Source: http://www.webmd.com/sleep-disorders/tc/nighttime-leg-cramps-topic-overview#1

This reaction to the hormone THP tends to go away as teens approach adulthood, possibly contributing to fewer episodes of moodiness during stressful times.
Source: https://www.verywell.com/why-is-my-teen-so-moody-3200842

Hormonal changes are at work, too. The adolescent brain pours out adrenal stress hormones, sex hormones, and growth hormone, which in turn influence brain development. The production of testosterone increases 10 times in adolescent boys. Sex hormones act in the limbic system and in the raphe nucleus, source of the neurotransmitter serotonin, which is important for the regulation of mood.
As long as the brain is still in formation, things can go wrong in many ways, and some of them involve the onset of psychiatric disorders. Stress can retard the growth of the hippocampus, which consolidates memories.
Source: http://www.health.harvard.edu/mind-and-mood/the-adolescent-brain-beyond-raging-hormones

At Zygote two celled stage, where each cell has 46 Chromosomes, these two cells separate and undergo division independent of each other as if they are two separate Zygotes giving rise to Identical Twins and hence the Chromosome Number is maintained.
Source: Verywell.com
To look at a diagram of what normal fertilization looks like, click here

Drugs affect three primary areas of the brain:

Prescription drugs can cause dangerous short- and long-term health problems when they are not used as directed or when they are taken by someone other than the person they were prescribed for.

Abusing opioids like oxycodone and codeine can cause you to feel sleepy, sick to your stomach, and constipated. At higher doses, opioids can make it hard to breathe properly and can cause overdose and death.

Abusing stimulants like Adderall or Ritalin can make you feel paranoid (feeling like someone is going to harm you even though they aren’t). It also can cause your body temperature to get dangerously high and make your heart beat too fast. This is especially likely if stimulants are taken in large doses or in ways other than swallowing a pill.

Abusing depressants like barbiturates can cause slurred speech, shallow breathing, sleepiness, disorientation, and lack of coordination. People who abuse depressants regularly and then stop suddenly may experience seizures. At higher doses depressants can also cause overdose and death, especially when combined with alcohol.

Abusing depressants like barbiturates can cause slurred speech, shallow breathing, sleepiness, disorientation, and lack of coordination. People who abuse depressants regularly and then stop suddenly may experience seizures.

In addition, abusing over-the-counter drugs that contain DXM can also produce very dangerous effects.

Abuse of any of these types of medications can lead to addiction. And, abusing any type of drug that causes changes in your mood, perceptions, and behavior can affect judgment and willingness to take risks.

Prescription drugs can increase risk for health problems when combined with other prescription medications, over-the-counter medicines, illicit drugs, or alcohol. For example, combining opioids (painkillers) with alcohol can make breathing problems worse and can lead to death.

Source: https://teens.drugabuse.gov/drug-facts/brain-and-addiction