Biology Class-Xll (CBSE)

11/28/2011 CBSE

1. Why is human male referred to
as heterogametic?

Human male has an X and a Y chromosome in its karyotype. Thus to make
a male zygote, gametes of two different karyotypes are required. Therefore human
male is called heterogametic.

2. What are the events that take
place at the point of stimulation of an axon?

An unexcited axon has the Resting Membrane Potential on its membrane
and is in the polarised State. In this state the inner side of the membrane is
negative as compared to the outer membrane. A threshold stimulus causes the
resting membrane potential to become the Action Potential, i.e. the inner side
of the membrane becomes electropositive to its outside. This action potential
propagates along the membrane of the nerve fibre as the nerve impulse. As the
impulse has moved away, the membrane becomes Depolarised i.e., it becomes
negatively charged on the inner side.

3. Write the transcribed m-RNA
from the DNA strand with the base sequence of TAG TAC ACT. What is the specific
term used for the last codon of the transcribed m-RNA in this case?

The transcribed mRNA from the given sequence will be AUC AUG UGA.
Last codon UGA is one of the nonsense Stop-codons and would terminate the
translation of the polypeptide after it is encountered during the translation.

4. Why is blood group
identification not required while transfusing serum?

The serum is a cell-free fluid, which forms when the blood has
clotted and the thrombus has separated from it. Since the blood group deciding
antigens reside on the RBCs which are not present in the serum, hence it can be
transfused to anyone without the customary requirement of the blood group

5. The primitive earth had reducing atmosphere to enable abiotic origin of
life. What put an end to it?

The emergence of green bacteria and plants with the ability to
produce photosynthetic oxygen caused heavy build up of oxygen in the earth’s
atmosphere, which caused the reducing environment to be changed into an
oxidising environment.

6. How a sickle-cell carrier has
an advantage over the rest of the human population in a Malaria ridden area?

A defective form of haemoglobin is found in the RBCs of the
Sickle-cell carrier. They have sickle-shaped RBCs, which are highly inefficient
in binding oxygen. However, a sickle shaped RBC effectively kills the malaria
parasite too, which harbors in it. Thus individuals homozygous for Sickle-cell
trait have a better survivability in the event of malaria epidemics and thus

selection has not eliminated this trait in those geographical locations in the
world, where there is a prevalence of malaria.


7. What acts as an inducer in Lac
Operon? How does it switch on the operon?

Lactose acts as the inducer for Lac operon. When lactose is present,
the repressor protein structure is modified such that the repressor cannot bind
to the operator any more. This leads to the transcription of the operon and
induction of b-galactosidase and the other two enzymes.

8. A Chrysanthemum plant with
floral buds requires more than 12 hours of dark night time to flower. If the
night is interrupted by white light for duration of an hour, will the flowers
bloom the following morning? Name the pigment that perceives the light stimulus
for flowering. Where is the pigment present in the plant and what are its two

Chrysanthemum is a short day plant if it needs a night longer than 12
hours. It will not bloom if the night of continuous 12 hours is not available,
and is interrupted by any length of light. The pigment, which perceives the
light response for flowering, is Phytochrome. Phytochrome is present in the
leaves, and it exists in the red absorbing Pr form and the Far-Red
absorbing Pfr forms.

9. Define Cancer. How are cancers
broadly classified pathologically? Name one major cancer in women and one major
cancer in men in India.

Pathologically, cancers may be broadly classified into three major
catagories :

(i) Carcinomas, which are malignant growths of the epithelial (ectodermal)

(ii) Sarcomas, which are malignant growths of the mesodermal tissues such as
bone, muscle, lymph nodes and the likes.

(iii) Leukaemias, which are unchecked proliferation of blood cells, and there
precursors in the bone marrow.

In India the major form of cancer in women is the uterine-cervical cancer and it
is the cancer of the mouth and throat in men.

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