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Name: Deborah
Status: Educator
Grade:  9-12
Location: WA
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.

Christopher Perkins

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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