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Genetics summary chapter 8 VU amsterdam
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This summary covers complete chaper 8 of genetics at the Vrije Universiteit Amsterdam.
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Chapter 8 summary- Genetic variation: genetic differences among members of the same species or among different
species.
- Allelic variation: variation of different genes.
- Cytogeneticist: scientist who studies chromosomes microscopically. They usually examine
chromosomes in actively dividing cells, since the chromosomes are then compacted and good
microscopically visible. In order to classify and identify chromosomes, they look at three features: the
location of the centromere, the size and the banding patterns.
Chromosomes are classified with regard to centromere location as follows:
- Metacentric: the centromere is near the middle
- Submetacentric: the centromere is slightly off the middle
- Acrocentric: the centromere is significantly off the middle but not at the end
- Telocentric: the centromere is at the end.
Because the centromere is never exactly at the centre of the chromosome, each chromosome has a short arm
and a long arm. The short arm is referred to as p and the long arm is referred to as q. In the case of the
telocentric centromere, the short arm may be nearly non-existent.
- Karyotype: photographic representation in which all of the chromosomes within a single cell have
been arranged in a standard fashion. The short arms are on top and the long arms are on the bottom.
The chromosomes are numbered according to their size, which the largest chromosomes having the
smaller numbers. For detailed identification, chromosomes are treated with stains to produce
characteristic banding patterns. -> G bands. In this procedure, chromosomes are treated with mild
heat or with proteolytic enzymes that partially digest chromosomal proteins. when exposed to the
stain called Giemsma, some chromosomal regions bind to stain molecules heavily and produce dark
bands. Other chromosomal regions that hardly bind to the stain molecules, produce light bands. A
large number of G bands can be observed in the prometaphase because the chromosomes are less
compacted compared to metaphase.
The banding pattern in chromosomes is useful due to:
- When stained, individual chromosomes can be distinguished from each other.
- Banding patterns are used to detect changes in chromosomal structure. Chromosomal
rearrangements or changes in total amount of genetic material are more easily detected in banded
chromosomes.
- Chromosome banding is used to assess evolutionary relationships between species.
The banding pattern of each chromatid changes when it condenses. The left chromatid of each pair of sister
chromatids shows the banding pattern of the metaphase and the right chromatid shows the banding pattern of
the prometaphase. In prometaphase, the chromatids are less condensed than in the metaphase.
The amount of genetic material can be rearranged without affecting the total amount of material. These
mutations are categorized as:
1. Deletions: a segment of chromosomal material is missing (nucleotide). The affected chromosome is
deficient in a significant amount of genetic material. -> deficiency.
2. Duplications: chromosome section is repeated more than once within a chromosome.
3. Inversions: change in the direction of the genetic material. An extra nucleotide for example.
4. Translocations: chromosome segment becomes attached to a different chromosome or to a different
part of the chromosome.
Deletions and duplications are changes in the total amount of genetic material within a single chromosome.
Inversions and translocations are chromosomal rearrangements. In a reciprocal translocation, two different
chromosomes exchange pieces, thereby altering both of them.
Deletions
A chromosomal deletion occurs when a chromosome breaks in one or more places and a fragment of a
chromosome is lost. The chromosome that lost a piece of segment is called a terminal deletion (one single
break). When a chromosome is broken into two places to produce three chromosome fragments, we call it a
, Chapter 8 summary
interstitial deletion. The central piece is lost and the other two pieces are then attached to each other. The
phenotypic consequences of chromosomal deletions depend on the size of the deletion and whether it includes
genes or portions of genes that are vital for the development of the organism. When deletions have a
phenotypic effect, it is usually detrimental (schadelijk). Larger deletions tend to be more harmful because more
genes are missing. An example is the cri-du-chat syndrome. Here a segment of the short arm is missing in
chromosome 5. Two other examples are the Angelman syndrome and Prader-Willi syndrome.
Duplications
Duplications result in extra genetic material. They may be caused by abnormal crossover events. During
meiosis, crossing-over usually occurs after the homologous chromosomes have properly aligned with each
other. On rare occasions, however, a crossover may occur at misaligned sites on homologs. In some cases, a
chromosome may carry two or more homologous segments of DNA that have identical or similar sequences.
There are called repetitive sequences, because they occur multiple times. An example of repetitive sequences
are transposable elements. The repetitive sequences have promoted the misaligning of the homologous
chromosomes. Therefore, it is called nonallelic homologous recombination because it has occurred at the
homologous sites, but the sites are the not the alleles of the same gene. The result is that one chromatid has
deletion and another chromatid has a duplication. The resulting chromosome with the extra genetic material
carries a gene duplication, because the number of copies of gene C has been increased by two. Gene
duplications do not happen that often.
The phenotypic consequences of duplications tend to be correlated with size. Duplications are more likely to
have phenotypic effects if they involve a large piece of chromosome. Small duplications often do not have
harmful effects. An example of a duplication is Charchot-Marie-Tooth syndrome.
However, majority of the gene duplication do not have phenotypic effects. Duplications do are very important,
because they provide raw material for the addition of more gene’s into a species’ chromosomes. Over the
course of time, this can lead to gene families, consisting of one or two genes in a particular species that are
similar to each other. When two or more genes are derived from a single ancestral gene, the genes are called
homologous. Homologous genes within a single species are called paralogs and constitute a gene family. The
globin gene family is composed of 14 paralogs that are derived from one single ancestral globin gene. The first
gene duplication produced two genes, one that encodes for myoglobin and a primordial hemoglobin gene that
duplicated several times to produce several α-chain and β-chain genes, which are found in chromosomes 16
and 11.
Although all globin polypeptides are subunits of proteins that play a role in oxygen binding, the accumulation of
different mutations in the gene family of the globin has produced globins that are more specialized in their
function.
- Copy number variation (CNV) : when the number of copies of a particular gene varies from one
individual to the next. For example, one individual can have gene A twice, whereas another individual
can have the same gene three times. A CNV mat be due to duplications. The homolog with the two
copies of a gene is said to have undergone segmental duplication. (the one with three times gene A
for example). One common cause might be due to non-allelic homologous recombination. This event
can produce a chromosome with a duplication or deletion, thereby altering the copy number of genes.
Researchers believe that the proliferation (explosive) of transposable elements might increase the
copy number of genes. A third mechanism might be the involvement of DNA replication errors.
In many cases, CNV does not have phenotypic consequences. However, recent studies suggest that CNV might
be associated with diseases. Anne Kallioniemi. Daniel Parrkins and other have come up with a method in order
to develop a more sensitive methods for identifying changes in chromosome structures called comparative
genomic hybridization. This method is used to determine if cancer cells have changes in chromosome
structure, such as deletions or duplications.
The DNA is isolated from the breast cancer cells and from normal breast cells. The DNA from the breast cancer
cells was used a template to make green fluorescent DNA, and the DNA from normal cells was used to make
red fluorescent DNA. These red and green DNA were denatured and mixed with each other, applying them
eventually to the metaphase chromosomes that do not have deletions or duplications. The fluorescent labelled
DNA strands can bind to complementary regions on the metaphase chromosomes (hybridization), visualizing
them under a fluorescent microscope.
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