Tag: reproductive health of humans

Howard and Pelc divided cell cycle in four stage. The cell cycle is an ordered set of events, culminating in cell growth and division into two daughter cells. Non-dividing cells not considered to be in the cell cycle. The stages  are G$ _1$-S-G$ _2$-M. The G$ _1$ stage stands for "GAP 1". The S stage stands for "Synthesis". This is the stage when DNA replication occurs. The G$ _2$ stage stands for "GAP 2". The M stage stands for "mitosis", and is when and cytoplasmic division occur.

The movement of pollen tube towards embryosac though style is chemotactic as it secrets pectinases and other hydrolytic enzymes to create a passage for its entry into style .

12, 36, 24; because pollen grains are haploid while endosperm has triploid chromosome number and integument is diploid in nature.

In porogamy, filiform apparatus of synergids secretes, some chemotropically active sunstances, which direct the pollen tube towards micropyle of ovule.

This type of tapetum can be seen easily in Alisma, Butomus, Tradescantia, and Typha, etc. The tapetal cells fuse among themselves for forming a tapetal plasmodium. The protoplast of the fused tapetal cells then moves into the locule, where they surround the pollen mother cells or pollen grains. This protoplast movement into the locule is taking place during meiotic prophase. 

Thus the correct answer is option D.

The pollen wall actually is the multi-layered structure that includes an intine, which is composed mainly of polysaccharide, and an exine, which consists mainly of sporopollenin. The exine can be further divided into the sexine and nexine. Sexine is the single layer of bacula which is covered by a layer of the tectum.

So the correct answer is C.