Study GuideCytogenetics

Lecturer: Ms Theresa RuppeltSemester 1

2012

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Index

1. Cytogenetics: What and why? 3

2. The history of cytogenetics 4

3. The basic human cell and it’s components 6

4. Cell division 11

5. Cell cycle 12

6. Mitosis 13

7. Meiosis 14

8. Gametogenesis 16

9. The chromosome 17

10. Chromosomes in different species 20

11. Normal and abnormal chromosome number 21

12. Uniparental disomy 26

13. References 30

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Cytogenetics: What and why?

Cytogenetics is the study of normal and abnormal chromosomes. This study includes the culture and processing, analysis and reporting of different kinds of samples for different reasons. Cytogenetics is used for diagnosis, prognosis and monitoring of treatment. A large range of clinicians may be interested in these results. These include medical geneticists and genetic counselors, pediatricians, cardiologists, obstetricians/gynaecologists, endocrinologists and oncologists.

Cytogenetics is quite unique in that living cells are required for the more traditional methods. Some also think of cytogenetics as an “art” combined with science. Cytogenetics is a very much hands-on discipline whereas many other disciplines are becoming more and more automated.

Traditional cytogenetics has been expanded with the introduction of molecular cytogenetics and molecular genetics. Some traditional tests have been replaced by the newer technologies e.g. Fragile X. The molecular test for Fragile X is much less time consuming and much more reliable. Another example is fluorescence in situ hybridization (FISH) for microdeletions which is a molecular cytogenetic test. As the name implies these deletions are too small to even see on a traditional light microscope, fluorescent DNA probes are therefore hybridized to patient DNA and analysed under a fluorescent microscope. A deletion is present when only one signal of a specific colour is seen.

The reasons for cytogenetic referral are varied and include some of the following:

Suspected congenital abnormalities e.g. in a newborn baby with specific clinical features.

Family history of abnormal children A previous child with an abnormality e.g. trisomy 21 (Down syndrome) Abnormal ultrasound in pregnancy If one of the parents is a known translocation carrier Recurrent miscarriage/infertility Advanced maternal age Consanguinity Known recessive condition in the family Intellectual disability

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The history of cytogenetics

The year 1956 is considered to mark the beginning of modern human cytogenetics. Until this time the number of chromosomes in the normal human cell was considered to be 48. Due to improvements in techniques it was discovered that the correct number was 46 (Tjio and Levan, 1956). Following this further technological improvement allowed the identification of individual chromosomes and the association of specific genetic disorders with specific chromosomes.

Historians have divided the discipline of human cytogenetics into five "eras": the "Dark Ages", the "Hypotonic Period", the "Trisomy Period", the "Banding Era", and the "Molecular Era".

During the "Dark Ages" (prior to 1952) mammalian tissue culture techniques were developed, as were techniques for arresting cells during division, which allowed chromosomes to be visualized. Early studies reported the number of chromosomes per cell to be 48, and staining techniques allowed for limited differentiation of specific chromosomes, based on darkly vs. lightly stained areas.

The "Hypotonic Era" (started in 1952 by TC Hsu) denotes the use of a solution with a lower salt concentration than the cells it contains. This causes the cells to absorb water through their membranes and swell (but not burst). The swollen cells allow the chromosomes to readily separate, making them easier to count. Thus the correct chromosome number, namely 46, was established.

During the "Trisomy Period" cytogeneticists turned their attention to patients with congenital abnormalities. Patients with Down syndrome were discovered to have an additional copy of a small chromosome, chromosome number 21 (Lejeune et al., 1959). The syndrome is therefore associated with a trisomy 21 genotype. Other trisomies were also discovered during this period, namely trisomy 13 (Patau syndrome) and trisomy 18 (Edward syndrome). Numerical abnormalities involving sex chromosomes (the X and Y chromosomes) were also described for the first time and associated with specific clinical phenotypes (such as Turner syndrome and Klinefelter syndrome).

Further advances in technology led to banding techniques (hence the "Banding Era"), which brought out horizontal bands of differential staining intensity (first employed in fluorescence microscopy). The pattern of bands was specific for individual chromosomes, and allowed the identification of each chromosome. This in turn made possible the recognition of structural abnormalities associated with specific genetic syndromes. Nowadays laboratories employ high-resolution banding techniques; this increases the number of bands visible, and therefore the level of resolution at which chromosomes can be studied.

The most recent developments in cytogenetics have led to the "Molecular Era". Advances in the use of DNA probes have allowed cytogeneticists to hybridize these probes to chromosomes and determine if a specific DNA sequence is present on the target chromosome. This has been

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useful in detecting abnormalities beyond the resolution level of studying banded chromosomes at the microscope, and also in determining the location of specific genes on chromosomes.

Cytogenetic analysis has been an invaluable tool in screening for and diagnosing genetic disorders. In the future cytogenetic methods will become more and more linked to molecular techniques, and will continue to play an important role in medical service and research.

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The basic human cell and it’s components

WHAT IS A CELL?

Cells are the structural and functional units of all living organisms. Some organisms, such as bacteria, are unicellular, consisting of a single cell. Other organisms, such as humans, are multicellular, or have many cells—an estimated 100,000,000,000,000 cells! Each cell is an amazing world unto itself: it can take in nutrients, convert these nutrients into energy, carry out specialized functions, and reproduce as necessary. Even more amazing is that each cell stores its own set of instructions for carrying out each of these activities.

Cell Organization

Before we can discuss the various components of a cell, it is important to know what organism the cell comes from. There are two general categories of cells: prokaryotes and eukaryotes.

Figure 1. History of life on earth.

Prokaryotic Organisms

It appears that life arose on earth about 4 billion years ago. The simplest of cells, and the first types of cells to evolve, were prokaryotic cells - organisms that lack a nuclear membrane, the membrane that surrounds the nucleus of a cell. Bacteria are the best known and most studied form of prokaryotic organisms, although the recent discovery of a second group of prokaryotes, called archaea, has provided evidence of a third cellular domain of life and new insights into the origin of life itself.

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Prokaryotes are unicellular organisms that do not develop or differentiate into multicellular forms. Some bacteria grow in filaments, or masses of cells, but each cell in the colony is identical and capable of independent existence. The cells may be adjacent to one another because they did not separate after cell division or because they remained enclosed in a common sheath or slime secreted by the cells. Typically though, there is no continuity or communication between the cells. Prokaryotes are capable of inhabiting almost every place on the earth, from the deep ocean, to the edges of hot springs, to just about every surface of our bodies.

Prokaryotes are distinguished from eukaryotes on the basis of nuclear organization, specifically their lack of a nuclear membrane. Prokaryotes also lack any of the intracellular organelles and structures that are characteristic of eukaryotic cells. Most of the functions of organelles, such as mitochondria, chloroplasts, and the Golgi apparatus, are taken over by the prokaryotic plasma membrane. Prokaryotic cells have three architectural regions: appendages called flagella and pili—proteins attached to the cell surface; a cell envelope consisting of a capsule, a cell wall, and a plasma membrane; and a cytoplasmic region that contains the cell genome (DNA) and ribosomes and various sorts of inclusions.

Eukaryotic Organisms

Eukaryotes include fungi, animals, and plants as well as some unicellular organisms. Eukaryotic cells are about 10 times the size of a prokaryote and can be as much as 1000 times greater in volume. The major and extremely significant difference between prokaryotes and eukaryotes is that eukaryotic cells contain membrane-bound compartments in which specific metabolic activities take place. Most important among these is the presence of a nucleus, a membrane-delineated compartment that houses the eukaryotic cell’s DNA. It is this nucleus that gives the eukaryote—literally, true nucleus—its name.

Eukaryotic organisms also have other specialized structures, called organelles, which are small structures within cells that perform dedicated functions. As the name implies, you can think of organelles as small organs. There are a dozen different types of organelles commonly found in eukaryotic cells. In this primer, we will focus our attention on only a handful of organelles and will examine these organelles with an eye to their role at a molecular level in the cell.

The origin of the eukaryotic cell was a milestone in the evolution of life. Although eukaryotes use the same genetic code and metabolic processes as prokaryotes, their higher level of organizational complexity has permitted the development of truly multicellular organisms. Without eukaryotes, the world would lack mammals, birds, fish, invertebrates, mushrooms, plants, and complex single-celled organisms.

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Eukaryotes and prokaryotes.

This figure illustrates a typical human cell (eukaryote) and a typical bacterium (prokaryote). The drawing on the left highlights the internal structures of eukaryotic cells, including the nucleus (light blue), the nucleolus (intermediate blue), mitochondria (orange), and ribosomes (dark blue). The drawing on the right demonstrates how bacterial DNA is housed in a structure called the nucleoid (very light blue), as well as other structures normally found in a prokaryotic cell, including the cell membrane (black), the cell wall (intermediate blue), the capsule (orange), ribosomes (dark blue), and a flagellum (also black).

Cell Structures: The Basics

The Plasma Membrane - A Cell's Protective CoatThe outer lining of a eukaryotic cell is called the plasma membrane. This membrane serves to separate and protect a cell from its surrounding environment and is made mostly from a double layer of proteins and lipids, fat-like molecules. Embedded within this membrane are a variety of other molecules that act as channels and pumps, moving different molecules into and out of the cell. A form of plasma membrane is also found in prokaryotes, but in this organism it is usually referred to as the cell membrane.

The Cytoskeleton - A Cell's ScaffoldThe cytoskeleton is an important, complex, and dynamic cell component. It acts to organize and maintains the cell's shape; anchors organelles in place; helps during endocytosis, the uptake of external materials by a cell; and moves parts of the cell in processes of growth and motility. There are a great number of proteins associated with the cytoskeleton, each controlling a cell’s structure by directing, bundling, and aligning filaments.

The Cytoplasm—A Cell's Inner SpaceInside the cell there is a large fluid-filled space called the cytoplasm, sometimes called the cytosol. In prokaryotes, this space is relatively free of compartments. In eukaryotes, the cytosol is the "soup" within which all of the cell's organelles reside. It is also the home of the

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cytoskeleton. The cytosol contains dissolved nutrients, helps break down waste products, and moves material around the cell through a process called cytoplasmic streaming. The nucleus often flows with the cytoplasm changing its shape as it moves. The cytoplasm also contains many salts and is an excellent conductor of electricity, creating the perfect environment for the mechanics of the cell. The function of the cytoplasm, and the organelles which reside in it, are critical for a cell's survival.

Genetic MaterialTwo different kinds of genetic material exist: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Most organisms are made of DNA, but a few viruses have RNA as their genetic material. The biological information contained in an organism is encoded in its DNA or RNA sequence.

Interestingly, as much as 98 percent of human DNA does not code for a specific product.

Prokaryotic genetic material is organized in a simple circular structure that rests in the cytoplasm. Eukaryotic genetic material is more complex and is divided into discrete units called genes. Human genetic material is made up of two distinct components: the nuclear genome and the mitochondrial genome. The nuclear genome is divided into 24 linear DNA molecules, each contained in a different chromosome. The mitochondrial genome is a circular DNA molecule separate from the nuclear DNA. Although the mitochondrial genome is very small, it codes for some very important proteins.

OrganellesThe human body contains many different organs, such as the heart, lung, and kidney, with each organ performing a different function. Cells also have a set of "little organs", called organelles, which are adapted and/or specialized for carrying out one or more vital functions. Organelles are found only in eukaryotes and are always surrounded by a protective membrane. It is important to know some basic facts about the following organelles.

The Nucleus - A Cell's CenterThe nucleus is the most conspicuous organelle found in a eukaryotic cell. It houses the cell's chromosomes and is the place where almost all DNA replication and RNA synthesis occur. The nucleus is spheroid in shape and separated from the cytoplasm by a membrane called the nuclear envelope. The nuclear envelope isolates and protects a cell's DNA from various molecules that could accidentally damage its structure or interfere with its processing. During processing, DNA is transcribed, or synthesized, into a special RNA, called mRNA. This mRNA is then transported out of the nucleus, where it is translated into a specific protein molecule. In prokaryotes, DNA processing takes place in the cytoplasm.

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The Ribosome - The Protein Production Machine

Ribosomes are found in both prokaryotes and eukaryotes. The ribosome is a large complex composed of many molecules, including RNAs and proteins, and is responsible for processing the genetic instructions carried by an mRNA. The process of converting an mRNA's genetic code into the exact sequence of amino acids that make up a protein is called translation. Protein synthesis is extremely important to all cells, and therefore a large number of ribosomes—sometimes hundreds or even thousands—can be found throughout a cell.

Ribosomes float freely in the cytoplasm or sometimes bind to another organelle called the endoplasmic reticulum. Ribosomes are composed of one large and one small subunit, each having a different function during protein synthesis.

The following organelles are also found in the cell

Mitochondria and Chloroplasts—The Power Generators Lysosomes and Peroxisomes—The Cellular Digestive System

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Cell division

Haploid and diploid are terms referring to the number of sets of chromosomes in a cell. Gregor Mendel determined his peas had two sets of alleles, one from each parent. Diploid organisms are those with two (di) sets. Human beings (except for their gametes), most animals and many plants are diploid. We abbreviate diploid as 2n. Ploidy is a term referring to the number of sets of chromosomes. Haploid organisms/cells have only one set of chromosomes, abbreviated as n. Organisms with more than two sets of chromosomes are termed polyploid. Chromosomes that carry the same genes are termed homologous chromosomes. The alleles on homologous chromosomes may differ, as in the case of heterozygous individuals. Organisms (normally) receive one set of homologous chromosomes from each parent.

Sexual reproduction occurs only in eukaryotes. During the formation of gametes, the number of chromosomes is reduced by half, and returned to the full amount when the two gametes fuse during fertilization.

Mitosis maintains the cell's original ploidy level (for example, one diploid 2n cell producing two diploid 2n cells; one haploid n cell producing two haploid n cells; etc.). Meiosis, on the other hand, reduces the number of sets of chromosomes by half, so that when gametic recombination (fertilization) occurs the ploidy of the parents will be re- established.

Meiosis is a specialized type nuclear division that segregates one copy of each homologous chromosome into each new "gamete". Although meiosis may seem much more complicated than mitosis, it is really just two cell divisions in sequence. Each of these sequences maintains strong similarities to mitosis.

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The cell cycle

The average mammalian cell cycle lasts about 17 to 18 hours and is the transition of a cell from interphase through cell division and back to the interphase.

There are 3 stages:

Gap 1(G1): this is the longest and last about 9 hours in mammalian cells. The chromosomes are in the form of elongated chromatids. The cells are metabolically active and protein synthesis takes place.

Synthesis: this lasts about 5 hours in mammalian cells. DNA is synthesized during this phase. The DNA replicates itself. The chromosomes then consist of two identical sister chromatids.

Gap 2 (G2): lasts about 3 hours. The cell prepares to undergo cell division. G1, Synthesis and G2 forms part of interphase. The final stage in the cell cycle is mitosis. This lasts about 1 to 2 hours in mammalian

cells.

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Mitosis

Every time a cell divides, it must ensure that its DNA is shared between the two daughter cells. Mitosis is the process of "dividing up" the genome between the daughter cells. The two daughter cells are genetically identical to one another and to the parent cell.

Interphase is the phase before a cell enters mitosis, the state of a eukaryotic cell when not undergoing division. Every time a cell divides, it must first replicate all of its DNA. Because chromosomes are simply DNA wrapped around protein, the cell replicates its chromosomes also. These two chromosomes, positioned side by side, are called sister chromatids and are identical copies of one another.

Prophase is the phase in which, the sister chromatids are separate from one another. To do this, the chromosomes have to condense.

Next, the nuclear envelope breaks down, and a large protein network, called the spindle, attaches to each sister chromatid. The chromosomes are now aligned perpendicular to the spindle in a process called metaphase.

Next, "molecular motors" pull the chromosomes away from the metaphase plate to the spindle poles of the cell. This is called anaphase.

During telophase the cells divide; the nuclear envelope reforms, and the chromosomes relax and decondense. The cell can now replicate its DNA again during interphase and go through mitosis once more.

Overview of the major events in mitosis.

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Meiosis

Meiosis I refers to the first of the two divisions and is often called the reduction division. This is because it is here that the chromosome complement is reduced from diploid (two copies) to haploid (one copy).

Interphase in meiosis is identical to interphase in mitosis. At this stage, there is no way to determine what type of division the cell will undergo when it divides. Meiotic division will only occur in cells associated with male or female sex organs.

Prophase I is virtually identical to prophase in mitosis, involving the appearance of the chromosomes, the development of the spindle apparatus, and the breakdown of the nuclear membrane.

Metaphase I is where the critical difference occurs between meiosis and mitosis. In mitosis, all of the chromosomes line up on the metaphase plate in no particular order. In Metaphase I, the chromosome pairs are aligned on either side of the metaphase plate. It is during this alignment that the chromatid arms may overlap and temporarily fuse, resulting in what is called crossovers.

During Anaphase I, the spindle fibers contract, pulling the homologous pairs away from each other and toward each pole of the cell.

In Telophase I, a cleavage furrow typically forms, followed by cytokinesis, the changes that occur in the cytoplasm of a cell during nuclear division; but the nuclear membrane is usually not reformed, and the chromosomes do not disappear. At the end of Telophase I, each daughter cell has a single set of chromosomes, half the total number in the original cell, that is, while the original cell was diploid; the daughter cells are now haploid.

Meiosis II is quite simply a mitotic division of each of the haploid cells produced in Meiosis I. There is no Interphase between Meiosis I and Meiosis II, and the latter begins with Prophase II. At this stage, a new set of spindle fibers forms and the chromosomes begin to move toward the equator of the cell.

During Metaphase II, all of the chromosomes in the two cells align with the metaphase plate.

In Anaphase II, the centromeres split, and the spindle fibers shorten, drawing the chromosomes toward each pole of the cell.

In Telophase II, a cleavage furrow develops, followed by cytokinesis and the formation of the nuclear membrane. The chromosomes begin to fade and are replaced by the granular chromatin, a characteristic of interphase.

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When Meiosis II is complete, there will be a total of four daughter cells, each with half the total number of chromosomes as the original cell. In the case of male structures, all four cells will eventually develop into sperm cells. In the case of the female life cycles in higher organisms, three of the cells will typically abort, leaving a single cell to develop into an egg cell, which is much larger than a sperm cell. Most cells in the human body are produced by mitosis. These are the somatic (or vegetative) line cells. Cells that become gametes are referred to as germ line cells. The vast majority of cell divisions in the human body are mitotic, with meiosis being restricted to the gonads.

Overview of the major events in meiosis.

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Gametogenesis

Gametogenesis is the process of forming gametes (by definition haploid, n) from diploid cells of the germ line. Spermatogenesis is the process of forming sperm cells by meiosis (in animals, by mitosis in plants) in specialized organs known as gonads (in males these are termed testes). After division the cells undergo differentiation to become sperm cells. Oogenesis is the process of forming an ovum (egg) by meiosis (in animals, by mitosis in the gametophyte in plants) in specialized gonads known as ovaries. Whereas in spermatogenesis all 4 meiotic products develop into gametes, oogenesis places most of the cytoplasm into the large egg. The other cells, the polar bodies, do not develop this. All the cytoplasm and organelles go into the egg. Human males produce 200,000,000 sperm per day, while the female produces one egg (usually) each menstrual cycle.

Mitosis Meiosis

- 1 division - 2 divisions

- 2 progeny cells/cycle - 4 progeny cells/cycles

- progeny cell identical - progeny cells genetically different

- chromosome number of progeny - chromosome number of progeny cells same a parent cells (2n) cells ½ of parent (1n)

- occurs in somatic cells - occurs in primordial germ cells

- growth repair and sexual reproduction - sexual reproduction in which new gene combinations arise

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The Chromosome

Cytogenetics is the study of chromosomes and the related disease states caused by abnormal chromosome number and/or structure.

Chromosomes (in Greek chroma = colour and soma = body) are complex structures located in the cell nucleus, they are composed of DNA, histone and non-histone proteins, RNA and polysaccharides. They are basically the "packages" that contain the DNA. If you were to stretch out the entire DNA from one of your cells, it would be over 3 feet (1 meter) long from end to end. Normally, we have 46 chromosomes in each cell; we received 23 from our mother and 23 from our father.

Normally chromosomes can't be seen with a light microscope but during mitosis (cell division), During the Metaphase in mitosis chromosomes are condensed and called metaphasic chromosomes this is the only natural context in which individual chromosomes are visible with an optical microscope. To collect cells with their chromosomes in this condensed state they are exposed to a mitotic inhibitor which blocks formation of the spindle and arrests cell division at the metaphase stage.

Chromosomes in eukaryotes

Eukaryotes possess multiple linear chromosomes contained in the cell's nucleus. Each chromosome has one centromere, with one or two arms projecting from the centromere. Anatomy of chromosomes - there are four important parts in metaphase chromosomes (telomeres, centromeres, and heterochromatin & euchromatin):

Anatomy of chromosomes

Under the microscope chromosomes appear as thin, thread-like structures. They all have a short arm and long arm separated by a primary constriction called the centromere. The short

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arm is designated as p and the long arm as q. The centromere is the location of spindle attachment and is an integral part of the chromosome. It is essential for the normal movement and segregation of chromosomes during cell division. The ends of the chromosomes are special structures called telomeres. DNA replication begins at many different locations on the chromosome.

Human metaphase chromosomes come in three basic shapes and can be categorized according to the length of the short and long arms and also the centromere location. Metacentric chromosomes have short and long arms of roughly equal length with the centromere in the middle. Submetacentric chromosomes have short and long arms of unequal length with the centromere more towards one end. Acrocentric chromosomes have a centromere very near to one end and have very small short arms. They frequently have secondary constrictions on the short arms that connect very small pieces of DNA, called stalks and satellites, to the centromere. The stalks contain genes, which code for ribosomal RNA.

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Chromatin

Two types of chromatin can be distinguished:

Euchromatin, which consists of DNA that is active, e.g., expressed as protein. Heterochromatin, which consists of mostly inactive DNA. It seems to serve structural

purposes during the chromosomal stages. Heterochromatin can be further distinguished into two types:

o Constitutive heterochromatin, which is never expressed. It is located around the centromere and usually contains repetitive sequences.

o Facultative heterochromatin, which is sometimes expressed.

Different levels of DNA condensation. (1) Single DNA strand. (2) Chromatin strand (DNA with histones). (3) Chromatin during interphase with centromere. (4) Condensed chromatin during prophase. (Two copies of the DNA molecule are now present) (5) Chromosome during metaphase.

In the early stages of mitosis, the chromatin strands become more and more condensed. They cease to function as accessible genetic material and become a compact transport form. Eventually, the two matching chromatids (condensed chromatin strands) become visible as a chromosome, linked at the centromere. Long microtubules are attached at the centromere and two opposite ends of the cell. During mitosis, the microtubules pull the chromatids apart, so that each daughter cell inherits one set of chromatids. Once the cells have divided, the chromatids are uncoiled and can function again as chromatin. In spite of their appearance, chromosomes are highly structured. For example, genes with similar functions are often kept close together in the nucleus, even if they are far apart on the chromosome. The short arm of a chromosome can be extended by a satellite chromosome that contains codes for ribosomal RNA.

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Chromosomes in different species

Species # of chromosomes Species # of chromosomes

Fruit fly 8 Human 46

Rye 14 Ape 48

Guinea Pig 16 Sheep 54

Dove 16 Horse 64

Edible Snail 24 Chicken 78

Earthworm 36 Carp 104

Pig 40 Butterflies ~380

Wheat 42 Fern ~1200

Mouse 40 Rat 42

Hare 46 Rabbit 44

Dog 78 Cat 38

Cow 60 Syrian hamster 44

Normal members of a particular species all have the same number of chromosomes (Table 1). Asexually reproducing species have one set of chromosomes, which is the same in all body cells. Sexually reproducing species have somatic cells (body cells), which are diploid [2n] (they have two sets of chromosomes, one from the mother, one from the father) or polyploid [Xn] (more than two sets of chromosomes), and gametes (reproductive cells) which are haploid [n] (they have only one set of chromosomes). Gametes are produced by meiosis of a diploid germ line cell. During meiosis, the matching chromosomes of father and mother can exchange small parts of themselves (crossover), and thus create new chromosomes that are not inherited solely from either parent. When a male and a female gamete merge (fertilization), a new diploid organism is formed.

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Normal and Abnormal chromosome number

Diploid number

Normal human somatic cells have 46 chromosomes: 22 pairs, or homologs, of autosomes (chromosomes 1-22) and two sex chromosomes. This is called the diploid number. Females carry two X chromosomes (46,XX) while males have an X and a Y (46,XY). Germ cells (egg and sperm) have 23 chromosomes: one copy of each autosome plus a single sex chromosome. This is referred to as the haploid number. One chromosome from each autosomal pair plus one sex chromosome is inherited from each parent. Mothers can contribute only an X chromosome to their children while fathers can contribute either an X or a Y.

Polyploidy

Polyploid (in Greek: πολλαπλόν - multiple) cells or organisms contain more than two copies (ploidy) of their chromosomes. The polyploid types are termed triploid (3x or 69 chromosomes), tetraploid (4x or 92 chromosomes), pentaploid (5x), hexaploid (6x) and so on. Where an organism is normally diploid, a monoploid (1x or 23 chromosomes) may arise as a spontaneous aberration; monoploidy may also occur as a normal stage in an organism's life cycle.

Autopolyploids are composed of multiple sets of chromosomes from within one species, while allopolyploids are composed of chromosome sets from different species. Allopolyploids usually only form between closely related species, the chromosome of allopolyploids is described as homeologus since they are only partially homologous. Amphidiploid and allotetraploid mean having two chromosome sets from one species and two chromosome sets from another species. These are formed from the hybridisation of two separate species followed by their subsequent chromosome doubling.

Polyploidy occurs in animals but is especially common among flowering plants, including both wild and cultivated species. Wheat, for example, after millennia of hybridization and modification by humans, has strains that are diploid (two sets of chromosomes), tetraploid (four sets of chromosomes) with the common name of durum or macaroni wheat, and hexaploid (six sets of chromosomes) with the common name of bread wheat. Triploids are always sterile since one third of the chromosomes cannot pair. Polyploidy can be induced in cell culture by some chemicals: the best known is colchicine, which causes chromosome doubling.

Aneuploidy

Aneuploidy is a chromosomal state where abnormal numbers of specific chromosomes or chromosome sets exist within the nucleus.

A change in the number of chromosomes leads to a chromosomal disorder. These changes can occur during the formation of reproductive cells (eggs and sperm) or in early fetal development.

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In humans the most common form of aneuploidy is trisomy, or the presence of an extra chromosome in each cell. Monosomy, or the loss of one chromosome from each cell, is another kind of aneuploidy. Aneuploidy is also common in cancerous cells.The meiotic error that causes aneuploidy is called non-disjunction.

Trisomy

A trisomy is the presence of three, instead of the normal two, chromosomes of a particular numbered type in an organism. Thus the presence of an extra chromosome 21 is called trisomy 21. Most trisomies, like most other abnormalities in chromosome number, result in distinctive birth defects. Many trisomies result in miscarriage or death at an early age.

A partial trisomy occurs when part of an extra chromosome is attached to one of the other chromosomes. A mosaic trisomy is a condition where extra chromosomal material exists in only some of the organism's cells.

While a trisomy can occur with any chromosome, the most common types in humans are:

Trisomy 21 (Down syndrome) Trisomy 18 (Edward's Syndrome) Trisomy 13 (Patau's Syndrome)

Trisomy involving sex chromosomes include:

Trisomy X (Triple X Syndrome) Klinefelter's syndrome (XXY) XYY

Monosomy

Monosomy is the presence of only one chromosome from a pair in a cell's nucleus. Monosomy is a type of aneuploidy. Partial monosomy occurs when the long or short arm of a chromosome is missing.

Human genetic disorders arising from monosomy are:

Turner syndrome

Disomy

A disomy is the presence of a pair of chromosomes, or the normal amount for some organisms including humans. It is not a disorder, or aneuploid, but is the absence of aneuploidism.

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Non-disjunction

Definition(s)The failure of homologous CHROMOSOMES or CHROMATIDS to segregate during MITOSIS or MEIOSIS with the result that one daughter cell has both of a pair of parental chromosomes or chromatids and the other has none.

Sometimes there is a mistake in the sorting of the chromosomes during the production of the sperm or the egg.  This is called non-disjunction.  Non-disjunction can occur during meiosis I or meiosis II. In meiosis I, the error occurs when the homologous pairs both travel into the same daughter cell.  The result is two daughter cells that have two copies of the chromosome (called disomic cells) and two cells that are missing that chromosome (called nullisomic cells).  This is shown in diagram a).   In meiosis II, the error occurs when the sister chromatids will not separate and thus travel into the same daughter cell.   This is shown in diagram b) by the blue sister chromatids.  

An error in the proper segregation of the chromosomes during both meiosis I and II, are pictured below.

Non-disjunction in Meiosis I: Non-disjunction in Meiosis II:

What causes non-disjunction?

The cause of non-disjunction is unknown.  Non-disjunction seems to be a chance event.  Nothing that an individual does or doesn't do during their reproductive years can cause these chromosomal changes.  We do know that non-disjunction occurs more frequently in the eggs of women, as they get older.  So, how does trisomy, three copies of one chromosome in a baby, arise? At fertilization the egg (23 chromosomes) and the sperm (23

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chromosomes) fuse to create a conception, or zygote, which has 46 chromosomes.  If a sperm or egg carries an extra copy of one of the chromosomes, due to non-disjunction at meiosis I or meiosis II, there will be a total of 24 chromosomes instead of 23 in the reproductive cell. If this sperm or egg is fertilized by a normal sperm or egg the result will be a total of 47 chromosomes instead of 46. This is illustrated in the diagrams below where an egg carrying 24 chromosomes is fertilized by a sperm with 23 chromosomes.

I) Fertilization following Meiosis I error:  ii) Fertilization following Meiosis II error:

What is the difference between these diagrams?

There is a difference between the outcomes of errors, which occur during meiosis I when compared to the outcome of those, which occur during meiosis II.  In the diagrams above, the colour of the chromosomes illustrates the difference.  

i) Meiosis I errors: When the trisomic zygote is caused by an error in meiosis I, the zygote receives three different chromosomes.  Although both the yellow and the blue chromosomes were both contributed by the egg, they are homologous chromosomes where the actual genetic information contained within the chromosome is different.  Originally the yellow chromosome was the maternal copy and the blue chromosome was the paternal copy.

ii) Meiosis II errors: When the trisomy is caused by an error in meiosis II, two of the same chromosomes are inherited. Both of the blue chromosomes inherited from the egg are sister chromatids, which contain very similar genetic material.  However, there is a slight difference in the two blue chromosomes.  This is due to a process called recombination, which occurs prior to the first cell division in meiosis I. Refer back to the first diagram on this page to recall this time in cell division. During recombination the homologous pairs align themselves very closely and the two chromatids that are closest to one another exchange pieces of genetic material through a process called crossing over.  This is illustrated in the diagram on the right. The purpose of this process is to ensure variation in the gametes.  Due to the process of recombination each of the four gametes are unique. 

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A non-disjunction error during meiosis I or II leads to an individual with an extra chromosome in every cell of his/her body. 

Monosomy

Looking at the diagrams above which is illustrating non-disjunction, what happens with the reproductive cells which appear to be empty, that is, the sperm or egg which are lacking a chromosome?  If one of these gametes is fertilized by a normal gamete the result is monosomy of the chromosome involved.  Monosomy is a deficiency in number of chromosomes and is defined as only one copy of a chromosome that is normally present in two copies.  These eggs and sperm, which contain one less chromosome, have 22 chromosomes.  When fertilized, the outcome is 45 chromosomes in total.  In general, monosomies are less likely to survive when compared to trisomies. 

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Uniparental disomy (UPD)

In 1980 Engel introduced the concept of uniparental disomy (UPD).  Uniparental disomy (UPD) arises when an individual inherits two copies of a chromosome pair from one parent and no copy from the other parent. Recall that normally a baby inherits one copy of each chromosome from his/her mother and one copy of each chromosome from his/ her father. In the rare circumstance of UPD a baby may have two copies of one of his/ her mother’s chromosome and no copies of that chromosome from his/ her father.  This is called maternal UPD.  Paternal UPD is when a child inherits two copies of a specific chromosome from his/ her father and no copies of that chromosome from his/ her mother.

This abnormality in inheritance may lead to health concerns in a child.  UPD can result in rare recessive disorders, or developmental problems due to the effects of imprinting.  UPD may also occur with no apparent impact on the health and development of and individual.  We will discuss the effects of UPD in greater detail, but first we must understand how UPD occurs.

How does UPD happen? 

Three possible mechanisms have been proposed for the origin of UPD:

1. The loss of a chromosome from a trisomic zygote, "trisomic rescue" 2. The duplication of a chromosome from a monosomic zygote, "monosomic rescue" 3. The fertilization of a gamete with two copies of a chromosome by a gamete with no

copies of the same chromosome, called gamete complementation.

All of these mechanisms require two consecutive "mistakes".

UPD by Trisomic Rescue

Trisomic rescue is the most common mechanism producing UPD. The outcome will differ depending on the timing of the original error, or non-disjunction.  For example, did the original error, which gave rise to the trisomic zygote, occur during meiosis I or meiosis II?  Using the diagram below to illustrate the first example, consider that both the yellow and the blue chromosomes were inherited from the egg after an error in meiosis I.  Non-disjunction in meiosis I create a gamete with two homologous, non-identical chromosomes.  The green chromosome was inherited from the sperm.  The trisomic zygote contains three copies of the chromosome, 2 maternal copies and 1 paternal copy.  There are three equally possible options for trisomic rescue.  

The yellow chromosome can be eliminated, leaving the blue (maternal) and the green (paternal),

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The blue chromosome can be eliminated, leaving the yellow (maternal) and the green (paternal), or

The green chromosome can be eliminated, leaving the yellow (maternal) and the blue (maternal) chromosome. 

Trisomic rescue following an error in meiosis I.

In the last scenario both chromosomes are inherited from the mother, as seen in the diagram.  This is called maternal uniparental disomy (mat UPD).  The first two scenarios would lead to biparental disomy (BPD).  There is a one in three chance that a trisomic zygote which undergoes trisomic rescue will result in UPD.  Although we know that the actual genetic information is different on each of the chromosomes (the yellow one and the blue one) it is significant that they have both been inherited from the mother.  The inheritance of two homologous chromosomes from one parent is termed heterodisomy, since there are two copies of the chromosome (disomy), however the actual chromosomes are different (hetero) in genetic material.

Now consider the same situation of trisomic rescue, except the original imbalance in the egg was due to non-disjunction in meiosis II.  Again there are three equally possible options for loss of a chromosome.  Two would result in a bi parental situation as in the previous situation, with one maternal chromosome and one paternal chromosome.  Once again, loss of the paternally inherited chromosome, represented in green, would result in uniparental inheritance.  In this situation the two blue chromosomes are very similar.  This is termed isodisomy.  "Disomy" means two copies of the chromosome and "iso" means the same. 

UPD may cause health concerns in people for two possible reasons:

Parental imprinting in the case of heterodisomy and isodisomy The unmasking of recessive conditions in some cases of isodisomy

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What is imprinting?

Recall that the chromosomes are the packaging for our genetic material, our genes.  There are hundreds to thousands of genes on each chromosome.  Each gene has specific location on a chromosome.  Genes carry instructions that tell our bodies how to grow, develop and function.  Each gene gives specific instruction for the production of a particular protein, which has a job in the body.  Just like the chromosomes, there are two copies of each gene, one inherited from the mother (on the maternal chromosome) and one inherited from the father (on the paternal chromosome).  Usually the information from both copies is actively being used.  When a gene actively gives the instructions to create a protein, we say that it is being expressed.  Some genes are only expressed when inherited from the father.  Other genes are only expressed when inherited from the mother.  This phenomenon of differential expression depending on the parent of origin is called imprinting.  Some chromosomes, sections of chromosomes or genes are stamped with the parent of origin.  The stamping occurs during the formation of the egg and sperm.  Imprinting occurs in each generation.  Chromosomes, sections of chromosomes or genes can be turned on and off depending on the parent from which the component was inherited.  

Paternally imprinted genes are switched "off" when passed from father to child

Maternally imprinted genes are switched "off" when passed from mother to child

Imprinting and UPD

It is possible that concerns with imprinting may exist regardless of whether the original error occurred in meiosis I or meiosis II.  As described above, uniparental disomy is the inheritance of two copies of a chromosome from the same parent.  UPD causes concern with imprinted genes or regions of chromosomes because an individual with UPD only inherits either maternal copies of a chromosome or paternal copies of a chromosome.  In the case of paternal UPD, a chromosome may contain genes or regions that are paternally switched off.  This individual will have no working copies of these genes.  Alternatively, in the case of maternal UPD, a chromosome may contain genes or regions that are maternally switched off and this individual will have no working copies of these genes.  

Prader-Willi syndrome and Angelman syndrome provide an excellent example of the concept of imprinting.  Both conditions are the result of a deletion in the same area on chromosome 15.  If the deleted area is inherited from an individual's father the patient will have Prader-Willi syndrome (PWS).  The PWS gene is "switched off" on the maternally inherited chromosome 15, so this individual has no working copies of the PWS gene.  On the other hand, if the deleted area is maternal in origin the patient will have Angelman syndrome (AS).  The AS gene is "switched off" on the paternally inherited chromosome 15, thus this individual has no working copies of the AS gene.  About 20-30% of individuals with PWS do not have a deletion, but they have inherited two maternal copies of chromosome 15, maternal UPD15.  These individuals have no paternal contribution of chromosome 15 and thus no working copy of the PWS gene.  In 1992, the first case of UPD associated with CPM was reported. A patient with Prader-Willi

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syndrome, cause by maternal UPD for chromosome 15 was born after trisomy 15 was detected on CVS and a normal diploid karyotype was seen in amniotic fluid (Purvis-Smith et al, 1992). 

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References

1. NHLS GSH Cytogenetics Study Guide2. The Principles of Clinical Cytogenetics. Steven I. Gersen, Martha B. Keagle, 2nd edition,

2005ISBN 1-58829-300-9eISBN 1-59259-833-1

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