A: Some of the physiological effects of cytokinins in plants are: They
promote cell division in root apices, shoot buds and young fruits.
They help to produce new leaves and chloroplasts in leaves. They
promote adventitious shoot formation. Cytokinins help to overcome
apical dominance and promote lateral shoot growth. They promote
nutrient mobilisation and help in delay of leaf senescence.
Q: How are bacteria classified on the basis of number and distribution
of flagella? (4 Marks)
A: Based on the number and distribution of flagella the bacteria may
be classified as: Monotrichous having a single polar flagellum.
Amphitrichous having a single flagellum at each end of the cell.
Lophotrichous having two or more flagella at one pole of the cell.
Peritrichous having flagella distributed over the entire cell.
Q: Explain the lytic cycle with reference to certain viruses. (4 Marks)
A: Viruses like bacteriophages multiply by any of the two methods
lytic cycle and lysogenic cycle. Lytic cycle: Lytic cycle caused lysis
or death of host cell. T-even phages which cause lysis are called
virulent phages. Lytic cycle involves 5 steps:
Adsorption: It is the attachment of phage particle to the bacterial
host cell on complementary receptor sites.
Penetration: The tail sheath of the phage contracts and injects the
DNA into the bacterial cell. The capsid of the phage remains outside
the host cell as a ghost.
Bio synthesis: As the phage DNA enters the host cells it uses the cell
machinery and synthesizes phage DNA and capsid proteins. But the
complete virus particles do not appear hence called eclipse period.
Maturation: During this phase bacteriophage DNA and capsids assemble
into complete virions.
Release: At this stage the viral enzyme lysozyme produced in the host
cell dissolves the plasma membrane and breaks the cell wall to release
the newly synthesized
phage particles or virions. This phase is hence called lysis phase.
The number of newly synthesized phage particles released from a single
host cell is called 'burst size'. (50 to 200).
Q: Differentiate between the following: a) Dominant and recessive b)
Homozygous and heterozygous (4 Marks)
A: a) Dominant: It is the character that is expressed phenotypically
in both homozygous and heterozygous condition.
Recessive: It is the character which is not expressed phenotypically
in heterozygous condition.
b) Homozygous: An individual with two similar or identical alleles for
a single character. It produces only one kind of gametes with
reference to that character (gene).
Heterozygous: An individual with two different alleles for a single
character. It produces two different types of gametes with reference
to a gene.
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