Chapter 9

The passage of life’s organization and information from one generation to the next One way, but are there others?

How do organisms pass genetic information?

Are the contributions the same from males and females?

What kinds of mishaps occur and where do they originate?

General Life StrategiesAsexual reproduction

corms

bulbs

fragmentation

•No exchange of genetic material•Offspring are genetically identical to parents•No time ‘wasted” finding a mate•No courtship

Figure 9.8

Bacterial Duplication

Some Interesting Strategies

The life cycle of aphids can involve a mix of parthenogenetic (asexual) and sexual reproduction. Parthenogenetic reproduction provides the development of young from unfertilized eggs. The young are female and genetically identical to the parent. Eggs typically hatch in spring and develop into wingless females which then produce live young. After some generations of parthenogenesis, winged reproductive males and females are produced which mate and lay eggs.

Another Interesting Organism

In approximately 15 of the Cnemidophorus species there are no males. They reproduce by parthenogenesis.

Parthenogenesis is rare in vertebrates. The offspring of parthenogenic lizards are clones, identical to the mother.

Human Cloning

1997 Dolly

1998 Mice

2000 Monkey Business

United Nations (Nov. 20, 2001) - A key General Assembly committee backed a resolution calling for a treaty to ban the cloning of human beings, saying it was "contrary to human dignity.“ Under the draft resolution, a group would meet twice next year to define what should be negotiated in an international convention to ban reproductive cloning.

BACTERIAL CONJUGATION AND RECOMBINATION

Hfr cell Normal cell

Conjugation tube

1. Hfr cells containgenes that allow themto transfer some or all of their chromosometo another cell.

2. Conjugation tubeconnects Hfr cell to normal cell. Copy of Hfr chromosome begins to move to recipient cell.

3. Homologous sections of chromosome synapse.

4. Cells separate. Section of Hfr chromosome integrates into recipient chromosome by crossing over.

Box 9.3, Figure 1 But something else is happening: genetic recombination

Generation 1

Generation 2

Generation 3

Asexual reproduction Sexual reproduction

Figure 9.9

Some comparisons between asexual and sexual reproduction

So, what good are males???

Genetic Recombination: Sexual Reproduction

What are the benefits?

• Two copies of each gene (provides instructions)

• “Sharing” of beneficial genes

• “Infinite” number of combinations (variation)

Genetic Recombination: Sexual Reproduction

What are the Costs?

• Courtship expenses

• Two parents investing resources

• “Complicated” process to make gametes

• Dangerous!

Genetic Recombination: Sexual Reproduction

What are the Costs?

• Courtship expenses

• Two parents investing resources

• “Complicated” process to make gametes

• Dangerous!

Genetic Recombination: Sexual Reproduction

What are the Costs?

• Courtship expenses

• Two parents investing resources

• “Complicated” process to make gametes

• Dangerous!

Genetic Recombination: Sexual Reproduction

What are the Costs?

• Courtship expenses

• Two parents investing resources

• “Complicated” process to make gametes

• Dangerous!

Life Cycle Strategies Involving Sexual Reproduction

Diploid Dominant (two copies of each chromosome)

Haploid Dominant (one copy of each chromosome)

Alteration of Generations

Diploid adult

MITOSIS FERTILIZATION

MEIOSIS:

2n >> n

Haploidgametes (n)

Diploidzygote

Figure 9.7a

Diploid dominant

2n

FERTILIZATION

MITOSIS

MITOSISMEIOSIS

Diploid cell

Haploid cell

Haploid gametes

Haploid adult

Haploid dominant

Figure 9.7b

Diploid plant

Diploid cell

Haploid cells

Haploid gametes Haploid

plant

MITOSIS

MEIOSISMITOSIS

FERTILIZATIION

MITOSIS

Alternation of generations

Figure 9.7c, upper

Snails subject to parasitism by trematode worms (Lively)

Figure 9.10a

Evidence for the benefits of sexual reproduction: resistance

0.40

0.30

0.20

0.15

0.10

0.05

0.01

0.00 0.05 0.15 0.30 0.50

0.00

Mal

e fr

equ

ency

Frequency of infection by parasites

Figure 9.10b

Are genetically diverse populations more resistant to parasites?

Meiosis is a Special Type of Cell Division that Occurs in Sexually Reproducing Organisms

Meiosis reduces the chromosome number by half, enabling sexual recombination to occur.

• Meiosis of diploid cells produces haploid daughter cells, which may function as gametes. (Fig. 9.2a-c, 9.3)

A full complement of chromosomes is restored during fertilization.

Femalegamete

n = 23 in humans

Fertilization

Diploid offspringcontains homologouspair of chromosomes

Malegamete

n = 23 in humans

Figure 9.2c

Each chromosome replicates prior to undergoing meiosis.

Maternalchromosome

Centromere

Homologous pair of premeiotic chromosomes

Duplication in

S phase

Paternal chromosome

Sister chromatids

Figure 9.2a

(n = 23 in humans) (n = 23 in humans)

During meiosis, chromosome number in each cell is reduced.

Parent cellcontainshomologouspair of chromosomes

ME

IOS

IS I

Homologs separate

at meiosis I

Sisterchromatids separate at meiosis II

Daughtercellscontainjust one homolog

Four daughter cells contain one chromosome each. These cells become gametes.

ME

IOS

IS I

I

Figure 9.2b

PRIOR TO MEIOSIS MEIOSIS I

Homologous chromosomes separate.

Sister chromatids

Tetrad (4 chromatids from homologous chromosomes)

Chiasma

1. Chromosomesreplicate inparent cell.

2. Synapsis of homologous chromosomes. Crossing over of non-sister chromatids.

3. Tetrads migrate to middle of cell.

4. Homologsseparate.

Chromosomes replicate, forming sister chromatids.

Figure 9.3, left

MEIOSIS II

Sister chromatids separate

5. Cell divides. 6. Chromosomes begin moving to middle of cell.

7. Chromosomes line up at middle of cell.

8. Sister chromatids separate.

9. Cell division results in four daughter cells.

Figure 9.3, right

Meiosis is a Special Type of Cell Division that Occurs in Sexually Reproducing Organisms

Meiosis reduces the chromosome number by half, enabling sexual recombination to occur.

• Gametes undergo fertilization, restoring the diploid number of chromosomes in the zygote.

But what about the difference in sizebetween the egg and sperm?

Can be “extrachromosomal” factors in cytoplasm of egg:Mitochondria, chloroplasts, infectious agents, chemicals

23 pairs of chromosomes in humans

Box 9.1 Figure 1

12 types of chromosomes in the lubber grasshopper

Each type of chromosome has two homologs.

ab

c

d

X

e

fh

g

ij

c

X

ba j

e

k

kd

g

f

hi

Figure 9.1a,b

Meiosis is a Special Type of Cell Division that Occurs in Sexually Reproducing Organisms

Meiosis and fertilization introduce genetic variation in several ways:

Independent assortment of homologous pairs at metaphase I:

• Each homologous pair can orient in either of two ways at the plane of cell division. (Fig. 9.5a,b)

• The total number of possible outcomes = 2n (n = number of haploid chromosomes). (Fig. 9.6)

• Crossing over between homologous chromosomes at prophase I.

Hypothetical example

Eye color

Gene thatcontributes to browneyes

Gene thatcontributes to blueeyes

Maternalchromosome

Paternalchromosome

Hair color

Gene thatcontributes to black hair

Gene thatcontributes to red hair

Maternalchromosome

Paternalchromosome

Figure 9.5a

During meiosis I, tetrads can line up two different waysbefore the homologs separate.

OR

Brown eyesBlack hair

Blue eyesRed hair

Blue eyesBlack hair

Brown eyesRed hair

Figure 9.5b

2. Crossingover duringmeiosis I.

1. Parent cellwith four chromosomes.

3. Homologs separate.(Pairing of chromosomes depends on independentassortment.)

4. Gametes produced by meiosis II.

5. Offspring produced by selfing (only some of the possibilities shown.)

EVEN SELF-FERTILIZATION LEADS TO GENETICALLY VARIABLE OFFSPRING because of crossing over

Figure 9.6

Crossing over

Shape of chromosome 9 varies in two maize strainsKnob

Long

Strain 1 Strain 2

No knob

Short

Genes on chromosome 9 also vary

Colored kernels

Waxy kernels

Strain 1 Strain 2

Colorless kernels

Starchy kernels

Box 9.2, Figure 1a,b: Crossing over involves breakage and reunion of chromatids

Predictions of crossing over hypothesis

Products of meiosis

Chromosome shape:

Traits contributedto offspring:

Long withknob

Short withknob

Colored, waxy kernels

Longwithno knob

Shortwith no knob

Colored, starchykernels

Colorless, waxykernels

Colorless, starchykernels

If crossing over results in exchange ofgenetic material between twochromosomes, the products ofmeiosis will look like this:

Experimental results support these predictions

Box 9.2, Figure 1c

Figure 9.4c

Figure 9.4b

Figure 9.4d

The Consequences of Meiotic Mistakes

Nondisjunctions occur when homologous chromosomes fail to separate at meiosis I or when chromatids fail to separate at meiosis II.

• Fertilization can result in embryos that are 2n + 1 (a “trisomy”) or 2n - 1. (Fig. 9.11)

• Abnormal copy numbers of one or more chromosomes is usually, but not always, fatal (Example: Down syndrome). (Fig. 9.12)

• Human survivors: trisomics = 13, 18, 21

n + 1

n + 1

n – 1

n – 1

1. Meiosis I starts normally. Tetrads line up in middle of cell.

2. Then one set of homologs does not separate (= nondisjunction).

3. Meiosis II occurs normally.

4. All gametes have an abnormal number of chromosomes--either one too many or one too few.

NONDISJUNCTION at Meiosis I: most common cause, weak meiosis I

alignment checkpoint in females???

2n = 4  n = 2

Figure 9.11

Inci

den

ce

of

Do

wn

syn

dro

me

per

nu

mb

er o

f b

irth

s

20 24 28 32 37 4742

Age of mother (years)

12300

11600

11200

1100

146

1290

1880

Figure 9.12

Other Consequences of Meiosis

Polyploidy can occur when whole sets of chromosomes fail to separate at meiosis I or II.

• The resulting 2n gametes, if fertilized by normal sperm, create 3n zygotes (triploid).

• Organisms with an odd number of chromosome sets cannot produce viable gametes (Example: seedless fruits).

3n = 2X1 chromosome separation at meiosis I = unbalanced gametes, undeveloped seeds

So where does this take us?

How do mitosis and meiosis figure into the passage of genetic information?

What are “patterns of inheritance”?

How do genes determine organismic characteristics