a few damaged nucleotides, 1-5 can be excised. The result in both is damaged nucleotides excised and the resulting gap can be filled by DNA polymerase and sealed by DNA ligase. |
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Explain the relationship between DNA damage and cancer
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Failure of DNA repair causes increased numbers of mutations in genome. Protein p53 monitors repair of damaged DNA and if damage is too severe it triggers apoptosis- programmed cell death.
Mutations in genes encoding DNA repair proteins can be inherited – DNA no longer repaired efficiently. Tumours are large masses of cells formed from uncontrollable mitosis of a single, individual abnormal cell which lacks normal growth controls. Usually the cell is from a type that undergoes frequent cell division eg blood cells or bone cells. All cells in a tumour are identical and their behaviour in the tumour depends on the cell type. There is evidence that DNA damage is linked to cancer: 50 forms of cancer are inherited in some form, ames test proved that most carcinogens are mutagens, some viruses carry genes that promote cancer, there are specific chromosomal changes in some cancers. Oncogenes are involved in control of cell division. They are normally present as proto-oncogenes but mutations can cause formation of oncogenes from proto-oncogenes. Copies of the mutated gene are carried by viruses and can promote cancer, uncontrollable mitosis. The presence of virus means the gene doesn’t function as normal |
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Recognise the fundamental importance of PCR in the diagnosis of genetic disease.
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PCR is needed to allow detection of the mutation by amplifying a specific DNA segment, around the mutation, from a very small amount of DNA. Detecting the mutation could come from information regarding loss or gain of restriction site in the PCR product as in sickle cell where the loss of site for the enzyme MstII would show unbroken peptide. Information regarding the size of the PCR product could be used to detect mutation as in cystic fibrosis as a 3bp deletion occurs. presence of absence of a PCR product (eg PCR with allele specific primers) or DNA sequence of the PCR product.
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Provide an overview of the different genetic tests available for the detection of mutations in genes.
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PCR amplifies DNA segment around a mutation by repeated copying of the target DNA using a thermo-stable DNA polymerase and a pair of primers that uniquely define the region to be copied – forward and reverse. Prior knowledge of DNA sequence needed.
Not all mutations can be easily detected with PCR –based methods eg gene inversions – haemophilla A. Southern blotting allows investigation of an individual gene in a background of other genes and so is chosen when there is need to analyse larger segments of DNA within and around a gene. Southern blotting is also used to analyse triplet repeat disorders such as huntingtons disease. Array comparative genomic hybridisation ( array CGH) is used to detect sub-microscopic chromosomal deletions for which the location cannot be deduced from the patients phenotype. DNA probes covering the entire genome are attached to a solid surface matrix. The patient DNA and the normal control DNA are fluorescently labelled different colours. They are applied to the array in equal concentrations and undergo hybridisation with the probes. The hybridisation signals are detected and compared and for probes where the fluorescence intensity in normal DNA exceeded that in patient DNA then the patient has a chromosomal deletion from where that probe derived. CGH is used to detect changes in number of copies of DNA eg downs syndrome, cancer. Loss of DNA sequences contributes to the inactivation of tumour suppressor genes where as amplification may activate oncogenes – control cell division. |
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Show an appreciation of the ethical issues associated with genetic testing.
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Direct genetic test – examine individual mutations
Indirect genetic test – linked DNA markers tract the mutation in the family (linkage analysis) - Confirmation of diagnosis - Risk assessment in affected families - Prenatal testing – in high risk families or after ultrasonogram or biochemical tests, choice of termination or intervention - Predictive testing – removes uncertainty and can plan for future - Positive result – screening or prevention may be possible - Negative result – peace of mind, screening not neede - Offered in autosomal dominant disorders to asymptomatic people with 50% chance of having it – pre and post counselling needed-> affects on children - Might not want to know if they are likely to get a disease, worry, stress, if they already have children they may feel guilt at the chance of passing it on. If parents don’t get tested but they might pass on to children. If they are genetically predisposed to a condition will be discriminated against by insurance companies or employers. |
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Explain the potential pitfalls of using linking markers.
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The linked marker must be very close to the gene on the chromosome to reduce chance of separation by recombination. Samples from appropriate family members will be needed including affected and unaffected members.
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