Chapter 6 How Cells Divide

Cell Division

Chapter 6

General Outcome

8.0 The students should be able to describe in detail the phases of mitosis and their significance.

Specific Learning Outcomes:

Upon successful completion of this unit, the students should be able to:

8.1 List essential life processes that depend on mitotic production of new cells identical to the parent cell.

8.2 Explain the role mitosis plays in the cell cycle.

8.3 Explain one strategy for curing cancer.

8.4 Discuss the two cell divisions of meiosis and their effect on the chromosome number.

8.5 Describe the importance of these 2 events of meiosis I and II: 1) crossing over, and 2) independent assortment.

8.6 Define the following terms:

Mitosis of eukaryotes

Somatic cells

Chromosomes

Homologous chromosomes

Diploid cells

Sister chromatids

Centromere

Cell cycle

3 Phases

Interphase

M- mitosis

C- Cytokinesis

Stages of mitosis

Cytokinesis

Programmed cell death

Example: Developmental cell death

Cancer = cell division out of control

Tumor

Metastases

Strategies for cures

Meiosis

Germ cells

Chromosome number

Diploid

Haploid

Sexual reproduction

Gamete

Zygote

Meiosis reduces the number of chromosomes in gametes to half.

Meiosis I separates the 2 parental versions of each chromosome

Crossing over occurs during synapsis in prophase I

Creates 2 collage chromosomes containing parts of both parental versions. 2

parental chromosomes remain unchanged.

Independent assortment of each parental chromosome versions occurs in anaphase I

Possibilities = 2 to the power of the number of chromosome pairs

Humans = 2²³ = 8,388,608

No replication of chromosomes occurs between Meiosis I and II, so a reduction in

chromosome numbers occurs.

Meiosis II separates the 2 replicas of each parental chromosome

4 Gametes are produced, each haploid

Random fertilization results in 2²³ x 2²³ = 70 trillion possible outcomes

Generating diversity provides choices for survival in a changing environment

Cell Division (p. 136)

6.1 Eukaryotes Have a Complex Cell Cycle (p. 137; Fig. 6.3)

A. In eukaryotes, cell division, along with replication of segments of DNA called chromosomes, is more complex.

B. In duplicated form, eukaryotic chromosomes appear as an “X” still attached in the middle at the centromere.

C. Somatic, or body cells undergo mitosis, while germ cells in reproductive organs undergo meiosis. The life cycle of a cell is called the cell cycle.

1. The first, or G₁ phase, is the primary growth phase and takes up most of the life span of the cell.

2. The S, or synthesis phase follows, during which DNA is replicated.

3. Next is the G₂ phase when the cell readies itself for cell division.

4. These three phases together are known as interphase.

5. Mitosis (M phase) follows next, during which the nucleus and chromosomes of the cell are divided.

6. During cytokinesis (C phase) the cytoplasm is cleaved, resulting in two daughter cells.

6.2 Chromosomes (p. 138; Figs. 6.4, 6.5, 6.6, 6.7; Table 6.1)

A. Chromosome Number

1. Eukaryotic somatic cells have two copies of each chromosome, known as homologous chromosomes.

2. When cells have both copies of each type of chromosome, they are called diploid; when cells have only one of each type, such as after meiosis, they are called haploid.

3. When chromosomes replicate, each replicated copy is known as a chromatid.

4. Humans have 46, or 23 pairs, of chromosomes.

B. Chromosome Structure

1. Eukaryotic cells have histones associated with their DNA to help hold the shape of the large DNA molecule and coil it into a tightly compacted chromosome.

2. Chromosomes exist in somatic cells as homologues, or homologous chromosomes.

3. Cells that have both homologues are diploid.

4. Before cell division, each homologue replicates into two identical copies called sister chromatids.

5. In the duplicated condition, there are 92 chromatids in a human somatic cell.

6.3 Cell Division (p. 140; Figs. 6.8, 6.9, 6.10)

A. Interphase

1. During interphase, chromosomes replicate and begin condensation.

B. Mitosis

1. The first stage of mitosis is prophase, during which condensation of chromosomes and nuclear membrane breakdown occur, centrioles move toward the poles, and kinetochore fibers extend outward from chromosomes.

C. Cytokinesis

Meiosis (p. 148)

6.4 Discovery of Meiosis (p. 148; Figs. 6.17, 6.18)

A. Meiosis

1. A reduction division was required to reduce the number of chromosomes to half so sexual reproduction could occur.

2. This reduction division is known as meiosis.

B. The Sexual Life Cycle

1. Adult cells are diploid and gametes are haploid.

2. Sexual reproduction joins haploid gametes to produce a new diploid individual.

6.10 The Sexual Life Cycle (p. 149; Figs. 6.19, 6.20)

A. Somatic Tissues

1. In the sexual life cycle, there is an alternation of diploid and haploid generations.

2. Normal diploid body cells are called somatic cells.

3. Somatic cells arise from the zygote and are all genetically identical.

B. Germ-Line Tissues

1. Cells that produce gametes are called germ-line tissues.

2. Germ-line cells will undergo meiosis to produce haploid gametes.

6.11 The Stages of Meiosis (p. 150; Figs. 6.21, 6.22, 6.23, 6.24)

A. Meiosis has two divisions: meiosis I serves to divide the two versions of each chromosome; and meiosis II separates the two replicas of each chromosome.

B. Meiosis

1. Meiosis is similar to mitosis, except that it involves two divisions, meiosis I and meiosis II, and the resulting cells are haploid, rather than diploid like those produced by mitosis.

2. Also, a phenomenon called crossing over occurs during prophase I of meiosis I when pieces of nonsister chromatids exchange places to promote new genetic combinations in the offspring.

3. Prior to meiosis I, during interphase, the DNA replicates and the cell readies itself for cell division.

4. During meiosis I, the alignment of homologous pairs along the center of the cell is random, with different combinations of parental chromosomes possible for each daughter cell.

5. This process is known as independent assortment.

C. Meiosis II

1. Meiosis II, which also has four stages, follows after meiosis I, and the result is the separation of the sister chromatids to form four haploid gametes.

D. The Important Role of Crossing Over

1. Because of crossing over, no two haploid cells are the same.

5.12 The Evolutionary Consequences of Sex (p. 156; Fig. 6.27)

A. Sexual reproduction has the capacity to generate new genetic combinations.

B. Independent Assortment

1. The random alignment of homologues during meiosis I results in an astounding number of possible kinds of gametes that can be produced.

C. Crossing Over

1. Crossing over allows for combinations of genes that may never have existed previously.

D. Random Fertilization

1. Since the new zygote is the product of two gametes, each with new variation within them, fertilization adds even more genetic diversity.

E. Importance of Generating Diversity

1. The evolutionary process is revolutionary in that the pace of evolutionary change is quickened by genetic recombination.

2. Sexual reproduction adds genetic versatility to the offspring, a trait favored in the vertebrates.

28 Reproduction and Development

The last three learning objectives will not be covered until the section on Animal organization and evolution. The arrows indicate the sections that will not be on the second exam. This material will be include in the section for the final exam.

LEARNING OBJECTIVES

· Differentiate between sexual and asexual reproduction, and list methods of asexual reproduction.

· Describe the evolutionary trends in reproduction among the vertebrates.

· Understand the evolutionary significance of internal fertilization and development.

· Know the structures involved in the production of sperm and eggs

· Understand why fertilization must occur in the fallopian tubes for mammals.

Vertebrate Reproduction (p. 662)

28.1Asexual and Sexual Reproduction (p. 662; Figs. 28.1, 28.2)

A. Asexual reproduction is accomplished in certain animals by various means, such as budding or fission.

B. Sexual reproduction occurs as sperm and eggs are joined to produce a new individual.

C. Gametes are formed by meiosis in the gonads.

D. Different Approaches to Sex

1. Parthenogenesis is common to many arthropods and involves the production of eggs that develop into identical copies of the mother without fertilization.

2. Hermaphroditism occurs when one individual houses both ovaries and testes and can fertilize itself.

3. Numerous fish exhibit the tendency to change from one sex to the other due to environmental influences; protogyny (first female, then male) and protandry (first male, then female) are fairly common.

E. Sex Determination

1. The SRY gene on the Y chromosome determines whether or not a developing embryo is male.

2. Once testes form in the embryo, they secrete testosterone that influences other male traits.

This following material will be included on the final but not the second exam.

28.2 Evolution of Reproduction Among the Vertebrates (p. 664; Figs. 28.3, 28.4, 28.5, 28.6, 28.7, 28.8)

A. External fertilization occurs in many animals and involves the female releasing eggs from her body (usually into the water), and a male fertilizing them with sperm.

B. Internal fertilization occurs as the male deposits sperm inside the body of the female.

C. Internal fertilization may be classified as oviparous, ovoviviparous, or viviparous.

D. Fish and Amphibians

1. In fish, fertilization is external, as it is in amphibians.

2. Eggs lack shells and undergo development in an aquatic habitat.

E. Reptiles and Birds

1. Birds and most reptiles lay water-tight eggs able to withstand desiccation on dry land.

2. Reptiles lay leathery eggs and then abandon them.

3. All birds practice internal fertilization and lay shelled eggs.

4. Birds generally show parental care of eggs and young.

F. Mammals

1. Female mammals undergo cycles in which they are fertile (“in heat”), called estrus.

2. Monotremes lay water-tight eggs, but all other mammals are viviparous, giving birth to live young.

3. Marsupials give birth to fetuses that are incompletely developed and complete development within a pouch.

4. Placental mammals retain their young for a longer period of development and nourish the growing fetus by a placenta.

The Human Reproductive System (p. 668)

28.3Males (p. 668; Figs. 28.9, 28.10, 28.11)

A. The male reproductive system is made up of the testes located in the scrotum, which produce haploid sperm through meiosis, and a number of accessory structures.

B. Males Gametes Are Formed in the Testes

1. Since sperm production cannot occur successfully at internal body temperatures, the testes hang outside the body in a sac called the scrotum.

2. Within the testes are large numbers of coiled tubes called seminiferous tubules; within these tubules sperm production occurs.

3. Special cells within the testes secrete the male hormone testosterone, which, along with sperm production, is under the influence of the anterior pituitary hormones FSH and LH.

4. After their production, sperm mature in a coiled tube called the epididymis that sits atop each testis.

5. From there, sperm are stored in the vas deferens, the tube leading from the epididymis to the penis.

1. About 5 milliliters of semen are ejaculated at one time, containing tens of millions of sperm cells per milliliter.

2. Any man with less than 20 million sperm per milliliter is considered infertile.

28.4Females (p. 670; Figs. 28.12, 28.13, 28.14)

A. Human females produce oocytes, or egg cells, within ovaries that lie deep within the pelvic cavity.

B. Only One Female Gamete Matures Each Month

1. At a female's birth, the egg cells in her ovaries are primary oocytes, arrested in the first stages of meiosis.

2. After puberty, with the release of FSH from the anterior pituitary, each month one of the primary oocytes is stimulated to develop further.

3. When fully mature, an egg cell is a haploid ovum.

4. When an ovum is released during ovulation, it journeys from the ovary to the fallopian tubes leading to the uterus. The trip down the fallopian tube requires 5 to 7 days.

C. Fertilization Occurs in the Oviducts

1. Fertilization must take place high up in the fallopian tubes not only because the ovum rapidly loses its ability to participate in development, but also so the zygote will be in the proper stage of development for implantation in the uterus.

2. The inner lining of the uterus, the endometrium, is made up of two layers.

3. The innermost layer will house the developing embryo or, if fertilization does not occur, will be shed during menstruation.

4. The outer layer of the endometrium gives rise to the layer that is shed once monthly.