Crossing over takes place at a stage between

Which of the following is a sulphur containing amino acid

$A$ : $SNPs$-pronounced "snips" are common in human genome.
$R$ : It is minute variations that occurs at a frequency of one in every $300$ bases

The human chromosomes are divided into $7$ groups, $B$-chromosomes are

A $DNA$ strand is directly involved in the synthesis of following except