B. some species die out when environmental changes occur
Answer: option "A" One seed develops from each ovule.
Explanation:
Ovules are the female reproductive part of the plant which is internally covered by the nucellus and integuments further helped in fertilization and development of seed.
After fertilization, ovule is the part of the plant develops into seed, after a period of time food tissue is developed inside it consisting the seed coats called integuments.
pollen grains or sperms are being transferred through the pollen tube to the embryo sacs from integuments only, which results into the fertilization and development of seed.
Answer:
B) Tissue is made of different types of cells.
D) Organs are made of different types of tissue.
Explanation:
The tissues are made of different types of cells. The organs are also made of different types of tissue. There is no need of same types of tissues to make organs. Thus, option (B) and (D) is correct answer.
Answer:
Watson and Crick proposed the double helical model of DNA.
Explanation:
- This model says that DNA exist as double helix. It has two polynucelotide strands running in opposite direction i.e the strands are anti-parallel.
- Each polynucleotide strand is made up of several nucleotide liknked together by phophodiester bonds.
- The two strands are not coiled upon each other but the double strand is coiled upon itself around a common axis like the spiral stair case. Here base pairs forms the stairs and backbone of two strands form railing. Back bone contains sugar and phosphate.
- The base pairing is specific, adenine always pairs with thymine and guanine always pairs with cytosine. Thus if we see any base pair, it consists of one purine and one pyrimidine.
- The two strands of DNA thus show complementary base pairing.
- The strands are held together because of hydrogen bonding between the bases in opposite strand.
- One full helical turn involves ten base pairs and the base pairs are stacked 3.4 A apart.
- This form is called as B-form of DNA.