Which organism can change its sex?

In human females,an event that reflects the onset of the reproductive phase is:

What is the effect of $DNA$ copying,which is not perfectly accurate,on the reproduction process? How does the amount of $DNA$ remain constant though each new generation is a combination of $DNA$ copies of two individuals?

What is the number of chromosomes in a human somatic cell?

$(a)$ Give the functions of the following :
$(i)$ Pollen Tube
$(ii)$ Ovary
$(b)$ List any two changes observed in the body of a female during puberty.

What are chromosomes? Explain how in sexually reproducing organisms,the number of chromosomes in the progeny is maintained?