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

Gametogenesis in animals

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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/tutorials/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)

S I II

one too many chromosomes

one too few chromosomes

Normal meiosis

S I II

Non-disjunction

Fertilization of aneuploid gametes with normal gametes produces aneuploid zygotes

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Trisomic

2N + 1 chromosome

Monosomic

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

• 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.

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Aneuploidy in plants

• For example, in Datura, trisomies are viable.

• For each of the 12 chromosomes, the presence of one additional chromo-some leads to a difference in fruit morphology.

• Plants usually tolerate aneuploidy better than animals do.

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Polyploidy in plants: Extra chromosome sets

• Autopolyploidy: chromosome sets derived from the same species

• Allopolyploidy: chromosome sets derived from different species

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Some familiar polyploids

soybean (4n)

potato (4n)

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.

Triploids are usually sterile

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Triploids we know and love

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• 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.

Banana

Seedless watermelon

Allopolyploidy

• Chromosome sets derived from different species

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• 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

Bag-O’-BrassicasFun 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.