Transcript Maintaining Chromosome Number

Maintaining Chromosome Number
Processes that affect
chromosome number
• Meiosis
– chromosome number is reduced by half
• Aneuploidy
– addition or deletion of one or a few
chromosomes
• Polyploidy
– addition of complete sets of chromosomes
Meiosis
• Meiosis is the type of cell division that
accompanies gamete formation
• Key features:
– requires two cell divisions
– produces cells in which chromosome number
is reduced by half
– produces cells that are genetically variable
Pierce, B. 2005. Genetics, a conceptual aplproach. 2nd Ed. WH Freeman.
Gametogenesis in animals
Gametogenesis and
fertilization in plants
Pierce, B. 2005. Genetics, a conceptual
aplproach. 2nd Ed. WH Freeman.
Steps in first meiotic cell division
(Meiosis I)
• Interphase: DNA duplicates.
• Prophase I: Chromosomes condense;
homologous chromosomes pair; crossing over
occurs; nuclear membrane dissolves
• Metaphase I: Homologous chromosomes line
up on equator of cell; spindles form
• Anaphase I: Homologous chromosomes are
pulled to opposite poles of cell.
• Telophase I: Chromosomes arrive at poles.
Cells divide.
Product is 2 cells, each with a single copy of each
homologous chromosome.
Steps in second meiotic cell division
(Meiosis II)
• Prophase II: Chromosomes condense
• Metaphase II: Chromosomes line up on equator
of cell; spindles form at opposite poles
• Anaphase I: Chromatids are pulled apart at the
centromeres, to opposite poles of the cell.
• Telophase I: Chromatids (now called
chromosomes) arrive at the poles. Nuclear
membranes reform. Cells divide.
Product is 4 cells, each with half the number of
chromosomes as the original cell.
Meiosis Animations
• http://bcs.whfreeman.com/pierce2e
• http://www.biology.arizona.edu/cell_bio/tut
orials/meiosis/page3.html
Result of Meiosis
• Meiosis produces four cells that are
genetically distinct from the original cell.
• The daughter cells have half the number
of chromosomes as the original cell.
• One diploid (2N) cell gives rise to four
haploid (N) cells.
Purposes of meiosis
• Reduces chromosome number, so that
when fertilization occurs, the zygote will
maintain its proper chromosome number.
• Introduces genetic variability
– Independent assortment of chromosomes
– Crossing over
– http://bcs.whfreeman.com/pierce2e
Mistakes in meiosis can lead to aneuploidy
(wrong number of chromosomes)
Normal meiosis
S
I
II
Non-disjunction
S
I
II
one too many chromosomes
one too few chromosomes
Fertilization of aneuploid gametes with
normal gametes produces aneuploid zygotes
+
Monosomic
2N - 1 chromosome
Trisomic
+
2N + 1 chromosome
Effects of aneuploidy in animals
• Usually lethal.
• Lethality due to imbalance in dosage of
chromosome expression.
• Most genes are expressed from both
chromosomal copies.
• If there are too many or too few copies of
a gene, expression levels are abnormal.
Trisomy in humans
Pierce, B. 2005. Genetics, a conceptual
aplproach. 2nd Ed. WH Freeman.
http://www.ncdsn.com/resources.html
http://www.denverdsclinic.org/patients.htm
• Most trisomies are lethal.
• Trisomy for 13, 18 or 21 can survive to birth.
• Trisomy 13 and Trisomy18 infants do not usually live
past a year.
• Trisomy 21 (Down Syndrome) individuals can live
productive lives.
• Plants usually tolerate aneuploidy better than
animals do.
• For example, in
Datura, trisomies are
viable.
• For each of the 12
chromosomes, the
presence of one
additional chromosome leads to a
difference in fruit
morphology.
Pierce, B. 2005. Genetics, a conceptual aplproach. 2nd Ed. WH Freeman.
Aneuploidy in plants
Polyploidy in plants:
Extra chromosome sets
• Autopolyploidy: chromosome sets derived from
the same species
• Allopolyploidy: chromosome sets derived from
different species
Some familiar polyploids
potato (4n)
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soybean (4n)
QuickTime™ and a
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strawberry (8n)
Autoploidy
• Can occur as result of error in mitosis
• Non-disjunction or failure of cell division
• Autopolyploids with even numbers of
chromosome sets are fertile.
Pierce, B. 2005. Genetics, a conceptual aplproach. 2nd Ed. WH Freeman.
Autopolyploidy
• Can occur as result of error in meiosis
Pierce, B. 2005. Genetics, a conceptual aplproach. 2nd Ed. WH Freeman.
Pierce, B. 2005. Genetics, a conceptual aplproach. 2nd Ed. WH Freeman.
Triploids are usually sterile
Triploids we know and love
Quick Time™ a nd a
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ar e n eed ed to s ee this pi ctur e.
Banana
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see t his picture.
Seedless
watermelon
• The fruits of triploid plants
have very small. very few
or non-existent seeds.
• Banana plants are
propagated vegetatively.
• Seedless watermelons are
propagated by crossing a
tetraploid female by a
diploid male to produce
triploid seeds.
Allopolyploidy
• Example: The
Triangle of U is
proposed as a theory
to explain the
evolution of many of
the Brassica species,
which are thought to
have hybridized with
one another to
produce new species
http://en.wikipedia.org/wiki/Triangle_of_U
• Chromosome sets derived from different species
Bag-O’-Brassicas
Fun with Vegetables
• Brassica rapa
(AA = 20)
• Brassica oleracea
(CC=18)
• Brassica juncea
(AABB = 36)
• Brassica napus
(AACC = 38)
Consequences of polyploidy
• Polyploid plants are bigger than diploid
relatives. Consequently, many crops have
been bred to be polyploid.
– Larger plant mass
– Bigger and / or more numerous fruits
• Polyploids with odd numbers of
chromosome sets produce few seeds.
• Polyploids with even numbers of
chromosome sets produce normal seed.
Summary of processes
affecting chromosome number
• Meiosis is the cell division process that
reduces chromosome number by half and
introduces genetic variability.
• Aneuploidy (wrong number of
chromosomes) is usually lethal in animals,
but can be tolerated by plants.
• Many plants are polyploids and have extra
sets of chromosomes, derived either from
the same or different species.