3. genetics

• Pre-natal diagnosis of chromosomal abnormalities

  1. chorionic villus sampling
  2. cells of the chorionic villus collected via cervix/needle through abdomen wall
  3. preferred method before 15 weeks of gestation
  4. risk of miscarriage: 1%
  5. 2. amniocentesis
  6. sample of amniotic fluid collected via needle through abdomen wall
  7. amniotic fluid contains cells from foetus
  8. risk of miscarriage: >0.5%
• What are plasmids and which organisms contain them?

  1. Plasmids are present in most prokaryotes.
  2. They are small loops of naked DNA.
  3. Plasmids can be passed from one bacteria to another via the pili.
  4. This is how antibiotic resistance genes spread in bacterial populations.
  5. Eukaryotes do not have plasmids.
• Sickled red blood cells may result in the symptoms noted above because. . .

  • Red blood cells cannot flow through / cause blockages of capillaries/blood vessels
  • Less oxygen is carried by the red blood cells OR blockages restrict blood flow around the body
  • Less oxygen is delivered to respiring tissues (including the brain, causing dizziness)
  • Heart rate increases to try and increase oxygen delivery
  • Lower rates of aerobic respiration means less energy released (leading to fatigue)
• Sickled red blood cells may result in the symptoms noted above because. . .

  • Red blood cells cannot flow through / cause blockages of capillaries/blood vessels
  • Less oxygen is carried by the red blood cells OR blockages restrict blood flow around the body
  • Less oxygen is delivered to respiring tissues (including the brain, causing dizziness)
  • Heart rate increases to try and increase oxygen delivery
  • Lower rates of aerobic respiration means less energy released (leading to fatigue)
• Cairns autoradiography technique can be outlined as follows:

  • Bacteria/ E. coli were kept in a nutrient broth with radioactive thymine/tritiated thymidine
  • During DNA replication, the radioactive thymine/tritiated thymidine was incorporated into the bacterial DNA
  • Lysozyme / an enzyme was used to lyse/break down the cell walls to access the DNA
  • The DNA was fixed to a membrane and submerged into photographic emulsion containing silver ions
  • Silver ions were converted to silver atoms by the (radioactive DNA)
  • The results were studied using an electron microscope
• Cairns autoradiography technique allowed the following discoveries…

  • Scientists could study the length and shape of DNA
  • Cairns found that the chromosome of coli was 1100 μm long
  • Cairns found that E.coli had a single, circular chromosome
  • Cairns made discoveries about the process of DNA replication in prokaryotes
• Sickle cell anaemia:

  • Mutation occurs in the gene that codes for the alpha-globin polypeptide in haemoglobin
  • Within this gene, the base thymine/T is replaced by adenine/A in the sixth codon
  • This causes the DNA triplet GAG to mutate to GTG and transcribed into the mRNA codon GUG, instead of GAG
  • During translation, the amino acid valine/VAL replaces the original (sixth) amino acid glutamic acid/GLU
  • The protein haemoglobin S is produced instead of haemoglobin A
  • This causes a distortion in the shape of the red blood cells into sickle shapes
• The daughter cells will be genetically different because….

  • Random orientation of homologous chromosomes occurs during meiosis
  • The bivalents/homologous chromosomes are randomly lined up across the equator of spindle
• State two features that allow nucleic acids or proteins to be separated by gel electrophoresis:

  • Net (overall) charge
  • Size/mass/length of fragment
•  During gel electrophoresis, DNA moves towards…

  • The anode / positive (+ve) electrode
  • Because DNA is negatively charged

  The component of DNA that gives it its charge is…

  • Phosphate groups / backbone
• The steps of somatic cell nuclear transfer that led up to implantation:

  • The nucleus is removed/enucleated from a donor egg cell
  • A somatic (body) cell is taken from the donor animal to be cloned
  • Somatic cell is introduced into the enucleated egg cell;
  • An embryo develops
• Human recombinant insulin can be produced in the following way…

  • Human insulin mRNA is extracted from human pancreatic cells
  • Reverse transcriptase is used to make complementary DNA (cDNA)
  • cDNA is amplified by PCR
  • Amplified gene is cloned/ligated/inserted into a plasmid
  • Recombinant plasmid is introduced into host cells/bacteria
  • Transformed cells are grown in culture usually in a fermenter
  • Expressed insulin is separated and purified from the host cells
• Risks of GMO:

  • Antibiotic resistance genes that are used as markers could be transferred to bacteria leading to antibiotic resistance of pathogenic bacteria
  • Non-native proteins produced by the modified crops could be toxic or lead to allergic reactions
  • Toxins produced to control pests could affect non-target organisms
  • Biodiversity could be reduced that could impact wildlife
  • Cross-pollination could lead to ‘super-weeds'
  • Genetic material could be transferred between the GM crop and related plant species
• Benefits of GMO:

  • Crops can be developed that are resistant to pests or herbicides
  • Nutritional value of foods can be improved
  • Crops can be developed that are resistant to extreme conditions e.g. drought, cold, high salinity
  • Crops can be developed that yield useful products such as vaccines, hormones
  • Crops can be developed with improved yields that leads to improved food supply and better use of land and other resources
  • GM crops reduces the need for ploughing and spraying agrochemicals
  • Fruit and vegetables can be developed that have a longer shelf life resulting in less wastage
• The steps of PCR are…

  • Denaturation - DNA is heated to a high temperature to separate the two strands
  • Annealing - DNA is cooled allowing primers/short lengths of DNA to anneal to target sequences
  • Elongation/Extension - Double-stranded DNA is synthesised by Taq DNA polymerase, using the single strands with primers as templates
• The method by which DNA profiling is undertaken:

  • Isolate DNA, which can be extracted from a biological sample
  • DNA sequences that are highly variable between individuals are selected
  • Copy, increase / amplify the target DNA through PCR
  • Use restriction enzymes/endonucleases to cut the amplified DNA into fragments
  • DNA fragments are separated using gel electrophoresis
  • Explanation of electrophoresis e.g. mixture put into wells in gel AND an electric current passed through
  • The DNA fragments form a distinct pattern of bands that are unique to an individual
• Why DNA profiling is a useful techniques:

  • DNA profiling can be used by forensic scientists to identify suspects of crimes
  • DNA profile of a sample from the crime scene can be compared to a sample of DNA taken from the suspect
  • DNA profiling can be used in paternity investigations to find out if a man is the father of a child
• A gene can be isolated in the following ways:

  • Isolate mRNA (for factor VIII)
  • (Then use) reverse transcriptase
  • To build (double stranded) DNA/ cDNA (from mRNA)
  • Use restriction endonuclease/enzyme
  • To cut out the gene for factor VIII at specific base sequences / restriction sites/recognition sites
• The first meiotic division is a reduction division (diploid → haploid) in which homologous chromosomes are separated 

  • P-I: Chromosomes condense, nuclear membrane dissolves, homologous chromosomes form bivalents, crossing over occurs
  • M-I: Spindle fibres from opposing centrosomes connect to bivalents (at centromeres) and align them along the middle of the cell
  • A-I: Spindle fibres contract and split the bivalent, homologous chromosomes move to opposite poles of the cell
  • T-I: Chromosomes decondense, nuclear membrane may reform, cell divides (cytokinesis) to form two haploid daughter cells
• The second division separates sister chromatids (these chromatids not identical due to crossing over in prophase I) 

  • P-II: Chromosomes condense, nuclear membrane dissolves, centrosomes move to opposite poles (perpendicular to before)
  • M-II: Spindle fibres from opposing centrosomes attach to chromosomes (at centromere) and align them along the cell equator
  • A-II: Spindle fibres contract and separate the sister chromatids, chromatids (now called chromosomes) move to opposite poles
  • T-II: Chromosomes decondense, nuclear membrane reforms, cells divide (cytokinesis) to form four haploid daughter cells
• • Division – Mitosis involves only one cell division, but meiosis requires two cell divisions
  • Independent assortment – Homologous pairs are randomly separated into separate cells in meiosis, but not mitosis
  • Synapsis – Homologous pairs form bivalents in meiosis, but not mitosis
  • Crossing over – Non-sister chromatids of homologous pairs may exchange genetic material in meiosis, but not mitosis
• • Outcome – Mitosis results in the formation of two daughter cells, while meiosis produces four daughter cells
  • Ploidy – Daughter cells produced by mitosis are diploid, while daughter cells produced by meiosis are haploid
  • Use – Mitosis is used to clone body cells, while meiosis is used to generate sex cells (gametes)
  • Genetics – Cells produced by mitosis are genetically identical (clones), while cells produced by meiosis are genetically distinct