Equilibrium constant

Equilibrium constant of salt MX:


For homogenous equation A + B --> C + D (i.e. all in the same state):

Formatted in LaTeX using the CodeCogs editor.

What do you understand by the term 'Dynamic equilibrium'?

Dynamic equilibrium refers to a reversible reaction in which the rate of the forward reaction equals the rate of the backward reaction and there is no net change in the concentration of ions.

Principles of evolution

1. Natural populations have great reproductive potential
2. But numbers remain about constant as many offspring fail to survive
3. Due to environmental factors, competition for resources, predators, other selection pressures, that impose a limit on their number and organisms struggle to survive
4. Individuals within a population show variation for natural selection to act on
5. Variants with a selective advantage/which are best adapted to their environment are more likely to survive and reproduce
6. Producing viable fertile offspring which pass on their favourite traits, thus favourable genotypes accumulate over time, leading to increased allele frequencies of favourable alleles
7. Over many generations, evolutionary changes by natural selection and may form new species if reproductive isolation occurs which is necessary for speciation
8. Include examples which may include: Darwin's finches, giraffes and how they got their long necks, peppered moths

Note: don't neglect points 1-4! 

Reproductive isolation is key to explaining speciation

Suggest why the cytochrome b gene is used to measure changes in DNA sequences in closely related species.

1. Cytochrome b is a homologous gene meaning that it is conserved in all the species being compared 
2. was found in their common ancestor; forms the basis of comparison 
3. Yet there are sufficient differences in the DNA of cytochrome b for scientists to distinguish between closely related species 
4. It is found in the mitochondria and hence do not undergo recombination and any mutation accumulates at a regular rate in the maternal line --> can be used for molecular clock

Characteristics of stem cells

1. Are unspecialised/undifferentiated
2. Able to differentiate
3. Can undergo extensive proliferation and self-renewal

Standard electrode potential

The standard electrode potential, E^⦵, of a half-cell is the electromotive force measured at 298 K between the half-cell and the standard hydrogen electrode, in which concentration of any reacting species is 1M/gaseous species is at 1 atm.

Remember to indicate the surrounding temperature when drawing half-cells!

The standard cell potential, E^⦵_cell is the maximum potential difference between two half-cells under standard conditions.

Some definitions

What is recombinant DNA?
DNA that contains genes/sections of DNA from two species/two types of organisms.

What is a vector?
A carrier such as a virus or plasmid used to carry the gene of interest into another host/gene

Why are plant tissue cultures initially grown in sterile conditions?

1. Prevent contamination by bacteria/fungi
2. which may produce toxins which will inhibit growth of plant cells;
3. as culture medium is suitable for the growth of mirobes, may outgrow the cells of the plant and deplete nutrients from the culture

Note: idea of 'outgrowing' is stronger than merely 'competing'.

Creating a cDNA library

1. Isolate mRNA from [tissue] 
2. Use reverse transcriptase to synthesise a cDNA strand using RNA as a template
3. Use DNA polymerase to make double-stranded cDNA and amplify using PCR
4. Cut plasmid with restriction enzyme that leaves blunt ends
5. Use terminal transferase to add extra cytosine nucleotides to the blunt end of DNA strands and add guanine nucleotides to that of plasmids to form sticky ends
6. Mix plasmid with cDNA; complementary sticky ends anneal to one another by hydrogen bonding;
7. Add DNA ligase to seal nicks by forming covalent phosphodiester bonds to form recombinant plasmid

What causes a unique banding pattern in Southern blots?

1. Polymorphic nature of DNA in individuals → variations in number/sequence of restriction sites/RFLPs
2. Resulting in unique banding pattern between individuals

Could also be for  gel electrophoresis.

Standard answer for chi-square test

1. At chi-squared value of [value], p is greater than [value]. Since p is more than 0.05/p is less than 0.05 @ 5% significance level, 
2. We do not reject the null hypothesis that there is no significant difference between observed and expected ratios and any difference is due to random chance.

Note: our teacher says to only state what the difference in observed phenotype is if you absolutely sure! 

What causes genetic variation in meiosis?

• Chiasma formation and crossing over- portions of non-sister chromatids of homologous chromosomes may break and rejoin. Exchange of alleles between homologous chromosomes ⇒ new combination of alleles 
• In metaphase I – (independent assortment) orientation of homologous chrm pairs at equator is random and how one homologous pair orientates does not affect how other homologous pair orientates. Metaphase II- orientation of non-identical sister chromatids of one chromosome at equator is random. Each daughter cell will receive a random mix of maternal and paternal chromosomes. 
• Random fusion of two gametes will result in 223x223 different possible types of offspring in humans. Can also be non-disjunction of homologous chromosomes in mei I or non-disjunction of sister chromatids during mei II resulting in polyploidy.

How does Southern Blot work?

1. DNA fragments separated by size using gel electrophoresis
2. Gel slab containing DNA fragments laid under nitrocellulose membrane, set in alkaline medium to denature dsDNA into ssDNA. ssDNA is drawn upwards onto the nitrocellulose membrane and binds to the membrane.
3. Membrane incubated with probe complementary to the target sequence in the DNA fragments, which contains radioactive marker. Hybridization via complementary base pairing occurs.
4. The bands formed are then visualized using autoradiography. Radioactive regions expose the film, forming image that shows where the bands have formed complementary base pairs with the probe.

How gel electrophoresis works

1. DNA fragments are pipetted into the wells at the top of the gel furthest from the anode;
2. Dense loading buffer mixed with DNA sample to help it sink to the bottom of the well
3. Loading dyes added to allow visualization of separation process
4. Negatively charged DNA migrates out of well, toward anode when subjected to an electric field
5. Fragments migrate through an agarose gel matrix made out of a meshwork of polysaccharides, which impedes movement of longer fragments more than shorter fragments.
6. Longer fragments migrate slower compared to shorter fragments.

How mutations result in differences in [protein]

1. Single base substitution → change in one codon → different amino acid incorporated in the polypeptide chain. Protein → different conformation that can no longer bind to [protein]
2. Deletion → frameshift mutation → viable [protein] not produced, could result in premature termination due to a new STOP codon /newly synthesized mutant protein quickly targeted for breakdown