Lectures 43 Changes in Chromosome Structure

(version 19 Sept 2001)

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Chromosome structure

Classification by centromere position

While most species use a single centromere (b) , a few have very diffuse centromeres (a).

Other visible features

Heterochromatin -- tightly packed regions

Euchromatin --- Regions where DNA is more openly packed, location of most functional genes

Chromosome banding (requires special stains)

Structural Changes

Types of Deletions

Deletions are very useful in locating genes

Deletions are common in cancerous cells

pseudodominance -- expression of a recessive allele when placed over a deletion

Pericentric vs. paracentric inversions

pEricentric inversions involve the cEntromere

pAracentric inversions involve chromosome Arms

Inversions supress recombination within the inverted region

Recombination within paracentric inversion gives a dicentric bridge

Resulting gametes show duplications and deletions (see Figure 17-16).

Recombination in a pericentric inversion (Fig 17-17)

Different species handle inversions at meiosis differently

Drosophila can tolerate paracentric inversions

No crossing over in males

only single meiotic product used in females

Translocations

Alternative segregation produces viable gametes (fig 17-23)

Adjacent segregation produces inviable gametes (Fig 17-23)

Translocation involving the X chromosome can significantly alter gene expression.

Chromosome Evolution

Human, Chimp, Gorrila and Organatang share large regions of synteny, where gene order is preserved between species.

More generally, even very distinct species share large regions of synteny

This shared synteny is very powerful for using genetic maps from one species to mape genes in another. For example, a gene mapped in rice provides information for the gene location in maize and wheat.