PBL Week 1 Flashcards
What is the biochemical basis of genetics?
DNA - made up of Deoxyribose Sugar and Phosphate backbone, with 4 bases (Adenine, Guanine, Cytosine, Thymine). Is double stranded in a helix shape; one chain runs in the 3’ to 5’ direction and one runs in the 5’ to 3’ direction. Chains are connected by the bases (A-T, C-G).
RNA - Same as DNA, but has a Ribose sugar and Uracil instead of Thymine.
What is the concept of a polypeptide chain encoded by a gene and how are they formed?
Proteins are made of a chain of amino acids, also known as a polypeptide chain. A gene is a section of DNA that codes for one of these. Made up of codons; 3 bases that each code for one amino acid (20 types).
Transcription - The enzyme DNA polymerase attaches to a gene when activated, moving along and attaching complementary nucleotides to a primer to form Messenger RNA (mRNA).
Translation - mRNA moves out of nucleus and into cytoplasm, where they pass through Ribosomes. Transfer RNA (tRNA), each of which are connected to an amino acid and have 3 complementary bases, connect along the mRNA one at a time, forming the amino acid chain. Stops when it reaches the stop codon.
What is Mitosis and what are the stages?
Mitosis is the division of 1 diploid cell to 2 identical diploid cells. Before Mitosis is interphase, where the cell grows (G1), replicates its genetic material (S) and grows more (G2). During S phase, centrosomes are also replicated. Interphase has various checkpoints to check for defects in the cell.
The first stage of Mitosis is Prophase. Chromatin condenses into Chromosomes, Spindle fibres + Microtubules begin to form between centrosomes and the Nuclear Envolope disappears.
Next is Metaphase. Chromsomes line up along middle of cell, Spindle Fibres connect to the Centromeres on the Chromosomes and the Centrosomes move to the poles of the cell.
In Anaphase, the chromosomes get pulled apart with one arm going to each pole.
Telophase sees the nuclear envelope reform around the 2 sets of chromosomes. The cell also begins to split along the middle, eventually breaking apart into 2 cells in Cytokinesis.
What is Meiosis and what are the stages?
Meiosis is made up of 2 divisions, with a diploid cell dividing into 2 haploid cells, which further divide into 4 haploid cells. These are called gametes (sex cells). Unlike Mitosis, all 4 are genetically different. In the germ/beginning cell, there are homologous chromosomes; chromosomes that code for the same genes but are different - one from each parent.
Prophase 1 is identical to Mitosis. Metaphase 1 is also similar, except that when the homologous chromosomes line up in the middle of the cell, they can join up at certain points called Chiasmata. When this happens, the ends of the chromosomes break off and rejoin, giving the paternal chromosome some maternal genes and vice versa. This is called recombination.
Anaphase 1 is also similar, except it’s random whether the maternal or paternal chromosomes are pulled to each pole, creating further genetic variation. Finally, Telophase 1 and Cytokinesis are also essentially identical, except that the daughter cells are haploid. The cells then divide again in Meiosis 2, which is identical to Mitosis.
What is the one gene one protein hypothesis and why is it an over-simplification?
The one gene one protein hypothesis states that each gene codes for one protein. It’s an oversimplification because not all genes code for proteins; only around 1% of DNA are protein-coding genes. The rest codes for regulatory elements, RNA molecules and Telomeres. Also non-coding regions (Introns) which are removed after transcription.
How do you draw family trees and how are they used?
A square represents a male, circle a female and diamond is unknown gender. If the shape is shaded, the person has the disease. If there is a number in the shape, it represents mutiple individuals. If there is a line through the shape, the individual is deceased. If there is a P in the shape, it represents an unborn child. An arrow pointing to the shape represents the person providing the info.
A line between 2 shapes is a marriage, a line with 2 diagonal stripes is a divorce. A double line represents an incestuous relationship. Lines down from the joining line represent offspring, with a 120 degree split representing twins (with an extra line making a triange showing identical twins).
The consultand is the person seeking the genetic counselling and the proband is the person who’s prompted the need for genetic counselling. The family trees are used to identify and advise about genetic diseases. Can be used to calculate odds of someone having a genetic disease. At the bottom of the diagram, date of pedigree and name of consultand should be noted.
Define aneuploidy, monosomy and trisomy and describe how it arises during cell division.
Aneuploidy is the presence of an abnormal number of chromosomes in a cell; monosomy is one too few and trisomy is one too many. Can happen due to a non-disjunction event, where homologous chromosomes/sister chromatids don’t seperate during cell division.
Give examples of autosomal dominant and autosomal recessive diseases and how the phenotype of these diseases relate to the cell biology.
Autosomal means not a sex cell. Dominant examples include Huntingdon’s disease while Recessive examples include C.F and Sickle Cell Anaemia.
For Huntingdon’s disease, a faulty gene causes parts of the brain to damage over time, leading to neurological problems. For Sickle Cell, the faulty gene causes a different form of haemoglobin to be created, leading to sickle-shaped red blood cells.
What is FISH and what are the stages?
FISH is fluoresence in situ hybridisation. It is used to detect chromosomal mutations using fluorescent probes that bind to certain parts of the chromosome.
First, the cell is fixated. Then, the endonuclease enzyme DNase removes certain bases and they are replaced with fluorescent tagged bases instead. This is the probe. The probe and targed DNA is then denatured at 95 degrees and is split into individual strands. The temperature is lowered during hybridisation, allowing the single stranded probe to bind to a specific gene (depending on the base order of the probe). If there is a mutation on the gene, the probe cannot bind and therefore there is no fluorescence.
What is PCR and what are the stages?
PCR is polymerase chain reaction. It is used to amplify/duplicate a region of DNA. First, high heat (94 degrees) denatures and splits the DNA into single strands, then the temperature is cooled (50-60 degrees) to allow for the primer to bind. The primer is a short sequence of bases that binds to the borders of the amplicon (the piece of DNA being duplicated) to distinguish it from the rest of the DNA. The temperature then raises to the optimum temperature (72 degrees) for DNA Polymerase to synthesise the new sequence by adding nucleotides.
After 3 cycles, you have a single copy of the amplicon. For every successive cycle, the number of DNA molecules almost double.
What are the pre-natal and post-natal tests available for chromosomal abnormalities?
Pre-natal: Amniocentesis, where a needle is pushed through the membrane of the amniotic sac and a sample of the amniotic fluid is taken. This contains foetal DNA which can be used to detect chromosomal abnormalities. Can also determine the karyotype of the baby. Furthermore, FISH is also a pre-natal test using foetal DNA from blood or amniotic fluid.
Post-natal: Similar tests can be done as well as whole genome sequencing, which sequences the whole genome and detects any chromosomal abnormalities present. Heel prick test involved taking a spot of blood from a newborn’s heel and testing for 9 different inherited diseases, including C.F and sickle cell.
What are single gene disorders, how can they be diagnosed and how do they happen?
A single gene disorder is when a disease is caused due to a mutation on just one gene. They can be diagnosed using processes such as whole genome sequencing or FISH. They occur due to a faulty gene; as the sequence of bases on the gene is wrong, the corresponding polypeptide can be different. Depending on the function of the polypeptide, this can have a range of consequences, including the symptoms of the diseases. Examples include Cystic Fibrosis, Sickle Cell Disease and Huntingdons Disease.
What are new technologies that could modify human embryos and what are the ethical consequences of this?
The main new technology is CRISPR, a revolutionary new tech that allows scientists to edit a genome. This could be used to edit foetal DNA, altering genes to theoretically change anything about the baby. There are many ethical consequences to this, including the worry that some might pay extra to give their kids more desirable traits, ostracisation of those who dont have said traits, the possiblity of governments using this to create specially-designed soldiers and so on.
Why are some genetic diseases more common in certain societies and why do we suspect that they might provide a evolutionary advantage?
One way some genetic diseases are more common in societies are due to inbreeding; for example, haemophilia was very common amongst European royals due to inbreeding as a way to “keep the bloodlines pure”. However, sometimes they can even be an evolutionary advantage; people of Afrocarribean descent are far more likely to have sickle cell anaemia than other races. This is due to the fact that sickle cell actually provides a resistance to malaria; as people
of Afrocarribean descent are from areas where malaria is more prevalent, it is thought that natural selection caused them to retain the trait, as it protected against a deadly disease and therefore made them more likely to have offspring.
What is NGS and what are the stages?
NGS stands for next generation screening and can be used for sequencing whole genomes. First, the DNA is chopped into fragments a few hundred base pairs long. Short DNA tags are added to the ends. Then, the fragments are added to a glass slide called a flow cell. This is loaded into the sequencing instrument alongside various reagents. Then, cluster generation occurs where each fragments is replicated on a specific point of the flow cell, forming a monoclonal cluster. Finally, sequencing by synthesis can happen where fluorescent nucleotides are washed over the fragments and bind to each cluster. A laser pulse causes each nucleotide to fluoresce. The wavelength of the fluorescence is used to identify the base order. This data can then be processed and analysed.
