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What Are Chromosomes?

Mitosis is a very orderly regular process. It produces two identical cells in a very predictable way. But it is monotonous. If this were the only way in which cells could divide, then all organisms in a species would be identical copies of their parents (except for mutations).

Sex is a great solution but it isn't as easy as it seems. How is it possible that two individuals can contribute genetic material to make a new one? You can not pool the genes. Your genes are on your 46 chromosomes. Combine them with someone else's and the result is a double set of genes and 92 chromosomes.

Even one extra chromosome results in fetal death or severe birth defects. Therefore, there has to be some reduction in chromosome number before sex.

This is what you start with:

Each black band represents one gene.

We start this reduction proess with a double set of chromosomes (one from dad and one set from mom) and reduce it down to one set.

The double set consists of 23 pairs of homologous chromosomes, or homologues,, one of each pair from one parent, the other from the other parent.

The genes are arranged along both homologues in the same order but carry different types of information---blue vs brown eyes and so on.

In the context of chromosomes, "gene" is not used but instead "locus" and "allele" are the correct terms.

So what makes up the gene? Each allele is a packet of information made of sugars, phosphates, nitrogen bases and hydrogen bonds!

Practice:

Use your cursor to pull the alleles on the right onto the "empty" chromosomes. There are four pairs of alleles. When the allele is in the correct position, it will snap into place.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
These pages authored and maintained by Marcella Dawson. Revised 3/2002 . Copyright 1995 CRPC GirlTECH. All rights reserved. . Email your comments. These pages were developed through GirlTECH '96, a teacher training and student technology council program sponsored by the Center for Research on Parallel Computation (CRPC), a National Science Foundation Science and Technology Center.