The Eukaryotic Chromosome

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The Eukaryotic Chromosome

• Chromosomal Structure
• Telomeres and Centromeres
• Chromosomal Packaging
• Changes in Chromosome Number

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The Protein Part of the Eukaryotic Chromosome

• Chromatin - Protein and DNA
• Histones are charged proteins bound to DNA
• Way of packaging DNA

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The Protein Part of the Eukaryotic Chromosome

• Nonhistone Proteins all the rest
• Diverse functions
• - structural scaffold of the chromosome
• - part of replication
• - help move chromosomes along spindle

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The Nucleosome Fundamental packaging unit

• DNA wraps Twice
• Condenses 7X
• Each nucleosome
• connected by linker DNA

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1000X
7X
42X
Compaction
Nonhistones fasten loops (Supercoiling)
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Supercoiling from above
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Chromosome Banding Patterns

• Highly reproducible
• Location of genes in relation to bands
• Differences between species and individuals

Gene for understanding genetics
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Importance of the Telomere
Protective Cap - Preserve the integrity of the
chromosome Telomeres have no genes Without them
chromosomes would fuse
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Importance of the Telomere
DNA Polymerase cannot reconstruct 5
end Telomeres protect the ends of chromosomes
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Importance of Centromeres
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Importance of Centromeres
Kinetochore -DNA and proteins -Site of
spindle fiber attachment Sister Chromatids
-held together by centromeres
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Chromosomal Packaging Influences Gene Expression

• Selected areas of chromatin decompact
• Opened areas are highly transcribed

Insulator prevents further decompaction
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Decompaction Influences Tissue Differentiation

• DNA hypersensitive sites vary from tissue to
  tissue

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Extreme Compaction Means No Gene Expression

• Heterochromatin vs. Euchromatin

Euchromatin contains sites of transcription of
almost all genes
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Evidence of heterochromatins Influence on Gene
Activity

• Moving a gene near heterochromatin
• Barr Bodies

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Changes in Chromosome Number

• Loss or gain of 1 or more chromosomes
• Monoploid to polyploid

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Ploidy Levels

• n the number of chromosomes in gamete
  (haploid)
• 2n normal diploid
• Aneuploid not exact multiple of haploid number
• Monosomic (2n 1)
• Trisomic (2n 1)
• Tetrasomic (2n 2)

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Autosomal Aneuploidy Is Harmful

• Usually lethal
• Chromosome 1 or 2 aborts spontaneously

Trisomy 18 Edwards Syndrome
Trisomy 13 (Patau Syndrome)
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Downs Syndrome (Trisomy 21)

• Genetic imbalance of only one or few genes may
  cause condition

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Humans Tolerate X Chromosome Aneuploidy
Kleinfelter Syndrome (XXY)
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Turner Syndrome (XO)
Sterility Short Stature Skeletal Abnormalities
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Aneuploids Result from Meiotic Nondisjunction
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Organisms with Different Ploidy Levels

• Most species are diploid (2n)
• Some not
• Haploid
• polyploid

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Monoploidy is Rare

• Males of ants and bees develop from unfertilized
  eggs

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Artificial Production of Haploid Plants
Treat germ cells Generate plant from mass of
tissue treated with hormones Restore diploid
organism by treating with colchicine
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Polyploidy is Common in Plants

• Polyloidy increases vigor and plant size
• Tetraploids include apple, peanut, alfalfa,
• Octaploids include strawberry

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Hybridization in Plants
Polyploidy increases the chances of fertile
hybrids between species
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