MBG 2040 Lecture Notes - Lecture 3: Sister Chromatids, Gene Duplication, Homologous Chromosome
2 May 2018
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Variation in Chromosome Number and Structure
Chapter Outline:
• Polyploidy
• Aneuploidy
• Rearrangements of chromosome structure
Outcomes:
• Define and use correctly the terms: chromosome, sister chromatid and homologue
• Differentiate between the events of mitosis and meiosis
• Describe the mechanism for maintaining association between paired homologous chromosomes
• Explain the model for nondisjunction in human meiosis
• Explain how chromosomal nondisjunction events can cause aneuploidy
• Describe how polyploidy is common in plants and rare in animals
• Identify aneuploidies resulting from nondisjunction events in humans
Mitosis
• Cell division initiated by doubling each chromatid (46 X 2)
o Before gene duplication: 2N = 46
o After gene duplication: 4N = 92
• Then chromatids are packaged and divided between daughter cells
• Result: two identical cells with 2N (=46)
Meiosis
• Meiosis I:
o Maternal and paternal chromatids stay together (bivalents) and segregate together
o Before gene duplication: 2N = 46
o After duplication: 4N = 92
• Meisosis II:
o No DNA duplication
o Segregation of sister chromatids (before division)
o After dividing, the end result is 4 gametes each with 23 chromosomes (N)
What will happen is the organism is triploid?
• Mitosis will be impaired
• Meiosis will be impaired
• The organism will die
• The organism will produce gametes with different numbers of chromosomes
• The organism will be sterile
Cytological Techniques
• Geneticists use stains to identify specific chromosomes and to analyze their structures
• Giemsa Banding
o Ex. dentification of human chromosome 5
• Chromosome painting
• Human Karyotype:
o G-banding
o Fluorescent probes
• Key points:
o Cytogenetic analysis usually focuses on chromosomes in dividing cells
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find more resources at oneclass.com
o Dyes such as quinacrine and Giemsa create banding pattern that are useful in identifying
individual chromosomes within a cell
o A karyotype shows the duplicated chromosomes of a cell arranged for cytogenetic analysis
Sets of Chromosomes:
• Many plants and most animal species are diploid (2N)
• Ants, bees and wasps and monoploid males (N)
• Polyploidy: >2 sets of chromosomes
o Ex. Triploid (3N)& Tetraploid (4N)
o Plant kingdom: 30-35% of flowering plants are polyploids
• Many seedless varieties of cirtus are triploids or pentapoloids
• Diploid watermelons have seeds
▪ seedless watermelons are triploids
• Diploid bananas have seeds
▪ Commerical bananas are triploid and seedless
• Seedless grapes are triploid
• Many commercial varieties of strawberries are 8N
• Sugarcane: 8N or 16N
• Oats: 6N
• Peanut: 4N
• Potato: 4N
• Coffee: 4N
o Polyploidy is rare in the animal kingdom
o Problems with polyploidy:
• Many polyploids are sterile due to problems with pairing and separation of homologous
chromosomes in meiosis
• Ex. Commercial varieties of bananas are 3N (=33) and they do not produce seeds
▪ Meiosis I: 11 bivalents and 11 univalents OR 11 trivalents
• Each set has to make a segregation decision (there are many options)
• The gamete pains that will re-created the 3N=33 plant is extremely
rare
▪ The number of chromosomes in a gamete from a banana cultivar can vary
• Gametes that have extra chromosomes or lack certain chromosomes are not
viable
▪ A banana could produce:
• Gametes with 2 copies of some chromosomes and 1 copy of other
chromosomes
• These are not viable
• Allopolyploids
o The result of crosses between two or more species (usually related)
o Their ploidy is best described as (1+1)
o The chromosomes of these plants are different
o These plants do not usually produce gametes; they reproduce asexually
• Autopolyploids
o Produced by duplication of the chromosomes of the plant
• If the original plant is an allopolyploid (1+1), a diploid (2N) plant is produced
• If the original plant is not a allopolyploid, a 4N plant is produced (the chromosome sets
are all identical)
o Autopolyploids usually have:
• More vegetative growth (larger cells, thicker leaves, bigger flowes, larger plants, bigger
fruits)
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Chapter outline: polyploidy, aneuploidy, rearrangements of chromosome structure. Identify aneuploidies resulting from nondisjunction events in humans. What will happen is the organism is triploid: mitosis will be impaired, meiosis will be impaired, the organism will die, the organism will produce gametes with different numbers of chromosomes, the organism will be sterile. Sets of chromosomes: many plants and most animal species are diploid (2n, ants, bees and wasps and monoploid males (n, polyploidy: >2 sets of chromosomes, ex. If the original plant is an allopolyploid (1+1), a diploid (2n) plant is produced. In animals, interspecies crosses can result in a sterile animal: chromosome doubling which potentially could restore fertility is not well tolerated, ex. Tissue-specific polyploidy and polyteny: endomitosis involves chromosome replication and separation of sister chromatids without cell division, this produces polyploid tissues. If sister chromosomes do not separate, the resulting chromosomes are polytene: ex.
