BIO 181 Cell Division

cell division

process by which cells make new cells

reasoning for celll division

1. Growth: multicellular organisms begin as a single fertilized cell 

2. Repair/replace damaged/worn out cells 

3. Reproduction: 

   1. Gamete (eggs and sperm) formation during sexual reproduction 

   2. Unicellular organisms divide to make new organisms. 

cell cycle

an ordered series of well-regulated events that a cell undergoes as it grows, replicates (makes another copy of) its DNA, and divides into two daughter(new) cells.

cell cycle phases

1. Interphase: The cell’s preparation for division, 90% of process

2. M Phase: the actual division

interphase

1. The cell’s preparation for division, 90% of process

   1. G1 (Gap 1): Growth and preparation for DNA replication

   2. S (DNA Synthesis): DNA replication 

   3. G2 (Gap 2): Final growth, DNA error check, preparation for cell division

chromatin

DNA remains in a loosely packed form

G₁ Gap

Cell growth and preparation for DNA replication 

1. The cell is accumulating building blocks of DNA, associated proteins, and energy reserves 

2. Some cells enter G0 phase, which is a resting/ pause state 

   1. Cells no longer divide 

   2. E.g. Neurons stay in G0 , Liver cells may reenter G1 if there is a damage

G₁ Checkpoint

At the end of the G₁ phase, checks whether conditions are favorable for the cell to enter the S phase. 

1. If not favorable, don't proceed to S phase

S Phase

DNA synthesis

DNA Replication

Before a cell divides, it must make an identical copy of its DNA so that each daughter cell receives a complete genome.

S-Phase Checkpoint

ensures accurate DNA replication and detects damage; the cell then moves on to the G2 phase of the cell cycle.

G₂

cells:

1. Continue to grow 

2. Some organelles are duplicated 

3. The cytoskeleton is dismantled

4. Make final preparations for cell division

G₂ Checkpoint

bars entry into the M phase (actual cell division phase) if certain conditions are not met.

Sister Chromatids

Two identical copies of a single chromosome, formed after DNA replication. They are joined at the centromere

Homologus Chromosomes

a pair of matching chromosomes (same genes in the same locations but different versions of those genes) 

1. In humans, one homologous chromosome comes from the mother, and the other comes from the father. 

2. E.g., 23 chromosomes from the mother and the remaining 23 from the father

Diploid Cell

a cell with two complete sets of chromosomes 

1. Normal body cell (somatic cell)

Haploid

 a cell with one complete set of chromosomes 

1. Gametes (eggs and sperm) are haploid

Karyotype

An organized display of an individual's chromosomes, arranged by size, shape, and banding pattern

1. Some conditions can be diagnosed based upon the chromosome appearance

M Phase

1. Division of the nucleus

   1. Mitosis 

   2. Meiosis 

2. Cytokinesis

Mitosis

1. Occurs in body (somatic) cells 

2. Produces 2 genetically identical daughter cells 

3. Chromosome number remains the same. This maintains the same genetic composition across the body. 

4. Used for growth, repair, and asexual reproduction. 

Meiosis

1. Occurs in reproductive organs 

2. Produces 4 genetically different daughter cells 

3. Chromosome number is reduced by half, so that when fertilization occurs, the offspring receives the correct number of chromosomes. 

4. Produces sperm and egg cells.

What is mitosis and its stages?

the process in which the nucleus of a eukaryotic cell divides to produce two genetically identical daughter cells. 

1. The duration of mitosis varies depending on the cell type and organism. 

   1. Skin cells: about every 2-4 weeks 

   2. Early embryonic cells: minutes

2. Stages of Mitosis • Prophase • Metaphase • Anaphase • Telophase

Prophase

1. Chromosomes condense and become visible. 

2. Nuclear membrane breaks down. 

3. Mitotic spindle is assembled in the cytosol. 

4. Microtubules assemble off the centrosome to prepare to eventually separate the sister chromatids

Metaphase

1. The mitotic spindle is fully developed, and centrosomes are at opposite poles. 

2. Chromosomes are aligned at the “equatorial plate”, and each sister chromatid rests on one side of the plate

M Checkpoint

The M checkpoint prevents the cell from entering anaphase until every chromosome is correctly attached. Also, known as the spindle checkpoint, because it determines if all sister chromatids are correctly attached to the spindle microtubules.

Anaphase

1. Sister chromatids separate. 

2. Chromatids move toward opposite poles

Telophase

1. Chromosomes arrive at opposite poles and start to decondense.

2. The nuclear envelope reassembles and begins to surround each new set of chromosomes. 

3. The mitotic spindle assembly breaks down, and the division of the cytoplasm begins via cytokinesis.

Cytokinesis

1. The physical separation of the cytoplasmic components into two daughter cells 

2. Results in two genetically identical daughter cells

Binary Fission

form of asexual reproduction: one parent cell divides into two identical daughter cells 

Binary Fission with Prokaryotes

Prokaryotic cells, such as bacteria, rely on binary fission for propagation.

Binary fission steps

1. The cell copies its DNA. 

2. The cell grows larger. 

3. The two DNA copies move apart. 

4. The cell membrane and cell wall pinch inward. 

5. The cell splits into two daughter cells. 

6. Important: Binary fission is not mitosis. Prokaryotes do not undergo mitosis because they lack a nucleus. They have circular DNA!