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Chapter 19 - Cell Cycles of Eukarya
- John Ringo, University of Maine, Orono
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Summary
Overview
In eukarya, as in bacteria and archaea, single cells reproduce asexually via a cell cycle. The cell cycle of eukarya, though, is more complex and elaborately regulated. The large, linear chromosomes of eukarya, whose DNA is packaged in chromatin, are replicated once per cycle, and copies are apportioned to daughter cells with great accuracy in a nuclear division phase, mitosis. The phases of the cell cycle and the main points of cell-cycle regulation are described in this chapter.
Keeping Track of Chromosome Number
Ploidy refers to the number of chromosome sets per cell nucleus. A euploid cell has an integer multiple of chromosome sets. Normal sex cells are haploid, having one set of chromosomes; sex cells unite to make zygotes, diploid cells with two sets of chromosomes.
In sexually reproducing eukarya, the haploid chromosome number of a species is N, and the diploid number is 2N. A gamete has N chromosomes, and most somatic cells of plants and animals have 2N chromosomes. Some cells, though, are polyploid – i.e., they have three or more sets of chromosomes. For example, in humans N = 23; a sperm has 23 chromosomes, a skin cell has 46 chromosomes, and a liver cell has 92 or more chromosomes. Mitosis works the same no matter what the cell's ploidy.
However – and this is important – the cell's ploidy normally does not change with the phase of the cell cycle, even though the number of copies of the genome does change.
Chapter 15 - Micromutations
- John Ringo, University of Maine, Orono
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No life form is a perfect machine, and from generation to generation, a genome may be subject to change – mutation. A mutation is any newly arisen, stably inherited alteration of a life form's genome. A mutation is not a transitory change, and it is not the same as damage, although damage to DNA may lead to mutation. A mutation may not impact the functioning or even the structure of gene products, but if it does, the effects can range from beneficial to fully lethal.
Micromutations are small, affecting at most a single gene and usually involving fewer than ~103 nucleotides. In this chapter, a classification of micromutations is followed by descriptions of their causes and effects. Large mutations – chromosomal abnormalities – are sufficiently different from micromutations that they are taken up later (Chapter 21).
Types of Micromutation
The five kinds of small mutation are substitution, deletion, insertion, duplication, and inversion. Most mutations are point mutations (single nucleotide change), and the most frequent kind of point mutation is a nucleotide substitution, also commonly called a base substitution. Micromutations can occur anywhere in a chromosome.
Substitution
Substitution is nucleotide replacement (Figure 15.1). There are two types of nucleotide substitution, transition and transversion. A transition is substitution of a purine nucleotide for a purine nucleotide or substitution of a pyrimidine nucleotide for a pyrimidine nucleotide. A transversion is a substitution of a purine nucleotide for a pyrimidine nucleotide, or vice versa. Substitutions occur throughout chromosomes.
Chapter 30 - Genomics
- John Ringo, University of Maine, Orono
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Genomics, the newest branch of genetics, is the study of genome structure and function: massive genome-wide mapping, determination of primary nucleotide sequence for whole genomes, analysis of spatial relationships of various sequences or classes of sequence within and between chromosomes, genomic inventory by the sequence or gene class, and global analysis of gene expression. Genomics emphasizes genes over nontranscribed, nonregulatory sequences. A major challenge in genomics is the analysis of very large amounts of information.
Genome Cloning
The first step in genomic analysis is construction of a fully representative, high-quality genomic library. A large, pure sample of the life form of interest is collected and treated physically to separate genomic DNA from other components of the life form. The DNA is extracted chemically, purified, and cleaved. The fragments are cloned in a suitable vector, commonly cosmid, bacteriophage P1, BAC, or YAC (Chapter 27). To ensure that the library contains overlapping clones that span the entire genome, the DNA is digested partially, and the cloned segments comprise a large random sample, typically an average of 10 to 30 copies per sequence. A set of overlapping, cloned, sequenced DNA segments is called a contig, because the sequence of the region spanned by the segments has no gaps (Figure 30.1). In genome sequencing, it is ideal to render each chromosome a contig – an array of fragments covering the chromosome's entire DNA molecule.
Chapter 29 - Genetically Engineered Organisms
- John Ringo, University of Maine, Orono
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A plant or animal that is genetically altered by the artificial introduction of foreign genes is transgenic, for genes have been transferred from one genome to another. Donor DNA is usually recombined in vitro with a vector before being introduced into recipient cells. The donor DNA may reside temporarily in the transgenic recipient, or it may become a permanent part of the recipient's genome, where it is stably inherited. Any species can be a recipient when workable methods of gene transfer are developed. It is also possible to inject mRNA into a recipient for transient expression there.
Transgenic organisms are useful for studying gene expression, phenotypic effects of mutations, and gene interactions as well as for discovering novel genes. Agricultural crop plants are modified genetically with the aim of improving the yield and quality of food and fiber. Gene therapy is the genetic modification of human cells to treat genetic disease; its enormous potential has yet to be realized.
Transient Gene Expression
Experiments with transient gene expression are useful for finding out what a protein does and determining the function of parts of the protein by inducing mutations in the gene that encodes it. The transgenic organism is a testing ground.
It often suffices to express a transgene or mRNA in vivo for a short time. This works if the foreign protein functions in transgenic cells and can be assayed quickly.
Chapter 34 - Locating Genes
- John Ringo, University of Maine, Orono
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Genetics often entails “detective work.” Detectives spend a lot of effort locating suspects, and geneticists spend a lot of effort locating genes. Finding a gene and putting it on a map has several applications:
To determine that a genetic factor maps to a point on a chromosome (Did one character do the deed, or is somebody in cahoots?)
To aid in further study of a gene
To help diagnose a human genetic disorder
This chapter tells about genetic, chromosomal, and physical maps. Genetic maps are based on genetic linkage. Chromosome maps place genes relative to chromosome landmarks. Physical maps are based on DNA sequencing.
Recombination and Chromosome Exchanges
General recombination occurs when paired homologous chromosomes undergo exchanges, or crossovers. Homologous chromosomes recombine in pachynema, the third stage of prophase I of meiosis. General recombination rarely happens in cells undergoing mitotic cycles, but that is not taken up here.
At any chromosome locus (location; a gene's address), a crossover is a random event – random in the sense that it is unpredictable and not predetermined. The farther apart two loci, the greater the chance for an exchange between them. Therefore, the probability of an exchange is positively correlated with distance. The probability of recovering a recombinant chromosome, r, measures genetic distance; express r as a percentage. One map unit separates two linked genes if 1% of the recovered chromosomes are recombinant for these two genes.
Chapter 40 - Quantitative Genetics
- John Ringo, University of Maine, Orono
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In natural populations of sexually reproducing organisms, individuals differ phenotypically in a quantifiable way. Most variable phenotypes of interest (e.g., size, fertility, and longevity) can be measured on a continuous scale and are influenced by many genes and a host of environmental factors. Usually, no single gene is a major source of natural variation for a phenotype (major gene), but it is possible to assess the magnitude and nature of the multigenic causes of natural variation for that phenotype, where many genes, each with a tiny effect, act together.
The goal of quantitative genetics is to measure the relative contributions of genotype and environment to phenotypic variation in a population of organisms, breaking down phenotypic variance into genetic and environmental components. The heritability of a trait is the fraction of phenotypic variance due to genes for a particular population of organisms in a particular environment. This genetic variation can be subdivided into components, notably additive effects, dominance, and epistasis. A further component of the environment is genotype-environment interaction. Natural selection can bring about evolutionary change in a trait if and only if a population has additive genetic variation for that trait. Genes that underlie natural quantitative variation for a phenotype can be mapped genetically to quantitative trait loci (QTL).
Quantitative Models
For any quantitative phenotype, there is usually natural genetic variation. Most of the genes contributing to this variation remain anonymous, as the effects of any one of them on quantitative variation are too small to observe directly, even though the concerted effects of many genes are big enough to measure.
Chapter 41 - Speciation
- John Ringo, University of Maine, Orono
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For all the sexually reproducing eukarya, the fundamental taxonomic unit is the species (s. and pl.). Biological species are populations of organisms between which reproduction is prevented biologically, by reproductive isolating barriers.
The birth of new species is speciation, during which one species splits into two reproductively distinct new species. The death of species is extinction. Speciation and extinction have produced, over the past two billion years, earth's diverse assemblage of extant species of eukarya, numbering ~107. The species of today are perhaps at most 1% of all species that have ever lived. Speciation is an inevitable by-product of genetic evolution. The exact genetic events that cause speciation are under intensive investigation.
Species Concepts
Phenotypic or Typological Species Concept
In the phenotypic concept, members of one species may vary quantitatively in form but are clearly distinct from members of another species. Individuals within species are considered variants of the same type. The phenotype is usually morphological, but it can be molecular. The Linnaean system of classification and binomial nomenclature for species (e.g., Homo sapiens) is purely phenotypic.
The main difficulty of this concept is that there is no evidence for the reality of a “true type,” from which individuals deviate, even though it is commonplace for species to appear distinct.
Biological Species Concept
According to this concept, biological species are reproductively isolated from each other. This concept applies only to sexually reproducing eukarya.
Chapter 7 - RNA Synthesis 1: Transcription
- John Ringo, University of Maine, Orono
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Every gene has a single role: to encode RNA via transcription. Transcription is the synthesis of a single strand of RNA – a transcript – whose nucleotide sequence is complementary to a portion of a gene. As molecular processes go, transcription is highly regulated, moderately accurate, and, once started, fast-paced. Transcription is performed by a small troop of large proteins, chief among which is RNA polymerase. RNA polymerase uses the template strand of DNA to synthesize a complementary strand of RNA in a 5′ to 3′ direction.
Transcription is a play in four acts: (1) binding of RNA polymerase to a gene's promoter, (2) initiation of the RNA chain, (3) elongation of the RNA chain, and (4) termination of transcription.
In many cases, RNA synthesis does not end with transcription, because some transcripts are processed into mature RNAs by enzymatic modifications. The next chapter describes posttranscriptional processing.
Polymerization Reaction
The key chemical reaction in RNA polymerization is an esterification, in which the α phosphate of ribonucleotide triphosphate (NTP) is added to the 3′ oxygen of the growing polynucleotide chain, yielding a lengthened chain plus diphosphate. The substrates for the reaction are the four NTPs: ATP, GTP, CTP, and UTP. Synthesis of an RNA molecule begins at its 5′ end, proceeds by adding one nucleotide at a time, and stops at its 3′-OH end. RNA polymerase catalyzes the polymerization reaction (Figure 7.1).
Chapter 16 - Repair of Altered DNA
- John Ringo, University of Maine, Orono
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Every cell's genome is vulnerable to change. DNA-modifying chemicals and electromagnetic radiation continually damage it, and the inherent inaccuracy of DNA synthesis erodes its perfect replication. There are two main lines of defense against DNA damage: prevention and repair. Preventive mechanisms (e.g., natural sunscreens, enzymatic degradation of mutagens) go beyond the scope of this book. The enzymes of DNA repair and their modes of action are described here.
Systems of Repair
Every organism has multiple systems to repair premutational alterations to DNA – both damage and mispaired nucleotides. For example, Drosophila has nearly 90 DNA repair enzymes. Systems of repair, some of which are universal among organisms and some of which are not, fall into broad categories:
Direct repair – the sugar-phosphate backbone remains intact (this is the only nonsurgical option for repairing DNA damage)
Repair of apurinic and apyrimidinic sites (AP repair) – excision of a short sequence of one strand containing an AP site (nucleotide missing its base)
Mismatch repair – a region containing a mismatched base is removed and replaced, usually repairing a newly synthesized strand.
Excision repair – uses exinucleases, which are endonucleases that nick a single strand of DNA on both sides of an altered site
Recombination repair – uses recombination to fill gaps in DNA
End-joining repair – joins double-strand breaks in DNA
Direct Repair
The two kinds of repair of altered nucleotides, which does not involve breaking the sugar-phosphate backbone of DNA, are photoreactivation of pyrimidine dimers and dealkylation of alkylated bases.
Chapter 37 - Genetic Variation in Populations
- John Ringo, University of Maine, Orono
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Natural populations of most plants and animals are genetically diverse, and a sizeable fraction of their genes have two or more common alleles. Population genetics is about the frequencies of alleles in populations and how these allele frequencies change from generation to generation. This chapter describes the following:
The relation between allele frequencies and genotype frequencies in populations
The magnitude of genetic variation in populations, and how it is measured
As with Mendelian theory and experiments, in population genetics one should focus attention first and foremost on gametes and haplotypes.
Populations
A population is a group of organisms of one species, living in one area. An asexual population is a clone, while a population of sexually reproducing, freely interbreeding eukarya is sometimes called a deme. In diplontic species, population size is defined as the number of all diploid individuals in that population, N. The copies of a gene in a population comprise a gene pool, and the size of the gene pool for diplontic species is 2N.
Predicting Genotype Frequencies from Allele Frequencies
Populations in Static Equilibrium
Consider gene A, an autosomal gene with two alleles, A1 and A2; the gametes in this population carry either A1 or A2. The frequency of A1 gametes is p and the frequency of A2 gametes is q, where p + q = 1.
Contents
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Chapter 3 - Proteins
- John Ringo, University of Maine, Orono
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Proteins are linear polymers of amino acids. Genes encode all proteins, and proteins perform essential roles in all genetic processes, including the synthesis of DNA, RNA, and proteins. Some proteins bind to DNA and RNA, and some proteins are enzymes that act on nucleic acids. Some proteins bind to specific nucleotide sequences, but others bind equally well to any nucleotide sequence. This chapter describes the main points of protein size and structure.
Amino Acids
Protein is a generic term for a linear polymer made of amino acids as well as an aggregate of these polymers. An amino acid is a small carboxylic acid with an amino group and a side group that defines it. There are hundreds of different amino acids, but only 22 that are known to be genetically encoded, and two of these – selenocysteine and pyrrolysine – are found in only a handful of proteins. The molecular masses of the 20 common, genetically encoded amino acids range between 75 and 204 Da (Figure 3.1). Notice that amino acids are smaller than nucleotides.
Peptides
A peptide is a short polymer of amino acids, usually 30 amino acids or fewer. Adjacent amino acids in peptides are held together by a peptide bond (Figure 3.2) – a covalent bond between the carboxyl carbon atom of one amino acid and the core amino nitrogen atom of the other. A polypeptide is a large polymer of amino acids, usually 100 amino acids or more.
Chapter 8 - RNA Synthesis 2: Processing
- John Ringo, University of Maine, Orono
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In many cases, newly transcribed RNA is not ready for duty; instead, it is a precursor of mature RNA and must undergo posttranscriptional RNA processing to become functional. Also known as RNA maturation, posttranscriptional processing falls into seven categories:
cleaving and trimming
3′ nucleotide addition
base modification
splicing
capping (modifying the 5′ end of pre-mRNA)
polyadenylation (adding a sequence of adenosines to the 3′ end of pre-mRNA)
editing (altering the coding sequence of pre-mRNA)
The exact nature of every maturation process depends on the life form and the type of RNA (rRNA, tRNA, mRNA). This chapter describes the seven types of processing in turn, noting differences among life forms. At chapter's end, some processing pathways are summarized.
Cleaving and Trimming
Some fresh transcripts contain extraneous sequences, and some contain two or more sequences that need to be separated. Endonucleases may cleave transcripts into smaller pieces to remove end sequences or internal spacers. In some instances, exonucleases trim the ends, removing nucleotides one by one.
Large rRNAs are encoded by genes whose transcripts are cleaved in multiple steps to yield smaller RNAs. In bacteria, each of the six copies of the rRNA gene contains 16S, 23S, and 5S rRNA sequences and one or two tRNAs; the functional and nonfunctional sequences are interspersed (Figure 8.1). Processing by ribonucleases (RNases) begins before transcription is complete.
Chapter 6 - Genome Content
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This chapter inventories classes of DNA sequences in the genomes of organisms, mitochondria, chloroplasts, and viruses.
Part of an organism's genome consists of functional sequences – genes, regulatory sequences, telomeres, centromeres, and origins of replication. However, genomes also include DNA sequences that either are clearly nonfunctional or appear to be nonfunctional. The genomes of bacteria and archaea are mostly free of nonfunctional sequences, but, in many eukaryal genomes, the junkyard is larger than the portion that encodes RNA.
Some genes are present in multiple, identical copies. There are also sets of functionally related, homologous genes, called gene families. Nonfunctional DNA sequences may also be present in multiple copies. Repeated DNA sequences are often arranged in tandem.
What Is a Gene?
Dear reader, I have bad news for you: geneticists cannot agree on what a gene is, even though we do agree that genes are fundamental biological objects. Worse still, gene can change its meaning with context. Though this situation wants a strong remedy, none is available. The best I can offer you is a simple, natural concept of gene, contrasted with widely used, alternative concepts.
Consistently in this book, a gene is defined as a chromosomal segment of nucleic acid that encodes an RNA transcript (a freshly synthesized piece of RNA), along with nearby regulatory sequences needed to initiate RNA synthesis. This applies even when the transcript codes for several peptides or polypeptides.
Chapter 22 - Life Cycles of Eukarya
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Sexually reproducing organisms alternate between haploid and diploid phases during one sexual generation, which extends from haploid phase to haploid phase (for animals and plants, from egg to egg).
During a sexual cycle, diploid cells produce haploid cells by meiosis, and haploid cells and their nuclei produce diploid cells by syngamy (fusion). Sexual life cycles are broadly classified according to whether mitotic nuclear division cycles occur in the diploid phase, the haploid phase, or both. These three types are as follows:
Haplontic – predominantly haploid; mitosis does not occur in the diploid phase
Diplontic – predominantly diploid; mitosis does not occur in the haploid phase
Haplodiplontic – mitosis occurs in both the haploid and diploid phases
Eukarya also reproduce asexually. The kinds of asexual reproduction are classified at the end of the chapter. Parthenogenesis, an asexual mode of reproduction that evolved from sexual reproduction, is given special attention.
Evolutionary Considerations
There are wags who never tire of asking children, “Which came first, the chicken or the egg?” While this nonsensical question has no answer, it leads to a valid evolutionary question, “What was the relationship between haploidy and diploidy in ancient eukarya?” It is reasonable to assume that mitosis evolved before meiosis, at the beginnings of eukarya, and that the earliest eukarya were monoploid. Perhaps monoploid cells fused from time to time, yielding diploid cells.
Chapter 17 - Reproduction of Bacteria
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At each turn of a cell cycle, a bacterium or archaeon converts molecules taken from the environment into its own components and divides into two daughter cells. Each daughter cell receives a copy of the genome as well as copies of plasmids and nongenetic components. Mitochondria and chloroplasts proliferate by a similar division process, and, although they are organelles within eukarya, they reproduce rather than being made de novo by the cell. A population of genetically identical life forms made by this process is a clone.
Under poor or stressful environmental conditions, some bacteria form spores – specialized resting cells – by the process of sporulation. A few species of bacteria develop into multicellular organisms containing different cell types. Both sporulation and the formation of multicellular bacteria are simple examples of cell differentiation, which happens as a result of regulated changes in gene expression.
Bacterial Reproduction
Cell Cycle
The cell cycle of bacteria has two phases, C (chromosome copying) and D (division). On average, cells double in size between divisions. The cell cycle is tightly regulated and division is precise, ensuring that the chromosomes, cytoplasm, and cell envelope are synthesized at the right rate and are equally apportioned between the two daughter cells. During the C phase the chromosome(s) and plasmids replicate, and components of the cytoplasm such as ribosomes, tRNAs, and enzymes are approximately doubled in number. During the D phase, the cell envelope grows, daughter chromosomes segregate (move apart), a contractile ring pinches the cell in two, and a septum forms between the incipient daughter cells.
Chapter 25 - Sex Determination and Dosage Compensation
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Most animal species and many plants are dioecious, meaning that there are two sexes, female and male. Sexual identity develops by genetically regulated pathways, but not by genes alone is any trait determined. Two principles are at work here. First, an environmental or genetic trigger channels development in a male or female direction. Second, the trigger initiates a cascade of gene-environment interactions that ends in the expression of mature sexual traits.
Species with XX females and XY males have a problem: both sexes have two copies of each autosomal gene, but females have twice as many copies of X-linked genes as do males. Dosage compensation solves the problem.
This chapter surveys mechanisms of sex determination and then compares both sex determination and dosage compensation in mammals and Drosophila.
Triggers for Sex Determination
Sex determination can be initiated by environmental factors, including temperature, chemical signals between individuals, and amount of light.
The developmental trigger in some fish and reptiles is incubation temperature. In some turtles, eggs incubated at above 30°C develop into females; eggs incubated at cooler temperatures develop into males. In other turtles, females develop at extreme incubation temperatures and males at intermediate incubation temperatures. Perhaps temperature-sensitive transcription factors bind to the enhancers or promoters of sex-determining genes.
Sex is determined by short-range chemical signals in some mollusks (e.g., the slipper limpet) and a marine worm Bonellia.
Chapter 4 - Simple Chromosomes
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Every life form's genetic material is packaged in one or more chromosomes. Its genome is the nucleic acid in one complete set of chromosomes, excluding nonessential ones. This chapter is about the composition, size, shape, and number of chromosomes in bacteria, archaea, mitochondria, chloroplasts, and viruses – all but the eukarya. The nucleic acid of chromosomes is double stranded DNA, except for a few single-stranded DNA plasmids and except for some viruses.
Bacteria and Archaea
Bacteria and archaea have two kinds of chromosomes, essential chromosomes, which are required for the survival and normal functioning of the cell, and plasmids, which are not absolutely necessary for survival and reproduction. Most bacteria and archaea whose genomes have been analyzed have only one essential chromosome, and in nondividing (nonreproducing) cells there is one copy of that chromosome. The number of different types of plasmids per cell varies from zero to several, and the number of copies of each plasmid ranges from 1 to ~102, depending on the plasmid. Usually, essential chromosome is simply referred to as chromosome.
In most bacteria the genome is located in an amorphous region, the nucleoid (Figure 4.1), which takes up from a quarter to half the cell's volume. It is not known whether plasmids are also restricted to the nucleoid. The parts of the chromosome where DNA replication begins and ends are attached to the cell's membrane. Unlike the nucleus of a eukaryal cell, a membrane does not bound the nucleoid region.
Chapter 21 - Chromosomal Abnormalities
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Most mutations are clearly either very small, involving <103 nucleotides, or big, involving many genes or whole chromosomes – chromosomal abnormalities are rearrangements (deletion, duplication, inversion, or translocation), or altered numbers of chromosomes (aneuploidy and polyploidy). Here, “chromosomal abnormality” means a mutation affecting two or more genes. This chapter deals with the causes and consequences of chromosome mutations in eukarya.
Chromosomal Rearrangement
Chromosomal rearrangements arise more rarely than do micromutations. Furthermore, having two identical copies of a chromosome rearrangement in a diploid genome is often lethal. Deletions and duplications are extremely rare in natural populations, but, surprisingly, inversions and translocations are relatively common in many species of plants and animals. The evolutionary basis of this seeming paradox is, in a nutshell, that diploid individuals having one rearranged copy and one normal copy of a chromosome often have a reproductive advantage (e.g., higher survival or fertility). Such a reproductive advantage leads to an increase in the population frequency of the chromosomal rearrangement (Chapter 39).
Rearrangements usually result from an exchange following two or more double-strand breaks in physically close chromosomes. Ionizing radiation, transposons, or oxidation by free radicals can induce double-strand breaks.
Deletion (Deficiency)
A chromosomal deletion is the loss of a segment of the chromosome (Figure 21.1). Large deletions, sometimes called chromosomal deficiencies, result from two double-strand breaks followed by loss of the segment between the breaks and a rejoining of the outside pieces.
Chapter 36 - Cytoplasmic Inheritance
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Inherited cytoplasmic factors, unlike nuclear genes, do not segregate or assort regularly in sexual reproduction. This category includes organelle genes, infectious agents, and mRNAs. Extranuclear genes reside in mitochondria, chloroplasts, and cytoplasmic intracellular parasites. Prions infect somatic cells of animals and thereafter are inherited cytoplasmically. This chapter focuses on the modes of cytoplasmic inheritance and methods of analysis.
Mitochondria and Chloroplasts
Characteristics of Mitochondria and Chloroplasts
The mitochondrion is the site of oxidative metabolism in eukarya. There are two membranes separated by a space; the innermost space of the mitochondrion is the matrix. Initial oxidation of fatty acids happens between the two membranes; the oxidation of acetyl-CoA, the citric acid cycle, and β-oxidation of fatty acids take place inside the matrix. The electron transport system and the enzymes of oxidative phosphorylation reside in the inner membrane. The number of mitochondria per cell may be 1 to ~105, depending on cell type and species.
Mitochondria reproduce clonally, by splitting. The mitochondrion has a small chromosome (14 to 2500 kb), present in multiple copies because its replication is not tightly coordinated with organelle division. Chromosomes segregate into daughter mitochondria. Mitochondria have their own machinery for protein synthesis, but nuclear genes encode most mitochondrial proteins.
Mitochondria are distributed to daughter cells during cell division in a haphazard way. Suppose a mutation arises in one of the many mitochondria in an actively cycling cell.