Download MITOSIS THE HEREDITARY MATERIAL OF ORGANISMS (PLANTS

MITOSIS
THE HEREDITARY MATERIAL OF ORGANISMS
(PLANTS) ARE ORDERED INTO CHROMOSOMES
I. KARYOTYPE – THE CHARACTER TRAITS OF
THE CHROMOSOMAL COMPLEMENT OF A CELL
WITH REFERENCE TO THE COMPARATIVE
SIZE, SHAPE, NUMBER AND MORPHOLOGY OF
THE DIFFERENT CHROMOSOMES IN A CELL,
INCLUDING THICKNESS, LENGTH, POSITION OF
THE CENTROMERE, ETC.
A. THESE TRAITS CAN DIFFER BETWEEN
SPECIES
B. MEASUREMENTS ARE DONE DURING
EITHER MEIOSIS OR MITOSIS
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C. WHAT IS THE DIFFERENCE BETWEEN
MITOSIS AND MEIOSIS? WHAT STAGE IS
THE BEST TO DETERMINE KARYOTYPIC
TRAITS?
II. CHROMOSOME NUMBER
A. CHROMOSOMES ARE PRESENT IN PAIRS IN
SEXUALLY REPRODUCING ORGANISMS
(PLANTS)
1. THE TWO MEMBERS OF EACH PAIR ARE
ESSENTIALLY IDENTICAL (KARYOTYPE)
AND ARE CALLED HOMOLOGOUS
CHROMOSOMES
a. ONE HOMOLOGUE IS PATERNAL
b. ONE HOMOLOGUE IS MATERNAL
2. THE TOTAL NUMBER OF CHROMOSOMES
IN EACH CELL IS THE CHROMOSOME
2
COMPLEMENT
3. THE CHROMOSOME COMPLEMENT IS
CONSTANT FOR ALL SOMATIC CELLS
OF AN INDIVIDUAL PLANT
B. HAPLOID = 1n = THE NUMBER OF CHOMOSOMES IN A GAMETE (EGG OR SPERM) AND
IS CALLED THE GENOME
C. DIPLOID = 2n = THE NUMBER OF CHROMOSOMES IN A ZYGOTE; HENCE THE NUMBER
IN SOMATIC CELLS OF PLANTS.
THEREFORE A DIPLOID HAS TWO CHROMOSOME SETS OR GENOMES
III. POLYPLOID – COMPOSED OF THAN TWO
GENOMES; TRIPLOID = 3x; TETRAPLOID = 4x;
HEXAPLOID = 6x; OCTOPLOID =8x.
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A. EUPLOIDY – THE GAIN OR LOSS OF
COMPLETE SETS OF CHROMOSOMES OR
GENOMES
B. EUPLOIDY IS VERY COMMON IN PLANTS
(RARE IN ANIMALS) AND CAN ARISE IN THREE
BASIC WAYS:
1. SOMETHING CAUSES A DOUBLING OF
THE CHROMOSOME NUMBER IN A CELL –
THIS USUALLY HAPPENS IN A SOMATIC
CELL, AND WHEN IT HAPPENS IN THE
MERISTEM THE PLANT CAN BE AFFECTED
(X-RAYS, UV LIGHT, CHEMICALS, ETC.)
2. SOMETHING GOES WRONG IN MEIOSIS,
RATHER THAN REDUCING THE NUMBER
OF CHROMOSOMES TO A 1n GAMETE,
GAMETES ARE NOW 2n
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3. TWO PLANTS WITH DIFFERENT
GENOMES HYBRIDIZE – AA X BB = AB
a. NORMALLY THE F1 HYBRID IS HIGHLY
STERILE BECAUSE OF THE INABILITY OF
NON-HOMOLOGOUS CHROMOSOMES TO
PAIR IN MEIOSIS – A AND B
b. IN NATURAL POPULATIONS STERILE
HYBRIDS ARE OFTEN COMMON
c. HOWEVER, IF THERE IS A DOUBLING OF
THE CHROMOSOME NUMBER TO GET
AABB NOW NORMAL PAIRING IN MEIOSIS
CAN OCCUR AND THE PLANT IS FERTILE
IV. ANEUPLOIDY – THE GAIN OR LOSS ON ONE
OR MORE CHROMOSOMES OF A SET – NO
LONGER EXACT MULTIPLES OF THE BASIC
CHROMOSOME NUMBER
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A. THIS IS USUALLY CAUSED BY NONDISJUNCTION – THE FAILURE OF
CHROMOSOMES IN A PAIR TO SEPARATE IN
THE MEITOIC PROCESS RESULTING IN
GAMETES WITH EITHER MORE OR FEWER
CHROMOSOMES
B. IN DIPLOIDS THIS IS USUALLY LETHAL, BUT
IN POLYPLOIDS, BECAUSE OF THE NUMEROUS
DUPLICATIONS OF CHROMOSOMES, IT IS
USUALLY SURVIVABLE BY THE PLANT
V. EFFECTS OF POLYPLOIDY
A. CHANGE IN CHROMOSOME NUMBER
CHANGES GENETIC SEGREGATION RESULTING
IN NEW GENETIC COMBINATIONS
B. ANY INCREASE IN CHROMOSOME NUMBER
PROVIDES A MASK OR COVER FOR
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DELETERIOUS RECESSIVE GENES, SO ANY
LETHAL GENES ALREADY IN THE GENOME CAN
BE HIDDEN
C. LOSS OF AN ENTIRE CHROMOSOME
(ANEUPLOIDY) OR SEGMENTS OF A
CHROMOSOME, CAN OCCUR WITHOUT
PHENOTYPIC EFFECT BECAUSE OF THE
DUPLICATION OF OTHER CHROMOSOMES
CARRYING THE SAME GENES
D. AN INCREASE IN DELETERIOUS MUTATIONS
(AN MOST ARE) CAN INCREASE GREATLY IN A
POLYPLOID BECAUSE OF REDUNDANCY
E. AN INCREASE IN CHROMOSOME NUMBER IS
USUALLY ACCOMPANIED BY AN INCREASE IN
CELL SIZE = LARGER CELLS
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THIS IS A PARTIAL REASON FOR THE SUPERIOR
AGRICULTURAL PERFORMANCE OF MANY
POLYPLOIDS – BIGGER CELLS – BIGGER
PLANTS – BIGGER FRUITS AND SEEDS
F. GAMETE STERILITY AND LOWERED
FECUNDITY CAN ACCOMPANY POLYPLOIDY
G. POLYPLOIDS OFTEN HAVE THE ABILITY TO
EXPLOIT OR OCCUPY NEW AND DIVERSE
ECOLOGICAL HABITATS – ESPECIALLY SITES
THAT ARE DISTURBED
H. SPECIATION – POLYPLOIDY IS ONE WAY
NEW SPECIES EVOLVE – WHEN TWO
DIFFERENT SPECIES ARE INVOLVED IN THE
ORIGINAL CROSS.
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POLYPLOIDY DOES NOT ALWAYS RESULT IN
SPECIATION, ESPECIALLY WHEN ONE SPECIES
MAY BE INVOLVED IN A CROSS OR DOUBLING.
THERE EXISTS INTRASPECIFIC NONHYBRID
POLYPLOIDY. THIS INVOLVES
MULTIPLICATION OF THE SAME
CHROMOSOMAL SET = AUTOPLOIDY. IT MAY
OR MAY NOT LEAD TO SPECIATION. (FESTUCA)
POLYPLOIDY ASSOCIATED WITH
INTERSPECIFIC HYBRIDIZATION RESULTS IN
THE DOUBLING OF STRUCTURALLY
DISSIMILAR CHROMOSOMES IN A SPECIES
HYBRID = ALLOPLOIDY. (TRITICUM)
VI. POLYPLOIDY & ANEUPLOIDY ARE
CONSPICUOUS FEATURES IN THE EVOLUTION
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OF PLANTS. ESTIMATES OF THE NUMBER OF
SPECIES THAT ARE POLYPLOID ARE:
ANGIOSPERMS
47%
DICOTS
43%
MONOCOTS
58%
GYMNOSPERMS
1.5%
PTERIDOPHYTES
95%
OF GREAT INTEREST IS THE BASIC
CHROMOSOME NUMBER – THE HAPLOID
NUMBER OF THE DIPLOID SPECIES OF A
POLYPLOID SERIES. ALL CHROMOSOME
NUMBERS IN A POLYPLOID SERIES ARE
DIVISIBLE BY THE BASIC NUMBER.
FESTUCA
2n = 14
n=7
diploid
2n = 28
n=14
tetraploid
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2n = 42
n=21
hexaploid
2n = 56
n=28
octoploid
2n = 70
n=35
decaploid
THE BASIC CHROMOSOME NUMBER IS X=7
THIS IS AN EXAMPLE OF A POLYPLOID SERIES.
CHROMOSOME NUMBERS IN THE
ANGIOSPERMS RANGE FROM 2n=4
(HAPLOPAPPUS GRACILIS, ZINGERIA) TO 2n= CA.
500 (KALANCHOE). IN THE FERNS THE NUMBER
OF CHROMOSOMES PER CELL REACHES ITS
HIGHEST NUMBER IN OPHIOGLOSSUM
RETICULATUM WITH 2n=1260.
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