Chapter 8 notes

PowerPoint Lectures forCampbell Biology: Concepts & Connections, Seventh EditionReece, Taylor, Simon, and Dickey

Chapter 8Chapter 8 The Cellular Basis of Reproduction and Inheritance

Figure 8.0_2Chapter 8: Big Ideas

Cell Division andReproduction

The Eukaryotic CellCycle and Mitosis

Meiosis andCrossing Over

Alterations of ChromosomeNumber and Structure

Figure 8.0_3

8.1 Cell division plays many important roles in the lives of organisms

Organisms reproduce their own kind, a key characteristic of life.

Cell division

– is reproduction at the cellular level,

– requires the duplication of chromosomes, and

– sorts new sets of chromosomes into the resulting pair of daughter cells.

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, and

– sperm and egg production.

Living organisms reproduce by two methods.

– Asexual reproduction

– produces offspring that are identical to the original cell or organism and

– involves inheritance of all genes from one parent.

– Sexual reproduction

– produces offspring that are similar to the parents, but show variations in traits and

– involves inheritance of unique sets of genes from two parents.

Figure 8.1A

Figure 8.1B

Figure 8.1C

Figure 8.1D

Prokaryotes (bacteria and archaea) reproduce by binary fission (“dividing in half”).

The chromosome of a prokaryote is

– a singular circular DNA molecule

– much smaller than those of eukaryotes.

8.2 Prokaryotes reproduce by binary fission

Binary fission of a prokaryote occurs in three stages:

1. duplication of the chromosome and separation of the copies,

2. continued elongation of the cell and movement of the copies, and

3. division into two daughter cells.

8.2 Prokaryotes reproduce by binary fission

Figure 8.2A_s1

Plasmamembrane

Cell wall

Duplication of the chromosomeand separation of the copies

Prokaryoticchromosome

Figure 8.2A_s2

Plasmamembrane

Cell wall

Continued elongation of thecell and movement of the copies

Figure 8.2A_s3

Plasmamembrane

Cell wall

Continued elongation of thecell and movement of the copies

3Division into

two daughter cells

Eukaryotic cells

– are more complex and larger than prokaryotic cells,

– have more genes, and

– store most of their genes on multiple chromosomes within the nucleus.

8.3 The large, complex chromosomes of eukaryotes duplicate with each cell division

Eukaryotic chromosomes are composed of chromatin consisting of

– one long DNA molecule and proteins

To prepare for division, the chromatin becomes highly compacted chromosomes

Figure 8.3A

DNA in the form of chromatin

DNA in the form of condensed chromosomes

Before a eukaryotic cell begins to divide, 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 each is called a chromosome

– Each chromosome (single chromatid) is distributed into separate daughter cells.

Figure 8.3B

Sisterchromatids

Chromosomes

Centromere

Chromosomeduplication

Sisterchromatids

Chromosomedistribution

to thedaughter

DNA molecules

An ordered sequence of events that divide a cell

The cell cycle consists of two stages, characterized as follows:

1. Interphase: duplication of cell contents

– G1—growth, increase in cytoplasm

– S—duplication of chromosomes

– G2—growth, preparation for division

2. Mitotic phase: division

– Mitosis—division of the nucleus

– Cytokinesis—division of cytoplasm

8.4 The cell cycle multiplies cells

Figure 8.4

(first gap)S

(DNA synthesis)

(second gap)

CytokinesisM

I N T E R P H A S E

Mitosis progresses through a series of stages:

– prophase,

– prometaphase,

– metaphase,

– anaphase, and

– telophase.

Cytokinesis often overlaps telophase.

8.5 Cell division is a continuum of dynamic changes

A mitotic spindle (spindle fibers) is

– required to divide the chromosomes, and composed of microtubules

– produced by centrosomes

– contain a pair of centrioles in animal cells.

Interphase

– The cytoplasmic contents double,

– two centrosomes form,

– chromosomes duplicate in the nucleus during the S phase, and

– nucleoli, sites of ribosome assembly, are visible.

Figure 8.5_left

INTERPHASEMITOSIS

Prophase Prometaphase

CentrosomeEarly mitoticspindle

Chromatin

Fragments ofthe nuclear envelope

Kinetochore

Centrosomes(with centriole pairs)

Centrioles

Nuclearenvelope

Plasmamembrane

Chromosome,consisting of twosister chromatids

Centromere Spindlemicrotubules

Prophase

– In the cytoplasm microtubules begin to emerge from centrosomes, forming the spindle.

– In the nucleus

– chromosomes coil and become compact and

– nucleoli disappear.

Figure 8.5_left

INTERPHASEMITOSIS

Chromatin

Kinetochore

Centrioles

Nuclearenvelope

Plasmamembrane

Prometaphase

– Spindle microtubules reach chromosomes, where they

– attach at kinetochores on the centromeres of sister chromatids and

– move chromosomes to the center of the cell through associated protein “motors.”

– Other microtubules meet those from the opposite poles.

– The nuclear envelope disappears.

Figure 8.5_left

INTERPHASEMITOSIS

Chromatin

Kinetochore

Centrioles

Nuclearenvelope

Plasmamembrane

Metaphase

– The mitotic spindle is fully formed.

– Chromosomes align at the cell equator.

– Kinetochores (protein structure at centromere) of sister chromatids are facing the opposite poles of the spindle.

Figure 8.5_right

MITOSIS

AnaphaseMetaphase Telophase and Cytokinesis

Metaphaseplate

Cleavagefurrow

Nuclearenvelopeforming

Daughterchromosomes

Mitoticspindle

Anaphase – Sister chromatids separate at the centromeres.

– Daughter chromosomes are moved to opposite poles of the cell

– The cell elongates due to lengthening of microtubules.

Figure 8.5_right

MITOSIS

Metaphaseplate

Cleavagefurrow

Daughterchromosomes

Mitoticspindle

Telophase– The cell continues to elongate.

– The nuclear envelope forms around chromosomes at each pole, establishing daughter nuclei.

– Chromatin uncoils and nucleoli reappear.

– The spindle disappears.

Figure 8.5_right

MITOSIS

Metaphaseplate

Cleavagefurrow

Daughterchromosomes

Mitoticspindle

During cytokinesis, the cytoplasm is divided into separate cells.

In animal cells, cytokinesis occurs as

1. a cleavage furrow forms from a contracting ring of microfilaments, interacting with myosin, and

2. the cleavage furrow deepens to separate the contents into two cells.

8.6 Cytokinesis differs for plant and animal cells

Figure 8.6A

CytokinesisCleavage

furrow Contracting ring ofmicrofilaments

Daughtercells

Cleavagefurrow

Animation: Cytokinesis

In plant cells, cytokinesis occurs as1. a cell plate forms in the middle, from vesicles

containing cell wall material,

2. the cell plate grows outward to reach the edges, dividing the contents into two cells,

3. each cell now possesses a plasma membrane and cell wall.

Figure 8.6B

Cytokinesis

Cell wallof theparent cell

Daughternucleus

Cell wall

Plasmamembrane

Vesiclescontainingcell wallmaterial

Cell plateforming

Newcell wall

Cell plate Daughtercells

Figure 8.10A

In humans, somatic cells have

– 23 pairs of homologous chromosomes

– one chromosome of each pair from each parent.

The human sex chromosomes X and Y differ in size and genetic composition.

The other 22 pairs of chromosomes are autosomes with the same size and genetic composition.

8.11 Chromosomes are matched in homologous pairs

Homologous chromosomes are matched in

– length,

– centromere position, and

– gene locations.

A locus (plural, loci) is the position of a gene.

Different versions of a gene may be found at the same locus on maternal and paternal chromosomes.

Figure 8.11

Pair of homologouschromosomes

Centromere

Sisterchromatids

One duplicatedchromosome

Humans and many animals and plants are diploid, with body cells that have

– two sets of chromosomes,

– one from each parent.

8.12 Gametes have a single set of chromosomes

Meiosis is a process that converts diploid nuclei to haploid nuclei.

– Diploid cells have two homologous sets of chromosomes.

– Haploid cells have one set of chromosomes.

– Meiosis occurs in the 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.

Figure 8.12AHaploid gametes (n 23)

Egg cell

Sperm cell

Fertilization

Meiosis

Ovary Testis

Diploidzygote(2n 46)

MitosisKey

Haploid stage (n)Diploid stage (2n)

Multicellular diploidadults (2n 46)

Figure 8.12B

A pair ofhomologouschromosomesin a diploidparent cell

A pair ofduplicatedhomologouschromosomes

Sisterchromatids

INTERPHASE MEIOSIS I MEIOSIS II

Meiosis I – Prophase I – events occurring in the nucleus.

– Chromosomes coil and become compact.

– Homologous chromosomes come together as pairs

– Each pair, with four chromatids, is called a tetrad.

– Nonsister chromatids exchange genetic material by crossing over.

8.13 Meiosis reduces the chromosome number from diploid to haploid

Figure 8.13_1

Centrosomes(with centriolepairs) Centrioles

Sites of crossing over

Spindle

Tetrad

Nuclearenvelope

Chromatin Sisterchromatids Fragments

of thenuclearenvelope

Chromosomes duplicate Prophase IINTERPHASE:

MEIOSIS I

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.

Figure 8.13_2

Centromere(with akinetochore)

Spindle microtubulesattached to a kinetochore

Metaphaseplate Homologous

chromosomesseparate

Sister chromatidsremain attached

Metaphase I Anaphase I

MEIOSIS I

Figure 8.13_left

Centrosomes(with centriolepairs) Centrioles

Sites of crossing over

Spindle

Tetrad

Nuclearenvelope

Chromatin Sisterchromatids Fragments

of thenuclearenvelope

Centromere(with akinetochore)

Spindle microtubulesattached to a kinetochore

Metaphaseplate Homologous

chromosomesseparate

Sister chromatidsremain attached

Chromosomes duplicate Prophase I Metaphase I Anaphase IINTERPHASE:

MEIOSIS I: Homologous chromosomes separate

Meiosis I – Telophase I

– Duplicated chromosomes have reached the poles.

– A nuclear envelope re-forms around chromosomes in some species.

– Each nucleus has the haploid number of chromosomes.

Figure 8.13_3

Cleavagefurrow

Telophase I and Cytokinesis

Meiosis II follows meiosis I without chromosome duplication.

Each of the two haploid products enters meiosis II.

Meiosis II – Prophase II

– Chromosomes coil and become compact (if uncoiled after telophase I).

– Nuclear envelope, if re-formed, breaks up again.

Figure 8.13_right

Cleavagefurrow

Telophase I and Cytokinesis Prophase II Metaphase II Anaphase II

MEIOSIS II: Sister chromatids separate

Sister chromatidsseparate

Haploid daughtercells forming

Telophase IIand Cytokinesis

Meiosis II – Metaphase II – Duplicated chromosomes align at the cell equator.

Meiosis II – Anaphase II

– Sister chromatids separate and

– chromosomes move toward opposite poles.

Meiosis II – Telophase II

– Chromosomes have reached the poles of the cell.

– A nuclear envelope forms around each set of chromosomes.

– With cytokinesis, four haploid cells are produced.

8.13 Meiosis reduces the chromosome number from diploid to haploid

Mitosis and meiosis both

– begin with diploid parent cells that

– have chromosomes duplicated during the previous interphase.

However the end products differ.

– Mitosis produces two genetically identical diploid somatic daughter cells.

– Meiosis produces four genetically unique haploid gametes.

8.14 Mitosis and meiosis have important similarities and differences

Figure 8.14

Prophase

Metaphase

Duplicatedchromosome(two sisterchromatids)

MITOSIS

Parent cell(before chromosome duplication)

Chromosomeduplication

Site ofcrossingover

Chromosomesalign at themetaphase plate

Tetrads (homologouspairs) align at themetaphase plate

Tetrad formedby synapsis ofhomologouschromosomes

Metaphase I

Prophase I

MEIOSIS I

AnaphaseTelophase

Sister chromatidsseparate during

anaphase

Daughter cells of mitosis

No furtherchromosomalduplication;sisterchromatidsseparate duringanaphase II

n n n n

Daughter cells of meiosis II

Daughtercells of

meiosis I

Haploidn 2

Anaphase ITelophase I

Homologouschromosomesseparate duringanaphase I;sisterchromatidsremain together

MEIOSIS II

Genetic variation in gametes results from

– independent orientation at metaphase I

– Each pair of chromosomes independently aligns at the cell equator.

– random fertilization

– Combination of a unique sperm with a unique egg

8.15 Independent orientation of chromosomes in meiosis and random fertilization lead to varied offspring

Figure 8.15_s1

Possibility A

Two equally probablearrangements ofchromosomes at

metaphase I

Possibility B

Figure 8.15_s2

Possibility A

metaphase I

Possibility B

Metaphase II

Figure 8.15_s3

Possibility A

metaphase I

Possibility B

Metaphase II

Gametes

Combination 3 Combination 4Combination 2Combination 1

Genetic recombination is the production of new combinations of genes due to crossing over.

Crossing over is an exchange of corresponding segments between separate (nonsister) chromatids on homologous chromosomes.

– Nonsister chromatids join at a chiasma (plural, chiasmata), the site of attachment and crossing over.

– Corresponding amounts of genetic material are exchanged between maternal and paternal (nonsister) chromatids.

8.17 Crossing over further increases genetic variability

Animation: Crossing Over

Figure 8.17A

Chiasma

Tetrad

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.

8.18 A karyotype is a photographic inventory of an individual’s chromosomes

Figure 8.18_s5

Centromere

Sisterchromatids

Pair ofhomologouschromosomes

Sex chromosomes

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.

8.20 Accidents during meiosis can alter chromosome number

Figure 8.20A_s1

Nondisjunction

MEIOSIS I

Figure 8.20A_s2

Nondisjunction

MEIOSIS I

MEIOSIS II

Normalmeiosis II

Figure 8.20A_s3

Nondisjunction

MEIOSIS I

MEIOSIS II

Normalmeiosis II

Gametes

Number ofchromosomes

Abnormal gametes

n 1 n 1 n 1 n 1

Figure 8.20B_s1

Normalmeiosis I

MEIOSIS I

Figure 8.20B_s2

Normalmeiosis I

MEIOSIS I

MEIOSIS II

Nondisjunction

Figure 8.20B_s3

Normalmeiosis I

MEIOSIS I

MEIOSIS II

Nondisjunction

Abnormal gametes Normal gametes

n 1 n 1 n n

Table 8.21

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.

8.23 CONNECTION: Alterations of chromosome structure can cause birth defects and cancer

Figure 8.23A

Deletion

Duplication

Inversion

Reciprocal translocation

Homologouschromosomes Nonhomologous

chromosomes

1. Compare the parent-offspring relationship in asexual and sexual reproduction.

2. Explain why cell division is essential for prokaryotic and eukaryotic life.

3. Explain how daughter prokaryotic chromosomes are separated from each other during binary fission.

4. Compare the structure of prokaryotic and eukaryotic chromosomes.

5. Describe the stages of the cell cycle.

6. List the phases of mitosis and describe the events characteristic of each phase.

7. Compare cytokinesis in animal and plant cells.

8. Describe the functions of mitosis.

You should now be able to

9. Explain how chromosomes are paired.

10. Distinguish between somatic cells and gametes and between diploid cells and haploid cells.

11. Explain why sexual reproduction requires meiosis.

12. List the phases of meiosis I and meiosis II and describe the events characteristic of each phase.

13. Compare mitosis and meiosis noting similarities and differences.

14. Explain how genetic variation is produced in sexually reproducing organisms.

15. Explain how and why karyotyping is performed.

16. Define nondisjunction, explain how it can occur, and describe what can result.

You should now be able to

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