Here Allah mentions the story of Maryam and her son `Isa, just after mentioning Zakariyya and his son Yahya, may peace be upon them all. He mentions the story of Zakariyya first, followed by the story of Maryam because the one is connected to the other. The former is the story of a child being born to an old man of advanced years, from an old woman who had been barren and had never given birth when she was younger. Then Allah mentions the story of Maryam which is even more wondrous, for in this case a child was born from a female without (the involvement of) a male. These stories also appear in Surah Al `Imran and in Surah Maryam. Here Allah mentions the story of Zakariyya and follows it with the story of Maryam, where He says:
(And she who guarded her chastity,) means, Maryam (peace be upon her). This is like the Ayah in Surah At-Tahrim:
(And Maryam, the daughter of `Imran who guarded her chastity. And We breathed into it her garment through Our Ruh) 66:12.
(and We made her and her son a sign for the nations. ) means, evidence that Allah is able to do all things and that He creates whatever He wills; verily, His command, when He intends a thing, is only that He says to it, "Be" -- and it is! This is like the Ayah:
(And (We wish) to appoint him as a sign to mankind) 19:21