Cell DivisionAQA GCSE Biology: Revision notes

The nucleus is the membrane-bound organelle that contains and stores all of an organism's genetic information. This information is organised into structures called chromosomes, which are made of DNA wrapped around histone proteins.

Key points:

• Each chromosome contains many genes, which are sections of DNA that code for proteins
• In humans, there are 46 chromosomes (23 pairs) in most body cells
• The nucleus controls all cellular activities by regulating gene expression
• Chromosomes become visible under a light microscope only during cell division when the DNA is highly condensed
• The genetic information in the nucleus is passed on to daughter cells during cell division

The cell cycle is the sequence of events that occurs between the formation of a cell and its division. It consists of three main stages:

1. Growth (Interphase)

• The cell increases in size and volume
• Organelles are replicated (such as mitochondria and ribosomes)
• The cell accumulates nutrients and energy in the form of ATP
• This is the longest phase of the cell cycle

2. DNA Replication (S Phase of Interphase)

• The DNA in the nucleus is replicated to produce two identical copies of each chromosome
• Each replicated chromosome consists of two sister chromatids joined at the centromere
• DNA replication is semi-conservative, meaning each new DNA molecule contains one original strand and one new strand
• This stage is crucial for ensuring genetic information is accurately passed to daughter cells

3. Mitosis and Cytokinesis

• The cell divides to produce two daughter cells
• Mitosis is described separately in detail (see next section)
• Cytokinesis is the division of the cytoplasm and organelles

Mitosis is the process of nuclear division that produces two genetically identical daughter cells, each with the same number of chromosomes as the parent cell. Mitosis is preceded by DNA replication, so each chromosome consists of two sister chromatids joined at the centromere.

The four stages of mitosis:

1. Prophase

• Chromosomes condense and become visible under a light microscope
• Each chromosome consists of two sister chromatids joined at the centromere
• The nuclear membrane breaks down
• The spindle apparatus (made of microtubules) begins to form from the centrioles

2. Metaphase

• Chromosomes line up along the metaphase plate (the cell's equator)
• Chromosomes are attached to spindle fibres at the centromere
• This is the most condensed stage of the chromosomes

3. Anaphase

• Sister chromatids separate at the centromere
• Individual chromatids (now called chromosomes) are pulled to opposite poles of the cell by spindle fibres
• The cell elongates

4. Telophase

• Chromosomes arrive at opposite poles and begin to decondense
• The nuclear membrane reforms around each set of chromosomes
• The spindle apparatus disappears
• Cytokinesis occurs: the cytoplasm divides to form two daughter cells
• In animal cells, a cleavage furrow forms; in plant cells, a cell plate forms

Result: Two genetically identical daughter cells, each with the same chromosome number as the parent cell (diploid → diploid)

Cell division, particularly mitosis, is essential for the growth and development of all multicellular organisms.

Growth:

• When an organism is young, cells divide by mitosis to increase the total number of cells
• This increases the size and mass of the organism
• Cell division continues throughout life, but at a slower rate in adults
• The rate of mitosis is higher in cells that need frequent replacement (e.g., skin cells, blood cells)

Repair and replacement:

• Mitosis replaces damaged or worn-out cells through the process of tissue repair
• Skin cells are constantly replaced by mitosis
• Red blood cells have a lifespan of about 120 days and are continuously replaced by mitosis in bone marrow
• When tissue is injured, mitosis accelerates to repair the damage

Asexual reproduction:

• Some organisms reproduce asexually by mitosis (e.g., bacteria, some plants)
• Identical daughter cells are produced, allowing an organism to create clones of itself
• This is faster than sexual reproduction but produces no genetic variation

Development:

• Cell division is controlled and programmed during embryonic development
• Different tissues and organs form through a combination of mitosis and differentiation (cells becoming specialised)
• Growth is regulated by growth factors and hormones that control the rate of mitosis

Cancer is a disease caused by uncontrolled cell division. In a healthy body, cell division is tightly regulated by genes that control the rate of mitosis. When these control mechanisms fail, cells divide excessively, leading to cancer.

Normal cell division control:

• Proto-oncogenes are genes that promote cell division in a controlled manner
• Tumour suppressor genes (such as p53) inhibit cell division and trigger apoptosis (programmed cell death) if damage is detected
• Growth factors signal cells to divide only when needed
• Cells have a limited number of divisions they can undergo (Hayflick limit)

What goes wrong in cancer:

• Mutations in proto-oncogenes can cause them to become oncogenes, which continuously trigger cell division
• Mutations in tumour suppressor genes disable the 'brakes' on cell division
• Cancer cells lose contact inhibition (the ability to recognise when they should stop dividing)
• The checkpoint mechanisms that detect damaged DNA are disrupted
• Telomerase enzyme is reactivated in cancer cells, allowing unlimited divisions
• Cancer cells avoid apoptosis even when damaged

Consequences of uncontrolled cell division:

• Excessive growth of abnormal cells forms a tumour (either benign or malignant)
• Malignant tumours are cancers that invade surrounding tissues and can metastasize (spread to other parts of the body)
• Normal tissue function is disrupted
• The organism's resources are diverted to support the cancerous growth

Risk factors for cancer:

• Exposure to carcinogens (cancer-causing substances) such as tobacco smoke, UV radiation, and certain chemicals
• Age (cancer risk increases with age)
• Genetic predisposition
• Chronic inflammation or infection

Meiosis is a type of cell division that produces four haploid daughter cells, each with half the chromosome number of the parent cell. This is essential for sexual reproduction and the production of gametes (sex cells: sperm and eggs).

Key difference from mitosis:

• Mitosis produces 2 diploid cells (same as parent) from 1 diploid cell
• Meiosis produces 4 haploid cells (half the chromosomes) from 1 diploid cell
• Meiosis involves two divisions: Meiosis I and Meiosis II

The two divisions of meiosis:

Meiosis I (Reduction division):

1. Prophase I: Homologous chromosomes pair up (synapsis) and form tetrads. Crossing over occurs at this point—the exchange of genetic material between homologous chromosomes
2. Metaphase I: Tetrads (pairs of homologous chromosomes) line up randomly at the metaphase plate. This random arrangement is called independent assortment
3. Anaphase I: Homologous chromosomes (not sister chromatids) separate and move to opposite poles
4. Telophase I: Nuclear membranes reform; cytokinesis occurs, producing two haploid cells

Meiosis II (Similar to mitosis):

1. Prophase II: Chromosomes condense; spindle apparatus forms
2. Metaphase II: Chromosomes line up at the metaphase plate
3. Anaphase II: Sister chromatids separate
4. Telophase II: Nuclear membranes reform; cytokinesis occurs

Result: Four genetically unique haploid cells (half the chromosome number of the parent cell)

How meiosis produces genetic variation:

1. Crossing Over (Recombination)

• During Prophase I, homologous chromosomes exchange segments of DNA
• This creates new combinations of alleles on each chromosome
• The more genes involved, the greater the genetic variation produced
• Each gamete has a unique combination of alleles

2. Independent Assortment

• During Metaphase I, homologous chromosome pairs randomly align at the metaphase plate
• Each pair can align in two different orientations
• With 23 chromosome pairs in humans, 2²³ different combinations are possible (over 8 million combinations)
• This random distribution of chromosomes ensures each gamete is genetically unique

Significance of genetic variation:

• Genetic variation in offspring increases the chances that some individuals will survive if the environment changes
• This variation is the raw material for natural selection
• Sexual reproduction produces more variation than asexual reproduction
• Variation provides evolutionary advantages to populations