BIOL1240 - Chapter 10

Functions of Cell Division

- Single-Celled Organisms: reproduction

- Multicellular Organisms: growth, maintenance, and repair of cells and tissues

- Also a part of sexual reproduction

Genome

All of an organism's genetic material; the way DNA is organized in organisms

Genome in Prokaryotes

DNA is one double-stranded circular molecule

Can also have smaller loops of DNA called plasmids

- not necessary for normal growth, but may have important information

- exchange of plasmids with other cells --> gene transfer in prokaryotes

Genome in Eukaryotes

Several double-stranded DNA molecules in the form of chromosomes

The number of chromosomes in the cell nucleus varies among species:

- Humans have 46, but some plants have over 1000

Somatic Cells

- Normal body cells

- Typically have 2 matched sets of chromosomes

- Diploid (2n)

Gametes

- Egg & Sperm cells

- Have half the number of chromosomes

- Haploid (1n)

Chromosome Arrangement

Arranging the chromosomes by size produces a karyotype

- The human genome has 23 pairs of chromosomes (46 total). Since we have two of each chromosome, we are diploid

Chromatid

One half of a duplicated chromosome

Homologous Chromosomes

Chromosomes that pair in reproduction of diploid cells

Hetereologous Chromosomes

Chromosome pairs that do not match (such as X and Y chromosomes in humans)

Eukaryotic DNA Condensed

Eukaryotic DNA must be condensed into compact chromosomes to fit into the nucleus:

1. Short stretches of DNA wrap around a core of 8 histone proteins (like a string of beads)

2. The histone-DNA complex (the bead) is called a nucelosome and the connecting DNA (string) is called linker DNA

3. This structure coils to form a chromatin fiber

4. Condensin binds to DNA and wraps it into coiled loops that are compacted into a chromosome

The Cell Cycle

An ordered series of events in the life of a cell. The cell cycle has 2 major phases: interphase & M (mitotic) phase

Interphase

This is the time for normal growth and preparation for cell division.

- The cell spends the majority of its life in interphase

- Chromosomes are not condensed so that genes can be expressed

- During the S-phase, DNA is duplicated, and a single chromatid chromosome becomes double or sister chromatid chromosomes

Sister/Double Chromatid Chromosome

- Two identical copies (chromatids) formed by the replication of a chromosome

- A single chromatid is one-half of a duplicated chromosome

- The protein cohesin holds sister chromatids together

Mitotic Phase

Karyokinesis (karyo=nucleus) is nuclear division, the first step of the mitotic phase:

- Prophase

- Prometaphase

- Metaphase

- Anaphase

- Telophase

The second portion of the mitotic phase is cytokinesis (cyto=cell), where the cytoplasmic components separate into two daughter cells

Prophase

- First step of mitosis

- Nuclear envelope breaks down

- Membranous organelles (eg. Golgi body, ER, etc) move toward edges of the cell

- Nucleolus disappears

- Centrosomes begin migration to poles

- Microtubules of the spindle form

- Sister chromatids coil together (aided by condensin proteins)

Prometaphase

- Second step of mitosis

- SIster chromatids develop a protein called kinetochore in the centromere region of the DNA

- Kinetochore attaches the chromatids to the spindle microtubules

Metaphase

- Third step of mitosis

- One sister chromatid is attached on one pole and the other sister chromatid is attached to the other pole

- Chromosomes line up along the metaphase plate or the middle of the cell

- Sister chromatids remain attached by cohesin proteins

Anaphase

- Fourth step of mitosis

- Cohesion proteins degenerate, allowing chromatids to separate

- Separated sister chromatids move in opposite directions toward the centrosomes to which their microtubules are attached

- Cell elongates

Telophase

- Fifth step of mitosis

- Chromosomes reach opposite poles & begin to unravel

- Spindle microtubules depolymerize into tubulin monomers that will form cytoskeletal components for the daughter cells

- Nuclear envelopes reform around the chromosomes

- Nucleoli appear within the nuclear area

Cytokinesis

- Not considered part of mitosis, but begins in telophase

- Division of the cytoplasm during cell division

In animal cells:

- An actinomyosin contractile ring constricts the plasma membrane, creating a cleavage furrow

In plants:

- Golgi vesicles gather at the cell midplane and fuse to form a cell plate that will become part of the cell wall

Regulation at Internal Checkpoints

Mistakes affecting function (such as mutated or incorrect number of chromosomes) are regulated at 3 checkpoints in the cell cycle:

1. Near the end of G1 (before replication in S-phase)

2. At the G2 to mitosis transition (verifies replication is done & checks for DNA damage)

3. In metaphase of mitosis (prevents chromosome mis-segregation)

External Triggers that Inittate/Inhibit the Cell Cycle

- Death of nearby cells

- Release of growth hormones

- Cell crowding

Cancer & The Cell Cycle

The term cancer refers to many different diseases characterized by uncontrolled cell growth:

- Begins with a gene mutation that results in a faulty protein that regulates cell reproduction

Two types of regulators are involved:

- Positive Regulators (Oncogenes): promote movement to the next step of the cell cycle

- Negative Regulators (Tumor Suppressors): stop advancement of the cell cycle

Tumors result when the reproduction of mutated cells surpasses the growth of normal cells:

- Results from lack of homeostasis between cell division & death

- Apoptosis: programmed cell death

Prokaryotic Cell Division

binary fission