Answer:
E2F transcription factors
Explanation:
The E2F transcription factors (TFs) encode intracellular factors associated with the control of the cell cycle. E2F are TFs that bind to promoter consensus sequences in order to activate transcription. These TFs control the expression of target genes involved in cell proliferation including, among others, genes for DNA replication and mitotic activation. Moreover, the E2F proteins also may act as a link between cell fate and the cell cycle. The retinoblastoma (Rb) is a protein that modulates the activity of the E2F family of TFs, and it has been shown that the Rb/E2F pathway is a key molecular mechanism associated with cell proliferation.
Answer:
B) There is an increase in the number of giraffes with long necks in areas of Africa where low-growing trees have died
Explanation:
The directional selection is one of the ways of the natural selection in which the natural selection selects or favours the most extreme trait of the species and the extreme trait show higher fitness than the normal or average trait.
In the given question, the case of Giraffe necks shows directional selection as the normal length of the neck is not favoured by the environment but the extreme trait that is long neck is favoured by the environment and the selection shifted to that level.
Thus, Option-B is the correct answer.
Only once because the DNA replication initiation is tightly linked to cell cycle progression,ensuring that replication occur only once per cycle
