Lecture 2
R. 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)

ALTERNATION OF GENERATIONS
Lower Plants Higher Plants
SHADED AREA REPRESENT RELATIVE TIME IN DIPLOID OR
SPOROPHYTIC PHASE; OPEN = RELATIVE TIME IN HAPLOID
OR GAMETOPHYTIC PHASE.

1
MOSS
FERN
A www.calflora.net
E
D
B silviahartmann.com
A
D C
C
B

Figure 6.1
Structure of DNA
Nitrogenous bases
Adenine
Thymine
Guanine
Cytosine
Phosphate (PO
4
)
Deoxyribose sugar

Figure 2.1
Mitosis
A. Chromosome Physical States
Nucleus
Nucleus
Relaxed Chromosome
B. Chromosome Conformations
Chromatid Sister Chromatids
Condensed Chromosome
C. Homologous Chromosomes
Centromere
DNA
Synthesis
Centromere
Locus 2
Locus 1
Chromosome 1 Chromosome 2
Chromosome 1 Chromosome 1

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

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

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 mitochondria, 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.

1 = Interphase , 2-5 = Prophase ; 6 = Early Metaphase ; 7 = Late
Metaphase ; 8 = Anaphase ; 9 = Telophase ; 10 = Completion of
Cytokinesis http://www.vcbio.science.ru.nl/images/cellcycle/mmitosis_onion_zoom.jpg

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.

Figure 2.8. Crossing- over
A
.
B
.
C
.
Synapse – pairing of homologous chromosomes
Chiasma - Exchange between homologous chromosomes--sister chromatids during Prophase I.
Results in recombination and increased 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.

Figure 2.7.
Mitosis
Metaphase
2N
Anaphase
Metaphase Plate
Metaphase I
Meiosis I
2N
Anaphase I
Metaphase Plate
Metaphase II
Meiosis II
N
Anaphase II
Metaphase Plate
N
Cytokinesis
N
Cytokinesis
N
Cytokinesis

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





SOME MORE DEFINITIONS
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 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 gamete formation 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)

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
A. Monohybrid Cross B. F 1 Self Fertilization
Parent 1 Parent 2 Parent 1 Parent 2
X
X
YY yy Yy Yy
Gametes: Y Y y y
F
1
Fertilization:
Parent 1
Y Y
Yy Yy y
Parent 2 y Yy Yy
F
1
Hybrid Plants: 100% yellow
Yy
Gametes: Y y Y y
F
2
Fertilization:
Parent 1
Y y
YY Yy Y
Parent 2 y Yy yy
F
2
Plants: 75% yellow
25% green
YY & Yy yy

TY ty
TtYy ty
TtYy
TY
TtYy TtYy
F1 Generation of Dihybid cross

Figure 2.5 Independent Assortment – Two or more traits (color and height).
A. Dihybrid Cross
Parent 1
TTYY X
Parent 2 ttyy
F
1
Fertilization:
TtYy
F
1
Plants: 100% tall/yellow
B. F 1 Self Fertilization
TtYy X TtYy
F
2
Fertilization:
Ty
Parent 1 tY TY ty
TY TTYY TTYy TtYY TtYy
Parent 2
Ty TTYy TTyy TtYy Ttyy tY
TyYY TtYy ttYY ttYy ty TtYy Ttyy ttYy ttyy
Ratios of
F
2
Plants:
T_Y_ = 9/16 tall/ yellow
T_yy = 3/16 tall/green ttY_ = 3/16 short/yellow ttyy = 1/16 short/green

Another Monohybrid Cross
RR = Red Flower Parent -- rr = White Flower Parent r r
Rr
R Rr
F
1
R Rr Rr
If Dominance (R) then all should be RED

r
R
R Rr r
R
Rr
R
RR rR r
Rr
Rr r
Rr rr
F
1
But co-dominance of R and r produces all pink flowers
F 2
1 Red; 2 Pink; 1 White

5 Flower Parts – Only Angiosperms!
SEP
PIST
PET
ST
Figure 36.5. Morphology of a dicot flower. The perfect flower of this Oxalis species

4
AN
ST
FL
PET
SEP
REC
ST
STL
OVY
PIST
OVL
RAF

Sexual 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 selfincompatibility, the diploid parent prevents germination of pollen grains that share an allele with the parent. (Adapted from Briggs and
Walters 1997).

Zygotic Embryo Ontogeny -- Dicot
Figure 4.4

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.

• 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.

• 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.

• 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.