Solution

HALF YEARLY EXAMINATION

Class 12 - Biology

ANSWER KEY SET C

Section A
1.a
2.a
3.b
4.c
5.d
6.d
7.d
8.a
9. c
10.d
11.c
12.c
13.c
14.a
15.c
16.a
Section B
17.The signals for child birth originate from the fully matured foetus and placenta which induce mild uterine
contractions called foetal ejection reflex.
18.Following are some of the advantages of natural methods of contraception over artificial methods:
i. They are cost effective.
ii. They do not have side effects.
iii. They do not disturb the normal sexual life of the couple.
19i. 5’ – T T A C G T C G A T A A C C – 3’
ii. 5’ – U U A C G U C G A U A A C C – 3’
20.Wing of a butterfly and the wing of a bat are the anatomically different structures, though they look alike and
have evolved for the same function and hence having a similarity. These structures are called analogous structures
and are a result of convergent evolution (presence of similar adaptive features in different groups of organisms but
towards the same function).
21.The strategy is based on the process of RNA interference.
It involves blocking of a specific mRNA due to complementary ds RNA molecule that binds to and prevent
translation of the mRNA. It is called silencing of mRNA.
Section C
22.After double fertilisation the zygote and primary endosperm nucleus are the two structures
produced. Primary endosperm cell produces the nourishing tissue, the endosperm. Only after the formation of
certain amount of endosperm, the zygote becomes active and forms embryo so
that developing embryo can get food. So the zygote remains dormant for some time and develops into
an embryo, after a gap.
 23.Mendel's Laws of inheritance: The offspring always exhibits a dominant trait from the two alleles received
from parents, the only dominant allele is expressed and other will be suppressed.
In a cross between tall (TT) plant and dwarf (tt) plant, the tall trait appears in F 1, tallness and dwarfness
appear in F2, In the ratio of 3 : 1.
24.i. During the pre-industrialisation all the white-winged moths survived due to white coloured lichens on trees,
but during post-industrialisation, white-winged moths were unable to survive as predators could spot the moth
against contrasting background, and the dark-winged
moths survived, this showed that in a mixed population of white and dark-winged moths those who can
adapt better will survive.
ii. The excess use of herbicides and pesticides has resulted in the selection of resistant varieties that
developed due to the chance or random mutation.
25.CT: Computed Tomography
MRI: Magnetic Resonance Imaging
CT MRI

It uses a magnetic field and radiofrequency

It uses X - rays. pulses.

It is an invasive technique. Non-invasive technique.

They are less useful in making images of soft They are more useful in making images of soft tissues
tissues and bones can be scanned. and bones cannot be scanned.

They are quieter and more comfortable. They are less comfortable and noisy.

26.Drawbacks of use of enzymes in Biotechnology:

iv. Extraction and purification of enzymes are not only laborious but also highly expensive.
v. Enzymes are not stable and are quite fragile.
vi. Enzymes are denatured at a slightly higher temperature than room temperature.
vii. Changes in pH deactivate the enzymes.
viii. Enzymes are influenced by a number of organic solvents, exposure to air or protein poisons.
However, there is no alternative to enzymes.
27.Baculoviruses belong to the genus-Nucleopolyhedrovirus. Their role
in IPM is as follows:
ix. Baculoviruses are pathogens that attack insects and other arthropods.
x. The majority of baculoviruses act as biocontrol agents.
xi. These viruses are excellent for species-specific, narrow spectrum insecticidal applications.
xii. They do not show negative impacts on plants, mammals, bird, fish or even non-target insects.
Therefore, they play an important role as biocontrol agents.
28.i. Palindromic nucleotide sequences in the DNA are the groups of letters that form the same words
when reading from both forward and backward. For example, the following sequence reads the same on the
two strands in 5'→3' direction as well as 3'→ 5' direction.
5'— GAATTC — 3'
3'— CTTAAG — 5'
Significance- These sequences act as recognition sites that are recognized by specific restriction
endonucleases.
ii. Use of restriction endonuclease- During recombinant DNA formation, these enzymes recognize and
make a cut at specific positions within the DNA and vector due to their this function, restriction
endonucleases are also called as molecular scissors.
 Section D
29.i. (a): Barr body is a structure consisting of a condensed X chromosome that is found in
nondividing nuclei of female mammals. The presence of Barr body is used to confirm the sex of athletes in
sex determination tests. It is named after the Canadian anatomist M.L. Barr, who identified it. The number of
Barr bodies is one less than total number of X chromosomes. In Turner’s syndrome genotype is 45 + X0, so,
the number of Barr body is 0.
ii. (a): Failure of segregation of chromatids during cell division result in the gain or loss of a chromosomes
called aneuploidy. For example, Turner’s syndrome results due to loss of X chromosome in human
females.
iii. (d): Turner’s syndrome is a chromosomal disorder that occurs due to absence of one
chromosome.
iv.(c): In Turner’s syndrome individual lacks one X chromosome. This situation is known as monosomy.
v. (d): Turner’s syndrome occurs due to absence of X chromosome. Individuals having a single X chromosome
22A + X0 (45) have female sexual differentiation but ovaries are rudimentary. Other associated phenotypes
of this condition are short stature, webbed-neck, broad chest, lack of secondary sexual characteristics and
sterility. Thus, any imbalance in the copies of the sex chromosomes may disrupt the genetic information
necessary for normal sexual
development.
vi.(d)
30. i. (b)
ii. (d)
iii. (c): Microorganisms have a high rate of multiplication, which means a large quantity of biomass can
be produced in a comparatively shorter duration.
iv.(d)
v. (a)
31. i. Oogenesis is markedly different from spermatogenesis in the following aspects-
Spermatogenesis Oogensis

It occurs in males, starting from puberty till the It starts before birth during embryonic
complete life cycle.
development and occurs till menopause.
A single spermatogonium after second meiotic division a single oogonium after second meiotic
forms four haploid spermatids, that
division produces one ovum and two non-
mature to form spermatozoa. functional polar bodies.

The process of spermatogenesis, i.e. second The second meiotic division of oogenesis
completes in Fallopian tube when sperm enters
meiotic division completes in testes and releases
the secondary oocyte.
mature sperms.
 ii. A diagrammatic sectional view of ovary showing different stages-

32.

33. Personal and public hygiene plays an important role in the prevention and control of many infectious
diseases. Many fungal infections can spread because of poor personal hygiene,
especially during the adolescent period. A person who maintains good hygiene is least likely to suffer from
common skin infections; like ringworm and eczema. Lack of public hygiene is often the biggest cause for the
spread of various communicable diseases; like cholera, malaria,
typhoid, etc. If we do not keep our surroundings clean then we stand at a higher risk of getting infected with
malaria, dengue or cholera. However, a clean neighborhood helps in preventing various communicable
diseases to a great extent.