Topic 6.  PLANT REPRODUCTION

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

Two phases of reproduction

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

        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)

    3. Codons

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

- Somatic cell division
- Ends up with the same number of chromosomes in the divided cells
 

    2N ----------------Mitosis----------------› 2N    +    2N
   (diploid)                                     ;      (diploid) (diploid)

   - Cell division leading to gamete formation
    - Results in a reduction of chromosome number
    - Involves 2 separate divisions

                1^st division              2^nd division.
  2N ------------------meiosis-------------------› N   +   N   +   N   +   N
(diploid)                                                         (gametes, haploid)

 2N  =  20X  =  10
    where N = gametic number of chromosomes
              X = ploidy number

              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)

        a. Euploidy - variation between chromosome sets

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

            Example:  Chromosome numbers of some horticultural crops

            How to To Make a Polyploids

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

                Colchicine treatment:

• interferes with spindle fiber function
• used to induce polyploids, seedless watermelon

 
 
b. Aneuploidy - variation within a chromosome set
 
A diploid plant (2N=2X=14) would contain the following chromosome numbers depending on aneuploidy conditions:
 

Aneuploidy	Somatic chromosome number	Examples
Trisomic	2N + 1 (extra)	11  22  33  44  55  66 777
Monosomic	2N - (deficient)	11  22  3_  44  55  66  77
Double-trisomic	2N + 1 + 1 (2 extra)	11  22  33  44  555  66  77
4 of o chromosome	2N + 2 (2 extra)	11  22  33  44  5555  66  77

Note:

   Down syndrome in humans - caused by a trisomic for chromosome # 21
                                                 (three chromosomes instead of two)
 
 

How To Obtain Seedless Watermelon:
 

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

        a.  Microgametophyte - pollen or male gametophyte

- formed in anthers
- usually has 3 nuclei (2 sperm nuclei + 1 tube nucleus)
 
 

Microgametophyte   - One tube nucleus    - Two sperm nuclei

- formed in ovules
- Commonly consists of 8 nuclei:
 
 

Megagametophyte   1 egg nucleus    2 polar nuclei   3 antipodal nuclei   2 synergids

 

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).