Cellular Reproduction

Fission versus Division

• Before cell can divide, it must replicate its nucleic acids and assure that each daughter cell receives an equal compliment
• Procaryotic cells employ the process of fission
• Eucaryotic cells undergo a divisional process

Procaryotic Fission

• Procaryotic cells have a less complex organization than eucaryotes
  • Attain sufficient size to undergo binary fission
    • Chromosome is attached to the cellular membrane
    • The chromosomeis replicated
    • New membrane and cell wall material is synthesized
    • The two chromosomal attachment site grow apart
    • Additional synthesis of cell wall and membrane separates the daughter cells

Why Eucaryotic Chromosomes Pose Problems

• Eucaryotic chromosomes are linear (except for organelle DNA)
• Nuclear DNA is wound into nucleosomes
• Chromosomes are organized into loop structures
• Eucaryotic organisms are primarily diploid (Two copies of each chromosome which represents a homologous pair)

Cellular Division

• Cellular division in eucaryotes involves the separation of both nuclear and cytoplasmic content

Nuclear division of somatic cells results in two nuclei with the same number of chromosomes as the parent cell is referred to as mitosis

• The division of the cytoplasm is called cytokinesis

The Cell Cycle

• The time between cellular divisions is referred to as the cell cycle
  • The cellular cycle is divided into a series of stages

These stages are a series of continuous events by are designate by specific names to identify the gradation processes which are occurring during each each stage

Stages of the Cell Cycle

• Interphase is the period between the division processes
  • Interphase is subdivided into series of sub-phases
    • G1 (Gap or Growth Phase 1)
    • S ( Synthesis Phase)
    • G2 (Gap 2 or Growth 2)
• M Phase is the divisional phase
  • M Phase can occur in both somatic and germ cells (either mitosis or meiosis, respectively)

Interphase

• G1 is the portion of interphase which immediately follows cellular division

The cell conducts the processes of growth including resource acquistion, respiration, protein synthesis, ribosome synthesis and cellular differentiation

• Centrioles begin to replicate (animal cells)
  • These are essential components of flagella and cillia
• S Phase is the segment of the cell cycle during which the chromosomes are replicated
  • DNA replication specifically occurs during this time period
  • Histone and non-histone protein that are associated with chromosomes are also synthesized
• G2 the cell is preparing for the divisional process that will immediately follow this stage
  • Continued protein synthesis
  • Oganization of the cellular machinery necessary for cytokinesis

Regulation of the Cellular Cycle

• Passage between each of the various sub-phases of Interphase is tightly regulated
  • Regulation is necessary to assure that the progeny cells will have all the necessary resources to continue the cell line
    • The time require to complete a cell cycle (division to division) varies
      • Plants in general require 10 - 30 hours
      • Animals require between 18 - 24 hours
      • Some cells can divide as quickly as 20 mins\
    • Each stage of the cycle can also vary in time

Control of Phase Transition

• Passage between interphase stages is not automatic
  • Some cells become arrested in either G1 or G2
    • The arrest arises from the failure of the cell to produce chemical substances that signal the transition to the next phase
  • The chemicals that control the cell cycle are cyclins
    • S-cyclins stimulate DNA synthesis
    • M-cyclins initiate mitosis
  • Cyclins interact with cell-division-cycle (cdc) proteins to form a Maturation Promoting Factor

M Phase

• Mitosis is a complex process which can be sub-divided into four stages
  • Prophase
  • Metaphase
  • Anaphase
  • Telophase

Prophase

• Cell readies itself for separation of replicated chromosomes
  • Chromosomes begin to condense
    • Characteristic "X-shaped" appearance
  • Mitotic spindle fibers become visable
    • Cytoskeleton components
    • Centrioles move apart and Aster appear
    • Polar microtubules form
  • Nuclear envelope disappears

Metaphase

• Chromosomal bodies begin to align at the cellular equator
  • Movement of the chromosomes is directed by the spindle fibers which are attached to the kinetochore
  • Metaphase ends when the centromeres between the chromotids separate
    • DNA content is the same but the number of chromosomes has temporarily doubled

Anaphase

• During anaphase the two sets of chromosomes move toward opposite poles of the spindle apparatus
  • The chromosomes are single chromatids at this point
    • Movement is accomplished in two ways
      • Digestion of the spindle fibers near the polar bodies
      • Extension of the polar microtubules

Telophase

• Telophase is essential the opposite of prophase
  • Chromosomes have reached their respective poles
  • Nuclear envelopes begin to surround the two sets of chromosomes
  • Chromosome begin to decondense
  • Cytokinesis is almost complete during this

Cytokinesis

• Some organisms can undergo mitosis without cytokinesis producing coenocytic cell (fungi and algae)
• Animals cytokinesis begins with a cleavage furrow
  • Location is at right angles to the spindle apparatus
  • Cleavage furrow deepen until it cut the cells into two
• Plants produce a cell plate (new cell wall) between the two nuclei

Making Gamates:Meiosis

• A divisional process that reduces the number of chromosomes in each new cell to half the normal number (Haploid)

If sexual gamates had not undergone meiosis then the fertilized zygote would have twice the number of chromosomes as the parents

Stages of Meiosis

• Meiosis is subdivided into a series of stages
• Prophase I
• Metaphase I
• Anaphase I
• Telophase I
• interkinesis

Prophase I

• Prophase I in may superficially resemble prophase of mitosis
  • Similar event occur (chromosomal condensation, spindle apparatus, nuclear envelope dissolution)
• Homologous chromosome pair through the formation of synaptonemal complex
  • Allows for recombination (chiasmata) to occur

Metaphase I

• During metaphase the paired homologous chromosomes move to the equatorial plate of the cell
  • These paired chromosomes have four chromatids referred to as a tetrad
    • Within a tetrad each chromosome typically consists of two hybrid chromatids

Anaphase I

• Anaphase I of meiosis differs from that of mitosis
  • In mitosis the two sister chromatids of a chromosome separate

In meiosis the separation occurs between homologous chromosomes rather than between chromatids (no separation at the centromere)

Telophase I and Interkinesis

Telophase I is essentially the same as mitosis with the exception that each newly formed nucleus has half as many chromosomes as the parent cell .

• Same DNA content as an a G1 cell

• Interkinesis is similar to interphase in mitotically dividing cells
  • May be very short in duration
  • No replication of DNA occurs

Second Set of Meiotic Divisions

• These division are mechcanically the same as mitosis with a different functional result
  • Chromosomes do not synapse (no homologous chromosomes)
  • The two-chromatid chromosomes move to the metaphasal plate together
  • Separation occurs between the chromatids
• End result is four haploid cells with new genetic combinations
  • Different combination of the genes

• This page is maintained by James C. Pushnik
• Last modified 12/3/96