Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne Chromosome Clarification
Name: Deborah
Status: Educator
Grade:  9-12
Location: WA
Country: United States
Date: January 2005


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?



Replies:
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


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.



Click here to return to the Molecular Biology Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (help@newton.dep.anl.gov), or at Argonne's Educational Programs

NEWTON AND ASK A SCIENTIST
Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory