Reproductive Phase of Life in Human
At puberty, the released egg (in women), and the thickened lining of the uterus along with its blood vessels get shed off in the form of bleeding known as menstruation. The first menstrual flow begins at puberty and is known as menarche. Menstruation occurs once in about 28 to 30 days. By the age of 45 to 50 years, the menstrual cycle stops, which is known as menopause. The thread-like structures in the fertilized egg are known as chromosomes. All human beings have 23 pairs or 46 chromosomes in the nuclei of their cells. In boys, out of 23 pairs of chromosomes, two chromosomes named X and Y are the sex chromosomes. In girls, out of 23 pairs of chromosomes, two chromosomes named X and X are the sex chromosomes. When a sperm carrying an X chromosome fertilizes with the egg, the zygote would have two X chromosomes that develop into a female child. When a sperm carrying the Y chromosome fertilizes with the egg, the zygote would have two chromosomes i.e. X and Y and such a zygote develops into a male child.
NCERT Notes Science for Class 8 Chapter – 7: Reaching The Age of Adolescence
NCERT Notes for Class 8 Science Chapter – 7: Reaching The Age of Adolescence: Growing up is a natural process. Growth begins from the day one is born. But upon crossing the age of 10 or 11, there is a sudden spurt in growth which becomes noticeable. The changes taking place in the body are part of growing up. They indicate that the individual is no longer a child but is on the way to becoming an adult. The period of life, when the body undergoes changes, leading to reproductive maturity, is called Adolescence.
NCERT Notes on Reaching The Age of Adolescence of Class 8 Science Chapter – 7: Adolescence begins around the age of 11 and lasts up to 18 or 19 years of age. Since this period covers the ‘teens’- 13 to 18 or 19 years of age, adolescents are also called ‘teenagers’. In girls, adolescence may begin a year or two earlier than in boys. Also, the period of adolescence varies from person to person. The human body undergoes several changes during adolescence. These changes mark the onset of Puberty. Puberty ends when an adolescent reaches reproductive maturity.