Genetics Chapter 2

Disjunction

(1) Shugoshin must be degraded, reversing its protective role (2) The cohesin complex holding the centromere region of each sister chromosome is then cleaved by separase (3) Sister chromatids of each chromosome are pulled toward the opposite poles of the cell.

Nucleus

A membrane-bound structure that houses the genetic material, DNA, which is complexed with an array of acidic and basic proteins into thin fibers. During nondivisional phases of the cell cycle, the fibers are uncoiled and dispersed into chromatin. During mitosis and meiosis, chromatin fibers coil and condense into chromosomes.

Daughter Chromatids

After disjunction occurs, each migrating chromatid is referred to as _____________________.

Plasma Membrane

All cells are surrounded by a ___________________, an outer covering that defines the cell boundary and delimits the cell from its immediate external environment. It is not passive but instead actively controls the movement of materials into and out of the cell.

Alleles

Alternative forms of the same gene.

Nucleolus

An amorphous component where ribosomal RNA is synthesized and where the initial stages of ribosomal assembly occur.

Kinetochore

An assembly of multilayered plates of proteins associated with the centromere. This structure forms on opposite sides of each paried centromere, in intimate association with the two sister chromatids.

Chromosomes

An organisms DNA, with its arrays of genes, is organized into structures called ___________________ which serve as vehicles for transmitting information.

Glycocalyx (Cell Coat)

Animal cells have a covering over the plasma membrane referred to as the ____________. Consisting of glycoproteins and polysaccharides, this covering has a chemical composition that differs from parable structures in either plants or bacteria. Provides biochemical identity at the surface of cells, and the components that establish cellular identity are under genetic control.

G0

At a point during G1, all cells follow one of two paths. They either withdraw from the cycle, become quiescent, and enter the ______________, or they become committed to proceed through G1, initiating DNA synthesis and completing the cycle. Cells that enter _______ remain viable and metabolically active but they are not proliferative. Cancer cells apparently avoid entering _____ or pass through it very quickly. Other cells enter _____ and never reenter the cell cycle. Still other cells in _______ can be stimulated to return to G1 and thereby reenter the cell cycle.

G2

By the end of this phase, the volume of the cell has roughly doubled, DNA has replicated, and mitosis is initiated.

Telocentric

Centromere is at the end

Submetacentric

Centromere is between the middle and end.

Acrocentric

Centromere is close to the end

Metacentirc

Centromere is in the middle

Prophase 1

Chromatin present in interphase thickens and coils into visible chromosomes. And, as in mitosis, each chromosome is a double structure, held together by the molecular complex called cohesin. Each homologous pair of chromosomes pair up, undergoing synapsis. Third, crossing over occurs between chromatids of synapsed homologs.

Metaphase

Chromosome configuration following migration

Zygonema

Chromosomes continue to shorten and thicken during this stage. During the process of homology search, homologous chromosomes undergo initial alignment with one another. This so-called rough pairing is complete by the end of _________________. Structures called lateral elements are visible between homologs. As meiosis proceeds, the overall length of the lateral elements along the chromosome increases, and a more extensive ultrastructural component called the synaptonemal complex begins to form between the homologs. At the end of this stage, the paired homologs are referred to as bivalents.

Anaphase 1

Cohesin is degraded between sister chromatids, except at the centromere region, which, as in mitosis, is protected by a shugoshin complex. Then, one half of each tetrad is pulled toward each pole of the dividing cell. This separation process is the physical basis of disjunction. At the end, a series of dyad equal to the haploid number is present at each pole.

Genome

Collectively, the genetic information contained in a haploid set of chromosomes constitutes the ___________________ of the species. This, of course, includes copies of all genes as well as a large amount of noncoding DNA.

Spindle Fibers

Consist of microtubules, which themselves consist of molecular subunits of the protein tubulin.

G2/M checkpoint

DNA is monitored prior to the start of mitosis. If DNA replication is incomplete or any DNA damage is detected and has not been repaired, the cell cycle is arrested.

Biparental Inheritance

Each diploid organism contains two copies of each gene as a consequence of _______________________, inheritance from two parents.

Prophase 2

Each dyad is composed of one pair of sister chromatids attached by the common centromeric region

Haploid Number (n)

Equal to one half the diploid number.

Anaphase

Events critical to chromosome distribution during mitosis occur during ____________, the shortest stage of mitosis. During this phase, sister chromatids of each chromosome, held together only at their centromere regions, disjoin frome one another (disjunction) and are pulled to opposite ends of the cell.

Reductional Division

First division in meiosis 1, when the number of centromeres, each representing one chromosome, is reduced by one half. Components of each tetrad-representing the two homologs-separate yielding two dyads. Each dyad is composed of two sister chromatids joined at a common centromere.

Receptor Molecules

Found on the surfaces of cells. Act as recognition sites that transfer specific chemical signals across the cell membrane into the cells.

Cohesin

Holds the sister chromatids together. Is originally formed between them during the S phase of the cell cycle when the DNA of each chromosome is replicated. Thus, even though we cannot see chromatids in interphase because the chromatin is uncoiled and dispersed in the nucleus, the chromosomes are already double structures, which becomes apparent in late prophase.

Early meiosis

Homologous chromosomes form pairs; that is, they synapse or undergo synapsis. Each synapsed structure, initially called a bivalent, eventually gives rise to a tetrad consisting of four chromatids. The presence of four chromatids demonstrates that both homologs have in fact duplicated. Therefore to achieve haploidy, two divisions are necessary.

Locus

Homologous chromosomes have important genetic similarities. They contain identical gene sites along their lengths; each site is called a ______________. Thus, they are identical in the traits that they influence and in their genetic potential.

Cell furrow

In animals, complete constriction of the cell membrane produces the ___________________ characteristic of newly divided cells.

Meiosis 2

Is essential if each gamete or spore is to receive only one chromatid from each original tetrad.

Cytokinesis

Is essential if two new cells are to be produced from one cell. The mechanism of this step differes greatly in plant and animal cells, but the end result is the same: two new cells are produced. In plant cells, a cell plate is synthesized and laid down across the region of the metaphase plate. Animal cells however undergo a constriction of the cytoplasm, much as a loop of sting might be tightened around the middle of a balloon.

Diplonema

It is even more apparent that each tetrad consists of two pairs of sister chromatids. Within each tetrad, each pair of sister chromatids begins to separate. However, one or more areas remain in contact where chromatids are interwined. Each such area, called a chiasma, is thought to represent a point where nonsister chromatids have undergone genetic exchange through the process referred to above as crossing over.

Prokaryotic Organisms

Lack a nuclear envelope and membranous organelles.

Mitosis

Leads to the production of two cells, each with the same number of chromosomes as the parent cell. The portion of the cell cycle during which hereditary components are equally partitioned into daughter cells.

Q arm

Longer arm below the centromere

G1/S checkpoint

Monitors the size the cell has achieved since its previous mitosis and also evaluates the condition of the DNA. If the cell has not reached an adequate size or if the DNA has been damaged, further progress through the cycle is arrested until these conditions are "corrected." If both conditions are normal, then the cell is allowed to proceed from G1 to the S phase of the cycle.

Zygotes

Multicellular diploid organisms begin life as single-celled fertilized eggs called _____________. The mitotic activity of the __________________ and the subsequent daughter cells is the foundation for the development and growth of the organism.

Separase

Once properly attached to the spindle fibers, cohesin is degraded by an enzyme, appropriately named _________________, and the sister chromatid arms disjoin, except at the centromere region.

Microtubules

Originate and "grow" out of the two centrosome regions at opposite poles of the cell. They are dynamic structures that lengthen and shorten as a result of the addition or loss of polarized tubulin subunits.

Prophase

Over half of mitosis is spent in this phase. This stage is characterized by several significant occurrences. The migration of two pairs of centrioles to opposite ends of the cell. After migration, the centrosomes, where the centrioles are localized, are responsible for organizing cytoplasmic microtubules into the spindle fibers that run between these poles, creating an axis along which chromosomal separation occurs. As the centrioles migrate, the nuclear enevelope begins to break down and gradually disappears. In a similar fashion, the nucleolus disintegrates within the nucleus. While these events are taking place, the diffuse chromatin fibers have begun to condense, until distinct threadlike structures, the chromosomes, become visible. It becomes apparent near the end of prophase that each chromosome is actually a double structure split longitudinally except at a single point of constriction, the centromere.

Cell Wall

Plant cells have an outer covering called the _________________ whose major component is a polysaccharide called cellulose.

Cytokinesis in plant cells

Plant cells, which are more regularly shaped and structurally rigid, require a mechanism for depositing new cell wall material around the plasma membrane. The cell plate laid down during telophase beceoms a structure called the middle lamellae. Subsequently, the primary and secondary layers of the cell wall are deposited between the cell membrane and middle lamella in each of the resulting daughter cells.

Nucleolus Organizer Region (NOR)

Portions of DNA that encode rRNA are collectively referred to as _______________.

Sugoshin

Protects cohesin from being degraded by separase at the centromeric region.

Meiosis

Reduces the genetic content and the number of chromosomes by precisely half. This reduction is essential if sexual reproduction is to occur without doubling the amount of genetic material in each new generation. Part of a special type of cell division that leads to the production of sex cells: gametes or spores.

Prometaphase

Referes to the period of chromosome movement

Crossing Over

Results in genetic exchange between members of each homologous pair of chromosomes. This process creates intact chromosomes that are mosaics of the maternal and paternal homologs from which they arise, further enhancing the potential genetic variation in gametes and the offspring derived from them.

Telophase 1

Reveals a nuclear membrane forming around the dyads. In this case, the nucleus next enters into a short interphase period. If interphase occurs, the chromosomes do not replicate because they already consist of two chromatids. In other organisms the cells go directly from anaphase 1 to meiosis 2.

Telophase 2

Reveals one member of each pair of homologous chromosomes present at each pole. Each chromosome is now a monad.

Equational Division

Second division that occurs during meiosis 2, the number of centromeres remains equal. Here each dyad splits into two monads of one chromosome each.

P arm

Shorter arm above the centromere

G1, S, G2, M

Steps of the cell cycle

Spermatogenesis

Takes place in the testes. The process begins with the enlargement of an undifferentiated diploid germ cell called spermatogonium. This cell grows to become a primary spermatocyte, which undergoes the first meiotic division. The products of this division, called secondary spermatocytes, contain a haploid number of dyads. The secondary spermatocytes then undergo meiosis 2, and each of these cells produces two haploid spermatids. Spermatids go through a series of developmental changes, spermiogensis, to become highly specialized, motile spermatozoa, or sperm. All sperm cells contain the haploid number of chromosomes and equal amounts of cytoplasm.

Metaphase 2

The centromeres are positioned on the equatorial plate.

Pachynema

The chromosomes continue to coil and shorten, and further development of the synaptonemal complex occurs between the two members of each bivalent. This leads to synapsis, a more intimate pairing. Homologs are now separated by only 100 nm. Each homolog is now evident as a double structure, providing visual evidence of the earlier replication of the DNA of each chromosome. Thus, each bivalent contains four chromatids. As in mitosis, replicates are called sister chromatids, whereas chromatids from maternal and paternal members of a homologous pair are called non-sister chromatids. The four membered structure, also referred to as a tetrad, contains two pairs of sister chromatids.

Metaphase 1

The chromosomes have maximally shortened and thickened. The terminal chiasmata of each tetrad are visible and appear to be the major factor holding the nonsister chromatids together. Each tetrad interacts with spindle fibers, facilitating its movement to the metaphase plate. The alignment of each tetrad prior to the first anaphase is random: half of the tetrad will subsequently be pulled to one or the other pole, and the other half moves to the opposite pole.

First Polar Body

The dyads at one pole are pinched off with very little surrounding cytoplasm. It may or may not divide again to produce two small haploid cells.

Cell Cycle

The events that occur from the completion of one division until the completion of the next division constitute the __________________.

M checkpint

The final checkpoint occurs during mitosis and is called ___________________. Here, both the successful formation of the spindle fiber system and the attachment of spindle fibers to the kinetochores associated with the centromeres are monitored. If spindle fibers are not properly formed or if attachment is inadequate, mitosis is arrested.

Telophase

The final stage of mitosis. At its beginning, two complete sets of chromosomes are present, one set at each pole. The most significant event of this stage is cytokinesis, the division or partitioning of the cytoplasm.

Diakinesis

The final stage of prophase 1. The chromosomes pull farther apart, but nonsister chromatids remain loosely associated at the chiasmata. As separation proceeds, the chiasmata move toward the ends of the tetrad. This process of terminalization begins in late diplonema and is completed during diakinesis. During this final substage, the nucleolus and nuclear envelope break down, and the two centromeres of each tetrad attack to the recently formed spindle fibers. By the completion of prophase 1, the centromeres of each tetrad structure are present on the metaphase plate of the cell.

Oogenesis

The formation of ova, or eggs occurs in the ovaries. The daughter cells resulting from the two meiotic divisions of this process receive equal amounts of genetic material, but they do not receive equal amounts of cytoplasm. Instead during each division, almost all the cytoplasm of the primary oocyte, itself derived from the oogonium, is concentrated in one of the two daughter cells. The concentration of cytoplasm is necessary because a major function of the mature ovum is to nourish the developing embryo following fertilization.

Karyokinesis

The genetic material is partitioned into daughter cells during nuclear division, or _______________. This process is quite complex and requires great precision. The chromosomes must first be exactly replicated and then accurately partitioned. The end result is the production of two daughter nuclei, each with a chromosome composition identical to that of the parent cell.

Interphase

The initial stage of the cell cycle, as the interval between divisions. Characterized by the absence of visible chromosomes.

Leptonema

The interphase chromatin material begins to condense, and the chromosomes though still extended, become visible. Hemology search, which precedes and is essential to the initial pairing of homologs, begins during leptonema.

Centromere

The location of this establishes the general appearance of each chromosome.

Equatorial Plate (Methaphase plate)

The midline region of the cell, a plane that lies perpendicular to the axis established by the spindle fibers.

Secondary Oocyte

The other daughter cell produced by this first meiotic division contains most of they cytoplasm.

Eukaryotic Organism

The presence of a nucleus and other membranous organelles is the defining characteristic of _______________.

S phase

The replication of the DNA of each chromosome; the period during which DNA is synthesized, occurs before the cell enters mitosis and is called _________________. The initiation and completion of synthesis can be detected by monitoring the incorporation of radioactive precursors into DNA.

Sister Chromatids

The two parts of each chromosome are called ___________________ because the DNA contained in each of them is genetically identical, having formed from a single replicative event.

Chromomeres

They are along each chromosome, and are localized condensations that resemble beads on a string.

Kinetochore Microtubules

They are most directly responsible for chromosome migration, make contact with and adhere to, kinetochores as they grow from the centrosome region. They have one end near the centrsome region (at one of the poles of the cell) and the other end anchored to the kinetochore.

Aptonemal Complex

This complex is believed to be the vehicle responsible for proper alignment during the pairing of homologs. In some diploid organisms, this synapsis occurs in a zipper like fashion, beginning at the ends of chromosomes attached to the nuclear envelope.

Cytokinesis

This less complex process requires a mechanism that partitions the volume into two parts, then encloses each new cell in a distinct plasma membrane. As the cytoplasm is reconstituted, organelles replicate themselves, arise from existing membrane structures, or are synthesized anew in each cell.

G1 and G2

Two periods during interphase when no DNA synthesis occurs, one before and one after the S phase. During both of these intervals, as well as during S, intensive metabolic activity, cell growth, and cell differentiation are evident.

Molecular Motors

Use the energy generated by the hydrolysis of ATP. Their effect on the activity of microtubules serves ultimately to shorten the spindle fibers, drawing the chromosomes to opposite ends of the cell. The centromeres of each chromosome appear to lead the way during migration, with the chromosome arms trailing behind. Several models have been proposed to account for the shortening of spindle fibers. They share in common the selective removal of tubulin subunits at the ends of the spindle fibers.

Chromatin

When cells are not undergoing division, the genetic material making up chromosomes unfolds and uncoils into a diffuse network within the nucleus, generally referred to as _______________.

Nondisjuntion

When errors in meiosis occur and separation is not achieved.

Fertilization

When gametes combine to reconstitute the diploid complement found in parental cells.

Homologous Chromosomes

When the lengths and centromere placements of all such chromosomes are examined, a second general feature is apparent. With the exception of sex chromosomes, they exist in pairs with regard to these two properties, and the members of each pair are called _____________________. In sexually reproducing organisms, one member of each pair is derived from the maternal parent (through the ovum) and the other member is derived from the paternal parent (through the sperm).

Anaphase 2

When the shugoshin complex is degraded, the centromeres separate and the sister chromatids of each dyad are pulled to opposite poles.