Topic 6.  PLANT REPRODUCTION

I.  INTRODUCTION

 Reproduction
- replication and perpetuation of organelles, cells, organisms, and species a fundamental property of life

Two phases of reproduction

          a. Asexual Reproduction
                -Somatic cell division (mitosis)
                -Results in genetically identical cells and tissues

          b. Sexual Reproduction
                -Gametic cell division (meiosis)
                -Results in segregation of genetic traits



II.  REPLICATION OF LIVING SYSTEMS

    1. The Central Dogma

        DNA-----------› RNA-------------› Protein (enzyme)

           Gene - a unit of heredity; a sequence of DNA nucleotides that codes for a protein
           Chromosome - a thread-like body made up of DNA and chromatin

    2. Sites of occurrence

        DNA replication .......nucleus
        RNA synthesis..........nucleus
        Protein synthesis.......cytoplasm (ribosomes)

Nucleotides (building blocks of DNA)             Purines - adenine (A), guanine (G)
            Pyrimidines - cytosine (C), thiamine (T)
            Base pairing - on double helix (A:T, G:C)

    3. Codons

        - the basic unit of genetic information
        - 3 bases/codon or an amino acid
                - i.e. AAG = lysine     GAG = asparagine


III.  CELL REPRODUCTION

    1. Mitosis - Somatic cell division
- Ends up with the same number of chromosomes in the divided cells
 
    2N ----------------Mitosis----------------› 2N    +    2N
   (diploid)                                     ;      (diploid) (diploid)

 
    2. Meiosis
   - Cell division leading to gamete formation
    - Results in a reduction of chromosome number
    - Involves 2 separate divisions
                1st division              2nd division.
  2N ------------------meiosis-------------------› N   +   N   +   N   +   N
(diploid)                                                         (gametes, haploid)
 2N  =  20X  =  10
    where N = gametic number of chromosomes
              X = ploidy number
    3.  Chromosome Numbers

              N number refers to halfness (gametic number)
              X number refers to oneness (genomic number)

- Chromosome number varies by species
- Genomic (X) number is a set of different chromosomes
    2N = number of chromosomes in somatic cells (somatic chromosome number)
      N = number of chromosomes in gametic cells (gametic chromosome number)
      X = Number of chromosomes in a genome complement (genomic chromosome number)
    4.  Ploidy

        a. Euploidy - variation between chromosome sets

                              4X---------------meiosis----------------› 2X + 2X
                                     (tetraploid)                                        &nb sp;          (diploid gametes)
 

Ploidy Somatic Gametic
Fertility
Monoploid
2N = 1X
N = ?
very low
Diploid
2N = 2X
N = X
very high
Triploid
2N = 3X
N = ?
very low
Tetraploid
2N = 4X
N = 2X
variable
Pentaploid
2N = 5X
N = ?
variable
Hexaploid
2N = 6X
N = 3X
variable
Heptaploid
2N = 7X
N = ?
variable
Octoploid
2N = 8X
N = 4X
variable
Nonaploid
2N = 9X
N = ?
variable
Decaploid
2N = 10X
N = 5X
variable

            Example:  Chromosome numbers of some horticultural crops

Species
Ploidy
Somatic chromosome number
(2N)
Gametic chromosome number
(N)
Genomic chromosome number 
(X)
Spinach
2X
12
6
6
Corn
2X
20
10
10
Potato
4X
48
24
12
Strawberry
8X
56
28
7

            How to To Make a Polyploids

                                                              Colchicine (chromosome doubling)
                    Diploid Plant (2X)   --------------------------------------------›  Tetraploid plant (4X)
                       Germinating seed,               
                        or meristem                                                             

                Colchicine treatment:



IV.   SEXUAL REPRODUCTION

    1. Life Cycle

                - Alternation of sporophytic (2N) and gametophytic (N) stages in cycles.
                - In plants, the gametophytic stage is brief.



        a.  Microgametophyte - pollen or male gametophyte

        b.  Megagametophyte - embryo sac, female gametophyte - formed in ovules
- Commonly consists of 8 nuclei:
 
 
Megagametophyte

  1 egg nucleus 
  2 polar nuclei
  3 antipodal nuclei
  2 synergids
 
 


 
 2.  Fertilization - Pollen tube penetration of embyo sac
- Formation of zygote (2N) by double fertilization
        Double Fertilization
A process of fertilization in which one male gamete (sperm nucleus, 1N) unites the egg nucleus (1N) to form embryo (zygote, 2N) and the other sperm nucleus unites polar nuclei (2n) to form endosperm (3N).