A chromosome is a strand of DNA.
What is mitosis?
Mitosis is the type of cell division that produces identical new cells.
Describe the 4 steps of mitosis.
1. Chromosomes in a parent cell make identical copies of themselves.
2. The chromosomes line up along the centre.
3. They move apart.
4. Two identical daughter cells form with identical chromosomes to the parent cell.
What are gametes?
Gametes are sex cells produced by meiosis, with only 1 set of chromosomes.
Describe the 3 steps of meiosis.
1. Chromosomes in a parent cell replicate.
2. The pairs of chromosomes divide- 1st division.
3. Chromosomes divide- 2nd division.
Why does meiosis produce variation?
There are lots of possible combinations of chromosomes, this makes variation. This is important for evolution.
What are stem cells?
Stem cells are unspecialised/undifferentiated cells that can become specialised.
Where are stem cells found?
Stem cells can be found in the human embryo and in adult bone marrow.
Humans have how many pairs of chromosomes?
Sex is determined by the X and Y chromosomes- how so?
Human females have two X chromosomes. Human males have an X and a Y chromosome.
Different forms of the same gene.
Characteristics expressed by an individual= observable. Physical appearance of the characteristic.
The genetic make-up of an individual.
A dominant allele always shows, even if an individual only has one copy. Allele expressed with only one copy of allele.
A recessive allele only shows if the individual has two copies.
What is polydactyly?
A genetic disorder where a baby is born with an extra finger or toe, caused by a dominant allele.
What is cystic fibrosis?
Cystic fibrosis affects cell membranes and causes the production of thick, sticky mucus which can affect the lungs. It is caused by a recessive allele.
Describe and explain some of the symptoms of cystic fibrosis.
-Not putting on weight ➡️ digestive enzymes blocked by mucus ➡️reduced absorption of broken down foods.
-Difficulty breathing ➡️mucus blocks trachea, bronchi and bronchioles.
-Infection➡️mucus too thick to be coughed up➡️pathogens sit in mucus and cause disease.
What are the ethical issues with embryo screening of pregnant women and IVF?
Pregnant- Sample of cells from embryo➡️Risk of miscarriage and difficult decision if diseased.
IVF- Still ethical issues because its selecting 'healthy' embryos➡️Unhealthy embryos destroyed➡️Slippery slope.
A gene is an instruction for making proteins to determine a characteristic.
Why is it important that gametes have only 23 chromosomes? (2 marks)
When gametes fuse (1), chromosome number is restored to full number of 23 pairs (1).
When Mendel published his work on genetics, other scientists at the time did not realise how important it was.
Suggest two reasons why.
1. They did not know about genetics or alleles.
2. Mendel only worked with pea plants.
(3. Mendel was not considered to be a scientist.)