Talk:ANAT2341 Lab 1 - Gametogenesis

From Embryology


Cell Division

Historic drawing of mitosis

Features Two Mechanical Processes

  • Mitosis - microtubule based segregation of chromosomes and formation of 2 nuclei
  • Cytokinesis - microfilament based splitting of the cell cytoplasmic contents as a whole into 2 daughter cells

Features Two Types

  • Mitosis - occurs in all cells, producing genetically identical progeny.
  • Meiosis - occurs only in germ cells (sperm=spermatozoa and egg=oocyte), producing genetically different progeny.
    • progeny = daughter cells, offspring

Cell Changes

Nucleus

  • Chromosome condensation
  • Nuclear envelope breakdown

Cytoplasm

  • Cytoskeleton reorganization
  • Spindle formation (microtubule, MT) Contractile ring (microfilament, MF)
  • Organelle redistribution

Mitosis

Mitosis Movie[1] See also MCB Movie - The stages of mitosis and cytokinesis in an animal cell
  • Based on light microscopy of living cells light and electron microscopy of fixed and stained cells
  • 5 Phases - prophase, prometaphase, metaphase, anaphase, and telophase
  • Cytokinesis 6th stage overlaps the end of mitosis

Note that DNA duplication has occurred earlier in the S Phase of the cell cycle.

Mitosis 01 icon.jpg
 ‎‎Mitosis
Page | Play

1. Prophase

  • Chromosome DNA has been earlier duplicated (S Phase)
  • Chromosomes begin condensing
  • Chromosome pairs (chromatids) held together at centromere
  • Microtubules disassemble
  • Mitotic spindle begins to form
  • Prophase ends when nuclear envelope breaks down

2. Prometaphase

  • Microtubules now enter nuclear region
  • Nuclear envelope forms vesicles around mitotic spindle
  • Kinetochores form on centromere attach to some MTs of spindle
  • Prometaphase ends when chromosomes move to metaphase plate

MCB Movie - Centromeric attachment of microtubules

3. Metaphase

Mitosis - Metaphase.jpg

Metaphase fluorescent image of Mitotic spindle and Chromosomes
  • Kinetochore MTs align chromosomes in one midpoint plane
  • Metaphase ends when sister kinetochores separate

4. Anaphase

  • Separation of sister Kinetochores
  • shortening of Kinetochore microtubules pulls chromosome to spindle pole
  • Anaphase ends as nuclear envelope (membrane) begins to reform

5. Telophase

  • Chromosomes arrive at spindle poles
  • Kinetochore MTs lost
  • Condensed chromosomes begin expanding
    • Continues through cytokinesis

Cytokinesis

  • Division of cytoplasmic contents
  • Contractile ring forms at midpoint under membrane
  • Microfilament ring Contracts forming cleavage furrow
  • Eventually fully divides cytoplasm

Cell Organelles

  • Mitochondria - Divide independently of cell mitosis, distributed into daughter cells
  • Peroxisomes - localise at spindle poles
  • Endoplasmic Reticulum - associated with the nuclear envelope vesicles.
  • Golgi Apparatus- Golgi stack undergoes a continuous fragmentation process, fragments are distributed into daughter cells, then reassembled into new Golgi stacks

Meiosis

Mitosis and meiosis.jpg

Comparison of Mitosis and Meiosis

Meiosis Germ cell division (haploid)

  • Reductive division generates the haploid gametes (egg, sperm)
  • Each genetically distinct from the parent
  • Genetic recombination (prophase 1)
    • Exchanges portions of chromosomes maternal/paternal homologous pairs
  • Independent assortment of paternal chromosomes (meiosis 1)

Homologous chromosomes pairing unique to meiosis

  • Each chromosome duplicated and exists as attached sister chromatids before pairing occurs
  • Genetic Recombination shown by chromosomes part red and part black
    • chromosome pairing in meiosis involves crossing-over between homologous chromosomes

Meiosis I and II

  • Meiosis I - separates the pairs of homologous chromosomes, reduces the cell from diploid to haploid.
  • Meiosis II - separates each chromosome into two chromatids (chromosome behavior in meiosis II is like that of mitosis).

Figure 14.32. Comparison of meiosis and mitosis

Prophase I - The homologous chromosomes pair and exchange DNA to form recombinant chromosomes. Prophase I is divided into five phases:

  1. Leptotene - chromosomes start to condense.
  2. Zygotene - homologous chromosomes become closely associated (synapsis) to form pairs of chromosomes consisting of four chromatids (tetrads).
  3. Pachytene - crossing over between pairs of homologous chromosomes to form chiasmata (form between two nonsister chromatids at points where they have crossed over)
  4. Diplotene - homologous chromosomes begin to separate but remain attached by chiasmata.
  5. Diakinesis - homologous chromosomes continue to separate, and chiasmata move to the ends of the chromosomes.

Prometaphase I - Spindle apparatus formed, and chromosomes attached to spindle fibres by kinetochores.

Metaphase I - Homologous pairs of chromosomes (bivalents) arranged as a double row along the metaphase plate. The arrangement of the paired chromosomes with respect to the poles of the spindle apparatus is random along the metaphase plate. (This is a source of genetic variation through random assortment, as the paternal and maternal chromosomes in a homologous pair are similar but not identical. The number of possible arrangements is 2n, where n is the number of chromosomes in a haploid set. Human beings have 23 different chromosomes, so the number of possible combinations is 223, which is over 8 million.)

Anaphase I - The homologous chromosomes in each bivalent are separated and move to the opposite poles of the cell.

Telophase I - The chromosomes become diffuse and the nuclear membrane reforms.

Cytokinesis I - Cellular cytoplasmic division to form two new cells, followed by Meiosis II.

Prophase II - Chromosomes begin to condense, nuclear membrane breaks down and spindle forms.

Metaphase II - Spindle fibres attach to chromosomes, chromosomes align in cell centre.

Anaphase II - Chromosomes separate and move to the opposite poles of the cell.

Telophase II - Chromosomes reach spindle pole ends and the nuclear membrane reforms.

Cytokinesis - Cellular cytoplasmic division to form new cells.

Comparison of Meiosis/Mitosis

McGraw-Hill Animation comparing Mitosis and Meiosis

  • After DNA replication 2 nuclear (and cell) divisions required to produce haploid gametes
  • Each diploid cell in meiosis produces 4 haploid cells (sperm) 1 haploid cell (egg)
  • Each diploid cell mitosis produces 2 diploid cells
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