chromatid

Examples of chromatid in the following topics:

• Meiosis II

  • During meiosis II, the sister chromatids within the two daughter cells separate, forming four new haploid gametes.
  • The sister chromatids are maximally condensed and aligned at the equator of the cell.
  • The sister chromatids are pulled apart by the kinetochore microtubules and move toward opposite poles.
  • In prometaphase II, microtubules attach to the kinetochores of sister chromatids, and the sister chromatids are arranged at the midpoint of the cells in metaphase II.
  • In anaphase II, the sister chromatids are separated.

• Meiosis I

  • In synapsis, the genes on the chromatids of the homologous chromosomes are aligned with each other.
  • The sister chromatids remain tightly bound together at the centromere.
  • In some organisms, the chromosomes decondense and nuclear envelopes form around the chromatids in telophase I.
  • Although there is only one chromosome set, each homolog still consists of two sister chromatids.
  • Crossover occurs between non-sister chromatids of homologous chromosomes.

• The Mitotic Phase and the G0 Phase

  • Each sister chromatid develops a protein structure called a kinetochore in the centromeric region.
  • The sister chromatids are still tightly attached to each other by cohesin proteins.
  • During anaphase, the "upward phase," the cohesin proteins degrade, and the sister chromatids separate at the centromere.
  • Each chromatid, now called a chromosome, is pulled rapidly toward the centrosome to which its microtubule is attached.
  • During prometaphase, mitotic spindle microtubules from opposite poles attach to each sister chromatid at the kinetochore.

• Eukaryotic Chromosomal Structure and Compaction

  • After replication, the chromosomes are composed of two linked sister chromatids.
  • The connection between the sister chromatids is closest in a region called the centromere.
  • The conjoined sister chromatids, with a diameter of about 1 µm, are visible under a light microscope.

• Regulation of the Cell Cycle at Internal Checkpoints

  • The M checkpoint is also known as the spindle checkpoint because it determines whether all the sister chromatids are correctly attached to the spindle microtubules.
  • Because the separation of the sister chromatids during anaphase is an irreversible step, the cycle will not proceed until the kinetochores of each pair of sister chromatids are firmly anchored to at least two spindle fibers arising from opposite poles of the cell.

• Disorders in Chromosome Number

  • The gain of one chromosome is called trisomy (2n+1).They are caused by nondisjunction, which occurs when pairs of homologous chromosomes or sister chromatids fail to separate during meiosis.
  • If sister chromatids fail to separate during meiosis II, the result is one gamete that lacks that chromosome, two normal gametes with one copy of the chromosome, and one gamete with two copies of the chromosome .
  • Nondisjunction occurs when homologous chromosomes or sister chromatids fail to separate during meiosis, resulting in an abnormal chromosome number.

• Interphase

  • In the S phase, DNA replication results in the formation of identical pairs of DNA molecules, sister chromatids, that are firmly attached to the centromeric region.

• Gene rearrangement within genomes

  • Crossing-over is an event that occurs during meiosis when chromatids are in contact with each other and may exchange segments.
  • Crossing over also accounts for genetic variation, because, due to the swapping of genetic material during crossing over, the chromatids held together by the centromere are no longer identical.

• Genetic Variation

  • crossing over (or recombination) between chromatids of homologous chromosomes during meiosis

• Comparing Meiosis and Mitosis

  • During anaphase II and mitotic anaphase, the kinetochores divide and sister chromatids, now referred to as chromosomes, are pulled to opposite poles.