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The nucleus is a membrane-bound compartment isolating the genetic material from the rest of the cell cytoplasm.
Most of this book deals with the genetics of eucaryotes. Bacteria will be considered in Chapter 12 The cells of a multicellular organism seldom look alike or carry out identical tasks. Thus there is no such thing as a typical cell type. Any subcellular structure that has a characteristic morphology and function is considered to be an organelle. Diagram of an animat cell the organelles such as the nucleus and mitochondria are membrane-bound; others such as the ribosomes and centrioles are not enclosed by a membrane.
Most organelles and other cell parts are too small t0 be seen with the light microscope, bur they can be studied with the electron microscope. The characteristics of organelles and other parts of eucaryotic cells are outlined in Table 1. In higher organisms, each somatic cell any body cell exclusive of sex cells contains one set of chromosomes inherited from the maternal female parent and a comparable set of chromosomes ho- mologous chromosomes or homologues from the paternal male parent.
The number of chromosomes in this dual set is called the diploid 2n number. Sex cells, or gametes, which contain half the number of chromosome sets found in somatic cells, are referred to as haploid cells n. A genome is a set of chromosomes corresponding to the haploid set of a species.
The number of chromosomes in each somatic cell is the same for all members of a given species. For example, human somatic cells contain 46 chromosomes, tobacco has 48, cattle 60, the garden pea 14, the fruit fly 8, etc.
A chromosome with a median centromere metacentric will have arms of approximately equal size. A submetacentric, or acrocentric, chromosome has arms of distinctly unequal size. The shorter arm is called the p arm and the longer arm is called the arm. If a chromosome has its centromere at or very near one end of the chromosome, it is called telocentric. Each chromosome of the genome with the exception of sex chromosomes is numbered consec according to length, beginning with the longest chromosome first.
Autosomes vs. Sex Chromosomes. In the males of some species, including humans, sex is associated with a morphologically dissimilar heteromorphic pair of chromosomes called sex chromosomes.
Such a chromosome pair is usually labeled X and Y. Genetic factors on the Y chromosome determine maleness. Females have two mor- phologically identical X chromosomes. The members of any other homologous pairs of chromosomes homologues are morphologically indistinguishable, but usually are visibly different from other pairs nonhomologous chromosomes.
All chromosomes exclusive of the sex chromosomes are called auto- somes. All somatic cells in a multicellular organism are descendants of one original cell, the fertilized egg, or zygote, through a divisional process called mitosis Fig. The function of mitosis is first to construct an exact copy of each chromosome and then to distribute, through division of the original mother cell.
Interphase is the period between successive mitoses Fig. The double-helix DNA molecule Fig. I , producing an identical pair of DNA molecules. Mitosis in animal cells. Dark chromosomes are of matemal origin: light chromosomes are of paternal origin, One pair of homologues is metacentric. A mitotic division has four major phases: prophase, metaphase, anaphase, and telophase, Within a chromosome, the centromeric regions of each chromatid remain closely associated through the first two phases of mitosis by an unknown mechanism perhaps by specific centromeric-binding proteins , a Prophase.
In prophase, the chromosomes condense, becoming visible in the light microscope first as thin threads, and then becoming progressively shorter and thicker. Chromosomes first become visible in the light microscope during prophase.
The thin chromatin strands undergo condensation Fig. A toy airplane can be used as a model to explain the condensation of the chromosomes. A rubber band, fixed at one end, is attached to the propeller at its other end, As the prop is tured, the rubber band coils and supercoils on itself, becoming shorter and thicket in the process. Something akin to this process occurs during the condensation of the chromo. At the next- higher level of condensation, the beaded string spirals into a kind of cylinder.
The cylindrical structure then folds back and forth on itself. Thus, the interphase chromosome becomes condensed several hundred times its length by the end of prophase see Fig.
By late prophase, a chromosome may be sufficiently condensed to be seen in the microscope as con- isting of two chromatids connected at their centromeres. During prophase, each pair of replicated centrioles migrates toward opposite polar regions of the cell and establishes a microtubule organizing center MTOC from which a spindle-shaped network of microtubules called the spindle develops.
Two kinds of spindle fibers are recognized. Kinetochore microtubules extend from a MTOC to a kinetochore.
A kinetochore is a fibrous, multiprotein structure attached to centromeric DNA. Polar microtubules extend from a MTOC tosome distance beyond the middle of the cell, overlapping in this middle region with similar fibers from the opposite MTOC.
By late prophase, the nuclear membrane has disappeared and the spindle has fully formed. Late prophase is a good time to study chromosomes e. Such treated cells cannot proceed to metaphase until the colchicine is removed. Its hypothesized that during metaphase a dynamic equilibrium is reached by kinetochore fibers from different MTOCS tugging in different directions on the joined centromeres of sister chro- matids.
This process causes each chromosome to move to a plane near the center of the cell, a position designated the equatorial plane or metaphase plate. Near the end of metaphase, the concentration of calcium ions increases in the cytosol.
Perhaps this is the signal that causes the centromeres of the sister chromatids to dissociate. Anaphase is characterized by the separation of chromatids. According to one theory, the kinetochore microtubules shorten by progressive loss of tubulin subunits, thereby causing former sister chromatids now recognized as individual chromosomes because they are no longer connected at their centromeres to migrate toward opposite poles.
According to the sliding filament hypothesis, with the help of proteins such as dynein and kinesin, the kinetochore fibers slide past the polar fibers using a ratchet mechanism analogous to the action of the proteins actin and myosin in contracting muscle cells. As each chromosome moves through the viscous cytosol, its arms drag along behind its centromere, giving it a characteristic shape depending upon the location of the centromere, Metacentric chromo- somes appear V-shaped, submetacentric chromosomes appear J-shaped, and telocentric chromosomes appear rod-shaped.
In telophase, an identical set of chromosomes is assembled at each pole of the cell. The chromosomes begin to uncoil and return to an interphase condition.
The spindle degenerates. In animals. The two products of mitosis are called daughter cells or progeny cells and may or may not be of equal size depending upon where the plane of cytokinesis sections the cell.
Thus while there is no assurance of equal distribution of cytoplasmic components to daughter cells, they do contain exactly the same type and number of chromosomes and hence possess exactly the same genetic constitution.
The time during which the cell is undergoing mitosis is designated the M period. The times spent in each phase of mitosis are quite different. Prophase usually requires far longer than the other phases; metaphase is the shortest. DNA replication occurs before mitosis in what is termed the S synthesis phase Fig. In nucleated cells, DNA synthesis starts at several positions on each chromosome, thereby reducing the time required to replicate the sister chromatids.
A long G, phase pre-DNA synthesis follows mitosis and precedes chromosomal replication. Interphase includes G,, S, and G2. The four phases M, Gj , S, Gz constitute the life cycle of a somatic cell. The lengths of these phases vary considerably from one cell type to another.
Diagram of a typical cell reproductive cycle 2. Sexual reproduction involves the manufacture of gametes gametogenesis and the union of a male and a female gamete fertilization to produce a zygote.
Male gametes are sperms and female gametes are eggs, or ova ovum, singular. Gametogenesis occurs only in the specialized cells germ line of the reproductive organs gonads. In animals, the testes are male gonads and the ovaries are female gonads. Gametes contain the haploid number n of chromosomes, but originate from diploid 2n cells of the germ line. The number of chromosomes must be reduced by half during gametogenesis in order to maintain the chromosome number characteristic of the species.
This is accomplished by the divisional process called meiosis Fig. Meiosis involves a single DNA replication and two divisions of the cytoplasm. The first meiotic division meiosis 1 is a reductional division that produces two haploid cells from a single diploid cell. Meiosis in plant cells mitotislike, in that sister chromatids of the haploid cells are separated.
Each of the iwo meiotic divisions consists of four major phases prophase. The DNA replicates during the interphase preceding meiosis I; it does not replicate between telophase [ and prophase Il. The prophase of meiosis I differs from the prophase of mitosis in that homologous chromosomes come to lie side by side in a pairing process called synapsis. Each pair of synapsed chromosomes is called a bivalent 2 chromosomes. Each chromosome consists of two identical sister chromatids at this stage.
During synapsis nonsister chromatids one from each of the paired chromosomes of a tetrad may break and reunite at one or more corresponding sites in a process called crossing over. The point of exchange appears in the microscope as a cross-shaped figure called a chiasma chiasmata, plural.
Thus, at a given chiasma, only two of the four chromatids cross over in a somewhat random manner. Generally, the number of crossovers per bivalent increases with the length of the chromosome. By chance, a bivalent may experience 0, 1, or multiple crossovers, but even in the longest chromosomes the incidence of multiple chiasmata of higher numbers is expected to become progressively rare. It is not known whether synapsis occurs by pairing between strands of two different DNA molecules or by proteins that complex with corresponding sites on homologous chromosomes.
It is thought that synapsis occurs discontinuously or intermittently along the paired chromosomes at positions where the DNA molecules have unwound sufficiently to allow strands of nonsister DNA molecules to form specific pairs of their building blocks or monomers nucleotides. A ribbonlike structure called the synaptonemal complex can be seen in the electron microscope between paired chromosomes. It consists of nu- cleoprotein a complex of nucleic acid and proteins.
A few cases are known in which synaptonemal complexes are not formed, but then synapsis is not as complete and crossing over is markedly reduced or eliminated.
By the breakage and reunion of nonsister chromatids within a chiasma, linked genes become recombined into crossover-type chromatids; the two chromatids within that same chiasma that did not exchange segments maintain the original linkage arrangement of genes as noncrossover- or parental-type chromatids.
A chiasma is a cytological structure visible in the light microscope Crossing over is usually a genetic phenomenon that can be inferred only from the results of breeding experiments, Prophase of meiosis I may be divided into five stages. During leptonema thin-thread stage , the Jong, thin, attenuated chromosomes start to condense and, asa consequence, the first signs of threadlike structures begin to appear in the formerly amorphous nuclear chromatin material.
During zygonema oined-thread stage , synapsis begins. In pachynema thick-thread stage , synapsis appears so tight that it becomes difficult to distinguish homologues in a bivalent. This tight pairing becomes somewhat relaxed during the next stage called diplonema double-thread stage so that individual chromatids and chiasmata can be seen.
Finally, in diakinesis the chromosomes reach their maximal condensation, nucleoli and the nuclear membrane disappear, and the spindle apparatus begins to form. During metaphase I, the bivalents orient at random on the equatorial plane. At anaphase 1, the centromeres do not divide, but continue to hold sister chromatids together. Because of crossovers, sister chromatids may no longer be genetically identical. Homologous chromosomes separate and move to opposite poles; i.
This is the movement that will reduce the chromosome number from the diploid 2n condition to the haploid n state. Cytokinesis in telophase I divides the diploid mother cell into 2 haploid daughter cells. This ends the first meiotic division. Depending on the species, interkinesis may be brief or continue for an extended period of time. Duringan extensive interkinesis, the chromosomes may uncoil and return toan interphaselike condition with reformation of a nuclear membrane.
At some later time, the chromosomes would again condense and the nuclear membrane would disappear. Nothing of genetic importance happens during inter- kinesis, The DNA does nor replicate during interkinesis!
In prophase II, the spindle apparatus reforms. By metaphase II, the individual chro- mosomes have lined up on the equatorial plane. During anaphase II, the centromeres of each chromosome divide, allowing the sister chromatids to be pulled apart in an equational division mitotislike by the spindle fibers.
Cytokinesis in telophase II divides each cell into 2 progeny cells Thus, a diploid mother cell becomes 4 haploid progeny cells as a consequence of a meiotic cycle meiosis and meiosis II.
The characteristics that distinguish mitosis from meiosis are summarized in Table 1. Characteristics of Mitosis and Meiosis Mitosis Meiosis 1, An equational division that separates sister chro- matids 2. One division per cycle, i. Covers statistics, probability, chemical equilibrium, acid-base reactions, precipitates, complex ion equilibria, titrations, phase separations, radioactivity, and chromatography.
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