Celbridge Tutorials offers a comprehensive course in all areas of Biology, with particular emphasis on areas causing difficulty to
enrolled students. We often have to go through typical examination questions to explain what is required in order to obtain
optimum marks. Below, we deal with some topics which have proven to be problematic in the past
Mendel's First Law - the law of segregation; during gamete formation each member of the allelic pair separates from the other member of that pair to form part of the genetic content of different gametes.
Mendel's Second Law - the law of independent assortment; during gamete formation the segregation of the alleles of one allelic pair is independent of the segregation of the alleles of another allelic pair – thus the assortment of alleles within the resultant gametes is random. and unpredictable.
ANSWER THE FOLLOWING?
(i) State the Law of Independent Assortment.
(ii) In cattle the allele for red coat (R) is dominant to the allele for black coat (r) and the allele for straight coat (S) is dominant to the allele for curly coat (s). When a bull with a red, straight coat was mated with cows with curly, black coats it was found that calves with four different phenotypes resulted. These four phenotypes occurred in equal numbers.
1. State the genotypes of the bull and of the cows.
2. State the phenotypes of the calves.
3. Which phenotypes of the calves suggest that independent assortment has taken place?
(iii) Would you expect different phenotypes if the genes for coat colour and coat type were located on the same chromosome? Explain your answer
(1) Mendel's Second Law - the law of independent assortment states: During gamete formation -- when genes are borne on different chromosooomes -- the segregation of the alleles of one gene is independent of the segregation of the alleles of another gene; thus the combination of alleles within the resultant gametes is random.
(ii :1) The cows were all homozygous for the recessive trait: only genes with homozygous recessive alleles can give rise to the recessive phenotype. Even so, the bull had to have been heterozygous for the dominant traits; otherwise he would have given dominant alleles in all cases and the calves would show the dominant phenotype. Thus the bull’s genotype was Rr/Ss, and the cows’ genotypes were rr/ss
(ii: 2) The calves’ phenotypes were: 25% red with straight hair. 25% black with straight hair. 25% red with curly hair. 25% black with curly hair. This ratio always occurs in di-hybrid crosses, when one parent is heterozygous for the dominant traits and the other parent is homozygous tor the recessive traits -- when the genes involved are borne on different chromosomes
(ii: 3) All the phenotypes, except those with red and straight coats demonstrate that random assortment has occurred in the production of their father’s gametes. In only 25% of his gametes has the bull’s dominant R/S gametes been passed on together – instead they have been mixtures: R/S R/s r/S r/s. This has been due to the random alignment of alleles during metaphase (1) in meiosis – the process which produces the “haploid” gamete.
(iii) If the genes for coat colour and coat type had been borne on the same chromosome, this bull’s gametes would have been (RS) and (rs)., while the cows’ gametes would have been (rs) (rs) The calves’ phenotypes would have been 50% red with straight coats, and 50% black with curly coats. This is always the ratio when two traits are carried by genes on the same chromosome, and when one parent is heterozygous for the dominant traits, and the other is homozygous for the recessive traits.
(a) State a function of the cell membrane ?
(b) State one feature that would allow you to identify an eukaryotic cell ?
(c) Name a human cell that is haploid ?
(d) What term is used to describe a cellular reaction in which large molecules are broken down
to smaller ones? .
(e) What term is used to describe an individual’s genetic make up?
(f) Name a scientist responsible for the Theory of Natural Selection ?
SUMMARY OF STAGES OF THE NITROGEN CYCLE
(1)Lightening makes Nitrogen from the air and carries it to the soil where it becomes available to plants
(2)Bacteria fix nitrogen in air for assimilation to proteins in certain plants
Other plants obtain Nitrogen through their roots from the soil
(3)Herbivorous animals receive their nitrogen as proteins from digestion of plants. Carnivorous animals receive Nitrogen from eating herbivorous animals.
(4)Decomposition of dead plants and animals return Nitrogen to the soil, where it can again become available to plants. However, some denitrifying bacteria can return Nitrogen to the air.
The nitrogen cycle describes the path of the element nitrogen through nature. Nitrogen is essential for life. It is found in amino acids, proteins, and genetic material. – such as DNA. Nitrogen is the most abundant element in the atmosphere (~78%). However, gaseous nitrogen N2 must be changed into another form so that it can be used by living organisms: this is called “Nitrogen Fixing”
There are two main ways through which nitrogen is 'fixed': (1) Fixing by Lightning and (2) Biological Fixation
FIXING BY LIGHTNING The energy from lightning causes nitrogen in the air N2 to combine with Oxygen O2 to form Nitrogen oxides which are carried to the earth by precipitation. Here these are converted to Nitrates NO3 which can be assimilated into amino acids and proteins by plants.
BIOLOGICAL NITROGEN FIXATION
About 90% of nitrogen fixation is done by bacteria which possess the enzyme nitrogenase that can convert N2 to N03 Azotobacter is such a Nitrogen Fixer: it is free-living – living within the soil. It is heterotrophic : requiring carbohydrate compounds within its environment in order to obtain its energy.
Bacteria that are symbiotic depend on a mutually beneficial relationship with plants: the plants supply the bacteria with carbohydrates and energy, while the bacteria supply the Nitrogen to the plants. These plants include legumes – like clovers and peas; which have mutually symbiotic bacteria living within special root swellings, called “root-nodule.“ Rhizobium is such a mutually symbiotic bacterium – possessing nitrogenase and assimilating N2 into organic compounds within its host.
Ammonia NH3 can be taken up directly by plants; however, most of the ammonia produced by decay is converted into nitrates – which can be more easily absorbed by plant roots and thereafter assimilated into organic compounds.
• Bacteria of the genus Nitrosomonas oxidize Ammonia NH3 to nitrites (NO2−). This bacterium obtains its energy from this chemical reaction alone: thus it is both aerobic and a chemotrophic autotroph.
• Bacteria of the genus Nitrobacter oxidize the nitrites to nitrates NO3− These are also aerobic and are chemotrophic autotrophs.
• Nitrification occurs by the following reactions:
2 NH3 + 3 O2 - > 2 NO2 + 2 H+ + 2 H2O
2 NO2- + O2 -> 2 NO3-
When plants and animals die, bacteria convert nutrients – such as amino acids R-CH(NH2)COOH-- into ammonia NH3. This conversion process is called ammonification Clostridium is such a bacterium. It is an anaerobic heterotroph – obtaining its energy from the breakdown of organic matter.
Denitrification returns nitrogen to the atmosphere, and thus it is undesirable in agriculture.. In this process the heterotrophic bacteria Pseudomonas and Serratia convert soil nitrates NO3 to gaseous Nitrogen N2