CHROMOSOME THEORY OF INHERITANCE: MITOSIS AND MEIOSIS

1/10/92, rvsd 1/9/95, 1/5/96, 1/10/97, 7 Jan 00, 8 Jan 01, 9 Jan 04, 7 Jan 05, 11 Jan 08

SGML, p. 40.-49, 7th, pp. 67-73 (to 99???), 9th: 42-

 

Mendel's laws led to chromosome theory:            1)         equal segregation

                                                                              2)         independent assortment

CHROMOSOME THEORY HISTORY:

C. Nägeli

1842

first observed chromosomes

W. Waldeyer

1888

first named chromosomes

Walther Flemming

1870s

documented behavior of chromosomes during cell division: mitosis.

Correns, von Tschermak

 and de Vries

1900

rediscovered Mendel's work

Sutton & Boveri             

1902

Saw Mendel's particles act just like chromosomes during gametogenesis:

 

CHROMOSOME                                  1) occur in pairs in adult (diploid sporophytic stage)

THEORY OF                                         2) segregate equally

HEREDITY (a la Mendel’s laws)          3) assort independently of other pairs

 

             meiosis thus generates variation (one of two genetic determinants)

             But are chromosomes identical, or different?

[omit?:]

Elinor Carothers, 1913      studied grasshopper chromosomes: one pair heteromorphic. (not identical). Could use as visible markers, showed non-homologous single assorted independently to these

 

Alfred Blakeslee, 1922      studying Datura (12 chromosomes normally) found 12 different phenotypes of fruit, each with different extra chromosome, suggested each chromosome different.

 

REVIEW OF MITOSIS: Division of somatic cells, produce clones (P 47)

 

Cell cycle, M, G-1, S, G-2 Mitosis only 5-10% of cycle, DNA synthesis in S phase

 

           PROPHASE:         Chromosome become distinct, condense, two halves = chromatids joined at centromere, nucleolus disappears

           METAPHASE:      spindle appears, chromosomes moved to equatorial plane

           ANAPHASE:        move to end of cell propelled by microtubules of spindle

           TELOPHASE:       membrane reforms, nucleoli reappear

 

This elaborate mechanism suggests that chromosomes are very important

 

MEIOSIS: (p 48) two divisions: reduction division (2N to 1N) and equational division (1N to 1N)

 

     PROPHASE I, critical stage where recombination occurs:

           LEPTOTENE   (weak, fine; ribbon): slender chromosomes appear with chromomeres (beads on necklace)

           ZYGOTENE    (join) homologous pairs synapse, first by telomeres, zip up together

           PACHYTENE  (thick) chromosomes in full synapse, chromomeres produce distinct pattern, some DNA synthesis occurs here.

           DIPLOTENE    (double) nature of chromosomes becomes apparent, each bundle consisting of four homologous chromatids (tetrad). Pairing is less tight, chiasmata apparent. At least one cross over per chromosome is required...

           DIAKINESIS   (apart; move) Further contraction, ease anaphase, more maneuverable.

 

     METAPHASE I:  move to equatorial plane, centromeres DO NOT DIVIDE

     Anaphase I, telophase I, as expected possibly followed by interkinesis, NO DNA synthesis.

           (confirm haploidy by counting centromeres)

 

Second phase of meiosis essentially like mitosis, produce 4 haploid gametes from single diploid progenitor cell