Chapter 22

Formation of New Cells by Cell Division

Cell division, also called cell reproduction, occurs in humans and other organisms at different times in their life.  The formation of gametes involves yet a special type of cell division. Gametes are an organism’s reproductive cells, such as sperm or egg cells.  When a cell divides, the DNA is first copied and then distributed.

 

 

Prokaryotic Cell Reproduction

 

Prokaryotes reproduce by a type of cell division called binary fission.  Binary fission is a form of asexual reproduction that produces identical offspring.  In asexual reproduction, a single parent passes exact copies of all of its DNA to its offspring.  Binary fission occurs in two stages: first, the DNA is copied (so that each new cell will have a copy of the genetic information), and then the cell divides.  Eventually the dividing prokaryote is pinched into two independent cells.

 

 

Eukaryotic Cell Reproduction

 

 A gene is a segment of DNA that codes for a protein or RNA molecule.  When genes are being used, the DNA is stretched out so that the information it contains can be used to direct the synthesis of proteins.  As a eukaryotic cell prepares to divide, the DNA and the proteins associated with the DNA coil into a structure called a chromosome.

 

 

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1. The two exact copies of DNA that make up each chromosome are called chromatids.

2. The two chromatids of a chromosome are attached at a point called a centromere.

3. The chromatids, which become separated during cell division and placed into each new cell, ensure that each new cell will have the same genetic information as the original cell.

    

 

 

 

 

 

How Chromosome Number and Structure Affect Development

 

Sets of Chromosomes

1. Homologous chromosomes are chromosomes that are similar in size, shape, and genetic content.
2. Each homologue in a pair of homologous chromosomes comes from one of the two parents.
3. The 46 chromosomes in human somatic cells are actually two sets of 23 chromosomes.

 

Sets of Chromosomes

1. When a cell, such as a somatic cell, contains two sets of chromosomes, it is said to be diploid.
2. When a cell, such as a gamete, contains one set of chromosomes, it is said to be haploid.
3. The fusion of two haploid gametes—a process called fertilization—forms a diploid zygote. A zygote is a fertilized egg cell.

 

 

Chromosome Number of Various Organisms

 

Sex Chromosomes

1. Autosomes are chromosomes that are not directly involved in determining the sex (gender) of an individual.
2. The sex chromosomes, one of the 23 pairs of chromosomes in humans, contain genes that will determine the sex of the individual.
3. In humans and many other organisms, the two sex chromosomes are referred to as the X and Y chromosomes.

 

 

 

 

Change in Chromosome Number

1. Humans who are missing even one of the 46 chromosomes do not survive.
2. Humans with more than two copies of a chromosome, a condition called trisomy, will not develop properly.
3. Abnormalities in chromosome number can be detected by analyzing a karyotype, a photo of the chromosomes in a dividing cell that shows the chromosomes arranged by size.

Karyotype

 

 

  

 

Change in Chromosome Structure

Changes in an organism’s chromosome structure are called mutations.

Breakage of a chromosome can lead to four types of mutations:

1. deletion mutation
2. duplication mutation
3. inversion mutation
4. translocation mutation

 

 

The Cell Cycle

 

The Life of a Eukaryotic Cell 

 

The Cell Cycle •         The cell cycle is a repeating sequence of cellular growth and division during the life of an organism.     •         A cell spends 90 percent of its time in the first three phases of the cycle, which are collectively called interphase.

 

 

 

The Cell Cycle

 

The five phases of the cell cycle are:

1. First growth (G1) phase During the G1 phase, a cell grows rapidly and carries out its routine functions.
2. Synthesis (S) phase A cell’s DNA is copied during this phase.
3. Second growth (G2) phase In the G2 phase, preparations are made for the nucleus to divide.
4. Mitosis The process during cell division in which the nucleus of a cell is divided into two nuclei is called mitosis.
5. Cytokinesis The process during cell division in which the cytoplasm divides is called cytokinesis.

 

 

 

 

 

Control of the Cell Cycle

The cell cycle has key checkpoints (inspection points) at which feedback signals from the cell can trigger the next phase of the cell cycle (green light).  Other feedback signals can delay the next phase to allow for completion of the current phase (yellow or red light).  Control occurs at three principal checkpoints:

1. Cell growth (G1) checkpoint This checkpoint makes the decision of whether the cell will divide.
2. DNA synthesis (G2) checkpoint DNA replication is checked at this point by DNA repair enzymes.
3. Mitosis checkpoint This checkpoint triggers the exit from mitosis.

 

 

When Control Is Lost: Cancer

• Certain genes contain the information necessary to make the proteins that regulate cell growth and division.
• If one of these genes is mutated, the protein may not function, and regulation of cell growth and division can be disrupted.
• Cancer, the uncontrolled growth of cells, may result.

 

Mitosis and Cytokinesis

 

Chromatid Separation in Mitosis

• During mitosis, the chromatids on each chromosome are physically moved to opposite sides of the dividing cell with the help of the spindle.
• Spindles are cell structures made up of both centrioles and individual microtubule fibers that are involved in moving chromosomes during cell division.

Forming the Spindle     •         When a cell enters the mitotic phase, the centriole pairs start to separate, moving toward opposite poles of the cell.     As the centrioles move apart, the spindle begins to form.

 

 

Separation of Chromatids by Attaching Spindle Fibers

• The chromatids are moved to each pole of the cell in a manner similar to bringing in a fish with a fishing rod and reel.
• When the microtubule “fishing line” is “reeled in,” the chromatids are dragged to opposite poles.
• As soon as the chromatids separate from each other they are called chromosomes.

 

Separation of Chromatids by Attaching Spindle Fibers

• The chromatids are moved to each pole of the cell in a manner similar to bringing in a fish with a fishing rod and reel.
• When the microtubule “fishing line” is “reeled in,” the chromatids are dragged to opposite poles.
• As soon as the chromatids separate from each other they are called chromosomes.

 

Mitosis

 

 

Step 1 Prophase The nuclear envelope dissolves and a spindle forms. 

Step 2 Metaphase During metaphase the chromosomes move to the center of the cell and line up along the equator. 

Step 3 Anaphase Centromeres divide during anaphase. 

Step 4 Telophase A nuclear envelope forms around the chromosomes at each pole. Mitosis is complete

 

Stages of Mitosis

 

 

 

  

Cytokinesis

• As mitosis ends, cytokinesis begins.
• During cytokinesis, the cytoplasm of the cell is divided in half, and the cell membrane grows to enclose each cell, forming two separate cells as a result.
• The end result of mitosis and cytokinesis is two genetically identical cells where only one cell existed before.

 

 

 

  

Meiosis

 

Formation of Haploid Cells

• Meiosis is a form of cell division that halves the number of chromosomes when forming specialized reproductive cells, such as gametes or spores.
• Meiosis involves two divisions of the nucleus—meiosis I and meiosis II.
• Before meiosis begins, the DNA in the original cell is replicated. Thus, meiosis starts with homologous chromosomes.
• The eight stages of meiosis are:

 

1. Prophase I The nuclear envelope breaks down. Homologous chromosomes pair. Crossing-over occurs when portions of a chromatid on one homologous chromosome are broken and exchanged with the corresponding chromatid portions of the other homologous chromosome.
2. Metaphase I The pairs of homologous chromosomes are moved by the spindle to the equator of the cell.
3. Anaphase I The chromosomes of each pair are pulled to opposite poles of the cell by the spindle fibers.
4. Telophase I Individual chromosomes gather at each of the poles. In most organisms, the cytokinesis occurs.
5. Prophase II A new spindle forms around the chromosomes.
6. Metaphase II The chromosomes line up along the equator and are attached at their centromeres to spindle fibers.
7. Anaphase II The centromeres divide, and the chromatids (now called chromosomes) move to opposite poles of the cell.
8. Telophase II A nuclear envelope forms around each set of chromosomes, and the cell undergoes cytokinesis.

 

Stages of Meiosis

 

 

 

  

  

Meiosis and Genetic Variation

• Meiosis is an important process that allows for the rapid generation of new genetic combinations.
• Three mechanisms make key contributions to this genetic variation:

                                                1. independent assortment

                                                2. crossing-over

                                                3. random fertilization

 

Independent Assortment   The random distribution of homologous chromosomes during meiosis is called independent assortment.

 

 

 

 Crossing-Over and Random Fertilization

• The DNA exchange that occurs during crossing-over adds even more recombination to the independent assortment of chromosomes that occurs later in meiosis.
• Thus, the number of genetic combinations that can occur among gametes is practically unlimited.
• Furthermore, the zygote that forms a new individual is created by the random joining of two gametes.

 

Crossing-Over of Chromosomes

 

 

 

Importance of Genetic Variation

• Meiosis and the joining of gametes are essential to evolution. No genetic process generates variation more quickly.
• The pace of evolution is sped up by genetic recombination. The combination of genes from two organisms results in a third type, not identical to either parent.

 

Meiosis in Males

• The process by which sperm are produced in male animals is called spermatogenesis.
• Spermatogenesis occurs in the testes (male reproductive organs), and produces male gametes called sperm.

 

 

Meiosis in Females

The process by which gametes are produ6-0ced in female animals is called oogenesis.

Oogenesis occurs in the ovaries (female reproductive organs) and produces female gametes called ova.

 

 

 

 

Meiosis in Male and Female Animals

 

 

Sexual Reproduction

 

Sexual and Asexual Reproduction

 

In asexual reproduction a single parent passes copies of all of its genes to each of its offspring; there is no fusion of haploid cells such as gametes.  An individual produced by asexual reproduction is a clone, an organism that is genetically identical to its parent.  In contrast, in sexual reproduction two parents each form reproductive cells that have one-half the number of chromosomes.

 

 

Types of Asexual Reproduction

During fission, a parent separates into two or more individuals of about equal size.  Some multicellular eukaryotes undergo fragmentation, a type of reproduction in which the body breaks into several pieces.  Other organisms undergo budding, in which new individuals split off from existing ones.

 

Genetic Diversity

Asexual reproduction allows organisms to produce many offspring in a short period of time, without using energy to produce gametes or to find a mate. However, the DNA of these organisms varies little between individuals.  Sexual reproduction provides a powerful means of quickly making different combinations of genes among individuals. Such genetic diversity is the raw material for evolution.

 

Evolution of Sexual Reproduction

Only diploid cells can repair certain kinds of chromosome damage, such as breaks in both strands of DNA.  Thus the process of meiosis and the pairing of homologous chromosomes may have allowed early protistan cells to repair damaged DNA.

 

Sexual Life Cycles in Eukaryotes

The entire span in the life of an organism from one generation to the next is called a life cycle.  The life cycles of all sexually reproducing organisms follows a basic pattern of alternation between the diploid and haploid chromosome numbers.  Eukaryotes that undergo sexual reproduction can have one of three types of sexual life cycles: haploid, diploid, or alternation of generations.

 

Haploid Life Cycle

The haploid life cycle is the simplest of sexual life cycles.  In this life cycle, haploid cells occupy the major portion of the life cycle.  This type of life cycle is found in many protists, as well as in some fungi and algae.

 

Haploid Life Cycle

 

 

Diploid Life Cycle

The outstanding characteristic of the diploid life cycle is that adult individuals are diploid, each individual inheriting chromosomes from two parents.  In most animals, including humans, a diploid reproductive cell undergoes meiosis to produce gametes.  The gametes join in a process called fertilization, which results in a diploid zygote.

 

Diploid Life Cycle

 

 

 

 

Alternation of Generations

• Plants, algae, and some protists have a life cycle that regularly alternates between a haploid phase and a diploid phase. 
• In plants, the diploid phase in the life cycle that produces spores is called a sporophyte. 
• Spore-forming cells in the sporophyte undergo meiosis to produce spores, haploid reproductive cell produced by meiosis that is capable of developing into an adult without fusing with another cell. 
• In the life cycle of a plant, the gametophyte is the haploid phase that produces gametes by mitosis. 
• The gametophyte produces gametes that fuse and give rise to the diploid phase. 
• Thus, the sporophyte and gametophyte generations take turns, or alternate, in the life cycle.

 

Alternation of Generations