What you'll learn
- How mitosis fits into the cell cycle and produces genetically identical cells.
- Why mitosis is important for growth, repair and asexual reproduction.
- How animals and plants grow differently.
- How percentile charts and stem cells link cell biology to medicine.
Starting point: DNA, chromosomes and body cells
Your cells contain DNA, the chemical instructions for making and controlling the organism. In most animal and plant cells, DNA is found in the nucleus, the control centre of the cell.
DNA is organised into chromosomes. A chromosome is a long DNA molecule, coiled up with proteins, carrying many genes. A gene is a section of DNA that helps control a characteristic or cell process.
Diploid body cell
A diploid cell has two sets of chromosomes, usually one set inherited from each parent. Human body cells are diploid and contain 46 chromosomes.
A body cell is any cell that is not a gamete. Skin cells, liver cells, root cells and muscle cells are examples of body cells.
Main outcome of mitosis
Mitosis produces two daughter cells, each with an identical set of chromosomes in the nucleus to the parent cell. The daughter cells are genetically identical diploid body cells.
Tracking chromosome number
A human skin cell divides by mitosis. Work out how many chromosomes are in each daughter cell.
- The parent skin cell is a diploid body cell, so it has 46 chromosomes.
- During interphase, each chromosome is copied, but the cell is still preparing to divide rather than becoming two cells yet.
- During mitosis, the copied chromosomes are separated so that each new nucleus receives one complete set of 46 chromosomes.
- After cytokinesis, there are two daughter cells, each with 46 chromosomes.
The cell cycle
The cell cycle is the series of events a cell goes through as it grows and divides. Mitosis is only one part of the cell cycle.
The cell cycle includes:
- Interphase — the cell grows, copies its organelles, and replicates its DNA.
- Mitosis — the nucleus divides.
- Cytokinesis — the cytoplasm and cell membrane divide to make two separate cells.
Interphase: preparation before division
Interphase is the stage before mitosis. The cell is active: it grows, makes more proteins, copies organelles such as mitochondria, and replicates its DNA.
After DNA replication, each chromosome consists of two identical copies called sister chromatids, joined at a region called the centromere.
Interphase is not mitosis
Interphase is part of the cell cycle, but it is not part of mitosis. Mitosis is the division of the nucleus.
The stages of mitosis
Prophase
In prophase, chromosomes become shorter, thicker and visible. The nuclear membrane breaks down, and spindle fibres begin to form.
Metaphase
In metaphase, chromosomes line up across the middle of the cell. This middle region is often called the cell equator. Spindle fibres attach to the chromosomes.
Anaphase
In anaphase, the sister chromatids are pulled apart to opposite ends of the cell by spindle fibres. Once separated, each chromatid is treated as a chromosome.
Telophase
In telophase, new nuclear membranes form around the chromosomes at each end of the cell. Two nuclei are formed.
Cytokinesis
Cytokinesis is the division of the cytoplasm. The cell membrane pinches in, producing two separate daughter cells.
Remember the order
The mitosis stages go P-M-A-T: prophase, metaphase, anaphase, telophase. Then cytokinesis finishes the split into two cells.
Identifying a mitosis stage
A microscope image shows chromosomes lined up across the middle of a cell, with spindle fibres attached. Identify the stage.
- The chromosomes are not randomly spread out; they are arranged at the centre of the cell.
- Lining up at the cell equator is the key event of metaphase.
- The stage is therefore metaphase.
Why mitosis matters
Mitosis is important because it makes new cells with the same genetic information.
Growth
Multicellular organisms grow by increasing their number of cells. This happens through repeated mitosis.
Repair
Damaged tissues can be repaired when cells divide to replace dead or damaged cells. For example, mitosis helps replace skin cells after a cut.
Asexual reproduction
Asexual reproduction is reproduction involving only one parent and no fusion of gametes. It produces offspring that are genetically identical to the parent, unless a mutation occurs. Some plants, bacteria-like organisms, fungi and simple animals can reproduce asexually.
Why identical cells are useful
For growth and repair, new cells usually need the same instructions as the original cells, so mitosis keeps the chromosome number and genetic information the same.
Cancer and uncontrolled cell division
The cell cycle is normally controlled carefully. Cells should only divide when new cells are needed.
Cancer can result from changes in cells that cause uncontrolled cell division. These changes may affect genes that normally control when cells divide. The cells keep dividing and may form a mass of cells called a tumour.
Cancer is not normal growth
Growth is controlled cell division for a useful purpose. Cancer involves uncontrolled cell division, where cells divide when they should not.
Growth in animals and plants
Growth in animals
In animals, growth mainly involves:
- cell division by mitosis to increase cell number
- differentiation, where cells become specialised for particular functions
Most animal cells differentiate at an early stage of development. In adults, many specialised cells divide less often, although some tissues still need regular replacement, such as skin and blood.
Growth in plants
In plants, growth involves:
- cell division by mitosis
- elongation, where cells increase in length
- differentiation into specialised plant cells
Plant growth happens mainly in regions called meristems, found in places such as root tips and shoot tips.
Differentiation
Differentiation is the process by which a cell becomes specialised for a particular function.
Specialised cells have structures that help them do their jobs. For example, a nerve cell has long extensions to carry electrical impulses, and a root hair cell has a large surface area to absorb water and mineral ions.
Stem cells
A stem cell is an unspecialised cell that can divide and differentiate into other cell types.
Embryonic stem cells
Embryonic stem cells are found in early embryos. They can differentiate into many different types of body cell. Their function is to help produce all the specialised cells needed as the embryo develops.
Adult stem cells in animals
Adult stem cells are found in some tissues of animals. They can divide to replace damaged or worn-out cells, but they usually differentiate into a limited range of cell types. For example, stem cells in bone marrow can produce different types of blood cell.
Meristems in plants
Meristems contain plant stem cells. They can divide and differentiate into many types of plant cell throughout the plant’s life. This is why plants can keep growing from their roots and shoots.
Stem cells in medicine: benefits and risks
Stem cells could be used to replace damaged cells or tissues. For example, bone marrow stem cell transplants are already used to treat some blood disorders. Scientists are also investigating possible treatments for conditions such as spinal cord injury and diabetes.
However, stem cell treatments also carry risks. The cells may divide uncontrollably and form tumours, may be rejected by the patient’s immune system, or may not differentiate into the intended cell type. There are also ethical concerns about using embryonic stem cells because embryos are destroyed to obtain them.
Balancing a stem cell treatment
A patient has damaged nerve cells. Scientists suggest using stem cells to produce replacement nerve cells. Explain one possible benefit and one possible risk.
- The damaged tissue contains specialised nerve cells, so ordinary unspecialised cells would not automatically perform the same function.
- Stem cells could be useful because they may differentiate into nerve cells and help replace damaged cells.
- A risk is that the stem cells might divide uncontrollably or differentiate incorrectly, which could harm the patient.
Using percentile charts to monitor growth
A percentile chart compares a child’s growth with a large group of children of the same age and sex. Charts may show height, mass or head circumference.
Percentile
A percentile shows position within a comparison group. For example, a child on the 50th percentile for height is taller than about 50% of children of the same age and sex, and shorter than about 50%.
Doctors and nurses plot measurements over time. A child does not need to be exactly on the 50th percentile to be healthy. What matters is whether their growth follows a consistent pattern and whether there are sudden changes.
Interpreting a percentile chart
A child’s height has been plotted near the 91st percentile for several years. What does this suggest?
- The 91st percentile means the child is taller than about 91% of children of the same age and sex.
- The points stay close to the same percentile curve over time, so the child’s growth pattern is consistent.
- This is usually less concerning than a sudden drop across several percentile lines, because the child is continuing to grow steadily.
Percentile does not mean percentage of normal
A child on the 9th percentile is not “9% healthy” or “9% grown”. It means their measurement is lower than most children of the same age and sex, but it may still be normal if their growth is steady.
In the exam
- When describing mitosis, include both the chromosome outcome and the cell outcome: two genetically identical diploid daughter cells.
- Keep the order clear: interphase prepares the cell, then prophase, metaphase, anaphase, telophase, then cytokinesis.
- For percentile charts, comment on the pattern over time, not just one measurement.
Check yourself
- Why does DNA need to be replicated before mitosis?
- How is plant growth different from animal growth?
- What are two possible benefits and two possible risks of stem cell treatments?
