Biology 5 Marks Questions

i. When an E.coli bacterium is transformed with a recombinant DNA bearing ampicillin-resistant gene in its plasmid, the recombinant plasmid will lose tetracycline resistance due to the insertion of foreign DNA but can still be selected out from non-recombinant ones by plating the transformants on ampicillin containing a medium. The transformants growing on ampicillin containing medium are then transferred to a medium containing tetracycline. The recombinants will grow on ampicillin containing medium but not one that containing tetracycline. But non-recombinant will grow on the medium containing both the antibiotics.
ii. In this case, one antibiotic gene helps in selecting the transformants whereas the other antibiotic resistance gene gets inactivated due to the insertion of alien DNA and helps in the selection of recombinants. Ampicillin resistant gene in the above case helps in selecting the transformants and act as a selectable marker.

A single base mutation in a gene may not always result in loss or gain of function. We know that a codon is composed of three nucleotides. In simple terms, a codon can be taken as a word that is composed of 3 letters. For making any meaningful sentence we need a complete word. The addition or deletion of a single letter may not result in a meaningful word. Hence, in most of cases,there is a need for mutation in three bases to affect loss or gain of function. This can be illustrated by the following example of a sentence being altered:
RAM HAS RED CAP
RAM HAS BRE DCA P
RAM HAS BIR EDC AP
RAM HAS BIG RED CAP
It is clear that a meaningful sentence is made only when at least three letters are inserted in this sequence.

i. Since, DNA molecules are hydrophilic, they cannot pass through cell membranes. For recombinant DNA to be integrated into the vector or host genome, it is necessary for the DNA to be inserted in the cell. Therefore, making the host cells competent is necessary for biotechnology experiments.
The two ways by which cells can be made competent to take up DNA are:
a. Chemical action -The host cell is treated with a specific concentration of divalent cation, i.e. calcium which increases the pore size in the cell membrane. DNA is then incubated with the treated bacterial cell at 42°C, thereby increasing the efficiency of DNA entering through pores in the cell wall.
b. Heat shock treatment- Incubating the cells with recombinant DNA on ice, followed by a brief treatment of heat at 42°C
and again putting them back on ice. ii. Biolistic guns or gene guns are used to bombarded rDNA loaded on gold or tungsten particles with high velocity. In this way,
the rDNA is delivered to the desired host cells.

The Watson and Crick observed that the nitrogenous bases form complementary pair between the two polynucleotide chains of DNA. Based on the X-ray diffraction data, they proposed that DNA consisted of a double helix with two chains having sugar phosphate on the outside and nitrogen bases on the inner side. Further, they proposed that the two chains are antiparallel with 5'-3' orientation of the other. The two chains are twisted helically just as a rope ladder with rigid steps twisted into a spiral. This property of double helix model of DNA led them to hypothesize semi-conservative mode of DNA replication, where the two strands separate and act as a template for the synthesis of new complementary strand.

Microsporogenesis comprises the events which lead to the formation of the haploid unicellular microspores. During microsporogenesis, the diploid sporogenous cells differentiate as microsporocytes (pollen mother cells or meiocytes) which divide by meiosis to form four haploid microspores. Each diploid meiocyte gives rise to a tetrad of four haploid microspores and microsporogenesis is complete with the formation of distinct single-celled haploid microspores. Microgametogenesis comprises events which lead to the progressive development of the unicellular microspores into mature microgametophytes containing the gametes. This phase begins with the expansion of the microspore which is commonly associated with the formation of a single large vacuole. Vacuolation is accompanied by the displacement of the microspore nucleus to an eccentric position against the microspore wall. In this position, the nucleus undergoes first pollen mitosis (pollen mitosis I) which results in the formation of two unequal cells, a large vegetative cell and a small generative cell each containing a haploid nucleus. The generative cell subsequently detaches from the pollen grain wall and is engulfed by the vegetative cell forming a unique 'cell within a cell structure. The engulfed generative cell divides once more by mitosis (pollen mitosis II) to form the two sperm cells completely enclosed within the vegetative cell cytoplasm either before pollen is shed (tricellular pollen) or within the pollen tube (bicellular pollen).