What is the relationship between chromaids and chromosomes?
Normally a cells chromosomes are single strands - a bit like a
pice of string. Just prior to cell division, a chromosome
replicates (=makes another copy of itself) and these two copies are
joined together at a point called the centromere. Each copy is now
called a chromatid and being joined at the centromere make the
familiar "X" shape, with each copy (chromatid) forming a side of
the X and will have exactly the same sequence of genes down its
length as the other chromatid (they are copies!). In cell division
the copies (chromatids) will be pulled apart (X becomes > <)
to become chromosomes in their own right.