Cell Cycle and Cell Division

Topics Review

Q: Electrons removed from H₂O molecules are transported through the photosystems and photosynthetic electron transport chain and are ultimately used to reduce NADP⁺ to NADPH. At what point of the electrons have the greatest amount of potential energy?

A: after capturing photon energy in the reaction center of photosystem I → recall the ‘Z Scheme’, where the energy drops from the first photosystem but rises as it enters the next photosystem

Lecture Question(s)

Q: Why do offspring of the same parents have different genetic information?

A: synapsis and random pairings

Core Concepts

1. During cell division, a single parental cell divides into two daughter cells

2. Mitotic Cell Division is the basis of asexual reproduction in unicellular eukaryotes and the process by which cells divide in multicellular eukaryotes

3. Meiotic Cell Division is essential for sexual reproduction, the production of offspring that combine genetic material from two parents

Cell Division

• the process by which cells make more cells

• Cell division occurs for:

  • growth

  • cell replacement

  • healing

  • reproduction

• a single parental cell divides into two daughter cells

• Requirements;

  • the two daughter cells must each receive the full complement of genetic material present in the single parent cell

  • the parent cell must be large enough to divide in two and still contribute sufficiently cytoplasmic components to each daughter cell

Binary Fissions

Steps 1-3

1. the circular bacterial DNA molecule is attached by proteins to the inner membrane

2. DNA replication begins at a specific location and proceeds bidirectionally around the circle

3. the newly synthesized DNA molecule is also attached to the inner membrane, near the attachment site of the initial molecule

Steps 4-6

4. as replication proceeds, the cell elongates symmetrically around the midpoint, separating the DNA attachment sites

5. cell division begins with the synthesis of new membrane and wall material at the midpoint

6. continued synthesis completes the constriction and separates the daughter cells

Mitotic Cell Division

Eukaryotes

• genome is large and linear

• DNA is found in the nucleus

Prokaryotes

• genome is small and circular

• DNA is found in the cytoplasm

Cell Cycle (Eukaryotes)

• M Phase: the time during which the parent cell divides into two daughter cells

• Interphase: the time between two successive M phases

• cell make many preparations during interphase for division

• these preparations include DNA replication and an increase in the cell size

Interphase

• G₁ and G₂

• takes up majority of the cell cycle

• G₀/G₁ = greatest DNA content

• S = lowest DNA content

Eukaryotic DNA Organization

• DNA is organized with histones and other proteins into chromatin

• chromatin can be looped and packaged to form chromosomes

• Karyotype: describes the number and shape of a typical human body

• Sister Chromatids: identical chromatid copies

• Centromeres: what holds together chromosomes

Mitosis Stages

• takes place in five stages;

  1. Prophase

     • chromosomes condense

     • centrosomes radiate microtubules and migrate to opposite poles

  2. Prometaphase

     • microtubules of the mitotic spindle attach to the chromosomes

     • the nuclear envelope breaks down to allow this - centrosomes form outside the nucleus which is why this is required

  3. Metaphase

     • chromosomes align in center of the cell

  4. Anaphase

     • sister chromatids (which become individual chromosomes when the centromere splits) separate and travel to opposite poles

  5. Telophase + Cytokinesis

     • nuclear envelope re-forms and chromosomes condense

     • telophase marks the end of mitosis

  6. Cytokinesis (Animal)

     • begins when a ring of actin filaments forms

     • it pinches the cytoplasm of the cell and divides it into two

  7. Cytokinesis (Plant)

     • the construction of a new cell wall happens to split the cells

Meiotic Cell Division

• meant for sexual reproduction

• meiotic cell division results in four daughter cells

• each daughter cell contains a half number chromosomes as in the parent cell

Meiosis I

• homologous chromosomes separate

  1. Prophase

     • chromosomes become visible as thin threads, DNA replication is already complete

     • homologous chromosomes continue to condense and undergo synapsis

     • Crossing Over;

       • meiosis allows homologous chromosomes of maternal origin and paternal origin to undergo an exchange of DNA segments

       • crossing over also helps hold together the bivalent for metaphase I

     • when synapsis is complete, each pair of homologous chromosomes form a bivalent

     • each chromosome consists of two sister chromatids

     • the chromosomes continue to shorten and thicken and the chiasmata between non-sister chromatids become apparent

     • the nuclear envelope begins to break down

  2. Prometaphase

     • spindles attach to kinetochores on chromosomes

  3. Metaphase

     • homologous pairs line up in center of cell, with bivalents oriented randomly with respect to each other

  4. Anaphase

     • homologous chromosomes separate, but sister chromatids do not separate

  5. Telophase + Cytokinesis

     • daughter cells are ready to move into prophase II

• Reductional division reduces the number of chromosomes in daughter cells by half

• the centromeres do not split during this stage of mitosis

Meiosis II

• sister chromatids separate

• the process resembles that of mitosis, except that the nuclei in prophase II have the haploid number of chromosomes, not the diploid number

• there is no DNA synthesis between the two meiotic divisions

• meiosis II is often called the equational division because cells in meiosis II have the same number of chromosomes at the beginning and at the end of the process

Comparing Mitosis and Meiosis

• during meiosis II, sister chromatids separate from each other, this process is similar to mitosis

• during meiosis I, maternal and paternal homologs separate form each other

Cytoplasmic Division

• in females, the cytoplasm is divided very unequally in both meiotic divisions

• in males, the cytoplasm divides fairly equally, resulting in products that all form functional sperm