Cell division

Chromosomes

What does the nucleus contain?

• Chromosomes made of DNA molecules
• Each chromosome carries a large no of genes

Why do most organisms have an even number of chromosomes in body cells?

In body cells, chromosomes are normally found in pairs

1.2.2 Mitosis and the cell cycle

Define cell cycle.

• Cells divide in series of stages
• During cell cycle, genetic material is doubled & divided into 2 identical cells

Describe stages of cell cycle involving mitosis.

Stage 1 (Before cell divides)

• Cell grows & increase no of sub-cellular structures

eg mitochondria & ribosomes

• DNA replicates to form 2 copies of each chromosome

Stage 2 (Mitosis takes place)

In mitosis, one set of chromosomes is pulled to each end of cell & nucleus divides

Stage 3 Describe what is happening in mitosis. (2)

• Cytoplasm & cell membrane dividing to form 2 identical daughter cells

When a cell divides by mitosis the new cells are genetically identical

What causes the cells to be genetically identical?

• DNA replicates

Why is the ability of body cells to divide important? (1)

• For growth & development of multicellular organisms
• For repairing of organism
• For asexual reproduction

1.2.3 Stem cells

Define stem cells (3)

• Undifferentiated cells that are able to divide & can differentiate to form lots of cells of the same type, & from which certain other cells can arise from differentiation

Describe the function of stem cells in human embryos. (2)

Can be cloned & made to differentiate into different types of human specialised cells eg nerve cells

Advantages	Disadvantages
Can develop into most other types of cells & can treat many diseases Each cell divides every 30 min, plentiful Low chance of rejection & painless	Cause death to embryo Unreliable procedure The embryos can’t give consent that poses ethical issues

Describe the function of stem cells in adult bone marrow. (2)

Can be cloned & form many different types of cells eg blood cells

Advantages	Disadvantages
Procedure is well tested & relatively safe Give consent for the procedure to take place, which removes any ethical issues Quick recovery	Risk of infection from operation Painful to donate stem cells so may deter donors Few types of cells

Why do some scientists have concerns about the use of stem cells?

• Could cause cancer

What are the sources of adult stem cells?

• Bone marrow, umbilical cord, blood, skin

Why might stem cells from embryos be more useful than adult stem cells?

• Can be cloned & form many different types of cells eg blood cells

Therapeutic cloning

How stem cells can be used for medical treatment through therapeutic cloning? (4)

• An embryo is produced with same genes as patient
• Stem cells from embryo aren’t rejected by patient’s body so may used for medical treatment
• Once inside patient, stem cells can differentiate to replace cells which aren’t working properly
• Can help conditions eg diabetes & paralysis

Advantages	Disadvantages
May cure diseases Produce replacement cells Treat diabetes & paralysis Cells unlikely to be rejected Cells & tissues of any type can be made Many cells are produced	Potential life is killed Shortage of egg donors May transfer viral infection

What are the potential risks of using stem cells? (2)

• Transfer of viral infection
• Ppl have ethical or religious objections

Plants

Define meristems in plants.

• Plant stem cells that can differentiate into any type of plant cell throughout the life of the plant

What can plant stem cells be used for?

• They can be used to make clones of plants quickly & economically

Describe and explain the functions of stem cells in meristem tissue in plants. (4)

• Differentiate into any type of plant cell, throughout life of plant
• Used to produce clones of plant quickly & economically
• Protect rare species from extinction
• To produce large no of identical plants for farmers eg disease resistance crops