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to make gametes, Exercises of Cell Biology

University of California - Berkeley Cell Biology

…segregating replicated chromosomes in somatic cells. Diploid cell… homologue pairs. Good. Bad! or any outcome where each daughter cell does not have.

Typology: Exercises

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- to make gametes: meiosis

each daughter cell should have half the number of chromosome sets as the parental cell

If parental cell was diploid (2N)… daughters should be haploid (1N)

Will a normal haploid cell undergo meiosis? No

What happens to the replicated chromosomes?

… depends on the goal of the division

- to make more “vegetative” cells: mitosis

daughter cells’ chromosome set should be identical to parental cell’s

Segregating the replicated chromosomes

Discover Exercises of Cell Biology University of California - Berkeley

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to make gametes: meiosis each daughter cell should have half the number of chromosome sets as the parental cell If parental cell was diploid (2N)… daughters should be haploid (1N) Will a normal haploid cell undergo meiosis? No What happens to the replicated chromosomes? … depends on the goal of the division
to make more “vegetative” cells: mitosis daughter cells’ chromosome set should be identical to parental cell’s Segregating the replicated chromosomes

Mitosis …segregating replicated chromosomes in somatic cells Diploid cell… homologue pairs Good Bad! or any outcome where each daughter cell does not have exactly one copy of each parental chromosome a (^) A

At Metaphase... Chromosomes line up at cell’s “equatorial plate” Mechanism? Spindle fibers exerting tension on kinetochores Mitosis (cont’d) kinetochore Centromere: DNA sequence on which kinetochore is built Centriole = Spindle pole body (yeast) = MTOC (microtubule organizing center)

Mitosis (cont’d) At anaphase… cohesion between sister chromatids dissolved, sisters pulled to opposite poles Anaphase Telophase

Monitoring correct attachment to spindle (cont’d) Correct attachment

Monitoring correct attachment to spindle (cont’d) Anaphase begins! Correct attachment

The anaphase entry checkpoint—genetic analysis separase (non-functional) mutation… phenotype? cohesin (non-functional) mutation… phenotype? *how to keep the strains alive? …use temperature sensitive mutants cells stuck in metaphase premature sister separation Double mutant phenotype? premature sister separation!

Checkpoints

Cellular surveillance systems to monitor the integrity of the genome and of cellular structures Enforce the correct order of execution of cellular events. Examples:

Chromosomes not attached to spindle → block onset of anaphase
DNA is damaged → halt the cell cycle to allow repair
Irreparable DNA damage → trigger cell death

Chromosome segregation-

Genome 371, 11 Jan 2010, Lecture 3 Mitosis Meiosis Inheritance of traits from parent to offspring

Meiosis—to halve the ploidy for gametes

Both parents are diploid (2N).
Unless something is done—gametes will be 2N and offspring will be 4N! So… a specialized form of cell division to cut the ploidy by exactly half

Meiosis—to halve the ploidy for gametes

The problem:

ensuring that homologues are partitioned to separate gametes The solution:
Hold homologous chromosomes together by some means
target homologues to opposite poles…
then separate the homologues Overview of meiosis replicate chromosomes separate the sister chromatids separate the homologues Meiosis I Meiosis II

“Homologue recognition is absolutely necessary for the subsequent correct segregation of the homologues and thus the production of viable gametes, yet we have very little understanding of how it actually occurs.” Improving the chances of finding the right partner G. Moore and P. Shaw (2009) Current Opinion in Genetics & Development 19: 99- How do homologues pair up? Roles for: double-stranded DNA breaks specific pairing sites, including centromeres & telomeres pairing in premeiotic S phase other mechanisms

Beyond the Basics

Meiosis I — reductional division Cohesin near centromeres is maintained Homologues are separated, so ploidy is halved Sister centromeres/kinetochores stay together through meiosis I Metaphase I Anaphase I Crossovers hold the homologues together—again, tension on kinetochores indicates proper attachment