Genetics Chapter 3

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  • Created on: 31-03-18 19:59
In prokaryotes, what shape is the DNA molecule in the one chromosome?
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What are the shapes of the DNA molecules in eukaryotes?
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Do eukaryotes have plasmids?
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What is the difference between eukaryote and prokaryote DNA in terms of proteins associated with it?
Eukaryotes have DNA associated with histone proteins while prokaryotes have naked DNA not associated with any proteins
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How many chromosome do eukaryotes have?
Two or more DIFFERENT chromosomes
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Overall, what are the differences between eukaryote and prokaryote DNA/ chromosomes?
Eukaryotes: 2 or more different chromosomes, no plasmids, histone proteins, linear DNA; Prokaryotes: Plasmids, One chromosome, naked DNA, circular DNA molecule
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At what stage of mitosis are chromosomes at their minimum length in eukaryotes?
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What is identical in sister chromatids?
Their DNA base sequences
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Why are the base sequences identical in sister chromatids?
Produced by replication during interphase
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What holds sister chromatids together?
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How does autoradiography work?
Thin sections of cells are coated with photographic film and left in darkness for days or weeks. The film is then developed and viewed under an electron microscope
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What does autoradiography show?
The structure of cells and where radioactively labelled substances are located
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What does each black dot show on an electron microscope?
Where a radioactive atom decayed and gave out radiation
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What did John Cairns do?
Researched chromosomes of E.Coli. He grew E.Coli in a medium containing radioactive thymine to label the DNA but not RNA. Placed cells on a membrane, digested cell walls so DNA spilled. Covered DNA in photographic film and left in dark for 2 month
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What did John Cairns discover?
DNA in E.Coli is circular and 1100 micrometres long. Discovered the length and shape of DNA in prokaryotes as well as that DNA replication is semi-conservative and starts at the origin and moves in opposite directions with two replication forks
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What did other researchers find who researched eukaryotic chromosomes using John Cairn's method?
Eukaryotic DNA is linear and much longer than prokaryotic DNA. Eukaryotic chromosomes contain one very long DNA molecule not many shorter molecules.
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What is the size of a genome?
The total amount of DNA in one set of chromosomes in that species
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What is genome size measured in?
Millions of base pairs (bp)
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What pH are histone proteins?
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What is chromatin?
A complex of DNA found in interphase and only seen with an electron microscope
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How can an organism have a large genome but a small number of genes?
It could have few protein-coding genes and lots of non-coding DNA
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Arrange the homo sapiens, T2 phage, Paris Japonica woodland plant, E.Coli gut bacteria and drosophila melanogaster fruit fly in order of increasing genome size
T2 phage, E.Coli, Drosophila melanogaster, homo sapiens, Paris Japonica
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What did John Cairns change to make radioactively labelled thymine?
Isotope of hydrogen with three as its mass number
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What are homologous chromosomes?
Chromosomes of the same type (one from father, one from mother)
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What is the same and different in homologous chromosomes?
Same genes in the same sequence but might have different alleles
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What does diploid nucleus mean?
Pairs of homologous chromosomes
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What does haploid nucleus mean?
One type of chromosome
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What is the diploid cell called when haploid male and female gametes fuse?
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What is a characteristic feature of species?
Chromosome number
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Which pair of chromosomes is the sex chromosomes?
23rd pair
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Which chromosome is bigger, the X or the Y chromosome?
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What is a karyotype?
The number and type of chromosomes present in a cell or organism
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What is a karyogram?
A photograph or diagram showing the homologous pairs of chromosomes of an organism in decreasing length
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Why are karyograms made?
To study the karyotype of an individual
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What are the functions of making karyograms of humans?
Determine the sex of the individual; Look for any chromosomal abnormalities if there are more or less than two chromosomes of each type
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What is Klinefelter syndrome?
A male having 2X chromosomes and one Y chromosome
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What is it called if someone has three chromosomes of a certain type?
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Why must the chromosome number be halved in meiosis?
Otherwise with each generation, there would be double the number of chromosomes
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What is meiosis?
A process that halves the chromosome number and allows a sexual life cycle with fusion of gametes
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What does the letter 'n' represent?
Haploid number of chromosomes
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What does '2n' mean?
Diploid number of chromosomes
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How many haploid nuclei are produced from meiosis of one diploid cell?
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How many chromatids in each chromosome are present after the first division of meiosis?
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How many chromatids in each chromosome are present after the second division of meiosis?
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What was present in a diploid cell before replication?
Two chromatids of each chromosome type
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What happens to chromatids when replication occurs?
Each chromosome is formed from two sister chromatids to create a pair of homologous chromosomes
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What halves the chromosome number in meiosis?
Separation of pairs of homologous chromosomes in the first stage of meiosis
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What happens in prophase 1?
Supercoiling and condensation makes the chromosomes visible. Homologous chromosomes pair up (synapsis). Centrioles migrate to opposite poles and spindle fibres form. Nucleolus and nuclear membrane disintegrate. Crossing over to form chiasmata
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What happens in metaphase 1?
Pairs of homologous chromosomes line up on the equator. Spindle microtubules attach to different homologous chromosomes in each pair by their centromeres so they are drawn to opposite poles. Gentle pulling checks tension and aligns chromosomes
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What happens in anaphase 1?
Spindle microtubules shorten to pull homologous chromosomes to opposite poles. This halves the chromosome number. Cell membrane pulled inwards to soon divide the cell
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When does the first meiotic division end?
When the homologous chromosomes arrive at opposite poles (telophase 1)
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What happens in telophase 1?
Chromatids uncoil. Nuclear membrane reforms around each nucleus.
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Which stage is the longest in meiosis?
Prophase 1
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Prophase 2 is the same as...
Prophase 1
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What is the difference between meiosis 2 and meiosis 1?
Meiosis 1 involves diploid cells, meiosis 2 involves haploid cells as chromosome number already halved
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In metaphase 1, how many poles was each centromere attached to?
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In metaphase 2 how many poles is each centromere attached to?
2 (both)
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What do spindle microtubules attach to?
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What is the only difference between metaphase 1 and 2?
The number of poles each centromere is attached to
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At what stage do sister chromatids start being called chromosomes?
Anaphase 2
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What happens in anaphase 2?
Centromeres divide to pull chromatids to opposite poles. Cell membrane pulled inwards again
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What happens in telophase 2?
Chromatids reach opposite poles and nuclear membrane reforms. Chromosomes uncoil and cytokinesis splits cell to form four haploid cells
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What is a tetrad/ bivalent?
A pair of homologous chromosomes (4 chromatids)
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At what stage is the DNA of a cell replicated?
S stage of interphase before the first meiotic division
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If there was a division in meiosis 1, how come there are haploid cells formed at the end of meiosis 1 and meiosis 2?
In meiosis 1, a chromosome is two chromatids but in meiosis 2, a chromosome is 1 chromatid
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Meiosis 1 is the separation of...
Homologous chromosomes
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Meiosis 2 is the separation of...
Sister chromatids
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What are the two ways genetic variation is introduced in meiosis?
Random orientation of bivalents in metaphase 1, crossing over of non-sister chromatids in prophase 1
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What does it mean by random orientation of homologous chromosomes?
Homologous chromosomes always stick together but which chromosome faces which pole and attaches to which spindle microtubule could differ and does not affect the orientation of other pairs of homologous chromosomes
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Why does random orientation not create genetic variation in meiosis 2?
Consists of single chromosomes with two sister chromatids that are genetically identical
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How is crossing over random?
Crossing over could occur anywhere along the length of the chromosome
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What is reduction division?
Reducing the number of chromosomes through cell division (meiosis 1 e.g.)
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What is a haploid cell?
Half the number of chromosomes
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How do you work out the number of combinations of chromosomes formed in random orientation in metaphase 1?
2 to the power of the haploid number
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How does crossing over lead to genetic variation?
New combinations of alleles formed in male and female gametes
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Crossing over occurs as a part of which process?
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What else creates genetic variation other than random orientation and crossing over?
Fusion of gametes is random leading to a wide number of allelic combinations
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What is non-disjunction and when does it occur?
Non-separation of chromosomes and can occur in the anaphase 1 or 2
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What are the two ways of getting cells for chromosome testing?
Amniocentesis and chorionic villus sampling
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How is amniocentesis done?
Insert hypodermic needle through abdomen and uterus wall to draw out amniotic fluid containing fetal cells into a syringe
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Where are the chorionic villi?
In the placental chorion
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What are the risks of miscarriage with a) amniocentesis and b) chorionic villus sampling
a) 1% b) 2%
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Are there any risks of infection from amniocentesis or chorionic villus sampling?
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Just because there is a correlation between mother age and Down's syndrome or trisomy occurrence, this doesn't mean...
There is direct causation
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What are the ethical implications of testing for chromosomal disorders?
Terminating the pregnancy if there is a disorder (or a specific gender in some cultures) or do you keep going with it?
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Mendel's large numbers of pea plants ensured...
Reliability and also repeating his experiments with seven different traits
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What is a genotype?
The alleles possessed by an organism
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Why do you only have one gamete in a monohybrid cross but two genotypes?
Gamete involves haploid cells with one type of each chromosome but because the plants are diploid, they have two of each gene
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If you cross homozygous dominant gametes with homozygous recessive gametes what do you get?
All offspring are heterozygous dominant
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What is segregation?
In meiotic division, the two alleles of each gene separate into different haploid nuclei. Separating the alleles into gametes
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In a monohybrid cross of two heterozygous dominant genotypes, what is the genotypic and phenotypic ratio?
Genotypic: 1:2:1 Phenotypic: 3:1
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How does segregation occur?
Two alleles are located on homologous chromosomes which are pulled to opposite poles in anaphase
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What is required for a 3:1 phenotypic ratio to occur?
One allele must be dominant and one recessive
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What is the effect of a dominant allele?
Masks the effect of the recessive allele in a heterozygote
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What does homozygous mean?
A pair of matching alleles for a particular characteristic
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What does heterozygous mean?
An individual or cell carrying non-identical alleles for a particular trait
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What causes cystic fibrosis?
Recessive alleles of autosomal genes
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What causes Huntington's disease?
Dominant alleles of autosomal genes
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What happens with cystic fibrosis?
Presence of recessive allele of chromosome 7 codes for CFTR gene which affects chloride channels in mucous membranes (sweat, mucous and digestive juices)
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What must both parents be for a child to get cystic fibrosis?
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Why do carriers not get the disease?
They also have the dominant allele to mask the effects of the recessive allele
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How does Huntington's disease form?
A dominant allele developed due to a mutation in the HTT gene on chromosome 4
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What are some symptoms of Huntington's disease?
Loss of muscle coordination, cognitive decline, psychiatric problems
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What does the HTT gene code for?
Huntingtin protein with an unknown function
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If only one parent has the gene for Huntington's disease, what is the probability of offspring getting the disease?
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What do pedigree charts determine?
The genotype of an individual and if a disease is caused by a dominant or recessive allele
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In pedigree charts, how do you represent males and females?
Females with a circle, males with a square
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What does a shape with a dot in it mean in a pedigree chart?
They are a carrier
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In a pedigree chart, if the disease is present in every generation what does that suggest?
The disease is dominant
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Why is sex linkage almost always dependent on the X chromosome?
The X chromosome is bigger and has important genes on it while the Y chromosome is smaller and has less genes
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Why are males more likely to develop a sex-linked disease?
If it is due to a recessive allele, females require two recessive alleles to develop the disease but males only require one because the Y chromosome does not carry an allele
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Who can be carriers of sex-linked diseases?
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Name two diseases based on recessive alleles on sex-linked genes?
Red-green colourblindness and hemophilia
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What does co-dominance mean?
If both alleles are present, they both affect the phenotype of the individual
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What is meant by multiple alleles? Give an example
More than two alleles for a particular gene. For example, the blood groups have the alles IA, IB and i
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How can you get a punnett grid of all blood groups?
Heterozygous IAi crossed with IBi parents
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When can you predict offspring ratios?
Large numbers of offspring
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Why might a predicted ratio be different to the actual ratio?
One allele combination could be lethal
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Are mutations random?
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What types of radiation are mutagens?
X-rays, short or medium wave UV, gamma rays and alpha particles from radioactive isotopes
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Give examples of mutagenic chemicals
Mustard gas used as a chemical weapon, nitrosamines in tobacco, solvent benzene
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Give two examples of nuclear events that can be studied to see the effects of radiation
Chernobyl nuclear accident and Hiroshima nuclear bomb
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Why did Chernobyl cause less deaths despite the fact it released more radioactive contents
More radiation (higher doses) was spread over a wider area for longer and isotopes have a longer half-life
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What happened at Chernobyl?
Fire and explosions in core of nuclear reactor in Ukraine
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Which isotopes/ radioactive materials were released at Chernobyl?
Iodine-131, caesium-134 and caesium-137 as well as uranium and other radioactive metals broke up in the explosions and escaped
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How many workers died in Chernobyl? What disease affected workers?
28 within 3 months and leukemia more prominent
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What was the effect of radioactive iodine release in Chernobyl?
Milk and drinking water with high levels of radioactive iodine. As many as 6000 got thyroid cancer and many cattle and horses nearby died from damage to their thyroids
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What was there bioaccumulation of and in what from Chernobyl?
Radioactive caesium in fish and lamb - consumption banned due to the long half-life of caesium-137
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What happened to woodland from Chernobyl?
4km squared of pine forest downwind of reactor went ginger brown and died. In the absence of humans in this area, populations of lynx and wild boar have thrived
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How many people died from Hiroshima?
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The higher the dose of radiation...
The greater the risk of cancer or leukemia by survivors
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What was a main health effect of the Hiroshima bomb?
Mutations leading to many stillbirths and malformations
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How did people feel stigmatised after Hiroshima?
Husbands reluctant to marry them for fear their children would get genetic diseases
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Is there much evidence of mutations in children who were fetuses at the time of Hiroshima?
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How do you define genetics?
The study of variation and inheritance
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What is a gene?
A heritable factor consisting of the length of DNA and influences specific characteristics
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Where is the gene located for the beta polypeptide of hemoglobin?
Near the end of the short arm of chromosome 11
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What has less genes that eukaryotes?
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What makes alleles?
A change in the base sequence of one or a vary small number of bases in a gene DNA sequence
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What causes the genetic disease sickle cell anemia?
The second base on the sixth codon in the beta polypeptide hemoglobin gene is thymine instead of adenine
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What is the name of the beta polypeptide hemoglobin gene?
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What codon is normally transcribed, what is the mRNA sequence and amino acid formed in sickle cell anemia?
Normally the codon is GAG, in sickle cell anemia it is GTG leading to GUG on mRNA. This codes for valine instead of glutamic acid
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What are databases currently used for?
Storing large amounts of base sequence information from genome research
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Why has searching and sharing gene information become easier?
Internet means it can be easily accessed and shared across the world and data can also be searched and extracted easier
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What are some uses of gene databases?
Finding the locus of a gene and the corresponding protein it codes for (protein product). Also comparing base sequences of different species to find how many differences there are, how long ago they diverged and the common ancestor
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What does PCR do?
Produces many copies of a DNA molecule
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Why would you want to do PCR?
If you have only a small sample and you need a larger sample of DNA for analysis
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DNA from what can be amplified in PCR?
Blood, semen or other tissue or a long-dead specimen
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What is the first stage of PCR?
DNA is heated to 95 degrees celcius to separate the DNA strands
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What is the second stage of PCR?
Temperature reduced to 53 degrees celcius to allow primers to bind to both strands of DNA next to the section to be replicated
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What is the third stage of PCR?
Temperature increased up to 73 degrees celcius to encourage Taq DNA polymerase to replicate both strands, starting at the primer and producing two-double stranded copies of the original DNA sequence
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What does gel electrophoresis do?
Separates mixtures of proteins of DNA fragments
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How does gel electrophoresis work?
Thin sheet of gel acts as a molecular seive. The mixture of proteins or DNA is placed over/in the gel and an electrical charge applied with electrodes at either end. DNA is negatively charged so will move to the positive electrode.
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What sort of fragments move faster through the gel in gel electrophoresis? Why?
Smaller fragments because there is less resistance
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What is the human genome project?
Research to find human genomes (DNA sequence or genes that comprise a human)
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What is the human genome project allowing the study of?
Genetic variation
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What has the human genome project led to?
The plant genome project to find genomes of 1000 plants, also found 100 prokaryote genomes and eukaryote genomes and other species
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What are short tandem repeats?
Shorter DNA sequences of 3-5 bases repeated many times instead of one long sequence
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Where are short tandem repeats used?
DNA profiling
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What is specifically looked at in DNA profiling?
Short tandem repeat alleles as there could be many different alleles possible at that chromosome locus
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What is a requirement for the DNA used for DNA profiling?
Must not be contaminated by anyone or something else's DNA
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What is done with the DNA for DNA profiling?
A selection of short tandem repeat loci are found and copied with a PCR. The copies of alleles are separated with gel electrophoresis to give a pattern of bands
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How many loci are used for DNA profiling?
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Why do they use multiple loci for DNA profiling?
Unlikely for individuals to have identical base sequences and alleles for the more loci there are
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In the gel electrophoresis for DNA profiling, when might two individuals have the same pattern of bands?
Identical twins
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As well as for forensic purposes, when else is DNA profiling used?
Paternity testing
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When were the first DNA profiling techniques used? What was discovered?
Enderby double murder case showed the DNA from the semen of the culprit did not match the prime suspect DNA who confessed to the murders so he was innocent
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How can you use DNA profiling for forensic analysis?
Use as evidence in court cases and to eliminate or prove the prime suspect is guilty
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What are the three stages of PCR called?
1) Denaturation 2) Annealing 3) Extension
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What sort of amplification is PCR?
Exponential amplification
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What sort of enzymes are used in DNA profiling to chop the short tandem repeats into small sections before gel electrophoresis?
Restriction enzymes (endonucleases)
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What are the names of organisms that have had a gene transferred to them?
Genetically modified organisms or transgenic organisms
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What does a vector do in genetic modification?
Carries the DNA
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What is the vector in the transfer of insulin genes to bacteria?
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What is a recombinant plasmid?
A plasmid with a gene inserted from another species
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What is a host cell in genetic modification?
The cell that receives the new gene
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What allows genetic modification and gene transfer?
Universality of the genetic code
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What is extracted from human pancreas cells for genetic modification?
mRNA coding for insulin
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How is the mRNA coding for insulin converted to DNA?
Reverse transcriptase enzyme
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Sticky ends composed of which base are made in the DNA section?
G nucleotides
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Sticky ends composed of which nucleotide are made in the plasmid?
C nucleotides
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How are plasmids cut open at precisely defined locations?
Restriction enzymes (endonucleases)
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What happens once the preparation of plasmid and insulin genes are complete?
They are allowed to mix together and complementary base pairing links the sticky ends together
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What is the role of DNA ligase in genetic modification?
Makes sugar-phosphate bonds to seal up nicks in the DNA once plasmids and genes have combined
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What happens when recombinant plasmids are mixed with host cells?
The E.Coli absorbs them
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How is insulin produced from the E.Coli?
The bacteria is cultured in a fermenter and starts to make insulin which is extracted and purified to be used by diabetics
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What quality is the insulin in when it is produced by E.Coli?
High purity and reliability
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How has genetic modification been used for crops?
A gene from a bacterium has been transferred to Bt maize. It codes for a protein called Bt toxin which kills any insect pests that feed off the plant e.g. corn borers
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What other genetic modification of plants has taken place?
Herbicide resistance, increased vitamin content, resistance to viral diseases, decreases allergen or toxin content, increases tolerance to drought, cold or saline soils
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What are the benefits of Bt maize?
Increased yield so more food and less pests; Less land for crops needed so some for wildlife conservation; less use of insecticide sprays which are expensive and harmful to workers or wildlife; lowers levels of toxic and carcinogenic mycotoxins
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What environmental benefit not mentioned yet is there from Bt maize?
Produces a cleaner, better environment because less pesticides are going into lakes and rivers (eutrophication). Cost of crop production reduced because less pesticides needed
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What are the risks of Bt maize?
Could harm other animals if toxin pollen gets to wild plants and those organisms feed on it (caterpillars and monarch butterfly). Detritivores also at risk. Could make wild plants toxic through cross pollination. Pests could get resistant
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What risks are there of Bt maize in terms of human health, organic growers, biodiversity and wild plants?
Humans could be allergic. Pollen with toxin could cross pollinate neighbouring organic plants, leading growers to lose their certificate and not get premium price for produce. Reduced biodiversity of insects and 'superweed' formation
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What is a natural form of cloning?
Asexual reproduction
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Give examples of asexual reproduction leading to cloning?
Plants produce tubers, runners, extra bulbs or other structures. Aphids give birth to young formed asexually from their own cells
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What is the simplest method of cloning?
Break up an embryo at early stage into multiple groups of embryonic stem cells which develop into genetically identical individuals
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What is the drawback of the simplest cloning method?
Do not know what characteristics organism will develop to have
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Name another method of cloning
Somatic-cell nuclear transfer
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What is a clone?
A group of genetically identical organisms derived from a single original parent cell
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Which cell nucleus is used to make Dolly?
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Why is the udder cell to make Dolly cultured for 6 days?
To make it undifferentiated so it would not form an udder cell again
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What other cell is used to make Dolly? What happens to it?
An unfertilised egg cell has its nucleus removed
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How are the donor nucleus and egg cell fused together?
A pulse of electricity which also stimulates it to divide
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Which sheep is Dolly genetically identical to?
The sheep with the udder cell taken (nucleus used to make Dolly)
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Why is Dolly not completely genetically identical to the udder cell mother sheep?
The egg cell contains mitochondrial DNA. The enucleated egg contributes cytoplasmic contents and organelles to the cloned embryo
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Why is it hard to clone adult animals?
Cells are at an advanced stage of differentiation so fusion does not necessarily lead to embryos
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What is a method of cloning plants?
Stem cuttings
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How do stems produce roots from a stem cutting?
When base is placed in water or a solid medium
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What could be the independent variables in a stem cutting investigation?
Whether a rooting powder is used, a plastic bag is over the cutting, how many leaves are on the cutting, how warm the cuttings are, age of plant, light, species of plant
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What could be the dependent variable of a stem cutting investigation?
If roots form or not, how many roots form, length of longest root, number of days before rooting occurs,
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What could be a control variable in a stem cutting investigation?
Same species of plant
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How should a stem cutting investigation be carried out?
Cut a cutting off the parent plant just below a node. Place in a well-aerated and moisture-retentive compost and place a plastic bag over the cutting
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Why would you place a plastic bag over a cutting?
Increase humidity to decrease transpiration
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What is the actual name for cloning plants using stem cuttings?
Artificial propagation
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What is a node on a stem?
A meristematic zone
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What were Mendel's three laws?
Law of segregation, law of independent assortment, law of dominance
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What was Mendel's law of segregation?
Inherited traits are controlled by a pair of alleles which are separated during meiosis to give one allele of each gene in a sex cell. Alleles are passed from one generation to the next as distinct units
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What is Mendel's law of independent assortment?
The inheritance of one characteristic is independent of the inheritance of another. The separation of alleles of one gene is independent of the separation of other alleles in gamete formation
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What is Mendel's law of dominance?
An organism with two dissimilar alleles will express the one that is dominant
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What are the shapes of the DNA molecules in eukaryotes?



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Do eukaryotes have plasmids?


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Card 4


What is the difference between eukaryote and prokaryote DNA in terms of proteins associated with it?


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How many chromosome do eukaryotes have?


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