Chapter 7

sister chromatids.

After DNA replication, each chromosome has two DNA molecules, known as sister chromatids Each of a pair of newly replicated chromatids.

cytokinesis

Cell seperation

Animal cells

Cytokinesis usually begins with a furrowing of the cell membrane, as if an invisible thread were cinching the cytoplasm between the two nuclei. This contractile ring is composed of microfilaments of actin and myosin, which form a ring on the cytoplasmic surface of the cell membrane. These two proteins interact to produce a contraction pinching the cell in two.

reproductive signals:

External cues that stimulate cells to divide or organisms to reproduce.

Metaphase

In metaphase the chromosomes line up at the midline of the cell (the equatorial position).

Interphase

Interphase is characterized by an intact nuclear envelope, nucleoli, and a barely discernable tangle of chromatin. During the S phase of interphase, the nucleus replicates its DNA and centrosomes.

7.4

Meiosis Halves the Nuclear Chromosome Content and Generates Diversity

recombinant:

Pertaining to an individual, meiotic product, or chromosome in which genetic materials originally present in two individuals end up in the same haploid complement of genes.

7.5

Programmed Cell Death Is a Necessary Process in Living Organisms

two reasons for apoptosis in animals

The cell is no longer needed by the organism. For example, before birth, a human fetus has weblike hands, with connective tissue between the fingers. As development proceeds, this unneeded tissue disappears as the cells undergo apoptosis in response to specific signals. The longer cells live, the more prone they are to genetic damage that could lead to cancer. This is especially true of epithelial cells on the surface of an organism, which may be exposed to radiation or toxic substances. Such cells normally die after only days or weeks and are replaced by new cells.

cell cycle checkpoints:

Points of transition between different phases of the cell cycle, which are regulated by cyclins and cyclin-dependent kinases (Cdk's). Each CDK has its own cyclin to activate it, and the cyclin is made only at the right time. After the CDK acts, the cyclin is broken down by a protease

There are three types of microtubules in the spindle:

Polar microtubules overlap in the middle region of the cell and keep the two poles apart. Astral microtubules interact with proteins attached to the cell membrane, and also assist in keeping the poles apart. Kinetochore microtubules attach to the kinetochores on the chromosomes. The two sister chromatids in each chromosome become attached to kinetochore microtubules from opposite sides of the cell. This ensures that the two chromatids will move to opposite poles.

necrosis

Premature cell death caused by external agents such as toxins. occurs when cells are damaged by mechanical means or toxins, or are starved of oxygen or nutrients. These cells often swell up and burst, releasing their contents into the extracellular environment. This process often results in inflammation

Zygote

The fertilized egg. The cell created by the union of two gametes, in which the gamete nuclei are fused. The earliest stage of the diploid generation.

Gamete

The mature sexual reproductive cell: the egg or the sperm.

Cytokinesis.

The process of cytokinesis in plant cells (which have cell walls) is different than in animal cells (which do not have cell walls). We describe both processes below.

In order for any cell to divide, the following events must occur:

There must be one or more reproductive signals. These signals initiate cell division and may originate from either inside or outside the cell. DNA replication (i.e., replication of the genetic material) must occur so that each of the two new cells will have a full complement of genes to complete cell functions. The cell must distribute the replicated DNA to each of the two new cells. This process is called DNA segregation. The cytoplasm must divide to form the two new cells, each surrounded by a cell membrane and a cell wall in organisms that have one. This process is called cytokinesis.

DNA segregation.

This is much more complicated than in prokaryotes, because first, there is a nuclear envelope, and second, there are multiple chromosomes. When a cell divides, one copy of each chromosome must end up in each of the two new cells—for example, each new somatic cell in a human will have all 46 chromosomes. In eukaryotes, the pairs of newly replicated chromosomes are initially attached to one another. They become highly condensed, and then the pairs are pulled apart before segregating into two new nuclei. The cytoskeleton is involved in this process

growth factor:

A chemical signal that stimulates cells to divide.

Tumor suppressors

A gene that codes for a protein product that inhibits cell proliferation; inactive in cancer cells. Tumor suppressors are negative regulators of the cell cycle in normal cells, but in cancer cells they are inactive.

oncogene:

A gene that codes for a protein product that stimulates cell proliferation. Mutations in oncogenes that result in excessive cell proliferation can give rise to cancer. Oncogene proteins are positive regulators of the cell cycle in cancer cells

Homologous pair

A pair of matching chromosomes made up of a chromosome from each of the two sets of chromosomes in a diploid organism. One chromosome of each pair comes from the organism's female parent, and the other comes from its male parent. For example, in humans with 46 chromosomes, 23 come from the mother and 23 from the father, with, for example, a chromosome 1 from each parent, and so

centriole

A paired organelle that helps organize the microtubules in animal and protist cells during nuclear division.

apoptosis

A series of genetically programmed events leading to cell death.

Somatic cells

All the cells of the body that are not specialized for reproduction.

spindle

Array of microtubules emanating from both poles of a dividing cell during mitosis and playing a role in the movement of chromosomes at nuclear division. Named for its shape.

Eukaryotic cells divide by mitosis followed by cytokinesis

As in prokaryotes, cell division in eukaryotes entails reproductive signals, DNA replication, DNA segregation, and cytokinesis. Some of the details, however, are quite different:

caspases

Both internal and external signals lead to the activation of a class of enzymes called caspases in animals or of a functionally similar class of enzymes in plants. These enzymes hydrolyze target proteins i

7.3

Cell Reproduction Is Under Precise Control

section 7.2

Cell division by either binary fission or mitosis produces two genetically identical cells. This is the basis of asexual reproduction in single-celled organisms: prokaryotes reproduce by binary fission, and single-celled eukaryotes reproduce by mitosis. In multicellular organisms, mitosis is a way to build tissues and organs during development and to repair damaged tissues once development is complete.

DNA Replication

Chromosome replication takes place as the DNA is threaded through a "replication complex" of proteins near the center of the cell. Replication begins at the ori site and moves toward the ter site. When replication is complete, the two daughter DNA molecules separate and segregate from one another at opposite ends of the cell. In rapidly dividing prokaryotes, DNA replication occupies the entire time between cell divisions.

Meiosis

Division of a diploid nucleus to produce four haploid daughter cells. The process consists of two successive nuclear divisions with only one cycle of chromosome replication. In meiosis I, homologous chromosomes separate but retain their chromatids. The second division meiosis II, is similar to mitosis, in which chromatids separate. I.E. a process of cell division (described in Concept 7.4) resulting in daughter cells with only half the genetic material of the original cell. During meiosis, the genetic material is randomly separated and reorganized so that the daughter cells differ genetically from one another. resulting in genetic diversity

Anaphase

During anaphase, the centromere pairs separate, and the new chromosomes (each containing one member of one of the sets of paired chromatids) begin to move towards the poles.

daughter chromosomes

During mitosis, the separated chromatids from the beginning of anaphase onward.

Prometaphase

During prometaphase, the nuclear envelope breaks down, and kinetochore microtubules appear and connect the kinetochores with the centrosomes.

tetrad:

During prophase I of meiosis, the association of a pair of homologous chromosomes or four chromatids.

Condensed Chromosomes

During prophase the chromosomes become much more tightly coiled and condensed. After DNA replication, each chromosome has two DNA molecules, known as sister chromatids. Until they are separated during anaphase (see below), the chromatids are held together at a region called the centromere. During prophase the chromosomes become so compact that they can be seen clearly with a light microscope after staining with special dyes

Prophase

During prophase, the chromatin coils and supercoils, becoming more and more compact and eventually condensing into visible chromosomes. The chromosomes consist of identical, paired chromatids.

Fertilization

During sexual reproduction, two haploid gametes fuse to form a zygote in a process called fertilization.

Telophase

During telophase, the separating chromosomes reach the poles. Telophase passes into the next interphase as the nuclear envelopes and nucleoli re-form and the chromatin becomes diffuse.

Chromosome separation and movement are highly organized

During the next three phases of mitosis—prometaphase, metaphase, and anaphase—dramatic changes take place in the cell and the chromosomes

Reproductive Signals

External factors such as environmental conditions and nutrient concentrations are common reproductive signals for prokaryotes. For example, the bacterium Bacillus subtilis can divide every 30 minutes under ideal conditions. But when nutrients in its environment are low, it stops dividing. It then resumes dividing when conditions improve.

nondisjunction:

Failure of sister chromatids to separate in meiosis II or mitosis, or failure of homologous chromosomes to separate in meiosis I. Results in aneuploidy.

There are three checkpoints during interphase and one during mitosis:

G1 checkpoint is triggered by DNA damage. S checkpoint is triggered by incomplete replication or DNA damage. G2 checkpoint is triggered by DNA damage. M checkpoint is triggered by a chromosome that fails to attach to the spindle.

Haploid

Having a chromosome complement consisting of just one copy of each chromosome; designated 1n or n.

Diploid

Having a chromosome complement consisting of two copies (homologs) of each chromosome. Designated 2n

As noted above, meiosis consists of two nuclear divisions, meiosis I and meiosis II. Two unique features characterize meiosis I:

Homologous chromosomes come together and line up along their entire lengths. No such pairing occurs in mitosis. The homologous chromosome pairs separate, but the individual chromosomes, each consisting of two sister chromatids, remain intact. (The chromatids will separate during meiosis II.)

INDEPENDENT ASSORTMENT

In addition to crossing over, meiosis provides a second source of genetic diversity. It is a matter of chance which member of a homologous pair goes to which daughter cell at anaphase I

Anaphase

In anaphase the chromatids separate, and the daughter chromosomes move away from each other toward the poles.

During crossing over in meiosis I, chromatids from homologous chromosome pairs break and rejoin. Occasionally this can happen between non-homologous chromosomes. The result is a translocation,

In genetics, a rare mutational event that moves a portion of a chromosome to a new location, generally on a nonhomologous chromosome.

Metaphase

In metaphase, the centromere regions connecting paired chromatids become aligned in a plane at the cell's equator.

Plant cells

In plant cells, the cytoplasm divides differently because plants have cell walls. As the spindle breaks down after mitosis, vesicles derived from the Golgi apparatus appear along the plane of cell division, roughly midway between the two daughter nuclei. The vesicles are propelled along microtubules by the motor protein kinesin and fuse to form a new cell membrane. At the same time they contribute their contents to a cell plate, which is the beginning of a new cell wall between the two daughter cells

Prometaphase

In prometaphase the nuclear envelope breaks down and the compacted chromosomes, each consisting of two chromatids, attach to the kinetochore microtubules.

interphase

In the cell cycle, the period between successive nuclear divisions during which the chromosomes are diffuse and the nuclear envelope is intact. During interphase the cell is most active in transcribing and translating genetic information. Interphase has three subphases called G1, S, and G2 (the G stands for gap). G1 is quite variable, and a cell may spend a long time in this phase carrying out its specialized functions. The cell's DNA is replicated during S phase (S for synthesis). During G2, the cell makes preparations for mitosis—for example, by synthesizing components of the microtubules that will move the segregating chromosomes to opposite ends of the dividing cell.

Meiosis

Meiosis consists of two nuclear divisions that reduce the number of chromosomes to the haploid number. Although the nucleus divides twice during meiosis, the DNA is replicated only once. Unlike the products of mitosis, the haploid cells produced by meiosis are genetically different from one another and from the parent cell.

Meiosis vs Mitosis

Meiosis involves two cell divisions, the first of which is very different from the single division of mitosis. Meiosis II is similar to mitosis, in that the centromeres separate during anaphase, allowing the chromatids of the two homologous pairs to separate into four daughter chromosomes that are genetically distinct from the parental chromosomes.

Cytokinesis is the division of the cytoplasm

Mitosis refers only to the division of the nucleus. Cytokinesis, the division of the cell's cytoplasm, is the final stage of cell reproduction. This process occurs differently in plants and animals.

Mitosis

Nuclear division in eukaryotes leading to the formation of two daughter nuclei, each with a chromosome complement identical to that of the original nucleus.

Telophase

The final phase of mitosis or meiosis during which chromosomes become diffuse, nuclear envelopes re-form, and nucleoli begin to reappear in the daughter nuclei.

Prophase

The first stage of nuclear division, during which chromosomes condense from diffuse, threadlike material to discrete, compact bodies. three structures that appear during prophase and contribute to the orderly segregation of the replicated DNA: the condensed chromosomes, the reoriented centrosomes, and the spindle.

asexual reproduction

The formation of new individuals without the union of genetic material from two different parents. Often takes place by budding or fragmentation. The offspring are genetically identical to their parent, (clones).

The cell cycle is controlled by cyclin-dependent kinases

The kinases involved in cell cycle regulation are called cyclin-dependent kinases (CDKs). They catalyze the phosphorylation of target proteins that regulate the cell cycle. As their name implies, CDKs are activated by binding to the protein cyclin. This binding changes the shape of a CDK such that its active site is exposed, and is an example of allosteric regulation

centrosome

The major microtubule organizing center of an animal cell.

crossing over:

The mechanism by which linked genes undergo recombination. In general, the term refers to the reciprocal exchange of corresponding segments between two homologous chromatids.

karyotype

The number, forms, and types of chromosomes in a cell.

Centomere

The region where sister chromatids join.

cell division:

The reproduction of a cell to produce two new cells. In eukaryotes, this process involves nuclear division (mitosis) and cytoplasmic division (cytokinesis).

cell cycle:

The stages through which a cell passes between one division and the next. Includes all stages of interphase and mitosis. Mitosis is the set of processes in which the chromosomes become condensed and then segregate into two new nuclei. Cytokinesis usually follows immediately after mitosis, and these two stages—mitosis and cytokinesis—are referred to as M phase. M phase is followed by a much longer period called interphase, when the cell nucleus is visible and typical cell functions occur—including DNA replication in cells that are preparing to divide.

To understand the process of meiosis and its specific details, it is useful to keep in mind the overall functions that meiosis has evolved to serve:

To reduce the chromosome number from diploid to haploid To ensure that each of the haploid products has a complete set of chromosomes To generate genetic diversity among the products (gametes)

DNA replication.

Unlike prokaryotes, eukaryotes have more than one chromosome. But the replication of each eukaryotic DNA molecule is similar to replication in prokaryotes, in that it is achieved by threading the long strands through replication complexes (see Concept 9.2). DNA replication occurs only during a specific stage of the cell cycle

Reproductive signals.

Unlike prokaryotes, eukaryotic cells do not constantly divide whenever environmental conditions are adequate. In fact, most cells in a multicellular organism are specialized and do not divide. In a eukaryotic organism, the signals for cell division are usually not related to the environment of a single cell, but to the function of the entire organism. We will discuss the signals that control eukaryotic cell division in

Although the details vary widely, organisms have two basic strategies for reproducing themselves:

asexual reproduction and sexual reproduction.

In most prokaryotic cells, almost all of the genetic information is carried on one single chromosome. In many cases the ends of the single DNA molecule are covalently joined, making the chromosome circular. Two regions of the prokaryotic chromosome play functional roles in cell reproduction:

ori: the site where replication of the circular chromosome starts (the origin of replication) ter: the site where replication ends (the terminus of replication)

Chromosomes

the DNA in eukaryotic cells is organized into multiple structures called chromosomes. Each chromosome consists of a double-stranded molecule of DNA and associated proteins.

Meiosis cycle

Prophase 1 - The first stage of the first meiotic division (meiosis I), during which genetic reassortment takes place. Can be very long occupying up to 90% of the entire duration of meiosis. Prophase 2 - The first, brief stage of the second meiotic division (meiosis II), during which condensed chromosomes are visible. Prometaphase 1 - The second stage of the first meiotic division (meiosis I), during which the nuclear envelope breaks down, allowing microtubule access to chromosomes. Prometaphase 2 - The second stage of the second meiotic division (meiosis I), during which microtubules attach to chromosomes. Metaphase 1 - The third stage of the first meiotic division (meiosis I), during which chromosomes align at the center of the cell by way of microtubule force. Metaphase 2 - The third stage of the second meiotic division (meiosis II), during which chromosomes align at the center of the cell by way of microtubule force. Anaphase 1 - The fourth stage of the first meiotic division (meiosis I), during which maternal and paternal homologous pairs are separated on microtubules. Anaphase 2 - The fourth stage of the second meiotic division (meiosis II), during which either maternal or paternal sister chromatids are separated on microtubules. Telophase 1 - The fifth and final stage of the first meiotic division (meiosis I), during which chromosomes arrive at the poles of the cell and begin to recondense. Telophase 2 - The fifth and final stage of the second meiotic division (meiosis II), during which chromosomes arrive at the poles of the cell, the nuclear envelope begins to reform, and the chromosomes begin to recondense.

DNA Segregation

Replication begins near the center of the cell, and as it proceeds, the ori regions move toward opposite ends of the cell (FIGURE 7.4). DNA sequences adjacent to the ori region bind proteins that are essential for this segregation. This is an active process, since the binding proteins hydrolyze ATP. Components of the prokaryotic cytoskeleton are involved in the segregation process. In particular, a bacterial protein that is structurally related to actin but functionally related to tubulin provides a filament along which the ori regions and their associated proteins move.

sexual reproduction

Reproduction in which the genes of two individuals are combined to produce offspring, typically involving the union of male and female gametes. can result in offspring with considerable genetic variation.

Binary fission

Reproduction of a prokaryote by division of a cell into two comparable progeny cells.

kinetochores

Specialized structure on a centromere to which microtubules attach.

Cytokinesis

The actual division of a single cell and its contents into two cells begins immediately after chromosome segregation. Initially, there is a pinching in of the cell membrane caused by the contraction of a ring of fibers on the inside surface of the membrane (similar to a drawstring on shorts being tightened). In this case, the major component of these fibers is structurally similar to eukaryotic tubulin (which makes up microtubules), but its function is analogous to that of actin in the contractile ring of an animal cell (see below). As the membrane pinches in, new cell wall materials are deposited, which finally separate the two cells.