Biology 2581B Chapter Notes - Chapter 8: Mendelian Inheritance, Centriole, Y Chromosome

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15 Jan 2018
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Saturday, January 13, 2018
Ch. 8 — Chromosome variation
8.1 — Chromosome mutations include rearrangements, aneuploidy,
polyploidy !
-each organisms has n number of chromosomes, and all cells have the same n
amount of chromosomes. !
-variations in n-number, or chromosome structure COULD arise— chromosome
mutations —> evolution !
Chromosome morphology
-each chromosome has 1 centromere, 2 telomeres (stabilizes ends).!
-classified into 4 types, depending on centromere location: !
1. Metracentric— centromere is approx. in the middle, 2 arms of eq. length
2. Submetacentric— centromere located towards 1 end, has a shorter arm (p), and a
longer arm (q).
3. Acrocentric — centromere is near 1 end, produces long arm, and a knob/ satellite.
4. Telocentric— centromere is at the very end
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Saturday, January 13, 2018
-karyotype is the complete set of an org.’s chromosome presented at metaphase in
descending size order. !
-prepared from a rapidly divided cell!
-chemicals were used to stop cells after metaphase, and before anaphase. !
-then burst open onto microscope!
-chromosomes are then stained and photographed. !
-dierent dyes are used to stain dierent regions of chromosomes (ex: a AT rich
region is reviewed by Giemsa dye)!
Types of chromosome mutations
1. Rearrangement
-alters structure of chromosome!
-either duplication, deletion, inversion !
2. Aneuploidy (2n + 1)
-number of chromosomes changes!
-individual chromosomes are added !
3. Polyploidy (3n/ 4n …etc.)
-1+ complete sets of chromosomes are added !
-many organisms are polyploids!
8.2 — Chromosome rearrangements alter structure !
-duplication, deletion, inversion, translocation!
-occurs when there are doubled-stranded DNA breaks!
-cells have mechanisms to counter this. But sometimes they’re not rejoined correctly. !
-also occurs during crossing over errors. !
Duplications
-part of chromosome has been doubled !
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Saturday, January 13, 2018
-tandem duplication = when the repeated segment is immediately adjacent to the
copied segment (ex: ABCDEFEFG)!
-displaced duplication = duplicated segment is located some distance from oriiginal
— either on same or dierent chromosome. (ex: ABCDEFGEF)!
-reverse duplication = repeated sequence is reversed (ex:ABCDEFFEG)!
-chromosome duplication could either by
homozygous (both homologues are duplicated),
or heterozygous (only 1 is duplicated)!
-in heterozygotes, problem occurs during
homologue pairing during prophase I!
-duplicated chromosome is longer and
results in looping for regions to line up!
-appearance of loop allows cells to recognize
the mutation!
-unequal crossing over, when chromosomes
misalign during prophase I of crossing over also
causes 1 chromosome with duplication, and the
other with deletion. !
How does chromosome duplication alter
phenotype?!
-if the sequences are still the same, no genetic information is altered or missing…
how does duplication lead to phenotypic mutations? !
-by having additional copies of normal sequence, unbalanced amounts of gene
products are formed— abnormal gene dosage.!
-amount of protein synthesized is related to the number of copies of its gene. !
-developmental processes require the interaction of proteins, therefore depends on
proper gene dosage— if 1 protein required is significantly higher than the other
protein required, problems result. !
-unbalanced gene dosage —> developmental abnormalities!
-chromosome duplications are important in evolution— allows new genes to evolve!
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