1.2 Cell division Flashcards
Chromosomes
In eukaryotic cells, the nucleus contains thread-like structures called chromosomes.
Chromosomes are made from highly coiled strands of relatively long DNA.
Each chromosome is made from one
DNA molecule.
In the body cells of diploid organisms, chromosomes are normally found in pairs.
One chromosome from each pair is inherited from the mother, the other from the father.
Different species of organisms have different numbers of chromosomes in their nuclei.
Humans have 46 chromosomes in the nucleus of all their body cells, found in 23 pairs.
Before a cell can divide, its genetic material needs to be doubled. This results in the characteristic x shaped chromosome.
Cell cycle
G1 – sub-cellular structures duplicate. Structures include mitochondria and ribosomes
S – DNA replicates to form 2 copies of each chromosome
G2 – DNA is checked for errors made during replication. Done using enzymes, and any errors can be fixed
Mitosis – the chromosomes move to opposite poles of the cell and 2 nuclei form
Cytokinesis – cytoplasm divides and cell membrane separates creating 2 new cells
G0 – Temporary cell resting. Some cells will never divide again, e.g. nerve cells
Mitoses
Mitosis is cell division that results in genetically identical diploid cells.
The main purpose of mitosis is to create 2 identical daughter cells from 1 parent cell. Daughter cells are identical to one another and to the parent cell.
The 4 stages of mitosis are prophase, metaphase, anaphase, telophase.
Prophase
Prophase is the first stage of mitosis
Chromosomes condense and become visible
Spindle fibres emerge from centrosomes
Nuclear envelope breaks down
Nucleus disappears
Metaphase
Mitotic spindle is fully developed, centrosomes are at opposite poles of the cell
Chromosomes are lined up at the metaphase plate
Each sister chromatid is attached to a spindle fibre originating from opposite poles
Anaphase
Cohesion protein binding the sister chromatids break down
Sister chromatids (now called chromosomes) are pulled towards opposite poles
Non-kinetochore spindle fibres lengthen, elongating the cell
Telophase
Chromosomes arrive at opposite poles and begin to decondense
Nuclear envelope material surrounds each set of chromosomes
The mitotic spindle breaks down
Cytokinesis
Plant cell - a cell plate separates the daughter cells
Animal cell – a cleavage furrow separates the daughter cells
Stem Cells
Stem cell is an undifferentiated cell which is capable of self-renewal to produce more stem cells or differentiate into specific specialised cells.
Plant stem cells
Many plant cells retain the ability to differentiate throughout their lives and have a layer of unspecialised stem cells (meristem) in the roots and shoots. They can become any type of plant cell.
They are often used to save species that are at risk of extinction.
Adult stem cells
Some stem cells remain in the bodies of adults as adult stem cells. Adult stem cells are rare and found at certain locations. They can only differentiate into cells from the type of tissue where they are found. Their role is to replace body cells that die through injury and disease. For example, bone narrow cells can differentiate into blood cells and cells of the immune system, but not other cell types
Embryonic stem cells
Embryos contain stem cells that can grow into any type of cell found in the body and are not specialised. Stem cells can be removed from human embryos that are 3-5 days old to cure diseases however, as the new cells are used to treat a different individual, immunosuppressants are required.
Umbilical cord stem cells
After a baby is born, stem cells may be collected from the umbilical cord. This is a rich source of stem cells that are formed from the placenta. These can be frozen and stored for possible use in the future by that child. These stem cells have been used to treat a variety of blood disorders as they can differentiate into different blood cells.
Therapeutic stem cells
In therapeutic cloning an embryo is produced with the same genes as the patient. Stem cells from the embryo are not rejected by the patient’s body so they may be used for medical treatment.
The use of stem cells has potential risks such as transfer of viral infection, and some people have ethical or religious objections.
Stem cells from meristems in plants can be used to produce clones of plants quickly and economically.
Rare species can be cloned to protect from extinction.
Crop plants with special features such as disease resistance can be cloned to produce large numbers of identical plants for farmers.
Stem cells in medicine
Stem cells can be used to treat a wide variety of diseases including diabetes and paralysis.
Organs developed from a patients own stem cells reduces the risk of rejection and the need to wait for an organ donation.
However stem cells cultured in the lab could become infected and potentially be transmitted to the patient. There is also a risk of stem cells mutating which can lead to them developing into cancer cells.
