Ask A Scientist Molecular Biology Archive

name         Deborah
status       educator
grade     9-12
location     WA

Question -   Please  clarify the definition of a chromosome. When a
single strand of DNA has  replicated itself during interphase, is the
chromosome the single  chromatid or is it the chromatid strand plus its
sister chromatid (the x  shape) joined by the centromere?
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It's both.  Sorry.  Sometimes chromosomes are double stranded  (from the S 
phase through metaphase of mitosis) and then single stranded from  anaphase 
through G1.  I tell my students that when a chromosome is double  stranded, it is 
carrying a xerox copy of itself so there is no increase in  genetic 
information.  (There IS an increase in the amount of DNA, but there  is no NEW 
information).  This is a concept that takes a long time for kids  to accept and digest, 
but once they do, everything falls into place.
 
vanhoeck
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Two sister chromatids, attached at the centromere, are considered to be one
chromosome. During anaphase of mitosis, and anaphase of meiosis II, the
centromere splits and the sister chomatids pull apart. They are then
considered to be separate chromosomes, or daughter chromosomes.

Sarina Kopinsky, MSc, CGC, H.Dip.Ed.
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Before replication, you have a single-chromatid chromosome; after
replication, it's called a double-chromatid chromosome.  They are both
referred to as one chromosome.  This is a slightly unfortunate bit of
terminology that often confuses students.

Christopher Perkins
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I would say that during late interphase, prophase and metaphase, each 
chromosome is made up of two identical sister chromatids.

Ron Baker, Ph.D.
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