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Chapter 8Cell Division

and Reproduction

Cell Division Ensures the Passage of

Genetic Information

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8.1 Cell division is involved in both asexual and sexual reproduction

Somatic cells - body cells make up most of the organism Asexual reproduction - increase in number of

somatic cells or the number unicellular organisms

Germ cells - found only in testes and ovaries, they produce sperm or eggs Sexual reproduction – requires the production of

eggs and sperm

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8.2 Prokaryotes reproduce asexually

In bacteria and archaea, reproduction consists of duplicating the single chromosome, located in the nucleoid, and distributing a copy to each daughter cell

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Figure 8.2 Prokaryotes use binary fission to reproduce

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Somatic Cells Have a Cell Cycle and Undergo Mitosis

and Cytokinesis

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8.3 The eukaryotic cell cycle is a set series of events

Interphase - the time when a cell performs its usual functions

Figure 8.3A Stages of the cell cycle. 8-7

M (Mitotic) Stage

Cell division occurs during the M stage and encompasses both division of the nucleus and division of the cytoplasm Mitosis - nuclear division in the cell cycle Cytokinesis - division of the cytoplasm

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Figure 8.3B Cytokinesis is a noticeable part of the cell cycle

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8.4 Eukaryotic chromosomes are visible during cell division

When a eukaryotic cell is not undergoing division, the DNA within a chromosome is a mass of thin threads called chromatin Before nuclear division chromatin condenses, 2 identical chromatids are sister chromatids

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Figure 8.4A A condensed duplicated chromosome

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Somatic Cells Are Diploid (2n)

Diploid (2n) - includes two chromosomes of each kind

During mitosis, a 2n nucleus divides to produce daughter nuclei that are also 2n A dividing cell is called the parent cell and the new

cells are called the daughter cells

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Gametes Are Haploid (n) Haploid (n) number of chromosomes, contains

only one chromosome of each kind Half the diploid number

Figure 8.4B When sister chromatids separate, each daughter nucleus gets a chromosome

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8.5 Mitosis maintains the chromosome number

Before mitosis, DNA has replicated, each double helix is in a chromatid and the chromosomes consist of sister chromatids attached at a centromere The centrosome - the microtubule-organizing center

of the cell divides before mitosis Spindle fibers separate the sister chromatids of the

duplicated chromosomes

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Figure 8.5 Phases of mitosis in animal cells and plant cells

Figure 8.5 Phases of mitosis in animal cells and plant cells

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8.6 Cytokinesis divides the cytoplasm

Cytokinesis follows mitosis in most cells Cytokinesis in plant cells occurs by a process

different than in animal cells Cell plate - newly formed plasma membrane that

expands outward

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Figure 8.6A Cytokinesis in an animal cell

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Figure 8.6B Cytokinesis in plant cells

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Cancer Is Uncontrolled Cell Division

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8.7 Cell cycle control occurs at checkpoints

The cell cycle has checkpoints that can delay the cell cycle until all is well Apoptosis - programmed cell death

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Figure 8.7 Cell cycle checkpoints

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8.8 Signals affect the cell cycle control system

Signaling molecules stimulate or inhibit Kinases remove phosphate from ATP and add it

to another molecule Cyclins combine with kinases Control system for cell division control whether

kinases and cyclins are present

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Figure 8.8A Internal signals of the cell cycle are kinases and cyclins

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Figure 8.8B A cell-signaling pathway activates the control system to produce kinases and cyclins

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Cell Cycle Inhibition

Contact Inhibition - In a culture, cells divide until they line a container in a sheet and then stop dividing

Cells “remember” number of divisions Telomere - repeating DNA base sequence

Each time a cell divides some portion of a telomere is lost

When telomeres become too short, chromosomes fuse and do not duplicate

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8.9 Cancer cells have abnormal characteristics

A mutation (a DNA change) causes a cell to divide uncontrollably or ignore apoptosis Carcinogenesis - development of cancer

Characteristics of cancer cells Lack differentiation Have abnormal nuclei Form tumors Metastasis - establishing new tumors Angiogenesis - formation of new blood vessels

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Figure 8.9 Development of breast cancer

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APPLYING THE CONCEPTS—HOW BIOLOGY IMPACTS OUR LIVES

8.10 Protective behaviors and diet help prevent cancer

Behaviors that help prevent cancer Don’t smoke Use sunscreen Avoid radiation Be tested for cancer Be aware of occupational hazards Carefully consider hormone therapy

Dietary guidelines to reduce cancer risk Increase consumption of foods rich in vitamins A and C Limit consumption of salt-cured, smoked, or nitrite-cured foods Include vegetables from the cabbage family Be moderate in the consumption of alcohol Maintain a healthy weight

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Meiosis Produces Cells That Become the Gametes in Animals and Spores in Other Organisms

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8.11 Homologous chromosomes separate during meiosis

Karyotype - picture of chromosomes in numbered pairs called homologous chromosomes or homologues X and Y chromosomes are the sex chromosomes

because they contain the genes that determine gender Autosomes - all the pairs of chromosomes except the sex

chromosomes

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Figure 8.11 A karyotype shows that the chromosomes occur as pairs

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Meiosis

Occurs during the production of the sperm and egg

Requires two divisions Meiosis I - the chromosomes of each homologous

pair separate Meiosis II - the sister chromatids of each duplicated

chromosome separate

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8.12 Synapsis and crossing-over occur during meiosis I

Synapsis - homologous chromosomes come together and line up side by side forming a tetrad

Crossing-over - during synapsis, nonsister chromatids exchange genetic material

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Figure 8.12A Synapsis of homologues

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Figure 8.12B Crossing-over of nonsister chromatids

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8.13 Sexual reproduction increases genetic variation

Fertilization, the union of a male and a female gamete, enhances genetic variation Sexual reproduction brings about genetic variation,

and some offspring may have a better chance of survival and reproductive success than others

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Figure 8.13 Independent assortment increases genetic variation

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8.14 Meiosis requires two division cycles

Prophase, metaphase, anaphase, and telophase occur during both meiosis I and meiosis II No replication of DNA occurs during a period

called interkinesis between meiosis I and II

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Figure 8.14A Phases of meiosis I

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Figure 8.14A Phases of meiosis I

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Figure 8.14B Phases of meiosis II

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Figure 8.14B Phases of meiosis II

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8.15 The life cycle of most multicellular organisms

includes both mitosis and meiosis

Life cycle - in sexually reproducing organisms, all the reproductive events from one generation to the next Spermatogenesis in males, occurs in the testes and

produces sperm Oogenesis in females, occurs in the ovaries and

produces eggs Zygote - product of the sperm and egg joining

during fertilization, has homologous pairs of chromosomes

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Figure 8.15A Life cycle of humans

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Figure 8.15B Life cycle of plants Figure 8.15C Life cycle of algae

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8.16 Meiosis can be compared to mitosis

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Chromosomal Abnormalities Can Be Inherited

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8.17 An abnormal chromosome number is sometimes traceable

to nondisjunction

Polyploidy - a eukaryote has three or more complete sets of chromosomes

Aneuploidy - an organism has more or less than the normal number of chromosomes Monosomy only one of a type of chromosome Trisomy three of a type of chromosome

Nondisjunction - in meiosis I homologues do not separate and both go into the same daughter cell, or in meiosis II sister chromatids fail to separate and both daughter chromosomes go to the same gamete

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Figure 8.17A Nondisjunction of chromosomes during meiosis I of oogenesis, followed by fertilization with normal sperm

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Figure 8.17B Nondisjunction of chromosomes during meiosis II of oogenesis, followed by fertilization with normal sperm

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8.18 Abnormal chromosome numbers cause syndromes

Trisomy 21 (Down Syndrome) Over 90% of individuals with Down syndrome have three

copies of chromosome 21

Abnormal Sex Chromosome Inheritance Turner syndrome females are born with only a single X

chromosome A male with Klinefelter syndrome has two or more X

chromosomes in addition to a Y chromosome

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8.19 Abnormal chromosome structure also causes syndromes

Various agents in the environment, such as radiation, certain organic chemicals, or viruses, can cause chromosomes to break Deletion - an end of a chromosome breaks off or two

simultaneous breaks lead to the loss of an internal segment

Duplication - the presence of a particular chromosome segment more than once in the same chromosome

Inversion - a segment of a chromosome is turned 180 degrees

Translocation - the movement of a chromosome segment from one chromosome to another nonhomologous chromosome

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Figure 8.19 Types of chromosomal mutations

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Connecting the Concepts:Chapter 8

All cells receive DNA from preexisting cells through the process of cell division Mitosis is part of the cell cycle, and there are negative

consequences if the cell cycle comes out of synchronization

Meiosis is part of the production of gametes, which have half the number of chromosomes as the parent cell

Sexual reproduction increases genetic variability

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