Chapter 8hhh.gavilan.edu/jcrocker/documents/Ch08_001.pdf · 17.Describe the proce ss of transcription. 18.Describe the proce ss of translation. 19.What are codons and how do they

Copyright © 2009 Pearson Education, Inc.

Lectures byGregory Ahearn

University of North FloridaAmmended byJohn Crocker

Chapter 8

The Continuity of Life: How Cells

Reproduce

Copyright © 2009 Pearson Education Inc.

Review Questions for Chapters 8-11 (as always answers must be full and complete)1. Describe the structure of DNA. Be sure to include what forms the skeleton and how are the strands

held together?2. Compare and contrast chromosomes, chromatids, genes, and alleles.3. Compare and contrast prokaryotic and eukaryotic cell division.4. Describe the process of asexual reproduction in eukaryotic cells.5. Compare and contrast animal and plant cell asexual reproduction.6. Compare and contrast mitosis and meiosis.7. Without genetic testing how could you determine if an organism is homozygous or heterozygous for a

specific trait (ie hair color)?8. Describe three ways that genetic variability is increased.9. Two fruitflies are bred. One is true breeding for red eyes and one is true breeding for white eyes. Red

eyes are dominant. What will the genotype and phenotype of the offspring be?10.If two of the offspring of the above match are crossbred what will the genotype and phenotype of their

offspring be?11.In the above examples how would the genotypes and phenotypes be different if red eye color was

partially dominant producing pink eyes when heterozygous?12.In the example above (#9) the red-eyed fruitfly has straight wings and the white-eyed fruitfly has

wrinkled wings. Straight wings are dominant. What would the genotype and phenotype of the offspring be?

13.What characteristics can make genetic disorders more likely to be passed from one generation to the next? (at least 3)

14.Describe the process of DNA replication. What is meant by semiconservative replication? How are continuous synthesis and discontinuous synthesis involved in the process?

15.How common are mistakes in replication? What safeguards are in place to prevent mistakes? What types of mistakes are relatively common?

16.Compare and cont rast DNA and RNA.17.Describe the proce ss of transcription.18.Describe the proce ss of translation.19.What are codons and how do they function in protein synthesis?20.Describe the w ays by which gene expression may be regulated.


8.1 Why Do Cells Divide?

Cells reproduce by cell division.• One cell gives rise to two or more cells, called

daughter cells.• Each daughter cell receives:

• a complete set of heredity information identical to that in the parent cell and

• about half of the cytoplasm.



Cell division transmits hereditary information to each daughter cell.• DNA contains the hereditary information in

each cell.• DNA is contained in chromosomes.• Nucleotides are the monomers of DNA.



A nucleotide consists of a phosphate, a sugar (deoxyribose), and one of four bases.• The four bases are

• adenine (A)• thymine (T)• guanine (G)• cytosine (C).

The nucleotides are held together by hydrogen bonding between the bases in the two strands, called a double helix.



The structure of DNA

Fig. 8-1

nucleotidephosphatebasesugar

A single strand of DNA The double helix(a) (b)

A

C

A

T

A

A

A

T

T

T

GC

AT

C G

G

G

C

C

Flash



Genes are the units of inheritance • made up of segments of different lengths

along a DNA molecule.

Each gene spells out the instructions for making a proteins or regulating the expression of another gene.

When a cell divides, it first replicates its DNA, and each copy is transferred into each daughter cell.

Flash



Cell division is required for growth and development.• Cell division in which the daughter cells are

genetically identical to the parent cell is called mitotic cell division.

• After cell division, the daughter cells may grow and divide again, or may differentiate, becoming specialized for specific functions.

• The repeating pattern of division, growth, and differentiation followed again by division is called the cell cycle.



Most multicellular organisms have three categories of cells.• Stem cells: retain the ability to divide and can

differentiate into a variety of cell types• Other cells capable of dividing: typically

differentiate only into one or two different cell types (progenitor cells)

• Permanently differentiated cells: differentiated cells that can never divide again



Cell division is required for sexual and asexual reproduction.• Sexual reproduction in eukaryotic organisms

occurs when offspring are produced by the fusion of gametes (sperm and eggs) from two adults.

• Gametes are produced by meiotic cell division, which results in daughter cells with exactly half of the genetic information of their parent cells.

• Fertilization of an egg by a sperm results in the restoration of the full complement of hereditary information in the offspring.



Reproduction in which offspring are formed from a single parent, without having a sperm fertilize an egg, is called asexual reproduction.• Asexual reproduction produces offspring that

are genetically identical to the parent.• Examples of asexual reproduction occur in

bacteria, single-celled eukaryotic organisms, multicellular organisms such as Hydra, and many trees, plants, and fungi.


Cell division in Paramecium

Dividing bacteria

Hydra reproduces asexually by budding A grove of aspens often consists of genetically identical trees produced by asexual reproduction

The trees in this grove have already lost their leaves

The trees in this grove are still green

The trees in this grove have begun to change color

bud

(b)

(a)

(c) (d)Fig. 8-2


8.2 What Occurs During The Prokaryotic Cell Cycle?

The prokaryotic cell cycle consists of a long period of growth, during which the cell duplicates its DNA.

Fig. 8-3a

The prokaryotic cell cycle

cell division by binary fission

cell growth and DNA replication

(a)


8.2 What Occurs During the Prokaryotic Cell Cycle?

Cell division in prokaryotes occurs by binary fission, which means “splitting in two.”

The prokaryotic chromosome is attached at one point to the plasma membrane of the cell.



The prokaryotic cell cycle

Fig. 8-3b(1)

cell wall

plasma membrane

circular DNA

attachment site

The circular DNA double helix is attached to the plasma membrane at one point.



During the growth phase of the cell cycle, the DNA is replicated, producing two identical chromosomes that become attached to the plasma membrane at two separate points.

As the cell grows, new plasma membrane is added between the attachment points of the chromosomes, pushing them apart.



The prokaryotic cell cycle (continued)

Fig. 8-3b(2)(3)

The DNA replicates and the two DNA double helices attach to the plasma membrane at nearby points.

New plasma membrane is added between the attachment points, pushing them further apart.



Once the cell has doubled in size, the plasma membrane in the middle of the cell grows inward between the two DNA attachment sites.




Fig. 8-3b(4)

The plasma membrane grows inward at the middle of the cell.



Fusion of the plasma membrane along the equator of the cell completes binary fission, producing two daughter cells, each with its own chromosomes

The two daughter cells are genetically identical to each other and to the parent cell




Fig. 8-3b(5)

The parent cell divides into two daughter cells.


8.3 How Is The DNA In Eukaryotic Cells Organized?

Unlike prokaryotic chromosomes, eukaryotic chromosomes are separated from the cytoplasm by a membrane-bound nucleus.

Eukaryotic cells always have multiple chromosomes.

Eukaryotic chromosomes contain more DNA than prokaryotic chromosomes.



The eukaryotic chromosome consists of DNA bound to protein.• Human chromosomes contain a single DNA

double helix that is 50 to 250 million nucleotides long, which would be about 3 inches long if the DNA were completely relaxed.



During cell division, proteins fold up the DNA into compact structures that are 10 times shorter than during the rest of the cell cycle.

Fig. 8-4



Duplicated chromosomes separate during cell division.• Prior to cell division, the DNA within each

chromosome is replicated.• The duplicated chromosomes then consist of

two DNA double helixes and associated proteins that are attached to each other at the centromere.



Duplicated chromosomes separate during cell division (continued).• Each of the duplicated chromosomes attached

at the centromere is called a sister chromatid.• During mitotic cell division, the sister

chromatids separate and each becomes a separate chromosome that is delivered to one of the two resulting daughter cells.


sister chromatids

centromere genes

duplicated chromosome (2 DNA double helices)

A replicated chromosome consists of two sister chromatids

Sister chromatids separate during cell division

(a)

(b)

independent daughter chromosomes, each with one identical DNAdouble helix

8.3 How Is the DNA In Eukaryotic Cells Organized?

Eukaryotic chromosomes during cell division

Fig. 8-5



Eukaryotic chromosomes usually occur in pairs.• An entire set of

stained chromosomes from a single cell is called a karyotype.

Fig. 8-6

sex chromosomes



Eukaryotic chromosomes usually occur in pairs (continued).• The nonreproductive cells of many organisms

have chromosomes in pairs, with both members of the pair being the same length.

• The chromosomes are the same length and have the same staining properties because they have the same genes arranged in the same order.



Chromosomes with the same genes are called homologous chromosomes, or homologues.

Cells with pairs of homologous chromosomes are called diploid.



Homologous chromosomes are usually not identical.• The same genes on homologous

chromosomes may be different from each other due to changes in the sequence of nucleotides in the DNA, called mutations.

• A given mutation may have occurred recently, or may have occurred generations ago and has been inherited ever since.



A typical human cell has 23 pairs of chromosomes.

22 of these pairs have a similar appearance and are called autosomes.

Human cells also have a pair of sex chromosomes, which differ from each other in appearance and in genetic composition.• Females have two X chromosomes.• Males have one X and one Y chromosome.



Not all cells have paired chromosomes.

The ovaries and testes undergo a special kind of cell division, called meiotic cell division, to produce gametes (eggs and sperm).• Gametes contain only one member of each pair of

autosomes, plus one of the two sex chromosomes.

• Cells with half the number of each type of chromosome are called haploid cells.

• Fusion of two haploid cells at fertilization produces a diploid cell with the full complement of chromosomes.



The number of different types of chromosomes in a species is called the haploid number and is designated n.• In humans, n = 23.

Diploid cells contain 2n chromosomes.• Humans body cells contain 2n = 46 (2 x 23)

chromosomes.


8.4 What Occurs During The Eukaryotic Cell Cycle?

The eukaryotic cell cycle is divided into two major phases: interphase and cell division.• During interphase, the cell acquires nutrients

from its environment, grows, and duplicates its chromosomes.

• During cell division, one copy of each chromosome and half of the cytoplasm are parceled out into each of two daughter cells.


cell growth

synthesis of DNA; chromosomes are duplicated

cell growth and differentiation

telophaseand

cytoki nesi s

anaphase

metaphase

prophase

mitotic cell

division

interphase


The eukaryotic cell cycle

Fig. 8-7



There are two types of division in eukarytic cells: mitotic cell division and meiotic cell division.• Mitotic cell division may be thought of as

ordinary cell division, such as occurs during development from a fertilized egg, during asexual reproduction, and in skin, liver, and the digestive tract every day.

• Meiotic cell division is a specialized type of cell division required for sexual reproduction.



Mitotic cell division• Mitotic cell division consists of nuclear division

(called mitosis) followed by cytoplasmic division (called cytokinesis) and the formation of two daughter cells.



Meiotic cell division• Is a prerequisite for sexual reproduction in all

eukaryotic organisms.• Meiotic cell division involves a specialized

nuclear division called meiosis.• It involves two rounds of cytokinesis,

producing four daughter cells that can become gametes.



The life cycle of eukaryotic organisms include both mitotic and meiotic cell division.• A new generation begins with the fusion of two

gametes.• Through mitosis and differentiation, the

fertilized egg grows and develops a multicellular body.

• Meiotic cell division generates new gametes that may unite with other gametes to produce the next generation.


haploid

diploid

meiotic cell division in testes

meiotic cell division in ovaries

adults

eggfertilized egg

fusion of gametes

sperm

embryo

baby

mitotic cell division, differentiation, and growth




Mitotic and meiotic cell division in the human life cycle

Fig. 8-8

Flash


8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells?

Mitosis is divided into four phases.• Prophase• Metaphase• Anaphase• Telophase

Flash


nuclear envelope chromatin

nucleolus

centriole pairs

beginning of spindle formation

kinetochore

spindle pole

spindle polecondensing chromosomes

spindle microtubules

Late InterphaseDuplicated chromosomes are in the relaxed uncondensed state; duplicated centrioles remain clustered.

Early ProphaseChromosomes condense and shorten; spindle microtubules begin to form between separating centriole pairs.

Late Prophase Thenucleolus disappears; the nuclear envelope breaks down; spindle microtubules attach to the kinetochore of each sister chromatid.

MetaphaseKinetochores interact; spindle microtubules line up the chromosomes at the cell’s equator.

(a) (b) (c) (d)


Interphase, prophase, and metaphase

Fig. 8-9a–d


chromosomes extending nuclear envelope

re-forming

Anaphase Sisterchromatids separate and move to opposite poles of the cell; spindle microtubules that are not attached to the chromosomes push the poles apart.

Telophase One set ofchromosomes reaches each pole and relaxes into the extended state; nuclear envelopes start to form around each set; spindle microtubles begin to disappear.

CytokinesisThe cell divides in two; each daughter cell receives one nucleus and about half of the cytoplasm.

Interphase of daughter cells Spindlesdisappear, intact nuclear envelopes form, chromosomes extend completely, and the nucleolus reappears.

unattached spindle microtubules

(e) (f) (g) (h)


Anaphase, telophase, cytokinesis, and interphase

Fig. 8-9e–h



During prophase, the chromosomes condense and are captured by the spindle microtubules.

Three major events happen in prophase:• The duplicated chromosomes condense.• The spindle microtubules form.• The chromosomes are captured by the

spindle.



The centriole pairs migrate with the spindle poles to opposite sides of the nucleus.• When the cell divides, each daughter cell

receives a centriole.

Every sister chromatid has a structure called a kinetochore located at the centromere, which attaches to a spindle apparatus.



Prophase

Fig. 8-9b–c



During metaphase, the chromosomes line up along the equator of the cell.• At this phase, the spindle apparatus lines up

the sister chromatids at the equator, with one kinetochore facing each cell pole.



Metaphase

Fig. 8-9d



During anaphase, sister chromatids separate and move to opposite poles of the cell.• Sister chromatids separate, becoming

independent daughter chromosomes.• The kinetochores pull the chromosomes

poleward along the spindle microtubules.



Anaphase

Fig. 8-9e



During telophase, nuclear envelopes form around both groups of chromosomes.• Telophase begins when the chromosomes

reach the poles.• The spindle microtubules disintegrate and the

nuclear envelop forms around each group of chromosomes.



Telophase

Fig. 8-9f



Cytokinesis occurs during telophase, separating each daughter nucleus into a separate cell that then begins interphase.



Cytokinesis

Fig. 8-9g



During cytokinesis, the cytoplasm is divided between two daughter cells.• Microfilaments attached to the plasma

membrane form a ring around the equator of the cell.

• During cytokinesis, the ring contracts and constricts the cell’s equator.

• Eventually, the constriction divides the cytoplasm into two new daughter cells.



During cytokinesis, the cytoplasm is divided between two daughter cells.

Fig. 8-10

Scanning electron micrograph of cytokinesis.

Microfilaments contract, pinching the cell in two

The microfilament ring contracts, pinching in the cell’s “waist.”

The waist completely pinches off, forming two daughter cells

Microfilaments form a ring around the cell’s equator.

(b)(a)



Cytokinesis in plant cells is different than in animal cells.• In plants, carbohydrate-filled vesicles bud off

the Golgi apparatus and line up along the cell’s equator between the two nuclei.

• The vesicles fuse, forming a cell plate.• The carbohydrate in the vesicles become the

cell wall between the two daughter cells.



Cytokinesis in a plant cell

Fig. 8-11


8.6 How Does Meiotic Cell Division Produce Haploid Cells?

Meiosis is the production of haploid cells with unpaired chromosomes derived from diploid parent cells with paired chromosomes.

Meiosis includes two nuclear divisions, known as meiosis I and meiosis II.• In meiosis I, homologous chromosomes pair

up, but sister chromatids remain connected to each other.

• In meiosis II, chromosomes behave as they do in mitosis—sister chromatids separate and are pulled to opposite poles of the cell. Flash



Fig. 8-12a–d

paired homologous chromosomes

recombined chromatids

spindle microtubule

kinetochoreschiasma

(a) (b) (c) (d)Prophase I Duplicated chromosomes condense. Homologous chromosomes pair up and chiasmata occur as chromatids of homologues exchange parts by crossing over. The nuclear envelope disintegrates, and spindle microtubules form.

Metaphase IPaired homologous chromosomes line up along the equator of the cell. One homologue of each pair faces each pole of the cell and attaches to the spindle microtubules via the kinetochore (blue).

Anaphase IHomologues separate, one member of each pair going to each pole of the cell. Sister chromatids do not separate.

Telophase ISpindle microtubules disappear. Two clusters of chromosomes have formed, each containing one member of each pair of homologues. The daughter nuclei are therefore haploid. Cytokinesis commonly occurs at this stage. There is little or no interphase between meiosis I and meiosis II.


(e) (f) (g) (h) (i)Prophase IIIf the chromosomes have relaxed after telophase I, they recondense. Spindle microtubules re-form and attach to the sister chromatids.

Metaphase IIThe chromosomes line up along the equator, with sister chromatids of each chromosome attached to spindle microtubules that lead to opposite poles.

Anaphase IIThe chromatids separate into independent daughter chromosomes, one former chromatid moving toward each pole.

Telophase IIThe chromosomes finish moving to opposite poles. Nuclear envelopes re-form, and the chromosomes become extended again (not shown here).

Four haploid cellsCytokinesis results in four haploid cells, each containing one member of each pair of homologous chromosomes (shown here in the condensed state).


Fig. 8-12e–i



Meiosis I separates homologous chromosomes into two haploid daughter nuclei.• During prophase I, homologues pair up.

• The two homologues in a pair intertwine, forming chiasmata (singular, chiasma).

• At some chiasmata, the homologues exchange parts in a process known as crossing over.



Prophase I

Fig. 8-12a



During metaphase I, paired homologues line up at the equator of the cell.• Interactions between the kinetochores and the

spindle microtubules move the paired homologues to the equator of the cell.



Metaphase I

Fig. 8-12b



During anaphase I, homologous chromosomes separate.• One duplicated chromosome (consisting of

two sister chromatids) from each homologous pair moves to each pole of the dividing cell.

• At the end of anaphase I, the cluster of chromosomes at each pole contains one member of each pair of homologous chromosomes.



Anaphase I

Fig. 8-12c



After telophase I and cytokinesis, there are two haploid daughter cells.• The spindle microtubules disappear and the

nuclear envelope may reappear.• Cytokinesis takes place and divides the cell

into two daugher cells; each cell has only one of each pair of homologous chromosomes and is haploid.

• Each chromosome still has two sister chromatids.



Telophase I

Fig. 8-12d



Meiosis II separates sister chromatids into four haploid daughter cells.

Meiosis II is virtually identical to mitosis, although it occurs in haploid cells.



Prophase II: the spindle microtubules re- form

Fig. 8-12e



Metaphase II: duplicated chromosomes line up at the cell’s equator

Fig. 8-12f



Anaphase II: sister chromatids move to opposite poles

Fig. 8-12g



Telophase II and cytokinesis: four haploid cells are formed

Fig. 8-12h–i



Flash


8.7 How Do Meiotic Cell Division And Sexual Reproduction Produce Genetic Variability?

Ways to produce genetic variability from meiotic cell division and sexual reproduction• Shuffling of homologues• Crossing over• Fusion of gametes



Shuffling of homologues creates novel combinations of chromosomes.• There is a random assortment of homologues

to daughter cells at meiosis I.• At metaphase I, paired homologues line up at

the cell’s equator.• Which chromosome faces which pole is

random, so it is random as to which daughter cell will receive each chromosome.


The four possible chromosome arrangements at metaphase of meiosis I

The eight possible sets of chromosomes after meiosis I

(a)

(b)


Random separation of homologues during meiosis produces genetic variability.

Fig. 8-13



Crossing over creates chromosomes with novel combinations of genetic material.• Exchange of genetic material during prophase

I, through crossing over, is a unique event each time.

• Genetic recombination through crossing over results in the formation of new combinations of genes on a given chromosome .

• As a result of genetic recombination, each sperm and each egg is genetically unique.



Crossing over

Fig. 8-14

pair of homologous duplicated chromosomes

sister chromatids of one duplicated homologue

chiasmata (sites of crossing over)

parts of chromosomes that have been exchanged between homologues



Fusion of gametes creates genetically variable offspring.• Because every egg and sperm are genetically

unique, and it is random as to which sperm fertilizes which egg, every fertilized egg is also genetically unique.

Documents

Chapter 8hhh.gavilan.edu/jcrocker/documents/Ch08_001.pdf · 17.Describe the proce ss of transcription. 18.Describe the proce ss of translation. 19.What are codons and how do they