Country: United States
Date: January 2005
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?
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.
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.
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.
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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Update: June 2012