Cell replication Cell division in Prokaryotes The cell cycle in Eukaryotes Apoptosis

Cell division Individual cells need to grow and divide Importantly all the genetic information in a parent cell need to be transferred to the new daughter cells in such a way that they contain a complete set of genetic information. Central to the process of cell division is DNA replication

Cell division in Prokaryotes Bacteria do binary fission! Hey, we have just become two! That felt so good I think I’ll do it again in 20 minutes.

What is Binary Fission? Bacteria can divide by fission easily and rapidly. Fission is a form of asexual reproduction. The bacteria cell simply divides into two approximately equal parts. A single DNA molecule (chromosome) is attached to the plasma membrane a specific point. The DNA molecule replicates Two copies separate by expansion of the plasma membrane. Plasma membrane and cell wall furrow inwards to divide the cytoplasm resulting in two daughter cells.

The cell cycle The cell cycle takes approximately 24 hours. The cell cycle is divided into a series of discrete stages or phases. Cells spend the majority of time (95%) in interphase. Mitosis is the division of the nucleus and is followed by cytokinesis which is the division of the cytoplasm.

Interphase During interphase the cell appears to be at rest, but it is anything but. During interphase most of the cellular contents are synthesised and this increases the cell mass. The genetic material in the nucleus is in the form of chromatins fibres. Discrete chromosomes are not visible. Interphase is split into G1, S and G2 phases.

The G1 Phase Interphase starts with G1 or Gap 1. This is the time for the cell to grow and carry out biochemical activities. The length of this time is highly variable, typically 8 to 10 hours. Some cells sit in G1 for weeks, months or years. Cells that are said to be arrested in G1 are said to be in a G0 state. Most nerve cells never leave G0. The decision to commit to cell division is made when the cell passes through the first check point.

The S Phase Once committed to cell division the cell enters the S phase – S stands for synthesis. This is the time for DNA replication. This typically takes 6-8 hours. DNA replication is, in a sense the purpose of the cell cycle. The S phase ends when the DNA content of the cell has doubled. The evidence for this becomes obvious when the chromosomes become visible at the end of the M phase. Each chromosome is now made up of two sister chromatids.

The G2 Phase Once the genetic material has doubled the cell now enters G2 – Gap 2. This phase is more fixed in its timing and usually lasts 4 – 6 hours for most cells. During this phase the cell actively prepares for cell division. It is a period of high metabolic activity and protein synthesis. The cells pass through another checkpoint at the end of G2 to ensure that all is ready for the division of the nucleus.

The M Phase The M phase encompasses the division of the nucleus (mitosis) and then a division of the cytoplasm (cytokinesis). This phase explains how the two copies of the chromosomal DNA formed in the S phase are separated into daughter cells. The M phase lasts for less than one hour. The M phase is divided into various phases that are characterised by chromosome behaviour.

Prophase Prophase begins when the individual chromosomes have condensed to become discrete objects under the light microscope. Each chromosome is composed of two sister chromatids attached to each other at a constricted region called the centromere. In the cytoplasm, adjacent to the nucleus, the centrosomes, (duplicated in S phase) move to opposite ends of the cell. In late prophase the nuclear envelope breaks down. Centrosomes are now at opposite ends of the cell and growing spindle microtubules enter the nuclear area and make contact with the chromosomes.

The relationship of the centromere and the spindles Contact between a chromosome and a spindle occurs at a protein-DNA complex known as the kinetochore, not actually the centromere.

Metaphase Chromosomes are now maximally condensed and lined up along the metaphase plate. Chromosomes can now be used in karyotype analysis. The chromosomes appear stationary, but each chromatid is being tugged towards the opposite poles by equally strong forces. In animal cells the centrosome contains a pair of centrioles. Metaphase is the longest phase and can take up to 20 minutes.

Anaphase The centromere holding the two chromatids abruptly separates. Each chromatid (now a single chromosome) begins moving to opposite spindle poles. Anaphase is the shortest phase and typically only lasts a few minutes.

Telophase Daughter chromosomes arrive at the poles and revert to extended fibres of chromatin. The spindle microtubules disassembles and the nuclear membrane forms around the two groups of daughter chromosomes. During this period the cell usually undergoes cytokinesis – an independent process – that results in the division of the cytoplasm.

Cytokinesis In an animal cell cytokinesis causes inward constriction of the plasma membrane. This starts from the outside and works in In plant cells due to the cell wall, cytokinesis cannot constrict the plasma membrane inwards so it works from the inside out. A new cell wall and plasma membrane is assembled across the cell plate.

Checkpoints – points that regulate the cell The cell cycle is highly regulated by intracellular signalling molecules and extracellular signalling proteins called growth factors that control the rate of cell growth and division.

G1 Checkpoint G2 Checkpoint Spindle Assembly Checkpoint Check for: DNA damage, Cell size, Nutrients, Growth factors Check for: Cell size and DNA damage Check for: Chromosome attachment to spindle

Cell cycle animations Cell cycle summary Cell cycle game edicine/2001/ edicine/2001/ Mitosis to music CDBio.htmlhttp:// CDBio.html

What happens if problems are detected?

Apoptosis Apoptosis, or programmed cell death or ‘cellular suicide’, is self-destruction by cells for the good of the whole organism. How does it differ from necrosis? Necrosis occurs if a cell is seriously damaged by some mechanical or chemical trauma. It results in the cell bursting and causing inflammation in nearby tissue. In apoptosis, the cell is broken down into parts and engulfed by phagocytes, no inflammation is involved.

What happens if problems aren’t detected?

Defective cell cycle control mechanisms When control mechanisms fail uncontrolled cell proliferation can produce a mass of cells called a tumour. Tumours can be benign or malignant (cancer). Mutations in the genes that express regulatory proteins accumulate. This leads to genetic instability and the development of cancer.

Glossary Chromosome: Thread-like structures composed of DNA and protein, visible in cells during mitosis and meiosis. Chromatin: The complex of DNA and protein that make up chromosomes. Chromatid: One of the two daughter strands of a replicated chromosome which are joined by a single centromere; separates and becomes daughter chromosome. Centromere: Point on a chromosome at which chromatids are held together and to which the spindle fibres attach during cell division. Centriole: Small cylindrical structure composed of microtubules; occurs in pairs and are involved in the separation of chromosomes in animal cells.