Cell Cycle and Chromosomes

$G _2$ phase is the phase of cell cycle occurs immediately prior to mitosis.
sequence of cell cycle is -- $G _1$ to $S$ to $G _2$ to $M$ to cytokinesis.

In angiosperms, one meiotic division produces 4 microspores, but only one functional megaspore is formed from one megaspore mother cell. For 100 grains, 100 meiotic divisions are needed for the male side (pollen) and 100 for the female side (ovule), totaling 200. However, if the question implies the standard formula (n + n/4), it is 125.

Presence of three or more sets of chromosomes in an organism is called polyploidy. It is often seen in plants and very rare in animals. It is caused due to the non disjunction of the sister chromatids during the recombination (at meiosis). It can be induced in plants and cell cultures with the help of some chemicals like colchicine. Polyploidy causes variation, as it adds new genomes into a species. It has been widely used in producing hybrids of wheat. Ploidy level also effects some characteristics of plants. For example, higher ploidy leads to the larger cell size. So, the correct answer is B.

In ferns spores are haploid hence they have half number of chromosomes where as embryo is diploid.

A deletion mutation occurs when part of a DNA molecule is not copied during DNA replication. This uncopied part can be as small as single nucleotide or as much as an entire chromosome. The loss of this DNA during replication can lead to genetic disease.

Satellite DNA consists of arrays of tandemly repeating sequences that are widely present (>30%) in the genome. Satellite DNA is mainly present in heterochromatin or the tightly packed regions of chromosomes in centromeres, telomeres, and sometimes even in the euchromatin region (active region of the genome).

In women, both sex chromosomes are the same and are therefore called as XX. In males, they are different and called as XY. When sperm and eggs (ova) are made during meiosis they separate to form different gametes. As women have XX, their eggs always carry the X chromosomes. As males have XY, approximately half their sperm carry the X chromosome while the remainder carries the Y chromosome.

If the sperm and egg cells of a butterfly have six chromosomes, 12 chromosomes will be there in the maggot larvae. This is because maggot larvae forms due to the fusion of egg and sperm cells. Since egg contains 6 and sperm cells contains 6 chromosomes, maggot larvae will get both sperm cell and egg cell chromosomes. Thus, the correct answer is option C.

The endosperm in angiosperms is triploid (3n). If 3n = 24, then n = 8. Gametes are haploid (n), so they contain 8 chromosomes.

Endosperm is formed by the fusion of one male gamete (n) and two polar nuclei (n+n). The female plant is 2n=24, so n=12. The male plant is 2n=12, so n=6. Endosperm = (n from male) + (n from female) + (n from female) = 6 + 12 + 12 = 30.

Repeated or repetitive sequences make up large portion of human genome. Repetitive sequences are nucleotide sequences that are repeated many times, sometimes hundred to thousand times. They have no direct coding function but provide information as to chromosome structure, dynamics and evolution. Approximately 1 million copies of short 5-8 base pair repeated sequences are clustered around centromeres and near the ends of chromosomes. They represent junk DNA.