Mendelian Genetics and Plant Sexual Reproduction

Lecture 2R. N. Trigiano and Neal Stewart

Alternation of Generations

Mitosis and Meiosis

Mendelian Genetics

Sexual Reproduction in Plants

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 GENERATIONSALTERNATION OF GENERATIONS

Lower Plants Higher Plants

A

1

B

CD

E

MOSS FERN

A

BC

Dwww.calflora.net silviahartmann.com

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 2.1 Mitosis

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

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

STAGES OF MITOSIS 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 “ cell 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://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

Meiosis 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.

A. B. C.

Figure 2.8. Crossing- over

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

Results in recombination and increased diversity

Synapse – pairing of homologous chromosomes

Stages of MeiosisProphase: 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.

Mitosis Meiosis I Meiosis IIMetaphase Metaphase I Metaphase II

AnaphaseAnaphase I Anaphase II

Cytokinesis Cytokinesis Cytokinesis

Metaphase PlateMetaphase PlateMetaphase Plate

Figure 2.7.

2N

N

2N

N

N

N

Figure 2.2.

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

Also known as transmission genetics

Traits are inherited across generations by sexual reproduction

Mendelian traits are typically simple qualitative traits

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 DEFINITIONSSOME MORE DEFINITIONS

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

• 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 (assume no close linkage)

Mendel’s Laws

Figure 2.4. 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 YParent 1

Parent 2

F1 Hybrid Plants: 100% yellow

Parent 1 Parent 2

Yy Yy

X

YY Yy

yyYy

Y

y

Y yParent 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:

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 2.5 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

F1

R

R

r r

Flowers and Sexual Reproduction

ST

PETSEP

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

ANST

FL

ST

STL

OVY

PIST

OVL

RAF

4

Sexual Reproduction DefinitionsSexual Reproduction 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 2.11 Self-incompatibility systems in plants may be gametophytic (a) or sporophytic (b). In gametophytic self-incompatibility, the pollen grain will not grow and fertilize ovules if the female plant has the same self-incompatibility (S) alleles. In sporophytic self-incompatibility, the diploid parent prevents germination of pollen grains that share an allele with the parent. (Adapted from Briggs and Walters 1997).

Figure 4.4

Zygotic Embryo Ontogeny -- DicotZygotic Embryo Ontogeny -- Dicot

Figure 2.12

Figure 2.12 Triangle of U (1935) shows the relationships between several diploid and polyploidy crop species within the Brassica genus.

Figure 2.13

Figure 2.13 Hybridization and genetic integration between closely related species allows for the incorporation of genetic material from one species to another.

Summary Slide 1

• All plants (and animals) exhibit alternation of generations

• For higher plants, the sporophytic (2N) generation is the dominant dominant phase

• Meiosis is the driving biological process behind alternation of generations.

Summary Slide 2

• Mitosis in an equal division in which the ploidy of the mother cell is maintained

• The phases of mitosis are Prophase, Metaphase, Anaphase, and Telophase followed by Cytokinesis.

• Meiosis is a reduction division followed by an equal division similar to mitosis. Results in four cells of reduced (haploid) ploidy

• Crossing over (exchange of sister chromatid segments occurs in Prophase I and results in genetic recombination.

Summary Slide 3• 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

• 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 (assumes no close linkage)

• Expression of alleles can be dominant, recessive or codominant.