Cell division, mitosis and stem cells · GCSE Biology

Chromosomes

The nucleus of a cell contains chromosomes, made of DNA. Each chromosome carries a large number of genes.
In body cells chromosomes are found in pairs. Human body cells have 23 pairs (46 chromosomes).
Gametes (sex cells) have only one chromosome from each pair: 23 in humans.

Cells divide in a series of stages called the cell cycle.
First the cell grows: it makes more sub-cellular structures, such as ribosomes and mitochondria, and copies its DNA to form two copies of each chromosome.
In mitosis, one set of chromosomes is pulled to each end of the cell and the nucleus divides.
Finally the cytoplasm and cell membrane divide to form two identical daughter cells.
Mitosis is needed for growth, for repairing damage and in asexual reproduction.

Differentiation is when a cell changes to become specialised for its job.
Most animal cells differentiate at an early stage and then mainly divide only for repair and replacement.
Many plant cells keep the ability to differentiate throughout life.

A stem cell is an undifferentiated cell that can divide to make more stem cells and can differentiate into other types of cell.
Embryonic stem cells can become any type of human cell.
Adult stem cells, such as those in bone marrow, can form many types of cell, including blood cells.
Meristem tissue at the tips of plant roots and shoots can differentiate into any type of plant cell, throughout the plant's life.

Stem cells might one day treat conditions such as diabetes and paralysis.
In therapeutic cloning, an embryo is made with the same genes as the patient, so its stem cells would not be rejected by the patient's body.
Concerns: some people have ethical or religious objections to using embryos, and stem cells grown in a lab could carry viral infections.
Meristems are used to clone plants quickly and cheaply, for example to save rare species or to produce large numbers of crop plants with useful features such as disease resistance.

Animals grow by cell division (mitosis) and differentiation. Plants grow by cell division, cell elongation (cells take in water and get longer) and differentiation.
Doctors use percentile charts to check a child's growth (mass, length or head size) against other children of the same age.
A baby on the 75th percentile for mass is heavier than 75% of babies of the same age. Moving a long way across the percentile lines can be a sign of a problem.
Cancer is the result of changes in cells that lead to uncontrolled cell division.

Chromosome: A long, coiled molecule of DNA that carries many genes.
Gene: A small section of DNA that codes for a particular protein.
Cell cycle: The series of stages in which a cell grows, copies its DNA and divides into two.
Mitosis: The stage of the cell cycle in which the nucleus divides to form two genetically identical nuclei.
Differentiation: The process by which a cell becomes specialised for a particular job.
Stem cell: An undifferentiated cell that can divide to make more cells like itself and differentiate into other types of cell.
Embryonic stem cell: A stem cell from an early embryo that can become any type of human cell.
Adult stem cell: A stem cell in adult tissue, such as bone marrow, that can form a limited range of cell types.
Meristem: Plant tissue at the tips of roots and shoots that can differentiate into any type of plant cell.
Therapeutic cloning: Making an embryo with the same genes as a patient so its stem cells can be used without rejection.