Figure 6.5
During mitosis, the nucleus divides, paving the way for two cells to be produced after cell division, each with a complete makeup of genetic material. has an excellent animation of mitosis.
Prophase is the first and longest phase of mitosis. During prophase, the DNA coils up into visible chromosomes, each made up of two sister chromatids held together by the centromere. The nucleus disappears as the nuclear envelope and nucleolus break apart. The centrioles begin to move to opposite ends, or poles, of the cell. As the centrioles migrate, the fiber-like spindle begins to elongate between the centrioles. The spindle is a thin, cage-like structure made out of microtubules. In plant cells, the spindle forms without centrioles. The spindle plays an essential role moving chromosomes and in the separation of sister chromatids.
During metaphase the spindle attaches to the centromere of each chromosome. Helped by the spindle, the chromosomes line up at the center, or equator, of the cell, also known as the metaphase plate. Each sister chromatid is attached to a separate spindle fiber, with one fiber extending to one pole, and the other fiber extending to the other pole. This ensures that the sister chromatids separate and end up in distinct cells after cell division.
Anaphase is the phase in which the sister chromatids separate. The sister chromatids are pulled apart by the shortening of the microtubules of the spindles, similar to the reeling in of a fish by the shortening of the fishing line. One sister chromatid moves to one pole of the cell, and the other sister chromatid moves to the opposite pole. At the end of anaphase, each pole of the cell has a complete set of chromosomes, identical to the amount of DNA at the beginning of G1 of the cell cycle.
Telophase is essentially the opposite of prophase. The chromosomes begin to unwind in preparation to direct the cell’s metabolic activities. The spindle begins to break down, allowing a new nucleus to form. This is followed by cytokinesis, the division of the cytoplasm, resulting in two genetically identical cells, ready to enter G1 of the next cell cycle. The phases of mitosis are summarized in Figure below.
Figure 6.6
Mitosis. The phases of mitosis are depicted. The second phase, metaphase, is shown with the chromosomes lined up at the equator of the cell and the microtubule spindle fibers extending from the centrioles to the centromeres of the chromosomes.
Cytokinesis (Figure below) differs between plant and animal cells. In animal cells, the plasma membrane pinches inward along the cell’s equator until two cells are formed. In plant cells, a cell plate forms along the cells equator. A new membrane grows along each side of the cell plate, with a new cell wall forming on the outside of each new membrane.
Figure 6.7
Cytokinesis. In this electron micrograph of a cell, two formation of two new cells is almost complete, as new membrane grows and divides the parent cell.
Control of the Cell Cycle
How does the cell know when to divide? How does the cell know when to replicate the DNA? The answers to these questions have to do with the control of the cell cycle. But how is the cell cycle controlled?
The cell cycle is controlled by a number of protein-controlled feedback processes. Two types of proteins involved in the control of the cell cycle are kinases and cyclins. Cyclins activate kinases. Cyclins are a group of proteins that is rapidly produced at key stages in the cell cycle. Kinases activate other target molecules. It is this precise regulation of proteins that triggers advancement through the cell cycle.
The cell cycle has key checkpoints. When the cell receives key signals or information (feedback regulation), the cell can begin the next phase of the cell cycle. The cell can also receive signals that delay passage to the next phase of the cell cycle. These signals allow the cell to complete the previous phase before moving forward. Three key checkpoints are the cell growth (G1) checkpoint, the DNA synthesis (G2) checkpoint, and the mitosis checkpoint.
The cell growth (G1) checkpoint allows the cell to proceed into the S phase of the cell cycle and continue on to divide. The cell spends most of the cycle in the G1 phase. G1 is where the cell carries out its main functions. If the cell has performed its functions and has grown to significant size to be divided in half, key proteins will stimulate DNA replication to begin. If the cells are not to divide, such as some muscle and nerve cells, the cell will stop at this checkpoint and move into a resting phase. Some cells may stay in this resting period permanently, never dividing.
The DNA synthesis (G2) checkpoint determines if the cell is ready for mitosis. DNA repair enzymes check the replicated DNA at this point. If the checkpoint is passed, the many molecular mechanisms and processes needed for mitosis will begin.
The mitosis checkpoint determines the end of one cycle and the beginning of the next. This checkpoint signals the end of mitosis, allowing the cell to prepare for the beginning of G1 of the next cell cycle.
Cancer and the Cell Cycle
Many cancers result from uncontrolled cell division, when the regulation of the cycle is lost (Figure below). Cancerous cells divide much more rapidly than healthy cells. These cells use the blood and nutrients that other cells need and they can stress the environment of the healthy cells. As cancerous cells do not provide any useful function to the organism, they are extremely harmful. If cancerous cells are allowed to grow uncontrolled, they will kill the host organism. Many cancerous cells are the products of normal cells that have lost the ability to regulate the cell cycle. The genes that encode the proteins involved in cell cycle regulation have mutations. One category of genes, called oncogenes, accelerate the cell cycle. Many cancers can be inherited, such as breast cancer. Others are triggered by an environmental stimulus, such as through the relationship between tobacco smoke and lung cancer, or ultraviolet radiation and skin cancer.
Figure 6.8
When normal cells are damaged beyond repair, they are eliminated. (A) diagrams damaged cells being destroyed. Cancer cells avoid elimination and, because of uncontrolled cell division, continue to multiply in an unregulated manner. (B) depicts damaged cells dividing in an uncontrolled fashion.
Lesson Summary
The cell cycle is a repeating series of events, characterizing the life of a eukaryotic cell.
Binary fission is a form of cell division in prokaryotic organisms that produces identical offspring.
As a eukaryotic cell prepares to divide, the DNA and associated proteins coil into a structure, known as a chromosome.
The DNA copies during the S phase of the cell cycle, resulting in a chromosome that consists of two identical chromatids, known as sister chromatids, attached at a region called the centromere.
Any cell containing two sets of chromosomes is said to be diploid; the zygote forms from the fusion of two haploid gametes.
The cell cycle has five phases: the first growth (G1) phase, the synthesis (S) phase, the second growth (G2) phase, mitosis, and cytokinesis.
Mitosis is the division of the nucleus; four distinct phases of mitosis have been recognized: prophase, metaphase, anaphase, and telophase.
Cytokinesis is the division of the cytoplasm.
The cell cycle is controlled through feedback mechanisms.
Cancer results from uncontrolled cell division, due to the loss of regulation of the cell cycle.
Review Questions
How does cell division in bacteria differ from mitosis in eukaryotes?
Describe the structure of a chromosome in prophase of mitosis.
What is cytokinesis and when does it occur?
What is a centromere?
Describe interphase.
Describe the main steps of mitosis.
What is binary fission?
Define a gene.
Further Reading / Supplemental Links
http://www.genome.gov
http://www.cellsalive.com/mitosis.htm
http://www.cellsalive.com/cell_cycle.htm
http://biology.clc.uc.edu/courses/bio104/mitosis.htm
http://nobelprize.org/educational_games/m
edicine/2001
Vocabulary
autosomes
Chromosomes that are not directly involved in determining the sex of an individual.
binary fission
Asexual reproduction in prokaryotic organisms; produces identical offspring.
cancer
Disease that can result from uncontrolled cell division, when the regulation of the cycle is lost.
cell cycle
A repeating series of events, during which the eukaryotic cell carries out its necessary functions, including metabolism, cellular growth, and division, resulting in two genetically identical daughter cells.
cell division
Process of cell formation from the division of older cells.
cell plate
Forms during cytokinesis in plant cells; a new membrane grows along each side of the cell plate, with a new cell wall forming on the outside of each new membrane.
centriole
Structure from which spindle fibers originate.
centromere
Region that attaches two sister chromatids; approximately near the middle of a chromosome.
chromosome
Coiled structure of DNA and histone proteins; allows for the precise separation of replicated DNA; forms during prophase of mitosis and meiosis.
cyclins
A group of proteins that is rapidly produced at key stages in the cell cycle; activate kinases; participate in the regulation of the cell cycle.
cytokinesis
Division of the cytoplasm, forming two daughter cells.
diploid
A cell containing two sets of chromosomes; in human cells, two sets contains 46 chromosomes.
DNA replication
Process by which the DNA is copied, resulting in two identical copies.
gametes
An organism’s reproductive cells, such as sperm and egg cells.
gene
A segment of DNA that contains the information necessary to encode an RNA molecule or a protein.
haploid
A cell containing one set of chromosomes; in human gametes, one set is 23 chromosomes.
homologous chromosomes
A pair of chromosomes (one from each parent) consisting of two chromosomes that are similar in size, shape, and genes; also known as homologues.
interphase
The first three phases of the cell cycle; the cell spends the majority of its time here.
kinases
Proteins involved in the regulation of the cell cycle; activated by cyclins; activate other target molecules.
metaphase plate
The center (equator) of a cell during mitosis; chromosomes line up at the metaphase plate to ensure the proper separation of the chromatids.
mitosis
The division of the nucleus into two genetically identical nuclei.
oncogene
Cancer causing gene; can accelerate the cell cycle.
resting phase
Phase associated with the G1 phase of the cell cycle; cells that do not divide are in a resting phase and do not continue to the S phase.
S phase
Synthesis phase; the phase of the cell cycle in which the DNA is replicated (copied).
sex chromosomes
Contain genes that determine the sex of an individual.
sister chromatid
Identical copies of a DNA molecule; a chromosome at the start of mitosis and meiosis has two sister chromatids.
spindle
Thin, cage-like fibers made out of microtubules; used to move chromosomes and to separate the sister chromatids during mitosis.
zygote
The first cell of a new individual.
Points to Consider
A human cell has 46 chromosomes, while a bacterial cell has only one chromosome. Would you think that the number of chromosomes relates to the complexity of the cell or organism?
Mitosis and cytokinesis produce two genetically identical daughter cells. Think about how a cell with half as much DNA, such as a sex cell, may form.
As not every species has members of the opposite sex, such as bacteria, yet all organisms must reproduce to stay alive, think about how these sexless organisms may reproduce.
Lesson 6.2: Meiosis
Lesson Objectives
Describe asexual reproduction; explain the genetic relationship between parent and offspring.
Describe sexual reproduction; explain the genetic relationship between parent and offspring.
Identify and describe the main steps of meiosis, distinguishing between the quantity of genetic material in the parent and resulting cells.
Describe gametogenesis and identify the key differences between oogenesis and spermatogenesis.
Distinguish between the three types of sexual life cycles.
Introduction
Some organisms look and act exactly like their parent. Others share many similar traits, but they are definitely unique individuals. Some species have two parents, whereas others have just one. How an organism reproduces determines the amount of similarity the organism will have to its parent. Asexual reproduction produces an identical individual, whereas sexual reproduction produces a similar, but unique, individual. In sexual reproduction, meiosis produces haploid gametes that fuse during fertilization to produce a diploid zygote (Figure below and Figure below).
Figure 6.9
Fertilization of an egg cell by a sperm cell. In sexual reproduction, haploid gametes fuse to produce a diploid zygote.
Asexual Reproduction
Are there male and female bacteria? How could you tell? Remember, bacteria have just one chromosome; they do not have an X or Y chromosome. So they probably have a very simplified form of reproduction. Asexual reproduction, the simplest and most primitive method of reproduction, produces a clone, an organism that is genetically identical to its parent. Haploid gametes are not involved in asexual reproduction. A parent passes all of its genetic material to the next generation. All prokaryotic and many eukaryotic organisms reproduce asexually.
There are a number of types of asexual reproduction including fission, fragmentation and budding. In fission, a parent separates into two or more individuals of about equal size. In fragmentation, the body breaks into several fragments, which later develop into complete adults. In budding, new individuals split off from existing ones. The bud may stay attached or break free from the parent. Eukaryotic organisms, such as the single cell yeast and multicellular hydra, undergo budding (Figure below).
Figure 6.10
Magnification of a budding yeast.
Sexual Reproduction and Meiosis
Why do you look similar to your parents, but not identical? First, it is because you have two parents. Second, it is because of sexual reproduction.
Whereas asexual reproduction produces genetically identical clones, sexual reproduction produces genetically diverse individuals. As both parents contribute half of the new organism’s genetic material, the offspring will have traits of both parents, but will not be exactly like either parent.
Organisms that reproduce sexually by joining gametes, a process known as fertilization, must have a mechanism to produce haploid gametes. This mechanism is meiosis, a type of cell division that halves the number of chromosomes. During meiosis the pairs of chromosomes separate and segregate randomly to produce gametes with one chromosome from each pair. Meiosis involves two nuclear and cell divisions without an interphase in between, starting with one diploid cell and generating four haploid cells (Figure below). Each division, named meiosis I and meiosis II, has four stages: prophase, metaphase, anaphase, and telophase. These stages are similar to those of mitosis, but there are distinct and important differences.
Prior to meiosis, the cell’s DNA is replicated, generating chromosomes with two sister chromatids. A human cell prior to meiosis will have 46 chromosomes, 22 pairs of homologous autosomes, and 1 pair of sex chromosomes. Homologous chromosomes are similar in size, shape, and genetic c
ontent. You inherit one chromosome of each pair from your mother and the other one from your father.
Figure 6.11
During meiosis the number of chromosomes is reduced from a diploid number (2n) to a haploid number (n). During fertilization, haploid gametes come together to form a diploid zygote and the original number of chromosomes (2n) is restored.
The 8 stages of meiosis are summarized below. The stages will be described for a human cell, starting with 46 chromosomes.
Prophase I: prophase I is very similar to prophase of mitosis, but with one very significant difference. In Prophase I, the nuclear envelope breaks down, the chromosomes condense, and the centrioles begin to migrate to opposite poles of the cell, with the spindle fibers growing between them. During this time, the homologous chromosomes form pairs. These homologous chromosomes line up gene-for-gene down their entire length, allowing crossing-over to occur. This is an important step in creating genetic variation and will be discussed later.
Metaphase I: In metaphase I, the 23 pairs of homologous chromosomes line up along the equator of the cell. During mitosis, 46 individual chromosomes line up during metaphase. Some chromosomes inherited from the father are facing one side of the cell, and some are facing the other side.
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