Chromosomes – Cytogenetics

By: Caolan Lyons and Eamon O’Mahony

Karyotype

A karyotype is a number and visual profile of the chromosomes in the cell nucleus of a organism.

The chromosomes are arranged by size from tallest to shortest.

Autosomes

An autosome is any chromosome that is not a sex chromosome.

Humans have 22 pairs of autosomes. Humans may not have 22 pairs of autosomes

if a mutations occurs. If a mutation occurs, it is possible for a

human to have one of numerous diseases.

Sex Chromosomes

For a female the sex chromosomes are XX and for a male the sex chromosomes are XY.

Chromosome Anatomy

Chromosome anatomy is the structure of a chromosome.

In particular, knowing the structure for chromosome 5 is important.

Chromosome 5 is responsible for numerous types of growth and development (cell divisions) changes may cause cancers.

Chromosome 5

The p-arm, which is the short arm, is placed up in the karyotype of chromosome 5.

The q-arm is the long arm, is placed low on the karyotype of chromosome 5.

Chromosome 5

Chromosome 5

Chromosome 5 is one of the largest chromsomes, but it has one of the lowest gene densities.

There is huge interest in understanding chromosome 5 because it not only causes diseases, but it gives many clues into deletions and duplications when the sequence is complete.

Centromere and Telomeres

The centromere of a chromosome represents a constricted region. This is where the two sister chromatids are most in contact.

Telomeres are bunches or packs of DNA . They help the chromosome from deterioration.

There is a telomere and the top and the bottom of every chromosome.

Centromere and Telomeres

Chromatids

Before a cell gets prepared to divide in mitosis, each chromosome is duplicated.

It is common to call these chromosomes that have been duplicated as sister chromatids.

These duplicated chromosomes a fused with banding.

There are light and dark bands called G bands.

Chromosome Banding Techniques In the 1960’s a Swedish geneticist, Torbjörn Caspersson, developed

ways to stain chromosomes differently. To obtain chromosomes to band, a scientist must take a culture of cells

in the process of meiosis, swell the cell in a hypotonic solution and spreading the chromosomes, seize the cells in metaphase 1 by mixing them with colchicine, squeeze the chromosomes into a single line, and then prepare them for specific banding techniques.

Caspersson discovered that the stain quinacrine mustard showed light and dark bands along a chromosome. This banding technique allowed for the most accurate identification of the 22 autosomes and the X and Y chromosomes. This technique also could identify abnormalities, missing chromosomes, and extra chromosomes in a cell. This technique is called Q-banding.

A similar, but more widely used technique is G-banding, also known as Giemsa banding. Giemsa is a stain that attaches to adenine-thymine bonds. These stains create bands along the chromosome that can be identified under a microscope.

Another technique, C-banding stains the heterochromatin, tightly packaged DNA, lying near the chromosome’s centromere.

High Resolution Banding

In this technique, the chromosomes are examined under higher resolution. The chromosomes are stretched farther than in typical banding techniques so more miniscule defects can be seen and identified.

Chromosomal Breakpoints

Breakpoints are places on the chromosome where there is a breakage during

recombination, also known as crossing over. One breakpoint is also known as a

locus, while multiple points are known as loci. These are the locations, or

addresses, for genes.

During recombination the chromosomes break and reattach. There are many

pinpoint breakpoints across the p-arm, the arm above the centromere, and the

q-arm, the arm below the centromere. Example: 1p33, the 1 tells which

chromosome it is on, the p indicates it is located on the p-arm, and the 33

indicates it is located 33 megabases from the centromere.

The point 7q31.2 is the location for the gene mutation that causes cystic fibrosis. This mutation causes mucus build up outside cells and makes it hard to breathe.

The point 1q21 is the location for two syndromes, a microdeletion syndrome and a microduplication syndrome. Also known as 1q21 copy-number variations. In the deletion syndrome a part of the DNA on the point 1q21 is missing, or deleted. In the duplication syndrome a part of the DNA on point 1q21 is copied too many times. These syndromes can cause disabilities such as autism.

The point 1p34 is the location for monosomy 1p34 p32. This is a rare deletion syndrome which causes mental problems and fingernail abnormalities.

The point 1p36 is also a location for one of the most common deletion syndromes, monosomy 1p36. This monosomy takes place on the outermost band of the p-arm on chromosome 1. A result of the syndrome is various developmental disabilities.

Translocations

During recombination, sometimes the pieces of chromosomes that were broken off reattach in the incorrect place, also known as a translocation. Example: Philadelphia Translocation-when a piece of chromosome 22 is attached to chromosome 9 and vice-versa.

These translocations cause abnormalities in genes that will result in problems for the offspring such as mental and developmental disabilities.

Florescence in situ Hybridization (FISH)

Process developed by biomedical researchers in the early 1980’s.

Locates the presence of or lack of specific DNA sequences on chromosomes by using florescent probes which band to their complementary base pair sequences.

Florescent microscopes can be used to identify where the probes bound to the chromosome.

Ideograms

An ideogram is a diagram that shows an estimate of the chromosome’s size and its banding pattern. An ideogram of chromosome 1, the largest chromosome in terms of size and amount of DNA, is shown below:

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