A-level Biology/Biology Foundation/nuclear division

All living organisms, grow, repair themselves and reproduce. Chapter 1 told us that all cells arise from pre-existing cells, and we are going to see how in this chapter. The nucleus is responsible for the cells activities and does so through DNA. All cells in an organism are genetically identical.

Chromosomes

Chromosomes are thread-like structures found within the nucleus, and the number of chromosomes in a cell is characteristic of the species - for example, we humans have 46 of them. A photograph of chromosomes taken from an electron microscope is known as a karyotype. Animals have two sets of matching chromosomes, and as such their cells (except for reproductive cells, gametes) are called diploid. Gametes, which only have one set of chromosomes are described as haploid cells. In humans, a diploid cell has 46 chromosomes and a haploid cell has 23.

Structure

Look at the image to the right - it is a simplified drawing of chromosomal structure - and it can be seen that a chromosome is a double structure, made from two identical structures called chromatids (1). This is so because during interphase, a period between nuclear divisions, each DNA molecule makes an identical copy of itself contained in a chromatid - these chromatids are held together by a centromere (2). The gene for a particular characteristic is always located at the same place on a chromosome.

Homologous Pairs

Every chromosome is part of a matching pair known as a homologous (because they are the same) pair. Each member of a pair posses genes for the same characteristic, known as an allele - they may differ in how they code for those characteristics. Eye colour, for example is coded for by one allele that may be different, but there are dominant and recessive alleles and that is how the eye colour is 'presented'. There are two separate chromosomes which determine the sex of the individual, known as sex chromosomes, all others are known as autosomes. Females have a double X chromosome, whereas males have a X-Y chromosome.

Cystic fibrosis is caused by a faulty allele in a gene that codes for a chloride channel protein that produces normal mucus, and the mutated allele causes very thick mucus to be produced, leading to cystic fibrosis. Only when both homologous chromosomes have a copy of the faulty allele will the person be a victim of the disease - if not, the person will simply be a carrier.

Nuclear Division

Nuclear division is used for three things, growth, asexual production or sexual reproduction - and two different types are used - mitosis and meiosis respectively.

Growth and Asexual Reproduction

When a diploid cell divides, it must maintain the same number of chromosomes as the original cell, this is known as mitosis

Sexual Reproduction

Meiosis is used for sexual reproduction, and the resultant cell is a gamete - a haploid cell with half the full number of chromosomes.

Mitosis

Mitosis is the type of nuclear division used for growth and repair within organisms - it produces genetically identical cells with the full number of chromosomes (diploid cells). It is a precisely controlled process known as the cell cycle.

Cell cycle

The cell cycle is the period between one cell division and the next - it's three phases are interphase, nuclear division and cell division. During interphase, the cell regrows to its normal size following cell division and then carries on its normal processes, whatever its task may be. When a signal is received to divide, the DNA replicates so that each chromosome consists of two identical chromatids. Nuclear division then follows, and the whole cell then divides. In animals cells, cell division also involves construction of cytoplasm - known as cytokinesis - between the two new nuclei.

IPMAT cycle

Interphase is stage I of the diagram.

• Prophase, Stage II,III- The chromatin condenses into a highly ordered structure called chromosomes and the nuclear membrane begins to break up. The nucleus also breaks up and forms part of several chromosomes. Centrioles move to opposite poles of nucleus.

• Metaphase, Stage IV - Condensed chromosomes, carrying genetic information, align in the middle of the cell at their centromeres before being separated into each of the two daughter cells.

• Anaphase - Stage V, VI Chromatids (identical copies of chromosomes) separate at their centromeres as they are pulled towards opposite poles within the cell. They are pulled because the microtubule fibres begin to shorten, pulling them along.

• Telophase - Nucleus, nucleolus reforms and chromosomes unravel into longer chromatin structures for reentry into interphase. Cytokinesis also occurs - the division of cytoplasm and cell into two by constriction from edges of cell.

This section is called the IPMAT section as it is an easy way to remember the stages of mitosis (Interphase-Prophase-Metaphase-Anaphase-Telophase)

Significance

• Replacement of cells and repair of tissues is done via mitosis - cells are constantly being replaced by mitosis. In some animals mitosis can regenerate entire sections of their bodies.
• Mitosis is also the basis for asexual reproduction. This is more prevalent in unicelluar organisms and plants. Plants bud off of their stem which simply grows into a new plant, becoming detached from the parent and then living independently.
• The nuclei of the two new cells have the same number of chromosomes as the parent nucleus and are genetically identical.

Cancer

Cancer is a common disease in developed countries, with over 140,000 deaths in the UK alone in 2005. There are many different forms of cancer. Cancer is a result of uncontrolled mitosis, that is to say that they divide repeatedly and are out of control. This uncontrolled mitosis produces an irregular mass of cells known as a tumour, and these cells are usually abnormal in shape.

Carcinogens

Cancers are as a result of mutation to the genes that control cell division, and divide continuously from there, passing the mutated gene to all its descendants. There are many different reasons that a gene may mutate, and any factor that may cause it to do so is known as a mutagen - several of them at once are thought to have to happen in one cell. A carcinogen is an agent that causes cancer, and some mutagens are carcinogenic.

Radiation

X-rays, radioactive decay elements and gamma rays can all form damaging ions inside cells which can break DNA strands. UV light can damage genes, potentially causing skin cancer.

Chemicals

Chemicals in tar from cigarettes or cigars is an extremely common killer - 25% of all cancer deaths are due to tar carcinogens. These chemicals can damage DNA molecules.

Viruses

More rarely, cancers can be caused by viruses, including Pallipoma, a virus that can be transmitted sexually and has been linked to cervical cancer.

Predisposition

You do not inherit cancer, but you can inherit faulty genes that cause it, or simply a great susceptibility to developing cancer.

Primary Growths

Primary growths are small groups of tumour cells. Both tumour types place a huge drain on the body's resources because of the constant cell division.

Benign

• Do not spread from origin
• Compress and displace surrounding tissues

Malignant

• Far more dangerous
• Spread through body, invade tissues and destroy them.
• Distributed around the body via blood stream
• Interfere with the functioning of the area where they started to grow.
• Cancerous cells can break off and spread through the blood and lymphatic system to other parts of the body to form secondary growths.