Ch. 8 The Cellular Basis of Reproduction and Inheritance

Ch. 8 The Cellular Basis of Reproduction and Inheritance
© 2012 Pearson Education, Inc. 1

Cell division plays many important roles in the lives of organisms
Living organisms reproduce by two methods. Asexual reproduction offspring are identical to the parent involves inheritance of all genes from one parent. Sexual reproduction offspring are similar to the parents, but show variations in traits involves inheritance of unique sets of genes from two parents. Student Misconceptions and Concerns 1. As the authors note in Module 8.1, biologists use the term daughter to indicate offspring and not gender. Students with little experience in this terminology can easily become confused. 2. Some basic familiarity or faint memory of mitosis and meiosis might result in a lack of enthusiasm for mitosis and meiosis in some of your students. Consider beginning such lectures with important topics related to cellular reproduction. For example, cancer cells reproduce uncontrollably, stem cells have the capacity to regenerate lost or damaged tissues, and the study of embryonic stem cells is variously restricted and regulated. Teaching Tips Sometimes the most basic questions can challenge students and get them focused on the subject at hand. Consider asking your students why we expect that dogs will produce dogs, cats will produce more cats, and chickens will only produce chickens. Why does like produce like? 2

Cell division is reproduction at the cellular level, Requires duplication of chromosomes, sorting new sets of chromosomes into the resulting pair of daughter cells. Student Misconceptions and Concerns 1. As the authors note in Module 8.1, biologists use the term daughter to indicate offspring and not gender. Students with little experience in this terminology can easily become confused. 2. Some basic familiarity or faint memory of mitosis and meiosis might result in a lack of enthusiasm for mitosis and meiosis in some of your students. Consider beginning such lectures with important topics related to cellular reproduction. For example, cancer cells reproduce uncontrollably, stem cells have the capacity to regenerate lost or damaged tissues, and the study of embryonic stem cells is variously restricted and regulated. Teaching Tips Sometimes the most basic questions can challenge students and get them focused on the subject at hand. Consider asking your students why we expect that dogs will produce dogs, cats will produce more cats, and chickens will only produce chickens. Why does like produce like? © 2012 Pearson Education, Inc. 3

Cell division is used for: Reproduction of single-celled organisms Growth of multicellular organisms from a fertilized egg into an adult, Repair and replacement of cells Sperm and egg production. Student Misconceptions and Concerns 1. As the authors note in Module 8.1, biologists use the term daughter to indicate offspring and not gender. Students with little experience in this terminology can easily become confused. 2. Some basic familiarity or faint memory of mitosis and meiosis might result in a lack of enthusiasm for mitosis and meiosis in some of your students. Consider beginning such lectures with important topics related to cellular reproduction. For example, cancer cells reproduce uncontrollably, stem cells have the capacity to regenerate lost or damaged tissues, and the study of embryonic stem cells is variously restricted and regulated. Teaching Tips Sometimes the most basic questions can challenge students and get them focused on the subject at hand. Consider asking your students why we expect that dogs will produce dogs, cats will produce more cats, and chickens will only produce chickens. Why does like produce like? Figure 8.1F A human kidney cell dividing © 2012 Pearson Education, Inc. 4

Prokaryotes reproduce by binary fission
Prokaryotes (bacteria and archaea) reproduce by binary fission “dividing in half” Binary Fission Occurs in Three Steps: duplication of the chromosome and separation of the copies, continued elongation of the cell and movement of the copies division into two daughter cells. The chromosome of a prokaryote is: A singular circular DNA molecule & associated proteins Much smaller than eukaryotic DNA Student Misconceptions and Concerns Some basic familiarity or faint memory of mitosis and meiosis might result in a lack of enthusiasm for mitosis and meiosis in some of your students. Consider beginning such lectures with important topics related to cellular reproduction. For example, cancer cells reproduce uncontrollably, stem cells have the capacity to regenerate lost or damaged tissues, and the study of embryonic stem cells is variously restricted and regulated. Teaching Tips 1. The principle that “every cell comes from another cell” is worth thinking through with your class. Students might expect that, like automobiles, computers, and cell phones, parts are constructed and cells are assembled. In our society, few nonliving products are generated only from existing products (try to think of such examples). For example, you do not need a painting to paint or a house to construct a house. Yet, this is a common expectation in biology. Further, students who think through this principle might ask how the first cells formed. They might wonder further whether the same environments that produced these cells are still in existence. The conditions on Earth when life first formed were very different from those we know today. Chapter 15 addresses the origin and early evolution of life on Earth. 2. Consider contrasting the timing of DNA replication and cytokinesis in prokaryotes and eukaryotes. In prokaryotes, addressed in Module 8.2, these processes are overlapping. However, as revealed in the next few modules, these events are separate in eukaryotes. © 2012 Pearson Education, Inc. 5

Division into two daughter cells
Figure 8.2A_s3 Plasma membrane Prokaryotic chromosome Cell wall Duplication of the chromosome and separation of the copies 1 Continued elongation of the cell and movement of the copies 2 Figure 8.2A_s3 Binary fission of a prokaryotic cell (step 3) Division into two daughter cells 3 6

THE EUKARYOTIC CELL CYCLE AND MITOSIS

The large, complex chromosomes of eukaryotes duplicate with each cell division
Eukaryotic cells have more genes, and store most of their genes on multiple chromosomes within the nucleus. Eukaryotic chromosomes are composed of chromatin = one long DNA molecule and proteins Before dividing chromatin becomes highly compact (into chromosomes) and visible with a microscope Student Misconceptions and Concerns 1. Students often seem confused by the difference between a DNA molecule and a chromosome. This is especially problematic when discussing DNA replication. 2. Students are often confused by photographs of chromosomes. Such photographs, such as Figure 8.3B, typically show duplicated chromosomes during some aspect of cell division. It remains unclear to many why (a) chromosome structure is typically different between interphase G1 and the stages of division and (b) why chromosomes are not photographed during interphase (the stage in which chromosomes are typically first discussed) before the chromosomes duplicate. Teaching Tips 1. Figure 8.3B is an important point of reference for some basic terminology. Consider referring to it as you distinguish between a DNA molecule and a chromosome, unreplicated and replicated chromosomes, and the nature of sister chromatids. 2. The authors make an analogy between the precise packaging of DNA into chromosomes and packing a home for a move to another home. Tap into the intuitive advantages of packaging DNA using this or any other analogy of highly packaged materials (perhaps a boxed “desk” that requires some assembly). 3. The concepts of DNA replication and sister chromatids are often obstacles for many students. If you can find twist ties or other bendable wire, you can demonstrate or have students model the difference between (1) a chromosome before DNA replication and (2) sister chromatids after DNA replication. One piece of wire will represent a chromosome before replication. Two twist ties twisted about each other can represent sister chromatids. We have doubled the DNA, but the molecules remain attached (although not attached in the same way as the wire). You might also want to point out that when sister chromatids are separated, they are considered separate chromosomes. © 2012 Pearson Education, Inc. 8

Chromosomes DNA molecules Sister chromatids Chromosome duplication
Figure 8.3B Chromosomes DNA molecules Sister chromatids Chromosome duplication Sister chromatids Centromere Figure 8.3B Chromosome duplication and distribution Chromosome distribution to the daughter cells 9

The large, complex chromosomes of eukaryotes duplicate with each cell division
Before a eukaryotic cell divides, it duplicates all of its chromosomes, resulting in two copies called sister chromatids joined together by a narrowed “waist” called the centromere. When a cell divides, the sister chromatids separate from each other, (now called chromosomes) and sort into separate daughter cells. Student Misconceptions and Concerns 1. Students often seem confused by the difference between a DNA molecule and a chromosome. This is especially problematic when discussing DNA replication. 2. Students are often confused by photographs of chromosomes. Such photographs, such as Figure 8.3B, typically show duplicated chromosomes during some aspect of cell division. It remains unclear to many why (a) chromosome structure is typically different between interphase G1 and the stages of division and (b) why chromosomes are not photographed during interphase (the stage in which chromosomes are typically first discussed) before the chromosomes duplicate. Teaching Tips 1. Figure 8.3B is an important point of reference for some basic terminology. Consider referring to it as you distinguish between a DNA molecule and a chromosome, unreplicated and replicated chromosomes, and the nature of sister chromatids. 2. The authors make an analogy between the precise packaging of DNA into chromosomes and packing a home for a move to another home. Tap into the intuitive advantages of packaging DNA using this or any other analogy of highly packaged materials (perhaps a boxed “desk” that requires some assembly). 3. The concepts of DNA replication and sister chromatids are often obstacles for many students. If you can find twist ties or other bendable wire, you can demonstrate or have students model the difference between (1) a chromosome before DNA replication and (2) sister chromatids after DNA replication. One piece of wire will represent a chromosome before replication. Two twist ties twisted about each other can represent sister chromatids. We have doubled the DNA, but the molecules remain attached (although not attached in the same way as the wire). You might also want to point out that when sister chromatids are separated, they are considered separate chromosomes. © 2012 Pearson Education, Inc. 10

Chromosomes DNA molecules Chromosome duplication Sister chromatids
Figure 8.3B_1 Chromosomes DNA molecules Chromosome duplication Sister chromatids Centromere Figure 8.3B_1 Chromosome duplication and distribution (part 1) Chromosome distribution to the daughter cells 11

The cell cycle multiplies cells
The cell cycle extends from the time a cell is first formed until its own division. Teaching Tips 1. The authors note in Module 8.4 that each of your students consists of about 10 trillion cells. It is likely that this number is beyond comprehension for most of your students. Consider sharing several simple examples of the enormity of that number to try to make it more meaningful. For example, the U.S. population in 2011 is about 312 million people. To give every one of those people about $32,000, we will need a total of 10 trillion dollars. Here is another example. If we gave you $32,000 every second, it would take 10 years to give you 10 trillion dollars. The US Debt Clock helps relate these large numbers to the US national debt at 2. In G1, the chromosomes have not duplicated. But by G2, chromosomes consist of sister chromatids. If you have created a demonstration of sister chromatids, relate DNA replication and sister chromatids to the cell cycle. © 2012 Pearson Education, Inc. 12

The cell cycle multiplies cells
The cell cycle consists of two stages, characterized as follows: Interphase: G1—growth, increase in cytoplasm S—duplication of chromosomes G2—growth, preparation for division Mitotic phase: nuclear division Mitosis—division of the chromosomes in nucleus Cytokinesis—division of cytoplasm Teaching Tips 1. The authors note in Module 8.4 that each of your students consists of about 10 trillion cells. It is likely that this number is beyond comprehension for most of your students. Consider sharing several simple examples of the enormity of that number to try to make it more meaningful. For example, the U.S. population in 2011 is about 312 million people. To give every one of those people about $32,000, we will need a total of 10 trillion dollars. Here is another example. If we gave you $32,000 every second, it would take 10 years to give you 10 trillion dollars. The US Debt Clock helps relate these large numbers to the US national debt at 2. In G1, the chromosomes have not duplicated. But by G2, chromosomes consist of sister chromatids. If you have created a demonstration of sister chromatids, relate DNA replication and sister chromatids to the cell cycle. © 2012 Pearson Education, Inc. 13

I N T E R P H A S G1 (first gap) S (DNA synthesis) M G2 (second gap)
Cytokinesis G2 (second gap) Mitosis Figure 8.4 The eukaryotic cell cycle T MI O IC PH A SE 14

Cell division is a continuum of dynamic changes
Mitosis progresses through a series of stages (nuclear division): prophase prometaphase metaphase anaphase telophase. Cytokinesis often overlaps telophase. Teaching Tips Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPPMAT: the first letters of interphase, prophase, prometaphase, metaphase, anaphase, and telophase are represented in this acronym. © 2012 Pearson Education, Inc. 15

(with centriole pairs) Fragments of the nuclear envelope Early mitotic
MITOSIS INTERPHASE Prophase Prometaphase Centrosomes (with centriole pairs) Fragments of the nuclear envelope Early mitotic spindle Centrosome Kinetochore Centrioles Chromatin Figure 8.5_1 The stages of cell division by mitosis: Interphase through Prometaphase Centromere Nuclear envelope Plasma membrane Chromosome, consisting of two sister chromatids Spindle microtubules 16

Prophase In the cytoplasm microtubules begin to emerge from centrosomes, forming the mitotic spindle. In the nucleus chromosomes coil and become compact and visible with a light microscope Nucleoli disappear. Teaching Tips Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPPMAT: the first letters of interphase, prophase, prometaphase, metaphase, anaphase, and telophase are represented in this acronym. © 2012 Pearson Education, Inc. 17

Prometaphase Spindle microtubules attach at kinetochores on centromeres of sister chromatids and move chromosomes to the center of the cell The nuclear envelope disappears. Teaching Tips Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPPMAT: the first letters of interphase, prophase, prometaphase, metaphase, anaphase, and telophase are represented in this acronym. © 2012 Pearson Education, Inc. 18

Telophase and Cytokinesis
MITOSIS Metaphase Anaphase Telophase and Cytokinesis Metaphase plate Cleavage furrow Figure 8.5_5 The stages of cell division by mitosis: Metaphase through Cytokenesis Nuclear envelope forming Mitotic spindle Daughter chromosomes 19

Metaphase The mitotic spindle is fully formed and chromosomes align at the cell equator. Kinetochores of sister chromatids are facing the opposite poles of the spindle. Teaching Tips Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPPMAT: the first letters of interphase, prophase, prometaphase, metaphase, anaphase, and telophase are represented in this acronym. © 2012 Pearson Education, Inc. 20

Anaphase Sister chromatids separate at centromeres and daughter chromosomes are moved to opposite poles of the cell The cell elongates due to lengthening of nonkinetochore microtubules. Teaching Tips Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPPMAT: the first letters of interphase, prophase, prometaphase, metaphase, anaphase, and telophase are represented in this acronym. © 2012 Pearson Education, Inc. 21

Telophase The cell continues to elongate and nuclear envelope forms around chromosomes at each pole, establishing daughter nuclei Chromatin uncoils and nucleoli reappear. The spindle disappears. Teaching Tips Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPPMAT: the first letters of interphase, prophase, prometaphase, metaphase, anaphase, and telophase are represented in this acronym. © 2012 Pearson Education, Inc. 22

Cytokinesis differs in animal and plant cells. In animal cells a cleavage furrow forms from a contracting ring of microfilaments, interacting with myosin; the cleavage furrow deepens to separate the contents into two cells. Teaching Tips Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPPMAT: the first letters of interphase, prophase, prometaphase, metaphase, anaphase, and telophase are represented in this acronym. © 2012 Pearson Education, Inc. 23

In plant cells a cell plate forms in the middle, from vesicles containing cell wall material; the cell plate grows outward to reach the edges, dividing the contents into two cells, each with a plasma membrane and cell wall. New cell wall Cell wall of the parent cell Cell wall Plasma membrane Daughter nucleus Vesicles containing cell wall material Cell plate Daughter cells Figure 8.6B Cell plate formation in a plant cell Cell plate forming Cytokinesis 24

Anchorage, cell density, and chemical growth factors affect cell division
Cell division is controlled by presence of nutrients density-dependent inhibition anchorage dependence growth factors The addition of growth factor Teaching Tips Students who closely examine a small abrasion on their skin might notice that the wound tends to heal from the outer edges inward. This space-filling mechanism is a natural example of density-dependent inhibition, which is also seen when cells in a cell culture dish stop dividing when they have formed a complete layer. © 2012 Pearson Education, Inc. 25

G1 checkpoint Receptor protein S G1 Control system M G2
EXTRACELLULAR FLUID Plasma membrane Growth factor Relay proteins G1 checkpoint Receptor protein Signal transduction pathway S G1 Control system Figure 8.8B How a growth factor signals the cell cycle control system M G2 CYTOPLASM 26

Cancer cells escape controls on the cell cycle.
Cancer cells divide rapidly, often in the absence of growth factors Metastasis Tumor Glandular tissue Figure 8.9 Growth and metastasis of a malignant (cancerous) tumor of the breast Cancer cells invade neighboring tissue Tumor cells can spread through lymph and blood vessels 27

Cancer Treatment Radiaiton Proton Therapy Chemotherapy
Radiaiton Proton Therapy Chemotherapy Student Misconceptions and Concerns Students do not typically know that all cancers are genetically based. Consider making this clear early in your discussions. Challenge your students to explain how certain viruses can lead to cancer. Teaching Tips Chemotherapy has some disastrous side effects. The drugs used to fight cancer may attack rapidly dividing cells. Unfortunately for men, the cells that make sperm are also rapidly dividing. In some circumstances, chemotherapy can leave a man infertile (unable to produce viable sperm) but still able to produce an erection. Many other approaches are under consideration to attack cancers. You may wish to explore these as sidelights to your lecture. Good resources include cell biology and development textbooks. © 2012 Pearson Education, Inc. 28

MEIOSIS AND CROSSING OVER

Gametes have a single set of chromosomes
Meiosis is a process that converts diploid nuclei to haploid nuclei. Diploid cells have two sets of chromosomes. Haploid cells have one set of chromosomes. Meiosis occurs in sex organs, producing gametes: sperm and eggs. Fertilization is the union of sperm and egg. The zygote has a diploid chromosome number, one set from each parent. Student Misconceptions and Concerns Students might not immediately see the need for meiosis in sexual reproduction. Consider an example of what would happen over many generations if gametes were produced by mitosis. The resulting genetic doubling is prevented if each gamete has only half the genetic material of the adult cells. Teaching Tips 1. Consider helping students through mitosis and meiosis by developing an analogy to pairs of shoes. In this case, any given species has a certain number of pairs of shoes, or homologous chromosomes. 2. In the shoe analogy, females have 23 pairs of matching shoes, while males have 22 matching pairs and 1 odd pair maybe a sandal and a sneaker! 3. You might want to get your students thinking by asking them why eggs and sperm are different. (This depends upon the species, but within vertebrates, eggs and sperm are specialized for different tasks. Sperm are adapted to move to an egg and donate a nucleus. Eggs contain a nucleus and most of the cytoplasm of the future zygote. Thus eggs are typically larger, nonmotile, and full of cellular resources to sustain cell division and growth.) © 2012 Pearson Education, Inc. 30

Multicellular diploid adults (2n  46) Diploid stage (2n)
Haploid gametes (n  23) n Egg cell n Sperm cell Meiosis Fertilization Ovary Testis Figure 8.12A The human life cycle Diploid zygote (2n  46) 2n Key Mitosis Haploid stage (n) Multicellular diploid adults (2n  46) Diploid stage (2n) 31

Chromosomes are matched in homologous pairs
In humans, body (somatic) cells have 23 pairs of homologous chromosomes: One member of each is from each parent. One pair = sex chromosomes = X and Y The other 22 pairs of chromosomes are autosomes; Student Misconceptions and Concerns Some students might conclude that sex chromosomes function only in determining the sex of the individual. As the authors note, sex chromosomes contain genes not involved in sex determination. Teaching Tips Students might recall some basic genetics, remembering that for many traits a person receives a separate “signal” from mom and dad. These separate signals for the same trait are carried on the same portion of homologous chromosomes, such as the freckle trait noted in Module 8.11. 32

Chromosomes are matched in homologous pairs
Homologous chromosomes are matched in length, centromere position, and gene locations (except X & Y) A locus is the position of a gene. Different versions of a gene may be found at the same locus on maternal and paternal chromosomes = alleles Pair of homologous chromosomes Locus Student Misconceptions and Concerns Some students might conclude that sex chromosomes function only in determining the sex of the individual. As the authors note, sex chromosomes contain genes not involved in sex determination. Teaching Tips Students might recall some basic genetics, remembering that for many traits a person receives a separate “signal” from mom and dad. These separate signals for the same trait are carried on the same portion of homologous chromosomes, such as the freckle trait noted in Module 8.11. One duplicated chromosome Sister chromatids © 2012 Pearson Education, Inc. 33

Sister chromatids A pair of homologous chromosomes in a diploid
INTERPHASE MEIOSIS I MEIOSIS II Sister chromatids 1 2 3 A pair of homologous chromosomes in a diploid parent cell A pair of duplicated homologous chromosomes Figure 8.12B How meiosis halves chromosome number 34

Meiosis reduces the chromosome number from diploid to haploid
Meiosis I – Prophase I Chromatin condensation Homologous chromosomes come together The homologous pairs, now form a tetrad. Nonsister chromatids exchange genetic material by crossing over at the chiasma Student Misconceptions and Concerns Students might not immediately see the need for meiosis in sexual reproduction. Consider an example of what would happen over many generations if gametes were produced by mitosis. The resulting genetic doubling is prevented if each gamete has only half the genetic material of the adult cells. Teaching Tips Challenge students to identify which stage of meiosis is most like mitosis. Comparing the specific events of mitosis, meiosis I, and meiosis II to each other allows students to identify essential differences. © 2012 Pearson Education, Inc. 35

Chromosomes duplicate Prophase I
MEIOSIS I INTERPHASE: Chromosomes duplicate Prophase I Centrosomes (with centriole pairs) Sites of crossing over Centrioles Spindle Figure 8.13_1 The stages of meiosis: Interphase to Prophase I Tetrad Chromatin Sister chromatids Nuclear envelope Fragments of the nuclear envelope 36

Meiosis I – Metaphase I – Tetrads align at the cell equator. Meiosis I – Anaphase I – Homologous pairs separate and move toward opposite poles of the cell. Student Misconceptions and Concerns Students might not immediately see the need for meiosis in sexual reproduction. Consider an example of what would happen over many generations if gametes were produced by mitosis. The resulting genetic doubling is prevented if each gamete has only half the genetic material of the adult cells. Teaching Tips Challenge students to identify which stage of meiosis is most like mitosis. Comparing the specific events of mitosis, meiosis I, and meiosis II to each other allows students to identify essential differences. © 2012 Pearson Education, Inc. 37

attached to a kinetochore Sister chromatids remain attached
MEIOSIS I Metaphase I Anaphase I Spindle microtubules attached to a kinetochore Sister chromatids remain attached Figure 8.13_2 The stages of meiosis: Metaphase I to Anaphase I Centromere (with a kinetochore) Metaphase plate Homologous chromosomes separate 38

Meiosis I – Telophase I –duplicated chromosomes have reached the poles, nuclear envelope re-forms around chromosomes, each nucleus has the haploid number of chromosomes. Student Misconceptions and Concerns Students might not immediately see the need for meiosis in sexual reproduction. Consider an example of what would happen over many generations if gametes were produced by mitosis. The resulting genetic doubling is prevented if each gamete has only half the genetic material of the adult cells. Teaching Tips Challenge students to identify which stage of meiosis is most like mitosis. Comparing the specific events of mitosis, meiosis I, and meiosis II to each other allows students to identify essential differences. © 2012 Pearson Education, Inc. 39

Meiosis II follows meiosis I without chromosome duplication. Each of the two haploid products enters meiosis II. Meiosis II – Prophase II Chromosomes condense (if uncoiled). If re-formed nuclear envelope breaks up again. Student Misconceptions and Concerns Students might not immediately see the need for meiosis in sexual reproduction. Consider an example of what would happen over many generations if gametes were produced by mitosis. The resulting genetic doubling is prevented if each gamete has only half the genetic material of the adult cells. Teaching Tips Challenge students to identify which stage of meiosis is most like mitosis. Comparing the specific events of mitosis, meiosis I, and meiosis II to each other allows students to identify essential differences. © 2012 Pearson Education, Inc. 40

Meiosis II – Metaphase II – Duplicated chromosomes align at equator. Meiosis II – Anaphase II sister chromatids separate, move towards opposite pole. Meiosis II – Telophase II - four haploid cells are produced Student Misconceptions and Concerns Students might not immediately see the need for meiosis in sexual reproduction. Consider an example of what would happen over many generations if gametes were produced by mitosis. The resulting genetic doubling is prevented if each gamete has only half the genetic material of the adult cells. Teaching Tips Challenge students to identify which stage of meiosis is most like mitosis. Comparing the specific events of mitosis, meiosis I, and meiosis II to each other allows students to identify essential differences. © 2012 Pearson Education, Inc. 41

Figure 8.13_right MEIOSIS II: Sister chromatids separate Telophase II and Cytokinesis Telophase I and Cytokinesis Prophase II Metaphase II Anaphase II Cleavage furrow Sister chromatids separate Haploid daughter cells forming Figure 8.13_right The stages of meiosis: Telophase I and Meiosis II 42

Mitosis and meiosis have important similarities and differences
Mitosis and meiosis both begin with diploid parent cells have chromosomes duplicated during interphase. However the end products differ. Mitosis produces two genetically identical diploid somatic daughter cells. Meiosis produces four genetically unique haploid gametes. Student Misconceptions and Concerns Students might not immediately see the need for meiosis in sexual reproduction. Consider an example of what would happen over many generations if gametes were produced by mitosis. The resulting genetic doubling is prevented if each gamete has only half the genetic material of the adult cells. Teaching Tips 1. How meiosis results in four haploid cells, yet mitosis yields two diploid cells, is often memorized but seldom understood. It can be explained like this. Consider a pair of chromosomes in a cell before any cell divisions. This pair of chromosomes duplicates such that two chromosomes become four (although each pair of sister chromatids are joined at their centromeres). Therefore, mitosis and meiosis each typically begin with four chromosomes. Mitosis divides once, producing two cells, each with two chromosomes. Meiosis divides twice, sorting the four chromosomes into four separate cells. 2. Consider emphasizing a crucial difference between the processes of mitosis and meiosis. In mitosis, sister chromatids separate at metaphase. In meiosis I metaphase, sister chromatids stay together, and homologous pairs of chromosomes separate. Consider sketching a comparison of the alignment of the chromosomes at mitosis metaphase and meiosis metaphase I. Figure 8.14 helps to make this important distinction. You might create a test question in which you ask students to draw several pairs of homologous chromosomes lined up at metaphase in mitosis versus meiosis I. © 2012 Pearson Education, Inc. 43

(before chromosome duplication)
Figure 8.14 MITOSIS MEIOSIS I Parent cell (before chromosome duplication) Prophase Site of crossing over Prophase I Duplicated chromosome (two sister chromatids) Tetrad formed by synapsis of homologous chromosomes Chromosome duplication Chromosome duplication 2n  4 Metaphase Metaphase I Chromosomes align at the metaphase plate Tetrads (homologous pairs) align at the metaphase plate Anaphase Telophase Anaphase I Telophase I Homologous chromosomes separate during anaphase I; sister chromatids remain together Figure 8.14 Comparison of mitosis and meiosis Sister chromatids separate during anaphase Daughter cells of meiosis I Haploid n  2 MEIOSIS II 2n 2n No further chromosomal duplication; sister chromatids separate during anaphase II Daughter cells of mitosis n n n n Daughter cells of meiosis II 44

Two equally probable arrangements of chromosomes at metaphase I
Possibility A Possibility B Two equally probable arrangements of chromosomes at metaphase I Metaphase II Figure 8.15_s3 Results of the independent orientation of chromosomes at metaphase I (step 3) Gametes Combination 1 Combination 2 Combination 3 Combination 4 45

A karyotype is a photographic inventory of an individual’s chromosomes
A karyotype is an ordered display of magnified images of an individual’s chromosomes arranged in pairs. Karyotypes are often produced from dividing cells arrested at metaphase of mitosis and allow for the observation of homologous chromosome pairs, chromosome number, and chromosome structure. Student Misconceptions and Concerns Before addressing karyotyping and nondisjunction events, consider reviewing the general structure and terminology associated with replicated chromosomes and the arrangement of chromosomes during metaphase of mitosis, meiosis I, and meiosis II. Figures 8.3B and 8.14 will be particularly helpful. A firm foundation in chromosome basics is necessary to understand the irregularities discussed in Modules 8.19–8.23. Teaching Tips The Human Genome Website is a tremendous asset for nearly every discussion related to human genetics. It can be accessed at © 2012 Pearson Education, Inc. 46

Packed red Hypotonic and white solution Fixative blood cells Blood
culture Stain White blood cells Centrifuge 2 3 Figure 8.18_s3 Preparation of a karyotype from a blood sample (step 3) Fluid 1 47

4 Figure 8.18_s4 Preparation of a karyotype from a blood sample (step 4) 5 48

Figure 8.19A A karyotype showing trisomy 21, and an individual with Down syndrome
49

Accidents during meiosis can alter chromosome number
Nondisjunction is the failure of chromosomes or chromatids to separate normally during meiosis. This can happen during meiosis I, if both members of a homologous pair go to one pole or meiosis II if both sister chromatids go to one pole. Fertilization after nondisjunction yields zygotes with altered numbers of chromosomes. Student Misconceptions and Concerns Before addressing karyotyping and nondisjunction events, consider reviewing the general structure and terminology associated with replicated chromosomes and the arrangement of chromosomes during metaphase of mitosis, meiosis I, and meiosis II. Figures 8.3B and 8.14 will be particularly helpful. A firm foundation in chromosome basics is necessary to understand the irregularities discussed in Modules 8.19–8.23. Teaching Tips 1. The Human Genome Website is a tremendous asset for nearly every discussion related to human genetics. It can be accessed at 2. Students might be confused by the term nondisjunction. But simply put, it is an error in the sorting of chromosomes during mitosis or meiosis. Figure 8.20 illustrates two types of nondisjunction errors in meiosis. © 2012 Pearson Education, Inc. 50

Nondisjunction MEIOSIS I MEIOSIS II Nondisjunction Normal meiosis II
Figure 8.20A_s3 Nondisjunction MEIOSIS I MEIOSIS II Nondisjunction Normal meiosis II Gametes Figure 8.20A_s3 Nondisjunction in meiosis I (step 3) n  1 n  1 n  1 n  1 n  1 n  1 n n # of chromosomes 51

Alterations of chromosome structure can cause birth defects and cancer
Chromosome breakage can lead to rearrangements that can produce genetic disorders or, if changes occur in somatic cells, cancer. These rearrangements may include a deletion, the loss of a chromosome segment, a duplication, the repeat of a chromosome segment, an inversion, the reversal of a chromosome segment, or a translocation, the attachment of a segment to a nonhomologous chromosome that can be reciprocal Student Misconceptions and Concerns Before addressing karyotyping and nondisjunction events, consider reviewing the general structure and terminology associated with replicated chromosomes and the arrangement of chromosomes during metaphase of mitosis, meiosis I, and meiosis II. Figures 8.3B and 8.14 will be particularly helpful. A firm foundation in chromosome basics is necessary to understand the irregularities discussed in Modules 8.19–8.23. Teaching Tips 1. The Human Genome Website is a tremendous asset for nearly every discussion related to human genetics. It can be accessed at 2. Challenge students to create a simple sentence and then modify that sentence to represent (a) a deletion, (b) a duplication, and (c) an inversion as an analogy to these changes to a chromosome. 52

Reciprocal translocation
Deletion Inversion Duplication Reciprocal translocation Homologous chromosomes Nonhomologous chromosomes Figure 8.23A Alterations of chromosome structure 53

Alterations of chromosome structure can cause birth defects and cancer
Chronic myelogenous leukemia (CML) is one of the most common leukemias affects cells that give rise to white blood cells (leukocytes) results from part of chromosome 22 switching places with a small fragment from a tip of chromosome 9. Chromosome 9 Chromosome 22 “Philadelphia chromosome” Student Misconceptions and Concerns Before addressing karyotyping and nondisjunction events, consider reviewing the general structure and terminology associated with replicated chromosomes and the arrangement of chromosomes during metaphase of mitosis, meiosis I, and meiosis II. Figures 8.3B and 8.14 will be particularly helpful. A firm foundation in chromosome basics is necessary to understand the irregularities discussed in Modules 8.19–8.23. Teaching Tips 1. The Human Genome Website is a tremendous asset for nearly every discussion related to human genetics. It can be accessed at 2. Challenge students to create a simple sentence and then modify that sentence to represent (a) a deletion, (b) a duplication, and (c) an inversion as an analogy to these changes to a chromosome. © 2012 Pearson Education, Inc. 54

Mitosis Meiosis Number of chromosomal duplications
Number of cell divisions Number of daughter cells produced Number of chromosomes in the daughter cells How the chromosomes line up during metaphase Figure 8.UN03 Connecting the Concepts, question 1 Genetic relationship of the daughter cells to the parent cell Functions performed in the human body 55

Ch. 8 The Cellular Basis of Reproduction and Inheritance

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Presentation on theme: "Ch. 8 The Cellular Basis of Reproduction and Inheritance"— Presentation transcript:

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