Continuation of Chapter 13

What happens to the sister chromatids during Meiosis II?

- shugoshin is removed - separase cleaves centromeric cohesin at anaphase II, sister chromatids can now segregate to opposite poles.

How do sister chromatids remain attached in Meiosis I?

- Cohesion complexes complexes with meiosis-specific subunit. - Interacts with shugoshin (protein) - protects cohesin at the centromere from being cleaved by separase. - shugoshin does not protect cohesin along the arms of the sister chromatids, therefore the cohesion is cleaved along the arms. - The cohesion along the arms keep homologous chromosomes together - and its removal allows for homologous chromosomes to segregate to opposite spindle poles during anaphase of meiosis I.

How do centromeres present in yeast?

- Consist of two highly conserved nucleotide sequences (10-15 bp) - A highly repetitive sequence (AT rich in between) - Not as complex - Bind through the kinetochore proteins to a singular spindle microtubule.

What occurs if pairing during meiosis in a deletion heterozygote? What technique can be used to visualize this event?

- Deleted homolog pairs with the normal homolog. - No pairing, nor recombination is possible, the region opposite to a deletion has nothing to pair with and form a deletion loop. Result: Two normal and two deletion gametes will be produced. Using the FISH technique to help identify deleted sequences using fluorescent probes with a specific DNA sequence.

What does being heterozygote and having an inversion lead too?

- If an individual has one inversion chromosome and a normal homolog it is an inversion heterozygote. - When inversion chromosome pairs with its homolog at meiosis I, forms an inversion loop. - One chromosomal region rotates to conform to the region that is looped. Issue: this would not be a problem if there is no crossing over taking place, however crossing-over within an inversion loop produces aberrant recombinant chromatids. - Slides 26-28 take a real-life example of chimps and humans - an example using fish and probes to see what happens during an inversion.

How do centromeres present in higher eukaryotic organisms?

- Larger - More complex - Centromeres are contained within repetitive, noncoding sequences - satellite DNA

How are deletions used to locate genes?

- Require a recessive loss-of-function mutation. - Possibility (1) m /Del heterozygote is mutant (the mutation and the deletion line up and fail to complement) the deletion has uncovered the mutated locus → Lies within the region of deletion. - Possibility (2) if the gene ends up being wild type in the heterozygote, the deletion and the mutation do not line up → the deletion has not un- covered the recessive allele and the gene lies outside the deleted region.

Describe the features of Heterozygote pericentric inversions?

- Single crossover occurs within the inversion loop - Recombinant chromatid will have a single centromere but will carry a duplication of one region and a deletion of a different region. - Gametes will have an abnormal dosage of some genes - after fertilization zygotes created by the union of these abnormal gametes with normal gametes are likely to die because of genetic imbalance.

Describe the features of Heterozygote paracentric inversions?

- Single crossover occurs within the inversion loop. - Recombinant chromatids will be unbalanced n gene dosage and centromere number. Produces: an acentric fragment lacking a centromere and reciprocal crossover product that is a dicentric chromatid with two centromeres.

How do translocations appear in heterozygotes?

- Translocation produces genetically unbalanced gametes. The two haploid sets do not carry the same arrangements of genetics information The possibilities depend on the arrangements of the four chromosomes on the metaphase plate (alternate segregation pattern): Orientation (1): balanced - surviving (N1 and N2) and (T1 and T2) Orientation (2): unbalanced - non-surviving (T1 and N2) and (T2 and N1) Both can occur equally and thus semi sterility is seen. - Rare event does occur: T2 and N2 - T1 and N1 - chromsomes are separate from each other (wont be tested)

How do duplications affect phenotypes?

- Usually have no obvious phenotypic consequences - additional dosages don't affect. Rare phenotypic changes: - Traits can be sensitive to differences in dosages. - Genes near a duplication can have altered expression because it is in a new environment.

How to sister chromatids remain attached during mitosis?

- cohesin complexes resist the forces pulling the chromatids at their kinetochores - Generates tension across the chromosome. - During Anaphase: called separase cleaves the cohesin complexes, allowing the sister chromatids to separate and move to opposite spindle poles.

What is duplication and deletion?

Breakage of two homologous chromosomes or sister chromatids at different locations - the broken chromosome ends change places before DNA repair can occur.

What is Reciprocal translocation?

Breakage of two nonhomologous chromosomes, and the broken ends switch places before DNA repair can occur. - Example on slide 38-39

How do translocations appear in homozygotes?

Chromosomes segregate normally regardless of rearrangements.

How can a translocation be mutagenic? Example (Fig. 14.15)

Chronic Myelogenous Leukemia (CML) can be caused by a translocation. Cancer caused by overproduction of certain white blood cells, the cells have reciprocal translocation between chromosomes 9 and 22. - Breakpoint occurs in chromosome 9 within an intron of a proto-oncogene c-abl. - Breakpoint in chromosome 22 occurs within an intron of the bcr gene. - An abnormal hybrid gene is made from parts of the c-abl gene and bcr gene. - hybrid gene encodes an abnormal fused protein that disrupts controls on cell division. Fused protein has: first 25 amino acids of c-abl replaced with~600 amino acids from bcr. - The promoter of bcr leads to the unregulated expression of the gene (c-abl)

Describe the rare aberrant crossover events that can occur? general information: Rare aberrant crossover events between repeated sequences on the same chromosome or on two different chromosomes can generate rearrangements. - Similarities that can be recognized by recombination enzymes.

Deletions and inversions: occur if repeats are in the same orientation or in opposite orientations, via crossovers between repeats of the same sequence at two locations on the same chromosome. Deletion and Duplication: If two homologous chromosomes misalign at repeated sequences and cross over. Reciprocal translocations: Crossovers at a repeated sequence on two nonhomologous chromosomes

Provide an example (2) of duplications and phenotypic changes

Example 2: Drosophila eye shape Tandem Duplication of a region (16A), of drosophila X chromosome can cause bar eyes. Tandem Triplication: causes double bar eyes. - Flies with double-Bar eyes have X chromo-somes carrying three tandem copies due to unequal crossing over. - Crossing over will lead to a fly with 3 copies (double-bar) and a fly with one copy, wildtype, when the female is homozygous for the mutant.

Provide an example (1) of duplications and phenotypic changes

Example: The Notch+ gene in drosophila - dosage sensitive Duplication heterozygotes have three copies. - flies with three copies of Notch+ have wing vein pattern defects.

What does this mean in terms of the zygote development if they have paracentric inversions?

If it is acentric: spindle cannot attach, and the chromosome is lost and not included in the gamete. If it is dicentric: spindle attached to both centromeres breaking the chromatid at random positions. - No viable zygotes are produced. Both produce two balanced gametes and two unbalanced gametes. - The larger the inverted region, the probability that crossing over occurs increases, the mre unbalanced gametes will result. - Inversion heterozygosity, especially when the inverted region is large, often results in reduced fertility.

What are the different types of inversions?

Pericentric: inversions that include the centromere. Paracentric: inversions that exclude the centromere. - Usually do not result in an abnormal phenotype. - Can cause mutations due to the break point: If one inversion breakpoint lies within a gene: a loss-of-function mutation in that gene will occur. If inversion breaks the gene into two parts, relocating one part to a distant region of the chromosome, and the other part remains, this will disrupt the gene's function.

What is a deletion?

Single chromosome suffers two double-strand breaks - loses a fragment between the breaks. - DNA repair fuses the the remaining broken ends resulting in a deletion.

What are the different types of duplications?

Tandem duplications: repeated copies lie adjacent to each other, either in the same order or in reverse order. Nontandem duplications: copies of the region are not adjacent to each other and may lie far apart on the same chromosome or on different chromosomes.

How does the structure of the kinetochore present?

The kinetochore-forming DNA - has a different chromatin structure and packaging than chromosomal regions. - histone H3 protein is replaced by a variant CENP-A in the nucleosome core. - Attached to different proteins. - Centromere is bound by a set of proteins that forms the kinetochore which then allow the microtubule to bind.

What is the purpose of centromeres?

To ensure that eukaryotic chromosomes distribute properly during cell divisions. - Act as segregation centers. - Contains kinetochores - where chromosomes bind to spindle fibers - Contains centromeric regions - where sister chromatids are tightly bound together

How to uncover recessive mutations on other homologs? - purpose of this experiment

Using → A deletion heterozygote: hemizygote for genes on the normal, non deleted chromosome that are missing from the deleted chromosome. Example with Drosophila: fly with genotype st /Del (lacks the st gene due to a deletion on one homolog and has st+ mutant on another) will display recessive eye color scarlet. - Think about it like - seems that they are homozygous recessive. Purpose: to reveal the phenotype of the recessive mutation.

What is the first artificial eukaryotic chromosome? How was it created

Yeast artificial chromosome (YAC) Combined in a single DNA molecule yeast, 3 versions of chromosomal elements centromeres, telomeres, and origins of replication.

What does Synteny between mice and humans reveal?

just to know in general: There are genes linked together in species A and in Species B There was an ancestor chromosome that was in a specific order, and it ended up in both species. - By comparing there is a clear history of chromosome rearrangement between them. - The closer in evolution a species is to another, the longer the syntenic regions.

What are Kinetochores? - mention how it presents in yeast vs in higher eukaryotic organisms

kinetochores: specialized structures composed of DNA and proteins that are sites where chromosomes attach to the spindle fibers. - Yeast: attach to 1 spindle fiber - higher eukaryotes: attach to many spindle microtubules

Explain Robertsonian translocation and inherited down syndrome?

reciprocal exchanges between two acrocentric (centromere near the top) chromosomes generate a large metacentric chromosome and a very small chromosome. The latter may carry so few genes that it can be lost without ill effect. - Breaks that occur near or at the centromeres of two acrocentric chromosomes. - Reciprocal exchange occurs → one large metacentric chromosome and one very small chromosome. This is observed in individuals heterozygous for Robertsonian translocation between chromosomes 14 and 21: One of the possibilities: producing gametes with two copies of part of chromosome 21 During fertilization: the zygote will have three copies and this may cause down syndrome.

what is inversion?

the loss fragment from the double stranded break is ligated back but (180°) from the original orientation.

General info card: who is Barbara McClintock (1902-1992)?

• Cytogeneticist working in Maize meiosis; discoveries included how inversions and translocations behaved during prophase I • Third woman elected to the National Academy of Sciences in 1944 • In early 1950s develops theory on mobile pieces of DNA called transposons • Nobel Prize 1983 - first woman to win unshared.