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Summary Genetics Chapter 8
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Chapter 8: variation inchromosome structure and number
Genetic variation = genetic differences among members of the same species or among different species.
Allelic variation = variation in specific genes.
Larger alterations may affect the expression of many genes and thereby influence phenotypes. Variation in
chromosome structure and number are of great importance because they are critical in the evolution of new
species.
The structure of eukaryotic chromosome can be modified:
- Altering total amount of genetic material
- Rearranging the order of genes along the chromosome
Microscopic examination of Eukaryotic chromosomes
To identify change, researchers need to start with a reference point: the chromosomal composition of most
members of a given species.
Cytogeneticists = scientists who study chromosomes microscopically -> examine the chromosomes from
many members of a given species to determine the common chromosomal composition and to identify rare
individuals that show variation in chromosome structure and/or number. Or they compare chromosomal
compositions of two or more species to see how similar and different they are.
The three most commonly features to identify and classify chromosomes are: Location of the centromere,
Size, Banding patterns that are revealed when the chromosomes are treated with stains
Centromere location
- Metacentric = centromere is near the
middle
- Submetacentric = the centromere is
slightly off center
- Acrocentric = centromere is significantly
off center but not at the end
- Telocentric = centromere is at one end
The centromere is never exactly in the center of a
chromosome, so each chromosome has a short and a long arm. In humans the short arm -> p and the long
arm -> q. in telocentric case the short arm is nearly nonexisting.
Karyotype = photographic representation in which all of the chromosomes within a single cell have been
arranged in a standard fashion. Chromosomes are numbered according to size, with the largest
chromosomes having the smallest numbers except the sex chromosomes. Short arms on top, long arms on
the bottom.
For detailed identification, chromosomes are treated with stains to produces characteristic banding patterns.
There are different staining technics for example the procedure that produces G bands. In this procedure,
chromosomes are treated with mild heat or with proteolytic enzymes that partially digest chromosome
proteins. When exposed to the stain called Giemsa, some chromosomal regions bind the stain molecules
, heavily and produce dark bands. The dark bands are thought to represent regions that are more tightly
compacted. The alternation pattern of G bands is a unique feature for each chromosome.
A larger number of G bands can be observed in prometaphase (chromosomes are less compacted) compared
to metaphase. The left chromatic in each pair of sister chromatids shows the expected banding pattern
during metaphase, and the right the banding pattern during prometaphase.
Why is banding pattern of eukaryotic chromosomes useful:
- Individual chromosomes when stained can be distinguished from each other, even if they have
similar sizes and centromeric locations.
- Banding patterns are used to detect changes in chromosomes structure. Chromosomal
rearrangements or changes in total amount of genetic material are more easily detected in banded
chromosomes.
- Is used to assess evolutionary relationships between species. Similarity of chromosomes banding
patterns is a good measure of genetic relatedness.
Changes in chromosome structure: an overview
The structure of normal chromosomes can be modified by
mutation. Some times the total amount of genetic
material within a single chromosome can be decreased or
increased. Or the genetic material in one or more
chromosomes may be rearranged without affecting the
total amount of material.
Deletions an duplications are changes in the total
amount of genetic material within a single chromosome.
When deletion occurs, a segment of chromosomal
material is missing -> the affected chromosome is
deficient in a significant amount of genetic material.
Deficiency is also used to describe the condition of a
chromosome that is missing a region. Duplication occurs
when a section of a chromosome is repeated more than
once within a chromosome.
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