The Cell Cycle

Asexual vs. Sexual reproduction

Asexual:

• requires a single unit

• resulting products are genetically identical to the original

Sexual:

• requires two units that combine their genetic material into a new version of the originals

Both processes require specialized cells called gametes.

Asexual reproduction in Prokaryotes

• known as binary fission

• their single, circular chromosome is copied without additional steps

Asexual reproduction in Eukaryotes

• mitosis

• more complicated because eukaryotes have much longer chromosomes than prokaryotes and eukaryotes have multiple linear chromosomes arranged in pairs

Pre-mitosis phases:

G1, S Phase, G2

G0 Phase

• a resting state where cells exit the cell cycle, stopping division and differentiation.

• It is a reversible, non-proliferative state used for conserving resources, repairing DNA, or specialized functioning.

G1 Phase

where cells grow and mature

S Phase

Duplication of genetic material after confirming it it without extensive errors or damage

G2 Phase

Cells confirm the duplication process has been successful and all non-mitotic functions of the cell all suspended.

Mitosis Phases

Prophase

Prometaphase

Metaphase

Anaphase

Telophase

Prophase

Condensation of the genetic material in the H-shaped structures identified as chromosomes

• includes the destruction of the nuclear membrane

• the cytoskeleton starts to produce the mitotic spindle

Prometaphase

the arrangement of the cytoskeleton around the centrioles

• As the centrioles migrated to opposite ends of the cell, the cytoskeleton fibers grow and grab each chromosomes’ centromere (link between sister chromatids)

Metaphase

the alignment of the chromosomes across the equator of the cell

• the microtubules start to shrink

Anaphase

the cytoskeleton fibers retract and pull apart the sister chromatids

• issues during this phase lead to abnormal numbers of chromosomes or broken ones

Telophase

• opposite of prophase

• the cellular membrane reappears and surrounds the now independent sets of chromosomes to form the new nucleus

Cytokinesis

• not considered part of mitosis

• represents the moment where the cell breaks in two

Mitosis purpose

Necessary for:

• the growth, maintenance, and repair of multicellular organisms

• in adults, most mitotic events are asymmetric, so one of the daughter cells can specialize while the other remains a stem cell

Meiosis

• cell division that results in up to four gametes

• two successful mitotic events, one that occurs after S phase and one that is not preceded by an S phase

First division of meiosis

• homologous chromosomes align on top of each other to exchange large sections of genetic material.

• known as crossover

• key to providing genetic variation

Second division of meiosis

• not preceded by an s phase, so the resultant cells (gametes) have half the total number of chromosomes the original mother cell had.

• when the gametes combine, the total number goes back to pairs