Mendelian Genetics, Plant Sexual Reproduction and Other Stuff

R. N. Trigiano, Professor EPP

17 January 2012

Alternation of Generations

Mitosis and Meiosis

Mendelian Genetics

Sexual Reproduction in Plants

ALTERNATION OF GENERATIONS

DEFINITIONS:

Diploid: Having two sets of chromomosomes or 2N – the sporophyte phase of plants. 2N = 2X = 22 (Flowering dogwood)

Haploid: Having one set of chromosomes or N – the gametophyte phase of plants. N = X=11 (Flowering dogwood)

Polyploid: Having more than two sets of chromosomes – 2N = 6X = 54 sporophyte; gametophyte N = 3X= 27 (Chrysanthemum)

How is Alternation of Generations Accomplished?

Mitosis (1 cell 2 cells) and Meiosis (1 cell 4 cells)

and Sexual Reproduction where 1 +1 = 2 = 1!

Gametophyte (N) mitosis Gametes (N) fuse

(sexual reproduction) to form zygote (2N) mitosis

Mature Sporophyte (2N) meiosis Spores (n)

SHADED AREA REPRESENT RELATIVE TIME IN DIPLOID OR

SPOROPHYTIC PHASE; OPEN = RELATIVE TIME IN HAPLOID

OR GAMETOPHYTIC PHASE.

ALTERNATION OF GENERATIONS

Lower Plants Higher Plants

A

1

B

CD

E

MOSS FERN

A

B

C

Dwww.calflora.net

silviahartmann.com

Mitosis and Meiosis

http://www.phschool.com/science/biology_place/biocoach/index.html

http://www.vcbio.science.ru.nl/images/cellcycle/mmitosis_onion_zoom.jpg

Figure 6.1

Structure of DNA

Nitrogenous bases

AdenineThymineGuanineCytosine

Phosphate (PO4)

Deoxyribose sugar

Chromatid Sister Chromatids

DNA Synthesis

Chromosome 1 Chromosome 1

Centromere Centromere

B. Chromosome Conformations C. Homologous Chromosomes

Chromosome 1 Chromosome 2

Locus 1

Locus 2

Condensed ChromosomeRelaxed Chromosome

A. Chromosome Physical States

Nucleus Nucleus

Figure 2.2

http://nobelprize.org/nobel_prizes/medicine/laureates/2001/cellcycle_eng.jpg

STAGES OF MITOSIS

PROPHASE: Shortening and thickening of chromosomes (actually sister chromatids: Movement to the metaphase plate; nucleous disappears; nuclear membrane disappears.

METAPHASE: Chromosomes lie on the “plate”.

ANAPHASE: Sister chromatids separate and are pulled toward the poles.

TELOPHASE: Last phase of mitosis; chromosomes become reorganized into nuclei nucleolus reappears and nuclear membrane is reformed.

CYTOKINESIS: Literally division of the cytoplasm (including organelles such mito- chondria, chloroplasts, etc.

Essentially, mitosis is “asexual division (formation of somatic cells and builds the plants. Theoretically, all cells resulting from mitosis are “clones” – not quite true, but close enough for our purposes.

http://tainano.com/Molecular%20Biology%20Glossary.files/image011.gif

http://www.vcbio.science.ru.nl/images/cellcycle/mmitosis_onion_zoom.jpg

1 = Interphase, 2-5 = Prophase; 6 = Early Metaphase; 7 = Late Metaphase; 8 = Anaphase; 9 = Telophase; 10 = Completion of Cytokinesis

Interphase

Figure 2.7. Mitosis

Nucleus

ProphaseMetaphase

Telophase

Anaphase

Cytokinesis

Metaphase Plate

OH NO ! MORE DEFINITIONS

Meiosis: Two successive divisions (reduction followed by an equal) that

reduces the ploidy level from 2n to n or diploid to haploid.

Homologous Chromosomes: Chromosomes that associate in pairs in

the first division of meiosis: each member of the pair was derived from a different parent.

Crossing Over: The exchange of corresponding segments of genetic material between chromatids of homologous chromosomes. See also Synapse and Chiasma

Recombination: The process by which offspring can gain combination of genes different from the combinations in either of their parents. Leads to genetic diversity.

Stages of Meiosis

Prophase: Same as in mitosis except that crossing over occurs here.

Metaphase I: Homologous chromosomes (each chromosome consists of 2

sister chromatids) line up “opposite” each other on the plate.

Anaphase I: One of each of the homologous pair of chromosomes are “drawn” to the poles. This essentially reduces the number of chromosomes that are in each cell by half. Diploid Haploid.

Telophase I: Haploid cells are established; chromosomes “disappear”.

Prophase II: Chromosome condense (sister chromatids are not identical to one another because of crossing over).

Metaphase II: Chromosomes (actually sister chromatids) align on the plate.

Anaphase II: One sister chromatid (now chromosome) are drawn to each pole.

Teleophase II and Cytokinesis: Essentially the same as in mitosis.

Meiosis is essentially a “reduction” division followed by an “equal” division. “Tetrad” (4) of cells or spores are formed.

A. B. C.

Figure 2.9. Crossing- over

Chiasma - Exchange between homologous chromosomes. Actually sister chromatids during Prophase I.

Results in recombination and increased diversity

Synapse – pairing of homologous Chromosomes

Mitosis Meiosis I Meiosis IIMetaphase Metaphase I Metaphase II

AnaphaseAnaphase I Anaphase II

Cytokinesis Cytokinesis Cytokinesis

Metaphase PlateMetaphase PlateMetaphase Plate

Figure 2.8.

2N

N

2N

N

N

N

A

B

C

D

E

F

G

H

RAF

3 Meiosis

Figure 2. 3.

Gregory Mendel working with peas made two innovations to the science of genetics: 1) developed pure lines 2) counted his results and kept statistical notes

Pure Line - a population that breeds true for a particular trait[this was an important innovation because any non-pure (segregating) generation would and did confuse the results of genetic experiments]

Allele - one alternative form of a given allelic pair; tall and dwarf are the alleles for the height of a pea plant; more than two alleles can exist for any specific gene, but only two of them will be found within any individual

Dominant - the allele that expresses itself at the expense of an alternate allele; the phenotype that is expressed in the F1 generation from the cross of two pure lines. Recessive - an allele whose expression is suppressed in the presence of a dominant allele; the phenotype that disappears in the F1 generation from the cross of two pure lines and reappears in the F2 generation

Co-dominance: a relationship among alleles where both alleles contribute to the phenotype of the heterozygote.

Allelic pair - the combination of two alleles that comprise the gene pair

SOME MORE DEFINITIONS

http://www.ndsu.edu/instruct/mcclean/plsc431/mendel/mendel1.htm

More Definitions !!!

• Homozygote - an individual which contains only one allele at the allelic pair; for example DD is homozygous dominant and dd is homozygous recessive; pure lines are homozygous for the gene of interest

• Heterozygote - an individual which contains one of each member of the gene pair; for example the Dd heterozygote

• Genotype - the specific allelic combination for a certain gene or set of genes

• Phenotype - literally means "the form that is shown"; it is the outward, physical appearance of a particular trait

• Mendel's pea plants exhibited the following phenotypes: • - round or wrinkled seed phenotype • - yellow or green seed phenotype • - red or white flower phenotype • - tall or dwarf plant phenotype

http://www.ndsu.edu/instruct/mcclean/plsc431/mendel/mendel1.htm

Mendel's First Law - the law of segregation; during gameteformation each member of the allelic pair separates from the other member to form the genetic constitution of the gamete

e.g. A a yields gametes with A a

Mendel’s Second Law -- the law of independent assortment; during gamete formation the segregation of the alleles of one allelic pair is independent of the segregation of the alleles of another allelic pair

Mendel’s Laws

Figure 2.5. Mendel's First Law - the law of segregation; during gamete formation each member of the allelic pair separates from the other member to form the genetic constitution of the gamete

Parent 1 Parent 2

YY yy

X

Yy Yy

YyYy

y

y

Y Y

Parent 1

Parent 2

F1 Hybrid Plants: 100% yellow

Parent 1 Parent 2

Yy Yy

X

YY Yy

yyYy

Y

Y

Y y

Parent 1

Parent 2

F2 Plants: 75% yellow 25% green

A. Monohybrid Cross

Gametes: Y Y y y Gametes: Y y Y y

Yy

B. F1 Self Fertilization

YY & Yy

yy

F1 Fertilization: F2 Fertilization:

TtYy

TTYY TTYy

TY

Parent 1

Parent 2

TtYY TtYy

Ty tY ty

TTYy TTyy TtYy Ttyy

TyYY TtYy ttYY ttYy

TtYy Ttyy ttYy ttyy

TY

Ty

tY

ty

Figure 2.6. Independent Assortment – Two or more traits.

TtYy

TTYY ttyy

F2 Fertilization:

T_Y_ = 9/16tall/yellow

T_yy = 3/16tall/green

ttY_ = 3/16short/yellow

ttyy = 1/16short/green

F1 Fertilization:

X

F1 Plants: 100% tall/yellow

TtYy

X

Ratios ofF2 Plants:

Parent 1 Parent 2

B. F1 Self Fertilization

A. Dihybrid Cross

TY

TY

ty ty

TtYy

TtYy

TtYy

TtYy

F1 Generation of Dihybid cross

TtYy

TTYY TTYy

TY

Parent 1

Parent 2

TtYY TtYy

Ty tY ty

TTYy TTyy TtYy Ttyy

TyYY TtYy ttYY ttYy

TtYy Ttyy ttYy ttyy

TY

Ty

tY

ty

Figure 2.6. Independent Assortment – Two or more traits (color and height).

TtYy

TTYY ttyy

F2 Fertilization:

T_Y_ = 9/16tall/yellow

T_yy = 3/16tall/green

ttY_ = 3/16short/yellow

ttyy = 1/16short/green

F1 Fertilization:

X

F1 Plants: 100% tall/yellow

TtYy

X

Ratios ofF2 Plants:

Parent 1 Parent 2

B. F1 Self Fertilization

A. Dihybrid Cross

Co-dominance –Variation to Mendel “laws”

Another Monohybrid Cross

RR = Red Flower Parent -- rr = White Flower Parent

R

R

r r

Rr Rr

Rr Rr

If Dominance (R) then all should be RED

F1

Rr Rr

Rr Rr

But co-dominance of R and rproduces all pink flowers

R

R

r

r

RR

rR

Rr

rr

1 Red; 2 Pink; 1 WhiteF2

Flowers and Sexual Reproduction

ST

PET

SEP

PIST

5

Figure 36.5. Morphology of a dicot flower. The perfect flower of this Oxalis species has five sepals (SEP), which are hidden from view in the open flowers by the five petals(PET), five stamens (ST) and one pistil (PIS). Compare to Figures 36.5 and 36.6.

Flower Parts – Only Angiosperms!

SEP

PET

REC

AN

STFL

ST

STL

OVY

PIST

OVL

RAF

4

OVL

OVY FL

PET

SEP

OVL

OVY

REC

STY

FILPETSEP

A B

CAR

LOCOVL

6

A

B C

D E

MMC

GN

PTNS

PTN

PT

T

RAF

8

IV

MSMC

A

B

FMSN

DN

N

C

AN

PN

M

E

HN

D

SM RAF

7

You’re Right!!! More Definitions

Self Incompatibility: Plant that cannot fertilize its female gametes with its own male gametes. Basically two types: Gametophytic in which the pollen grain will not grow when it shares the same incompatibility gene (s) and Sporophytic in which the diploid parent hinders germination and growth of the pollen tube. Both types prevent double fertilization.

Outcrossing Species: basically self-incompatible; requires a partner of a different genotype.

Pollination: The process in which the pollen is carried from the anther to the stigma.Basically two types: Cross (between different genotypes of plants) and Self (same plant).

Double Fertilization: one sperm nucleus unites with egg to form the zygote and restore the sporophytic phase (2N) of the plant and the other sperm nucleus uniting with the polar nuclei of the female gametophyte to form the primary endosperm nucleus.

Figure 2.12 Pollen parent genotypeS1S2

S1 S2 S1 S2 S1 S2

Pollen parent genotypeS1S2

S1 S3 S1 S3 S1 S3

S1S2 pistil

ES ES ES

ESESES

S1S2 pistil

S1S3 pistil

(a)

(b)

S2S3 pistil

S3S4 pistil

S2S4 pistil

Gametophytic Incompatibility

Sporophytic Incompatibility

B

C

D

PT

PN

SPE

12

PEN

ZY

PG

PT

OVY

OVL

A

PT

AN

N

E

PN

RAF

9 Double Fertilization

Figure 4.4

Zygotic Embryo Ontogeny -- Dicot

AA

YGE

S

EN

B

GE

S

C

I

D

VT

C

I

EN

C

SMH

RM

E

E

10

Zygotic embryogenesisCercis canadensis(Eastern Redbud)

A B

C D

SC

CL

EN

SM

RM

CR

SN

H

H

EN

SM

SMC

CI

11 Somatic Embryogenesis

Orchardgrass – a monocot

Eastern Redbud – a dicot