Biology 101 - Cell Biology Part 5

Cancer Cancer can be promoted by many factors, including carcinogens, DNA damage, failure of tumor suppressor genes, and failure of apoptosis, or a combination of these events.

After meiosis I. Meiosis I is known as "reductional division," since homologous chromosomes separate during this stage. This leaves two daughter cells, each with a single chromosome from each pair and its replicated copy. Meiosis II, or "equational division," separates these copies.

G1 S G2 In these phases, the cell grows, replicates its DNA, and prepares for mitosis.

The M phase (mitosis itself) is not a part of interphase. G0 is generally thought to be distinct when it is permanent (as in neurons, which do not divide).

The DNA, to ensure that it is in proper condition to be replicated. The G1 checkpoint is also known as the G1/S checkpoint or the restriction checkpoint.

Passage through this checkpoint is required to enter the S phase of the cell cycle.

It makes sure that sufficient cell growth has occurred and that the DNA was replicated properly.

The G2 checkpoint is also known as the G2/M checkpoint.

Passage through this checkpoint is required to enter the M phase of the cell cycle.

Microtubules are anchored at MTOCs, or microtubule-organizing centers. The fibers attach to the MTOC at one end and radiate toward other parts of the cell.

Centrosomes are a specific type of MTOC involved in cell division. Spindle fibers attach to the centrosomes, each of which is composed of two tubulin-based centrioles.

They are cylinder-shaped microtubular structures, generally found in pairs as part of the centrosome, which act to organize the spindle apparatus along which the chromosomes align.

At the centromere, sister chromatids attach in pairs. This region also contains the kinetochore, which binds to a spindle fiber and allows the chromatids to be pulled to opposite poles of the cell.

The centromere is the location on the chromosome that appears "pinched."

It is a complex arrangement of various proteins that forms on the centromere. Here, spindle fibers bind, allowing sister chromatids to be separated to opposite poles of the cell.

The spindle apparatus. Spindle fibers radiate from the centrosome, which contains centrioles, and bind at the kinetochore region of the centromere.

Loose chromatin condenses into visible chromosomes. The nuclear membrane disintegrates. The nucleolus disappears. The centrioles and spindle apparatus form.

• Sister chromatids are cleaved apart.

• Spindle fibers shorten, causing each sister chromatid to move to the opposite pole as its partner.

The cell elongates. Each daughter nucleus forms a new nuclear membrane and nucleolus. The chromosomes start to become less condensed.

1. crossing over

2. independent assortment

Crossing over is a method of genetic recombination that occurs during prophase I of meiosis. In this process, homologous chromosomes exchange genetic material, increasing total variation.

The principle of independent assortment, or Mendel's second law, states that genes for different traits separate into gametes independently of each other (as long as those traits are not linked).

Crossing over, or the trading of DNA segments between homologous chromosomes, occurs when these chromosomes are paired together in prophase I.

DNA replication

More specifically, the S, or "synthesis," phase of the cell cycle involves replication of all chromosomes to form identical copies. Together, a chromosome and its copy are termed "sister chromatids."

The G1 and G2 phases are marked by growth and the synthesis of proteins, including enzymes.

These phases help the cell prepare for cell division, or mitosis (the M phase).

Also known as programmed cell death, this generally occurs in damaged cells. However, it can also serve as a normal part of development.

Viral infection, UV-induced damage, and hypoxia can all induce apoptosis.

The cleavage furrow.

This indentation acts as a contractile ring, gradually closing due to the presence of actin filaments which pinch it shut. At the end of cytokinesis, the two daughter cells are fully separated.

1. prophase I

2. metaphase I

3. anaphase I

4. telophase I

5. prophase II

6. metaphase II

7. anaphase II

8. telophase II

Synapsis

This process occurs during prophase I of meiosis and allows for crossing over, or genetic exchange between the homologous chromosomes, to occur.

A structure that forms during prophase I of meiosis.

This structure consists of both sets of homologous chromosomes, each of which includes two sister chromatids, all in close proximity to each other.

Gametogenesis

Gametes, or sex cells, are produced via meiosis in animal cells. These gametes are haploid, meaning that they have half of the chromosomes held by a typical somatic (body) cell. When two gametes join during the process of fertilization, they create a zygote, or diploid cell that can grow into a new organism.

• Spermatogenesis

• Oogenesis

Biologically speaking, spermatogenesis occurs in male organisms and oogenesis occurs in females.

• cyclins

• cyclin-dependent kinases

Cyclins are proteins that change in concentration depending on a cell's location in the cell cycle. These proteins can bind to cyclin-dependent kinases (Cdks), which are enzymes that phosphorylate other proteins. The behavior of cyclins and Cdks is key in the regulation of the cell cycle.