BIOLOGY 4 Marks Questions

i. All angiosperms are characterised by a unique process called double fertilisation.
Double fertilisation is the occurrence of two types of fusion syngamy and triple fusion in an embryo sac of the angiosperm.
Syngamy- Fusion of one of the male gamete and the egg cell resulting in formation of a zygote (diploid).
Triple fusion- Fusion of another male gamete with two haploid polar nuclei to produce a (triploid) primary endosperm nucleus.
ii. a. The fruit of orange is an example of polyembryony. The egg is fertilized and leads to the formation of multiple embryos. This phenomenon is known as polyembryony. There are many embryos produced from the single fertilized egg of ovule.
b. In case of Cashew thalamus also contribute in fruit formation along with ovary/development of fruit after fertilisation from the part other than ovary, Guava fruit develops only from the ovary after fertilisation therefore it is true fruit.

Role of Heat - In PCR (in vitro), the DNA strands are separated by heating them at 95°C for 2 minutes. Heating causes the breakdown of H-bonds between the bases of two strands leading to their unwinding.
Role of Primers - Primers are short lengths of DNA about 20bp long that are required to start DNA polymerisation in PCR. The primers hybridise to their complementary sequence on the DNA strands at 40-50°C temperature and help in DNA polymerisation.
Role of Thermus aquaticus - An enzyme called Taq polymerase is isolated from Thermus aquaticus. Since this bacterium thrives in temperature as high as 95°C, this enzyme can also tolerate high temperature without undergoing denaturation. Therefore, this enzyme is used in PCR instead of normal DNA polymerase.

The Hershey And Chase Experiment:
i. Hershey and Chase cultivated viruses inside a solution medium containing 32P radioactive phosphorus.
ii. Then enabled them to infect E. coli while the mixture was stirred in a mixer.
iii. Viral coats and bacterial cells containing viral particles were isolated using centrifugation.
iv. The steps were repeated using radioactive sulfur 355, where no radioactivity was found inside the bacterium.
v. Because S was never incorporated in Genome.
vi. However, radioactivity was discovered in the supernatant with protein coats of viruses.
Conclusion:
i. They concluded that genetic information was DNA rather than proteins.
ii. Since the protein coat stays outside of the host genome and only the DNA penetrates.
iii. They discovered that hardly any protein, but only DNA, was accountable for their proliferation.
iv. The DNA was responsible for all of the radioactive substances that managed to enter the cell.

i. Amino acid activated in presence of ATP and attached to their cognate tRNA
When two charged tRNA are in close contact they form peptide bond/Peptide bond formation between two charged tRNA is favoured energetically therefore the charging of tRNA is essential in the process of translation.
ii. Function To enhance the rate of peptide bonds formation/help in formation of peptide bond formation.
iii. Small subunit encounter an mRNA, the process of translation begins, there are two sites in larger subunit for subsequent amino acid to bind to, and thus be close enough to each other for the formation of peptide bond.

Post pollination event up.to double fertilization, that occur in an angiosperm is as follows:
The Pollen grain germinates on the stigma to produce pollen tube through one of the germ pore, the contents of the pollen grain/vegetative cell/generative cell/two male gametes move into the pollen tube, the pollen tube grows through the tissues of stigma and pollen tube to reach the ovary, the pollen tube enters (through micropyle) enters the synergids through filiform apparatus, pollen tube releases two male gametes in the cytoplasm of the synergids, one of the male gametes fuses with egg cell/female gamete completing syngamy, to form (diploid) zygote, the other male gamete fuses with two polar nuclei in the
(central cell) to produce (triploid) primary endospermic cell, three haploid cells fuse called triple fusion, two type of fusion syngamy and triple fusion is called double fertilisation.

i. Palindromic sequence for
5'-GAATTC-3'
3'-CTTAAG-5'
ii. Restriction endonuclease Eco RI recognises the above palindromic sequence.
iii. Sticky ends on DNA are formed by the action of enzymes restriction endonucleases. These enzymes cut the strand of DNA a little away from the centre of the palindrome sequence between the same two bases on both the strands. This results in single- stranded stretches on both the complementary strands at their ends. These overhanging stretches are called sticky ends as they form hydrogen bonds with the complementary basepair sequences.
Role of the sticky ends -These sticky ends produced from hydrogen bonds with their complementary cut counterparts. This stickiness of the ends facilitates the action of the enzyme DNA ligase.