Biology Sample Question Paper Sheet - Class Xii, Central Board Of Secondary Education, 2010 Page 12
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39A24 Population of birds will decrease. High concentration of DDT disturbs calcium metabolism in birds
which causes thinning of eggshell and their premature breaking, eventually causing decline in bird
populations.
[ 3 Marks]
A25 (i)
‘a’ – Vector DNA; ‘b’ – Foreign DNA = ½
(ii)
EcoRI
(iii)
DNA ligase
[½ + 1 + 1½ = 3 Marks]
A26 (a)
Gel electrophoresis = ½
(b)
DNA fragments / bands = ½
(c)
Ethidium bromide = 1
(d)
The separated bands of DNA are cut out from agarose gel and DNA extracted from gel
piece = 1
[½ + ½ + 1 + 1= 3 Marks]
A27 (a)
Thermus aquaticus = 1
It remains active during the high temperature induced denaturation = 1
(b)
Very low concentration of a bacteria or virus can be detected by amplification of their
nucleic acid by PCR = 1
[1 + 1 + 1 =3 Marks]
Section - D
A28 •
After implantation the chorionic villi and uterine tissue become interdigitated to form placenta.
•
Placenta facilitates supply of O
& nutrients to the embryo and removes CO
& excretory
2
2
materials produced by the embryo.
•
Increased production of hormones like estrogens, progesterone, prolactin are essential for
supporting foetal growth, metabolic changes in the mother & maintenance of pregnancy.
•
The inner cell mass differentiates into three distinct germ layers (mesoderm, ectoderm & en-
doderm ) which given rise to all tissues (organs) in adults.
•
After one month of pregnancy the embryo’s heart is formed.
•
By th end of the second month of pregnancy the foetus develops limbs & digits.
•
By the end of 12 weeks (first trimester) most of the major organ systems are formed.
•
By the end of 24 weeks (second trimester) the body is covered with fine hair, eye-lids sepa-
rate and eyelashes are formed.
•
The signals for parturition originate from the fully developed foetus and the placenta which
induce mild uterine contractions called foetal ejection reflex.
•
This triggers release of oxytocin from maternal pituitary along with stimulatory reflex resulting
in stronger contractions leads to parturition. = ½ × 10 = 5
[5 Marks]
OR
•
Incompatibility is considered as the most widespread & effective device to prevent inbreeding
and outbreeding.
•
Pollen pistil interaction is a dynamic process involving pollen recognition followed by promo-
tion or inhibition of the pollen.
12
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