Overview of Nuclear Division as an Information Distribution System
Some of the basic terminology used to describe the genome was introduced in the first two chapters. We know, for example, that the nucleus of a diploid (2n) cell contains two copies of each gene. The subtleties of such a statement were discussed in Chapter 4 but do not affect our understanding of nuclear division. Each gene is located somewhere on one of the numerous strands of DNA we see in the microscope as chromosomes. Each chromosome is, therefore, a separate unit of information transmission, a copy of a linkage group. It might contain hundreds or even several thousand genes linked in a linear array on the same strand of DNA. There are as many different linkage groups in a species as there are genetically different types of chromosome, ignoring the minor differences that distinguish allelic forms of the same gene. A diploid individual carries two copies of each linkage group, i.e., a pair of homologous chromosomes. The goal of mitosis is to duplicate each chromosome and pass one copy of every chromosome into each of the two new nuclei of the diploid daughter cells.
The goal of meiosis, on the other hand, is more complex and in some ways more important. Meiosis is a reduction division involving two cycles. Instead of transmitting one copy of every chromosome, meiosis results in passing one copy of each kind of chromosome, i.e., one copy of each linkage group, to each haploid (1n) egg or sperm nucleus. The diploid (2n) nuclear composition is regenerated at fertilization, with one copy of each linkage group coming from each parent.
Although cancer, somatic mosaicism, and other outcomes involving chromosome-level changes can be significant, errors in mitosis will usually have only minor consequences, if they have any at all. The presence of one abnormal cell is hard to detect among so many normal ones in the body. Its abnormality and death go unnoticed. But an error in meiosis is far more serious, since it will affect the initial genome of the zygote produced at fertilization. Estimates range from 8% to 25% (with most authorities leaning toward the higher end of this range) of human fertilizations will result in spontaneous abortion, perinatal death, or severe developmental consequences due to changes in chromosome number from errors in meiosis or fertilization. After discussing the normal events of nuclear division, we will explore some of the consequences of errors in meiosis.
The Cell Cycle in Eukaryotes
Whether we are considering mitosis or meiosis, the phases of the cell cycle can be subdivided into two parts, interphase and the stages of nuclear division (Figure 5-2a, which shows the events for mitosis). Interphase is sometimes called the "resting phase," but that is a misnomer. It may be a resting stage in the sense that it occurs between rounds of active nuclear division. But functionally, it is the most active time.
The growth-duplication cycle of mitosis. (Reprinted with permission from Brooker RJ: Genetics: Analysis & Principles, 3rd ed. New York: McGraw-Hill, 2008.)
During the G1 phase of interphase, active genes are being transcribed and are controlling the biochemical life of the cell. To be accessible to the enzymes of transcription, the chromosomes are in various degrees of uncoiling. That is why a stained microscopic preparation of interphase simply looks like a dark organelle with little internal structure other than one or more nucleoli. At this stage, each chromosome is a single DNA double-helix molecule complexed with nucleosomal proteins. In G1 the cell typically grows by duplicating cell contents, except for the nuclear material. Then, in response to a signal, such as cell age, reaching a critical cell size, or receiving a molecular trigger like a growth factor, a restriction point is reached. The cell becomes committed to make a transition into the S, or synthesis, phase. An example of activating this G1-S checkpoint by a growth factor is shown in Figure 5-2b.
Activation of cell division by epidermal growth factor (EGF). EGF binds a pair of EGF receptors and causes them to form an active dimer and become phosphorylated. This attracts the GRB2 and other proteins intracellularly, ultimately activating the Ras protein by forming a Ras/GTP (i.e., guanosine triphosphate) complex. This complex activates the Raf-1 protein kinase that phosphorylates MEK, which then phosphorylates MAPK. This MAPK then activates transcription factors that initiate cell division. (Reprinted with permission from Brooker RJ: Genetics: Analysis & Principles, 3rd ed. New York: McGraw-Hill, 2008.)
Progress through the G1 (or G1-S) and the later G2 (or G2-M) checkpoints is regulated by the formation of complexes of specific cyclins and cyclin-dependent kinases (CDKs). CDKs regulate the activity of other proteins by phosphorylating them, thereby either activating or inactivating them, depending on the function of the target protein. The specific cyclins determine which target proteins are acted on. At the G1-S checkpoint, the cyclin-CDK complex activates proteins needed for DNA replication. At the later G2-M checkpoint, a different cyclin-CDK complex activates proteins responsible for condensation and other chromosomal changes. If damage such as a DNA break is detected, a checkpoint protein like p53 inhibits the formation of an active cyclin/CDK complex.
In some cell lineages, a division restriction point is delayed or stops. A nucleus can be temporarily inactive (Figure 5-3) so it is not preparing for a new cell division cycle, or it might be terminally differentiated and will never divide again. Such a cell is described as being in the G0 phase.
The G0 phase represents a cell that is no longer dividing, as in case of a terminally differentiated cell lineage. (Reprinted with permission from Brooker RJ: Genetics: Analysis & Principles, 3rd ed. New York: McGraw-Hill, 2008.)
In the S (synthesis) phase, DNA replication occurs. To accomplish this complex task, the chromosomes must of course remain uncoiled, or uncondensed. But if we could visualize them in a microscope, we would see that the two single-stranded templates of the replicating parent DNA molecule separate and a pair of new complementary strands is constructed as described in Chapter 2. The two strands remain connected at the centromere, so when they condense during nuclear division, we are able to see the two copies for the first time (Figure 5-4) as sister chromatids. From here to the middle of nuclear division, each chromosome has twice the usual amount of DNA.
Chromosomes become highly coiled, or compacted, during early nuclear division (metaphase) and clearly show the position of the centromere attachment of the copies or the separation of the sister chromatids. (Reprinted with permission from Brooker RJ: Genetics: Analysis & Principles, 3rd ed. New York: McGraw-Hill, 2008.)
The terminology concerning chromosome number and DNA content during this transition can be confusing, but it is simplified if one keeps a key definition in mind. The term "chromosome" literally means "colored body." No matter how many chromosome arms it carries, anything connected together at the same centromere is one unit and, thus, one chromosome. To count chromosomes, you must simply count the number of centromeres. Chromosome number does not change between the beginning and the end of interphase. What changes is the amount of DNA in the nucleus. The C value is the amount of DNA in a haploid (1n) nucleus. The diploid cell at G1, therefore, has a DNA content of 2C. During S, this doubles to 4C. Nuclear division reduces it back to 2C in each of the two daughter cells at the end of mitosis or to 1C in each of the four nuclei that result from the meiotic reduction division to produce haploid eggs or sperm.
During G2, the cell makes final preparations for division of the nucleus and cytoplasm. A key event that continues from S into G2 is the error correction in DNA repair. The checkpoint between G2 and mitosis or meiosis (M) is not passed until repair activities have been completed. Timing of the substages of interphase will differ among species and as a function of how actively the tissue is dividing, with the period of interphase before S being the most variable. One estimate for dividing mouse fibroblasts is 9.1 hours for G1, 9.9 hours for S, 2.2 hours for G2, and 0.7 hours for mitosis (M).
Mitosis: Somatic Cell Division
The stages of mitosis (Figure 5-5) are defined for the convenience of talking about the details of the process. Keep in mind, however, that it is actually a continuous process. At the start, the DNA and chromosomal proteins have already duplicated in interphase, and the two sister chromatids of each replicated chromosome are still attached at the centromere (Figure 5-4). An additional cell organelle also comes into play. The centrosome containing a pair of centrioles is located in the cytoplasm near the nucleus. They produce the array of microtubules that move chromosomes during nuclear division.
Stages of mitosis. (Photomicrographs © Dr. Conly L. Rieder, Wadsworth Center, Albany, New York 12201-0509. Reprinted with permission from Brooker RJ: Genetics: Analysis & Principles, 3rd ed. New York: McGraw-Hill, 2008.)
Prophase is a preparatory phase (pro = before). The chromosomes coil or condense into compact structures that can move easily within a cell and that can begin to be seen microscopically. The nuclear membrane breaks down, and the centrosomes divide and begin moving to opposite poles of the cell. As they separate, they generate an array of microtubules called the spindle, which is composed of tubulin.
In prometaphase, the sister chromatids become attached to the spindle by means of the kinetochore microtubules that extend from the kinetochore, a group of proteins that binds the centromeric DNA region (Figure 5-6). The microtubules from the spindle, called the polar microtubules, overlap near the equator of the cell and help keep the spindle poles separate. One reason this phase is important in medical genetics is that current cytogenetic standards use prometaphase chromosomes to establish the karyotype, or chromosome picture (e.g., Figure 5-23), for clinical evaluation. The chromosomes are less condensed then than they will be at later stages, so more detail can be seen with certain kinds of staining treatments.
The mitotic spindle is made up of kinetochore microtubules that bind to the chromosome's kinetochore and polar microtubules that help keep the poles separate and the spindle in position. The kinetochore is made up of the centromeric DNA and two layers of kinetochore proteins that bind the kinetochore microtubules. (Reprinted with permission from Brooker RJ: Genetics: Analysis & Principles, 3rd ed. New York: McGraw-Hill, 2008.)
During metaphase, the chromosomes line up at the equator between the centrosome poles. The chromosome still has two attached sister chromatids, with each attached to the opposite pole. Anaphase begins when the centromere divides and each chromatid is now a separate chromosome. Chromosome number has temporarily doubled, e.g., from 46 chromosomes (each with a pair of sister chromatids) to 92 in humans. The kinetochore microtubules shorten by dissociation of their tubulin subunits, so the chromosomes move toward the poles a bit like pieces in the classic Pac-Man computer game.
In telophase, generally the briefest phase of mitosis, events opposite to those of prophase occur. The chromosomes decondense; the spindle breaks down; and two new nuclear membranes form around the chromosomes at each pole. In addition, division of the cytoplasm, called cytokinesis, occurs when a contractile ring that includes actin and the motor protein myosin constricts the cell membrane to distribute the cytoplasm and its organelles into the two daughter cells (Figure 5-7).
Division of the cytoplasm in cytokinesis is seen in the formation of a cleavage furrow produced by a ring of actin filaments and myosin motor proteins. Cytokinesis divides the cytoplasm and its internal organelles into two daughter cells. (b: Reprinted with permission from Brooker RJ: Genetics: Analysis & Principles, 3rd ed. New York: McGraw-Hill, 2008.)
Meiosis: Producing Haploid Egg and Sperm Nuclei
In contrast to mitosis, which passes one duplicate copy of every chromosome to each of the daughter cells, meiosis reduces chromosome number by half. It does this in two cycles of division: prophase I, metaphase I, anaphase I, telophase I, followed by a second round with prophase II, and so forth (Figure 5-8). But it is not sufficient simply to cut chromosome number in half. Each haploid gamete must have one copy of each kind of chromosome, i.e., one copy of each linkage group. The process that allows this to occur centers on events in prophase I. In prophase I, as in the beginning of mitosis, the spindle forms, chromosomes condense, and the nuclear membrane breaks down. But instead of each chromosome attaching independently to the spindle microtubules, the homologous chromosomes pair together to form a bivalent. The process of pairing is called synapsis and involves the creation of a synaptonemal complex (Figure 5-9) that only forms between the chromatids of different homologous chromosomes. They do not form between sister chromatids. Consequently, there will be one bivalent for each type of chromosome, i.e., one bivalent for each linkage group. It is the bivalent that binds to the spindle so that at anaphase I, one of the chromosomes (still with the two sister chromatids attached at the centromere) moves to one pole while the other homologous chromosome moves to the opposite pole. Thus, chromosome number is reduced from the diploid set of chromosomes (2n) to two haploid (1n) nuclei by the end of the first meiotic division.
Meiosis involves two rounds of division and yields four haploid nuclei, each carrying one copy of each type of chromosome. Key events occur in prophase I of the first cycle including synapsis, or pairing, of homologous chromosome copies and recombination between them. (Reprinted with permission from Brooker RJ: Genetics: Analysis & Principles, 3rd ed. New York: McGraw-Hill, 2008.)
In prophase I, the synaptonemal complex forms between homologous chromosomes. (a) An electron micrograph of a synaptonemal complex. (b) Diagram of the elements that make up the synaptonemal complex between chromatids. (Reprinted with permission from Brooker RJ: Genetics: Analysis & Principles, 3rd ed. New York: McGraw-Hill, 2008.)
Synapsis has at least two important functions in prophase I. First, it puts all of the copies of each linkage group into a separate cluster. This enables the cell to distribute one complete set of genetic information to each cell at the end of the first division. Synapsis keeps the homologous chromosomes together in a group. Second, there is an exchange between homologous chromosomes, called crossing over or recombination, which shuffles the alleles that are carried by the two homologues. Recombination is a powerful force in generating the enormous range of genetic variation that can be found among the offspring from each pair of parents. Since so many important events occur during prophase I, it is divided into subphases described in detail in Figure 5-10.
Events that occur during prophase I of meiosis include synapsis and recombination. (Reprinted with permission from Brooker RJ: Genetics: Analysis & Principles, 3rd ed. New York: McGraw-Hill, 2008.)
Following prophase I, the chromosomes move to the equator of the cell in metaphase I (Figure 5-11), and the homologous chromosomes are pulled to opposite poles in anaphase I. As we will see in Chapter 6, this separation, or segregation, of any genetic differences in the alleles carried by homologues is the basis of one of the fundamental Mendelian rules of genetic transmission. After a brief telophase I and cytokinesis, the second division proceeds as in mitosis, except that chromosome number is now haploid. The reduction division occurs during the first meiotic division. In humans, the result from each primary cell is four haploid sperm nuclei in spermatogenesis or one haploid egg nucleus plus three small haploid polar bodies in oogenesis (Figure 5-12).
The kinetochore microtubules from one pole are attached to only one of the chromatid pairs in a bivalent Thus, the chromosomes in a homologous pair are attached to different poles. (Reprinted with permission from Brooker RJ: Genetics: Analysis & Principles, 3rd ed. New York: McGraw-Hill, 2008.)
Comparison of: (a) spermatogenesis, which can yield four haploid sperm, and (b) oogenesis, which yields one haploid egg cell and up to three haploid polar bodies, which degenerate. (Reprinted with permission from Brooker RJ: Genetics: Analysis & Principles, 3rd ed. New York: McGraw-Hill, 2008.)
A karyotype is a picture of chromosomal makeup (Figure 5-13). Chemicals like colchicine and its synthetic equivalent colcemid will bind and unassemble the microtubules of the spindle. Without a functioning spindle, cell division is halted at prometaphase and metaphase. Their condensed shapes clearly show relative chromosome sizes and centromere placements. These characteristics can then be used to arrange pairs of homologous chromosomes into a standardized pattern called a karyotype.
Karyotypes are ways of organizing and presenting the chromosomal makeup of an individual. (a: Reprinted with permission from Brooker RJ: Genetics: Analysis & Principles, 4th ed. New York: McGraw-Hill, 2012. b: Reprinted with permission, Dr. Warren G. Sanger, University of Nebraska Medical Center.)
Additional information can come from modifying the basic staining protocols before the chromosomes are visualized microscopically. Giemsa is a polychromatic stain that darkens chromatin material uniformly. Chromosomes in dividing cells show up clearly, but modifications of technique can enhance structural detail. One example is G-banding, mentioned in Chapter 4 (Figure 4-19), which involves pretreating the chromosomes with the proteolytic enzyme trypsin before staining with Giemsa. The resulting dark bands are areas of heterochromatin, which is highly condensed chromosomal material. The intervening light bands are euchromatin. This produces a chromosome-specific pattern of dark and light bands that allows some degree of resolution for intrachromosomal changes in structure. Although small changes cannot be detected with these techniques, information about the karyotype of an individual can identify changes in chromosome number and large changes in chromosome structure that have clinical significance.
Overview of Changes in Chromosome Number and Structure
Species differ in the way their genomes are distributed among chromosomes. There is no correlation between the number of chromosomes and the developmental complexity of an organism. Likewise, genes of similar function are scattered among the chromosomes. There is no correlation between a particular chromosome and a particular body part or metabolic process. Chromosomes are simply the structures that link together and distribute genetic information from one cell generation to the next during mitosis and meiosis. Euploid is the normal chromosomal makeup of an individual (eu = true or normal; ploid = multiple). Deviations involving the loss or gain of one or more chromosomes are aneuploid, or "not true" multiples. A polyploid has "many" multiples of chromosomes, e.g., 3n triploids and 4n tetraploids. When they occur, changes in chromosome number are almost always much more serious than a single gene, or point, mutation, because so many different genes, and thus many biochemical processes, are involved. Chromosome number can also be altered by fusion or fission of the centromeric regions, although this phenomenon is typically of more importance when comparing chromosome arm homologies in related species.
Chromosome aberrations, or changes in structure, occur when the linkage of genes within and between chromosomes is altered (Figure 5-14). Changes in chromosome structure are most commonly due to breaks that are incorrectly repaired during replication. Chromosome breaks are very common. One estimate is that an average of 55,000 single-strand breaks and 9 double-strand breaks occur in DNA molecules in each nucleus each day. The vast majority of these are repaired, but if several affected strands are near each other, the broken ends can be reattached incorrectly.
Chromosome aberrations are changes in chromosome structure. (a) Deletion is the loss of a segment of chromosome. (b) Duplication is the insertion of a section of chromosome so that two copies of each affected gene are present. (c) Inversions can occur when two breakpoints are reattached at alternate ends. (d) Simple translocation is the movement of a section of one chromosome to a different linkage group. (e) Reciprocal translocation involves the exchange of sections between nonhomologous chromosomes. (Reprinted with permission from Brooker RJ: Genetics: Analysis & Principles, 3rd ed. New York: McGraw-Hill, 2008.)
Three types of aberrations can affect the genetic content of an individual chromosome. If two breaks are repaired so that the intervening segment is left out, a portion of the chromosome is no longer attached to a centromere and is lost from the nucleus the next time it divides. This yields a deletion or deficiency. Various mechanisms can cause a portion of the chromosome to be present twice, duplication. The order of genes along a chromosome can also change. For example, if two breaks in the chromosome are repaired so that alternate ends are attached, the intervening segment is now reversed, creating an inversion. In addition to the obvious changes in genetic content caused by these aberrations, especially in duplications and deficiencies, point mutations occur if the DNA breakpoints happen to be within the coding region of a gene. Furthermore, topological relationships among synapsed chromosomes in prophase I of meiosis can cause secondary consequences for the genetic composition of a fertilized egg. These are described in more detail later.
Finally, aberrations can affect more than one chromosome at a time. When a portion of one chromosome is reattached to a chromosome from a different linkage group, the result is called a translocation. Simple translocations involve the movement of a piece of one chromosome to another. When this translocated chromosome is passed to an offspring, there are extra copies of the genes carried in the translocated region. Reciprocal translocations involve the complementary exchange of segments between two nonhomologous chromosomes. If both translocated chromosomes are passed to the offspring, there is no overall change in genome content. But if only one of them is passed on, the offspring will carry an unbalanced genome content. Thus, translocations change how genes are arranged in linkage groups and can have secondary consequences due to the way the altered chromosomes segregate in meiosis. We will look at the consequences of these abnormalities in more detail next.
Aneuploidy: Errors in Segregation
Aneuploidy is a deviation from the normal chromosome complement involving less than a full haploid set of chromosomes. The breakdown of a spindle microtubule, delayed division of the centromere connecting two sister chromatids, and other events can lead to the failure of chromosomes to segregate properly to opposite poles during division. Although this can occur in both mitosis and meiosis, our focus here will be on meiotic errors.
Failure to separate, or "segregate," normally can occur at either the first or the second meiotic division (Figure 5-15). The term used to describe this kind of error, nondisjunction, is actually a double negative. "Junction" (to join) means to go together, so disjunction is not to go together, i.e., to separate. Nondisjunction is, therefore, not to separate, thus go together. The result is a gamete with either two copies or no copy of one or more chromosomes. When such a gamete combines with a normal gamete at fertilization, the resulting genome will be unbalanced by having abnormal numbers of active genes coding for their protein products. This will affect a large number of independent developmental and physiological processes. Most such affected embryos will die early. If an extra copy of a chromosome is present, there will then be three copies of that linkage group, a trisomic ("tri" is three, "som" is body). Alternatively, if the abnormal gamete has lost its copy of that chromosome, fertilization will result in only one copy of the linkage group, a monosomic ("mono" is one), coming from the normal gamete fertilizing the gamete that is deficient. In humans, most trisomics and all but one type of monosomic typically die early in development. Most special cases involve aneuploidy of the X or Y chromosomes. Since females carry two X chromosomes but males have only one, it is normal to have a difference in the copy number, or dosage, of all X-linked genes when comparing the two genders. One mechanism that compensates for this difference in copy number, X-chromosome inactivation, will be discussed next. Here we will simply point out that the mechanism that allows males and females to develop normally with different numbers of X chromosomes can also allow development to proceed fairly normally if aneuploidy of a sex chromosome occurs.
Nondisjunction is a failure of the separation of homologues at the first or second meiotic division to yield cells that are missing or have extra copies of a chromosome. (Reprinted with permission from Brooker RJ: Genetics: Analysis & Principles, 3rd ed. New York: McGraw-Hill, 2008.)
X-chromosome Inactivation in Mammals
In mammals, the number of copies of X-linked genes will differ between males (with one X chromosome) and females (with two). To balance this difference, i.e., to accomplish "dosage compensation," almost all genes on one X chromosome in a cell are inactivated (Xi) by a process called Lyonization after its discoverer, Mary Lyon. Important exceptions will be discussed later. Lyonization involves the tight coiling of all X chromosomes except for one that is left genetically active (Xa). X inactivation is permanent in the somatic cells but has to be reversible in the development of the germ cells. In summary, X inactivation occurs early in embryogenesis, it is random, and it is clonal in that, once inactivated, that same X chromosome remains inactive in somatic daughter cells.
In interphase nuclei, the inactivated X can be seen as a dark spot or Barr body, named for Murray Barr, who first described them in cells from female cats. If additional X chromosomes are present due to errors in segregation, they are also Lyonized to produce additional Barr bodies. The nucleus shown in Figure 5-16, for example, has three Barr bodies in an abnormal cell containing a total of four X chromosomes in addition to the normal 22 pairs of autosomes (2n = 48).
Three Barr bodies in an abnormal cell containing a total of four X chromosomes. Barr bodies are formed by the tight coiling, or Lyonization, of all except one X chromosome, so normal female nuclei have one Barr body.
X-chromosome inactivation is an example of epigenetic modification. In this phenomenon, a gene or in this case a chromosome becomes inactivated during an individual's lifetime. X inactivation is passed on to daughter cells during cell division, yielding patterns like the black and orange patch pattern seen in calico cats (Figure 5-17). Inactivation occurs randomly, so a female is really a patchwork of genetic expressions, a "functional mosaic," for any genes that differ between her two X chromosome copies. But in a broader context, one should be careful when using the term "mosaic," because in medical genetics it is usually reserved to describe differences in genetic makeup, not simply expression.
Calico cats are females that are heterozygous for black and orange alleles carried on randomly inactivated X chromosomes to yield patches of orange and black fur, respectively. (Courtesy of Sarah M. Granlund.)
The mechanism of X inactivation involves a limited amount of blocking factor protein that binds to an X chromosome and blocks its inactivation. All other X chromosomes are left unprotected and are inactivated. The X inactivation center (XIC) is thought to control this chromosomal silencing process by binding the blocking factor protein. Indeed, if the XIC is translocated to an autosome, that autosome will become inactivated.
Polyploidy is a change in chromosome number that involves multiples of a full haploid set. It is commonly found in plants, where it is an important mechanism for speciation. Many crop plants are polyploids derived from wild ancestors. Examples include coffee (4×, 6×, 8×), bananas (3×), bread wheat (6×), and common tobacco (4×). Agricultural geneticists can artificially induce polyploidy to combine genomes from different plant species. For some reason, polyploidy is tolerated much less well in animal development. In humans polyploidy is usually fatal at an early stage of development. Among the possible causes, polyploidy will result if more than one sperm enters the egg simultaneously or if there is a failure of separation of haploid nuclei during meiosis in the developing egg cell.
Changes in Chromosome Content: Deletions and Duplications
The terms "deletion" and "deficiency" are interchangeable and refer to the loss of a section of DNA that can range in size from simply affecting a region of one gene to encompassing tens or even hundreds of linked genes. If it is limited to a single gene, it may be difficult to tell a deletion from a nucleotide substitution or other point mutation. One way to identify them is by DNA sequencing or measuring the size of fragments amplified by the polymerase chain reaction, PCR (see Chapter 2). A small deletion will yield an amplified DNA fragment that is smaller than one from a simple base substitution mutation in which all nucleotides are present.
Figures 5-18 and 5-19 show some of the ways a change can be made in chromosome content. Typically, homozygosity for a deletion is lethal (it is equivalent to being homozygous for a large number of damaging point mutations), and heterozygosity can have severe effects on development. A deletion can also affect the phenotype if it happens to be heterozygous with a recessive mutant allele on the "normal" homologue. Since the dominant is missing in the deleted chromosome, the sole recessive allele is expressed phenotypically. This phenomenon is sometimes called pseudodominance. In experimental organisms like Drosophila, deletion mapping is a powerful tool for analyzing linkage relationships and for manipulating developmental processes.
Loss of genetic material can occur from (a) terminal deletions in which the broken end of a chromosome is lost, or (b) interstitial deletions involving two breaks and the loss of the intervening section of the chromosome. (Reprinted with permission from Brooker RJ: Genetics: Analysis & Principles, 3rd ed. New York: McGraw-Hill, 2008.)
Crossing over between two improperly aligned homologues can generate deletions and duplications by recombination. (Reprinted with permission from Brooker RJ: Genetics: Analysis & Principles, 3rd ed. New York: McGraw-Hill, 2008.)
Duplications are regions of a chromosome that occur twice, so that the diploid has a total of three copies of each gene in the duplication. The duplicated copies can be directly adjacent, called tandem duplications, or can be located at a distance from each other as dispersed duplications. On balance, a duplication generally has a smaller effect on development than does a deletion of the same size. But in both cases, deletions and duplications essentially alter the gene dosage, which changes the amount of protein produced when the genes are active. This can cause an imbalance in biochemical processes throughout the body.
Changes in Chromosome Organization: Inversions
As is true for all aberrations, there are several mechanisms that will cause a change in the order of genes on a chromosome. One way is for two simultaneous breaks to occur on portions of a chromosome that happen to be coiled next to each other in the interphase nucleus. A change in order will occur if the broken ends are misattached (Figure 5-20). If the two breaks are in different arms so that the centromere is included within the inverted region, it is called a pericentric inversion (peri = around, as in perimeter). If both endpoints of the inversion are in the same arm, it is a paracentric inversion (para = beside, as in paramedic). A pericentric inversion will change chromosome appearance if the breaks are not symmetrical around the centromere. Other than possible point mutations at the breakpoints, the genetic content of the chromosome is not altered in either type. But that does not mean that inversions are without consequences for the carrier. The consequences are expressed in a different way, specifically as a reduction in genetically normal gametes. This is due to the physical looping of one chromosome to allow its genes to pair with their homologous copies on the other chromosome in an inversion heterozygote during synapsis. This will result in chromosome anomalies if crossing over occurs within the synapsed inverted region in prophase I of meiosis (Figure 5-21).
Inversions occur when the ends of chromosome segments are reattached incorrectly. The products can be classified as (a) pericentric or (b) paracentric, depending on whether the centromere is included or not included in the inversion. (Reprinted with permission from Brooker RJ: Genetics: Analysis & Principles, 3rd ed. New York: McGraw-Hill, 2008.)
Crossing over in an inversion heterozygote yields deletions and duplications in half the haploid products of meiosis. In a paracentric inversion heterozygote, the centromere will be included in the segment that is either duplicated or deleted. This will result in dicentric bridges or acentric fragments, respectively. (Reprinted with permission from Brooker RJ: Genetics: Analysis & Principles, 3rd ed. New York: McGraw-Hill, 2008.)
As we saw earlier, at synapsis in prophase I, the synaptonemal complex forms between the identical regions of the non-sister chromatids all along the length of the paired chromosomes. The only way for that to happen in an inversion heterozygote is for one of the strands to form a loop as shown in Figure 5-21. Recombination occurs during synapsis. So, if a recombination event occurs within the inversion loop, the strands become attached so that deleted and duplicated chromosomes are produced. You can demonstrate this yourself by tracing one of the chromosome strands beginning at the top arrow of Figure 5-21a; at the recombination point, the path crosses to the other chromosome and ends at the second arrow. The result is a chromosome that is duplicated for the normal sequence to the left of the first breakpoint and deleted for the region to the right of the second breakpoint. If the inversion is paracentric (Figure 5-21b), the centromere will be duplicated in one of the crossover products yielding a dicentric chromosome that forms a bridge between the separating nuclei when the centromeres move to opposite poles in anaphase I. The other crossover product has no centromere and is considered an acentric fragment, which is left behind when the nucleus divides. Neither type of gamete will yield a viable zygote, so fertility is reduced in inversion heterozygotes.
Translocations are the main type of chromosome aberration affecting two different chromosomes at the same time. If the exchange is reciprocal, so that a fragment from the first chromosome becomes attached to the centromere-bearing second chromosome, and vice versa, the genetic content of the cell is not affected except for possible point mutations at the breakpoints as in an inversion. But, as in inversion heterozygotes, reciprocal translocation heterozygotes can suffer severe reductions in fertility due to segregation in meiosis. The mechanism behind this is illustrated in Figure 5-22 and is not as complicated as it may initially appear. It essentially comes down to how the centromeres happen to attach to the meiotic spindle.
Translocation products depend on how the homologous centromeres attach to the spindle during meiosis. (Reprinted with permission from Brooker RJ: Genetics: Analysis & Principles, 3rd ed. New York: McGraw-Hill, 2008.)
In our earlier discussion of meiosis, we pointed out that independent assortment is the result of the randomness with which different pairs of synapsed chromosomes attach to the spindle. If one bivalent is heterozygous Aa and a second bivalent is heterozygous Bb, the two sets of centromeres could attach so that the two dominant alleles are toward the same pole. In that case one gamete gets both dominant alleles, AB, and the other gets both recessive alleles. Or they could attach with the dominant-carrying strands facing opposite poles, so each gamete gets one dominant allele and one recessive, that is, they will be either Ab or aB. Think of segregation of centromeres in a reciprocal translocation heterozygote the same way.
To interpret Figure 5.22, you must imagine that the centromeres are attached to the spindle and that the poles are at the top and bottom of the figure. The left-hand portion of the figure is equivalent to one bivalent, and the right-hand side is a second bivalent. They interact because parts of their structures are synapsed to different homologues, but the key is how the centromeres attach to the spindle and segregate. Let us consider the case of adjacent segregation first (central panel of Figure 5-22). That is what will result if the centromeres are laid out as in the top part of the figure. The upper centromere of each bivalent will move to the top pole and the lower centromere of each bivalent will move the opposite direction. At the end of that first meiosis, each haploid nucleus has one normal and one translocated chromosome, thus being duplicated for some regions and deficient for others. These will not yield viable zygotes. But now imagine that the centromeres of one of the bivalents are flipped, so that the upper left centromere segregates along with the lower right centromere, and the lower left goes with the upper right. This is alternate segregation and results in one haploid having the two normal chromosomes and the other haploid getting both of the translocated chromosomes. The genetic content is balanced and the zygotes they create will each have a complete diploid genome.
An important example of this type of aberration is the Robertsonian translocation, in which two nonhomologous acrocentric or telocentric chromosomes become fused at their centromeres to produce one linkage group. This results in a reduced chromosome number that may or may not have a phenotypic effect, depending on whether any coding DNA is lost in the fusion.
The changes in chromosome makeup described so far in meiosis affect every cell in the offspring's body. But changes in chromosome number and structure can also occur during mitosis. The effect of such somatic changes will be limited to the lineage of cells that derive from the original error. They lead to an individual that is a cellular mosaic of different genotypes. Indeed, at some level, we are all probably mosaics of slight genetic differences that have occurred during our development. Most will involve point mutations of genes that may not even be transcribed in the specialized cell type in which they are found. A few will involve changes in chromosome number or structure that can cause serious medical conditions like certain cancers.
Another related phenomenon should be mentioned here. It is autonomous gene expression.
"Autonomous" means self-contained. In the context of gene expression it refers to a gene that affects only the biochemical activities of the cell in which it acts. Its gene product is not diffusible, so mutant cells in such a mosaic patch cannot be helped by normal tissue surrounding them. The phenotype for a visible trait would, therefore, be a mosaic spot or patch.
The Unique Nature of the Y Chromosome
Comparatively few genes are located on the human Y chromosome. Those that are unique to the Y, so-called holandric genes, include the Sry (sex-determining region Y) gene that is necessary for normal male-specific development. It specifies testis determination and promotes the synthesis of testosterone. In addition, a few genes are found in small areas of homology between the X and Y chromosomes called pseudoautosomal regions. These promote pairing of the X and Y to assist in proper segregation during meiosis. Like genes on the human X chromosome, those in the pseudoautosomal region of the Y can show recombination. But they do not undergo Lyonization, or X chromosome inactivation, so this small region of X and Y homology behaves like an autosomal region.