How much do you remember about cell division? Meiosis insures that Each gamete receives the same genes Chromosome number is doubled in the gametes Zygotes produced by fertilization have the normal number of chromosomes All paternal chromosomes end up in the same gamete The correct answer is C. This is the key to meiosis – reduction division so that the fertilized egg restores the normal number of chromosomes. Answer A: Each gamete doesn’t receive the same genes because the homologous chromosomes are randomly assorted during meiosis I. Answer B: Chromosome number is halved in the gametes. Answer D: As in the answer for A, the homologous chromosomes are randomly assorted during meiosis I. 1 1

Which of the following is not true about interphase? The cell grows larger during interphase. Chromosomes are duplicated during interphase. Interphase is divided into three phases. Chromosomes segregate to daughter cells during interphase. The correct answer is D. This does not occur during interphase, but during mitosis. It is important to stress that interphase is composed of 3 phases in which the cell grows and the chromosomes are duplicated. Students often believe that G1, S, and G2 are in addition to interphase. Answers A – C: These are all true. Growth occurs during G1 and G2 phases and the chromosomes are duplicated during S phase. 2 2

© 2009 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 4/e Which of the following is true about homologous chromosomes? Both were received from the same parent. One of each segregates to each daughter cell during mitosis. Both stay together in meiosis I. All 23 pairs are always homologous. The correct answer is B. It is important that students understand what homologous chromosomes are. These are not identical, but homologous; one came from each parent and they contain slightly different genes. One of each segregates into each daughter cell during mitosis so that each daughter cell is identical. Answer A: As stated above, one came from each parent. Answer C: They are separated during meiosis I, resulting in haploid daughter cells. Answer D: Sex cells are not homologous in males, but are in females. © 2009 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 4/e 3 3

© 2009 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 4/e Cell Division Renewal and repair of tissues with stem cells Reproduction Prokaryotes divide through binary fission Eukaryote cell division is more complicated © 2009 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 4/e 4 4

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Prokaryotes divide through binary fission 6

Eukaryote cell division is more complicated: Cell cycle Series of events in life cycle of a cell Two main stages Interphase Most cells spend 90% of lifespan in this stage Cell division by two different mechanisms Mitosis: occurs in nonreproductive cells, called somatic cells Meiosis: occurs in sexual reproduction cells,called germ cells © 2009 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 4/e

© 2009 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 4/e The Cell Cycle © 2009 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 4/e 8

DNA Packaged as Chromosome DNA molecules are enormously long Double helix nearly 2 meters in length DNA is tightly packaged with proteins Chromatin DNA and proteins Chromosome Tightly packed © 2009 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 4/e 9

© 2009 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 4/e Karyotype Chromosomes are visible during mitosis Their number and shape can be studied Humans have 46 chromosomes © 2009 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 4/e 10

Homologous Chromosomes 46 chromosomes are arranged in 23 pairs One came from each parent 22 pairs are autosomes Both chromosomes are homologues 1 pair are sex chromosomes Can be homologous; XX for females Can be different; XY for males © 2009 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 4/e 11

Interphase: The Longest Stage The period between divisions: The cell prepares to divide Divided into 3 stages: G1: growth after mitosis S: synthesis of DNA G2: growth before mitosis © 2009 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 4/e

© 2009 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 4/e Mitosis Consists of Prophase Metaphase Anaphase Telophase Cytokinesis: division of the cytoplasm © 2009 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 4/e

© 2009 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 4/e Prophase Cell enters mitosis Chromosomes condense Centrosomes move apart Go to the poles of the cell Mitotic spindle begins to form © 2009 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 4/e

© 2009 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 4/e Metaphase Chromosomes line up Metaphase plate Align sister chromatids Equal and balanced segregation © 2009 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 4/e

© 2009 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 4/e Anaphase Chromatids separate Break free and dragged to opposite sides Microtubules shorten Result: Equal segregation of chromosomes in two daughter cells © 2009 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 4/e

Telophase and Cytokinesis Chromosomes reach the poles Mitotic spindle falls apart Chromosomes unfold Nuclear membrane reforms Cytokinesis Cytoplasm is divided Two cells are formed © 2009 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 4/e

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Cytokinesis Cytoplasm is divided Two cells are formed

© 2009 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 4/e Cancer Tumor: inappropriate proliferation of cells Benign tumor: confined to one site Malignant tumor: cells migrate and invade © 2009 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 4/e 3

Cancer and Lifestyle: Lung Cancer Environmental factors: Smoking‑related cancers Lung, oral, kidney, stomach, bladder Tobacco smoke 40 known carcinogens Lung cancer cells (300X) Portrait of a cancer

Retinoblastoma: mutation of a gene An inherited form of cancer Occurs in the retina Causes blindness Due to missing Rb gene A tumor suppressor gene © 2009 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 4/e 14

Cancer: A Multi-step Process Example: Colon cancer Early polyp: Loss of tumor suppressor gene Polyp: Mutation of proto-oncogene Late Polyp: Loss of two additional tumor suppressor genes Malignant tumor: Inactivation of p53 tumor suppressor © 2009 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 4/e 6

© 2009 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 4/e Meiosis Used to make gametes Eggs and sperm Chromosome number is halved (haploid) Zygote is diploid after fertilization Meiosis Fertilization Mitosis © 2009 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 4/e

involves two divisions, meiosis I DNA is replicated only before meiosis I meiosis I separates the homologues in a homologous pair meiosis II meiosis II separates the replicate sister chromatids when meiosis is complete, the result is that one diploid cell has become four haploid cells

Meiosis I divided into four stages Prophase I Homologues pair up and exchange segments: crossing over Metaphase I The paired homologous chromosomes align on a central plane in any combination: independent assortment Anaphase I Homologues separate from the pairing and move to opposite poles Telophase I Individual chromosomes gather at each of the two poles

Meiosis II basically a mitotic division of the products of meiosis I, except that the sister non-identical chromatids four stages 1. Prophase II new spindle forms to attach to chromosome clusters 2. Metaphase II spindle fibers bind to both sides of the centromere and individual chromosomes align along a central plane 3. Anaphase II sister chromatids move to opposite poles 4. Telophase II the nuclear envelope is reformed around each of the four sets of daughter chromosomes

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Practice questions Prokaryotes reproduce new cells by copying DNA then undergoing binary fission. splitting in half undergoing mitosis copying DNA then undergoing the M phase. 2. The eukaryotic cell cycle is different from prokaryotic cell division in all the following ways except: the amount of DNA present in the cells how the DNA is packaged in the production of daughter cells the involvement of microtubules. 3. In eukaryotes, the genetic material is found in chromosomes and: the more complex the organism, the more pairs of chromosomes it has a few organisms have only one chromosome most eukaryotes have between 10 and 50 pairs of chromosomes most eukaryotes have between 2 and 10 pairs of chromosomes. 4. Homologous chromosomes: are also referred to as sister chromatids are genetically identical carry information about the same traits located in the same places on the chromosomes are connected to each other at their centromeres.

5. In mitosis, when the duplicated chromosomes line up in the center of the cell, that stage is called: A) Prophase B) metaphase C) anaphase D) telophase. 6. The division of the cytoplasm in the eukaryotic cell cycle is called: interphase.B) karyokinesis.C) cytokinesis.D) binary fission. 7. The cell cycle is controlled by: a series of checkpoints an internal clock the completion of one phase triggering the next phase cell size-when it grows large enough the cell cycle is triggered. 8. When cell division becomes unregulated, and a cluster of cells begins to grow without regard for the normal controls, that is called: A) a mutation.B) cancer.C) metastases.D) oncogenes.

9. During which stage of meiosis does crossing over occur? A) prophase IB) anaphase IC) metaphase IID) interphase 10. The purpose of mitosis is to ____, while the purpose of meiosis is to _____. make diploid cells/make haploid cells make haploid cells/make diploid cells make cells which are either haploid or diploid/make cells which are haploid make cells which are haploid/make cells which vary in chromosome number 11. An egg and a sperm unite to form a new organism. In order to prevent the new organism from having twice as many chromosomes as its parents: half of the chromosomes in the new organism quickly die off, leaving the correct number half of the chromosomes from the egg, and half from the sperm, are ejected from the new cell the large egg contains all the chromosomes, the tiny sperm only contributes some DNA germ cells went through meiosis; the egg and sperm only have half the parental chromosomes.

12. The diploid number of chromosomes in humans is 46 12. The diploid number of chromosomes in humans is 46. The haploid number is: 138 B) 92 C) 46 D) 23. 13. In organisms that have sexual life cycles there is a time when there are: 1n gametes (haploid), followed by 2n zygotes (diploid) 2n gametes (haploid), followed by 1n zygotes (diploid) 2n gametes (diploid), followed by 1n zygotes (haploid) 1n gametes (diploid), followed by 2n zygotes (haploid). 14. The purpose of meiosis I is to: duplicate all chromosomes randomly separate the homologous pairs, called independent assortment separate the duplicated sister chromatids divide the original material into four complete haploid cells. 15. The purpose of meiosis II is to: