Chromosome Segregation

Germ Line (Germ Cells)

• Haploid

• 23 chromosomes (n) in humans

• e.g. sperm cells, egg cells

Somatic Cells

• Diploid

• 46 chromosomes (2n) in humans

• e.g. skeletal and muscle cells, blood cells, stem cells, organ and tissue cells, fat cells, neuron cells

Germ Line (Germ Cells)

• Haploid

• 23 chromosomes (n) in humans

• e.g. sperm cells, egg cells

Somatic Cells

• Diploid

• 46 chromosomes (2n) in humans

• e.g. skeletal and muscle cells, blood cells, stem cells, organ and tissue cells, fat cells, neuron cells

Chromosome Segregation

The process in eukaryotes by which two sister chromatids formed as a consequence of DNA replication (or paired homologous chromosomes) separate from each other and migrate to opposite poles of the nucleus

• It occurs during both mitosis and meiosis

• Also occurs in prokaryotes → in contrast to eukaryotic chromosome segregation, replication and segregation are not temporally separated Instead, segregation occurs progressively following replication

The Cell Cycle

TWO basic phases:

1. Interphase → occurs between cell divisions

2. M phase or Mitosis

Homologous Chromosomes

• They are very similar to one another and have the same size and shape

• They carry the same type of genetic information: that is, they have the same genes in the same locations

Prophase (M Phase)

• chromosomes condense

• nucleolus disappears

• centrioles move to poles

• spindles begin to form

• nuclear membrane breaks down

Late Prophase or Prometaphase

movement of chromosomes to centre of the cell

Metaphase

chromosomes align at the equatorial plane of the cell

• mitotic spindle formation complete

Anaphase

starts when sister chromatids split

• chromosomes move to the poles of the cell

• cell begins to elongate

• cleavage furrow starts

Telophase

begins when chromosomes reach the poles

• nuclear membrane reforms

• chromosomes de-condense

• nucleoli reform

• spindle fibres disappear

• cleavage furrow continues

Cytokinesis

• not part of mitosis but normally follows closely after mitosis

• equals the completion of the cleavage furrow and production of two cells

• normally follows mitosis and is considered part of the M phase of the cell cycle BUT this is not always true (e.g., Drosophila development)

Meiosis

It is a specialized type of mitosis

• occurs in the germ line/cells

• necessary for the production of gametes in diploid, sexually reproducing organisms

• involves two sequential cell
  divisions without DNA replication between divisions

• 8 stages in total (plus interphase)

• produces 4 haploid daughter cells

• chromosome number is halved in each daughter cell

• increases genetic variation

Meiosis I

• REDUCTION DIVISION

• Segregation of homologous chromosomes reduction, diploid to haploid

• More complex

• Subdivided into more stages

• The end result is TWO cells, each with a duplicated copy of one homologue of each chromosome

Meiosis II

• Equational division

• It is just like mitosis except that it starts with fewer chromosomes

Prophase I, Leptotene

• chromosomes begin to condense and become visible

• thickened regions (chromomeres) appear

Prophase I, Zygotene

Chromosomes continue to condense and there is an active pairing of the chromosome threads between non-sister chromatids

Prophase I, Pachytene

Chromosomes become fully aligned

Prophase I, Diplotene

• aligned homologous pairs become less tightly aligned

• chiasmata becomes apparent

Prophase I, Diakinesis

Compaction is completed, and the chromosomes are ready to be segregated

• The cell then enters metaphase

Prophase II

Chromosome condensation

Metaphase II

Chromosome alignment

Anaphase II

Sister chromatid separation/movement to ends

Telophase II

Reformation of nuclei

How meiosis differs from mitosis?

1. Reduction in the number of chromosomes (from diploid to haploid)

2. Recombination between chromosomes

Importance of Meiosis

Recombination increases diversity even more by reshuffling genetic information between the chromosomes

• Production of haploid cells by meiosis is a critical component of sexual reproduction

Independent Assortment of Chromosomes During Meiosis I (Mendel’s Fourth Law)

Produces diversity in offspring (two offspring will almost never have the same chromosome complement)

• 8 possible combinations with
  only 3 chromosomes

• 8,324,608 possible combinations of 23 chromosome pairs

Mitosis

• 4 stages in total (plus interphase)

• happens in somatic cells

• purpose is cellular proliferation

• produces two diploid daughter cells

• chromosome number remains the same

• genetic variation doesn’t change