Biology

Chapter 8

Cell Reproduction

Mitosis and Meiosis1

Objectives• Explain the nature of cell division

• Discuss the significance of mitosis

• Identify and describe the phases of mitosis

• Distinguish between sexual and asexual reproduction

• Describe the forms of asexual reproduction

• Define meiosis and its importance to sexual reproduction

• Describe the major differences between mitosis and meiosis

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Pennsylvania State Standards• S11.A The Nature of Science

• S11.A.1 Reasoning and Analysis

• Assessment Anchor

• S11.A.1.3 Describe and interpret patterns of change in natural and human-made systems.

• Eligible Content

• S11.A.2.1.3 Use date to make inferences and predictions, or to draw conclusions, demonstrating understanding of experimental limits.

• S11.A.3.2.1 Compare the accuracy of predictions represented in a model to actual observations and behavior.

• S11.B Biological Sciences

• S11.B.1 Structure and Function of Organisms

• Assessment Anchor

• S11.B.1.1 Explain structure and function at multiple levels of organization

• Eligible Content

• S11.B.1.1.3 Compare and contrast cellular processes (e.g., photosynthesis and respirations, meiosis and mitosis, protein synthesis and DNA replication).

• S11.B.2.2.1 Describe how genetic information is expressed(i.e. DNA, genes, chromosomes, transcription, translation, and replication)

• S11.B.2.2.2 Compare and contrast the functions of mitosis and meiosis in passing on genetic information.3

Chapter 8 Cell Reproduction

• 8-1 Chromosomes

• 8-2 Cell Division

• 8-3 Meiosis

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Focus Concept

Cell reproduction perpetuates life,

It allows for the growth and reproduction

of organisms, and passes genetic

information to future generations.

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8-1 Chromosomes:

• Chromosome Structure

• Chromosome Numbers

• Diploid and Haploid Cells

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The Chromosome• Threadlike structure within a cell which

contains the genetic information that is passed on from one generation of cells to the next.

• The are located in the nucleus of a cell.

• Human somatic cells contain 46 chromosomes (or 23 pairs)

• Somatic cells – normal body cells

• Gametes – reproductive cells (sperm and egg)

• Filling the activity of specific regions of the DNA 7

Chromosome Structure• Chromatin – the genetic material that makes

up chromosomes.

– Chromatin is composed of DNA and proteins

• The DNA is super-coiled into a very compact structure during cell division.

• Histones: proteins that help maintain the shape of the chromosome and aids in the tight packing of DNA

• Nonhistone: do not participate in the packing of DNA. Involved in contro

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Chromosome Formation• During cellular division, chromatin condenses or

coils to form the rod-like chromosomes.

– The DNA double helix begins to coil and wrap tightly around proteins(histones) to form a nuclesome.

– Nucleosome – DNA which is tightly wrapped around histones.

– The nuclesome then begins to twist up to form coils.

– The coils then begin to twist to form larger coils called super coils.

– Super coils – large coils of nuclesome coils which make up chromosomes.

– Each chromosome consists of two halves, each is referred to as a chromatid.

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Chromatids

• Each identical half of a chromosome

• Form as the DNA makes a copy of itself before cell division

• Each new cell receives one chromatid from each chromosome.

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Centromere

• Constricted area of each chromatid

• Helps hold the two chromatids together

• Aids in movement of chromosomes during cell division.

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Chromatin

• Less tightly coiled DNA-protein complex.

• Regions of DNA uncoil between cell divisions so information can be read and used to direct the activities of the cell.

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Prokaryote DNA

• Bacterial DNA is simpler than eukaryotes.

• Usually only one chromosome which is attached to the inside of the cell membrane

• Consists of a circular DNA molecule and associated proteins.

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Chromosome Numbers of Various SpeciesAdder’s tongue fern

1,262 Fruit fly 8

Carrot 18 Garden pea 20

Cat 32 Gorilla 48

Chimpanzee 48 Horse 64

Dog 78 Human 46 or 23 pairs

Orangutan 48 Lettuce 18

Earthworm 36 Sand dollar 52

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Chromosome Numbers

• Each species has a characteristic number of chromosomes in each cell.

• Chromosome number does not indicate species complexity.

• The human chromosome number is 46

or 23 pairs.

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Sex Chromosomes

• Chromosomes that determine the sex of an organism.

• May also carry genes for other characteristics.

• Either X or Y.

– Normal females XX

– Normal males XY

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Autosomes

• All of the other chromosomes in an organism

• Humans: 2 sex chromosomes + 44 autosomes = 46 total chromosomes

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Homologous Chromosomes• Occurs in Meiosis diploid cells

• Also called Homologues or homologous pairs

• Every cell of an organism produced by sexual reproduction has 2 copies of each autosome.

• One copy is received from each parent

• Same size, shape and carry genes for the same traits.

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Sister Chromatids• Each duplicated chromosome is made up of

two halves. Each half is referred to as a sister chromatid.

• Sister chromatids – (occur during mitosis) and the DNA they contain are exact copies of each other.

• Formed when DNA is copied during interphase.

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Karyotype

• A photomicrograph of the chromosomes in a dividing cell.

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Chromosomal Abnormalities

• Deletion: a portion of a chromosome is lost

• Duplication: the deletion becomes incorporated into its homologue so that the segment appears twice on the same chromosome.

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• Inversion

• Translocation

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Chromosomal Abnormalities

• Nondisjunction: the failure of chromosomes to separate properly during meiosis. Results in too many or too few chromosomes.

• Trisomy: an abnormality in which a cell has an extra chromosome or section of a chromosome

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Nondisjunction

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Turner Syndrome

• Female with only one X chromosome

• XO

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Klinefelter Syndrome

• Male with two X chromosomes

• XXY

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Other Sex Chromosome Aneuploidies

• XYY genotype - taller than average; after about age 35, extra Y often degenerates and is not passed on to offspring.

• XXX genotype - some developmental deficiencies; some instances of mental retardation

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Down Syndrome

• Trisomy 21

Link to website with other chromosomal abnormalities35

Diploid Cells (2n)

• Cells having 2 sets of chromosomes. Have both chromosomes from each homologous pair.

• All normal human cells (body cells) except reproductive cells are 2n

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Hapliod (1n)

• Contain only one set of chromosomes.

• Have only half the number of chromosomes that are present in diploid cells.

• Sperm and egg cells (gametes) are 1n

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Fertilization

• When a sperm cell (1n) and an egg cell (1n) combine to create the first cell of a new organisms it becomes 2n.

• If the reproductive cells were diploid, the new cell would have too many chromosomes and would not be functional.

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8-2 Cell Division

• Cell Division in Prokaryotes

• Cell Division in Eukaryotes

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Cell Division in Prokaryotes

• Binary Fission: the division of a prokaryotic cell into two different offspring cells.

• Consists of three stages.

(Fig.8-4 p148)

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Cell Division in Eukaryotes

• Mitosis and Cytokinesis: cellular division that results in new cells with genetic material identical to the original cell.

• Occurs in:

– the reproduction of unicellular organisms

– the addition of cells to a tissue or organ in a multicellular organism (growth, repair)

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The Cell Cycle• The repeating set of events that make up

the life of a cell from mitosis to mitosis..

• Cell division is one phase of the cell cycle. (includes both mitosis and cytokinesis)

• Interphase: the time between cell divisions. The cell spends most of its life in interphase.

• IPMATC

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Cell cycle animation44

Interphase

• G1 phase: offspring cells grow to mature size. The time “gap” following cell division and preceding DNA replication

• S phase: the cell’s DNA is copied, “synthesized”

• G2 phase: the time “gap” following DNA synthesis and preceding cell division. Growth and preparation for division.

• G0 phase: cells can enter this state when fully developed. Do not copy DNA and do not divide. Ex. Central nervous system cells.

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Mitosis / M phase• The equal division of the nucleus

• Continuous process that can be divided into 4 phases:

– Prophase

– Metaphase

– Anaphase

– Telophase

– PMAT47

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Prophase:

• 1st phase of mitosis

• Shortening and tight coiling of DNA into rod-shaped chromosomes which are visible with a light microscope

• The 2 copies of each chromosome stay connected to one another by the centromere

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• The nucleolus and nuclear membrane break down and disappear

• Centrosomes appear next to the disappearing nucleus.– In animal cells, each centrosome contains a pair of

small cylindrical bodies called centrioles. Not present in plants.

– The centrosomes move toward opposite poles of a cell

– Spindle fibers (made of microtubules) radiate from the centrosomes. Called mitotic spindles and help to equally divide the chromatids between the two offspring cells.

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Metaphase

• 2nd phase of mitosis

• Spindle fibers move the chromosomes to the center of the dividing cell

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Anaphase

• 3rd phase of mitosis

• The chromatids of each chromosome separate at the centromere and slowly move, centromere first, toward the opposite poles of the dividing cell

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Telophase

• 4th phase of mitosis

• Spindle fibers disassemble and the chromosomes return to a less tightly coiled chromatin state.

• Nuclear envelope reforms around each set of chromosomes and a nucleolus forms in each of the newly forming cells

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Cytokinesis

• Occurs at the end of telophase

• The division of the cytoplasm

– Animals: begins with a pinching inward of the cell membrane midway between the dividing cell’s 2 poles. Cleavage furrow uses the action of microfilaments.

– Plant cells: vesicles formed by the Golgi apparatus fuse at the midline of the dividing cell forming a membrane-bound cell wall called the cell plate.

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Cytokinesis

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• Link to Steps of Cell Cycle Animation

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Link to web site 70

8-3 Meiosis

• Stages of Meiosis I and Meiosis II

• Formation of Gametes

• Asexual and Sexual Reproduction

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Meiosis

• A process of nuclear division that reduces the number of chromosomes in new cells to half the number in the original cell

• The halving of the chromosome number counteracts a fusion of cells later in the life cycle of the organism.

• In humans, meiosis produces haploid reproductive cells called gametes (sperm, egg)

Link to meiosis movie72

Stages of Meiosis

• Cells undergo:

– G1, S, and G2 phases of interphase

– Meiosis I (Prophase I, Metaphase I, Anaphase I, Telophase I)

– Meiosis II(Prophase II, Metaphase II, Anaphase II,Telophase II)

• One cell goes through two divisions to produce 4 gametes.

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Prophase I

• DNA coils tightly into chromosomes, spindle fibers appear, nucleus and nucleolus disassemble.

• Synapsis: chromosomes line up next to its homologue. Each pair of homologous chromosomes is called a tetrad.

• Crossing over occurs: portions of a chromatid twist around one another, break off, and attach to adjacent chromatids on homologous chromosomes. Permits the exchange of genetic material between maternal and paternal chromosomes and results in genetic recombination by producing a new mixture of genetic material.

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Link to crossing over animation

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Metaphase I

• Tetrads line up randomly along the midline of the dividing cell.

• Spindle fibers from each pole attach to the centromere of one homologous chromosome in each tetrad.

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Anaphase I

• Each homologous chromosome of a tetrad moves to an opposite pole

• Random separation of homologous chromosomes is called independent assortment and results in the random separation of the maternal and paternal chromosomes which results in genetic recombination.

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Telophase I

• Chromosomes reach the opposite ends of the cell and cytokinesis begins.

• The new cells contain a haploid number of chromosomes of the original cell but each new cell contains two copies of the chromosome because the original cell copied its DNA before meiosis I.

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Link to meiosis II animation showing independent assortment

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Meiosis II

• Occurs in each cell formed during meiosis I and is not preceded by the copying of DNA

• Prophase II: spindle fibers form and begin to move the chromosomes toward the midline.

• Metaphase II: chromosomes line up at the midline

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Meiosis II

• Anaphase II: chromatids separate and move toward the opposite poles of the cell

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Meiosis II

• Telophase II: nuclear membrane forms around the chromosomes in each of the 4 new cells.

• Cytokinesis II: occurs during telophase II, resulting in 4 new cells, each of which contains half the original cell’s number of chromosomes. (1n)

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• Link to animation of meiosis

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Formation of Gametes

• Gamete: haploid reproductive cells produced during meiosis

• Meiosis occurs only within reproductive organs in humans

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Testes

• Involved in the production of sperm cells or spermatozoa.

• Meiosis produces 4 haploid spermatids that develop into mature sperm cells during spermatogenesis

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Ovaries

• Involved in the production of mature egg cells or ova.

• During oogenesis, a diploid reproductive cell divides meiotically to produce 1 mature egg cell (ovum) and 3 polar bodies.

• The one egg cell receives most of the cytoplasm and the 3 polar bodies degenerate.

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Asexual Reproduction

• The production of offspring from one parent.

• Does not usually involve meiosis or the union of gametes.

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Asexual Reproduction

• In unicellular organisms, new organisms are created either by binary fission or mitosis

• In multicellular organisms, new organisms form by budding off portions of their bodies or by some forms of regeneration. Offspring are genetically identical to the parent.

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Sexual Reproduction

• The production of offspring through meiosis and the union of a sperm and an egg.

• Offspring are genetically different from the parents because genes combined in new ways during meiosis.

• Evolutionary advantage: it enables species to adapt rapidly to new conditions.

– Example: if a disease strikes a grain crop, a few plants may have genetic variations that make them resistant to the disease. Many individuals die, but a few resistant plants survive and produce offspring.

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Review

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• Cell Division Tutorials

• Another Mitosis Animation

• On-line Onion Root Tip Activity

• Cell Cycle and Mitosis Tutorial

• Karyotype Activity

• Meiosis Tutorial

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THE END

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QUESTIONS?

Objectives• Explain the nature of cell division

• Discuss the significance of mitosis

• Identify and describe the phases of mitosis

• Distinguish between sexual and a sexual reproduction

• Describe the forms of asexual reproduction

• Define meiosis and its importance to sexual reproduction

• Describe the major differences between mitosis and meiosis

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REVIEW FOR CHAPTER TEST

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