5.1 Cell division and growth
Cell division is necessary for the growth of an organism or the repair of damaged tissues. In body cells, chromosomes are found in pairs.
Mitosis results in two identical "daughter" cells being produced
The chromosomes contain the genes (alleles) which must be passed on to a new cell
A copy of each chromosome is made before the cell divides and one of each chromosome goes to each new cell.
In early development of animal and plant embryos the cells are unspecialised and are called stem cells.
Most types of animal cells differentiate at an early stage of development, but many plants can differentiate throughout their life.
5.2 Cell division in sexual reproduction
Cells in the reproductive organs divide by meiosis to form the gatmetes (sex cells).
Body cells have two sets of chromosomes; gametes have only one set.
In meiosis the genetic material is copied and then the cell divides twice and forms four gametes, each with a single set of chromosomes and a different combination of genes.
Sexual reproduction gives rise to variation because genetic information from two parents is combined.
5.3 Stem cells
Embryonic stem cells (from human embryos) and adult stem cells (from adult bone marrow) are unspecialised, and can be made to differentiate into many different types of cell.
Stem cells have the potential to treat previously incurable conditions.
We may be able to grow nerve cells or whole new organs for people who need them.
5.4 From Mendel to DNA
Gregor Mendel was the first person to suggest separately inherited factors which we now call genes.
He worked out how characteristics are inherited.
Genes make up the chromosomes which are large molecules of DNA which control our characteristics.
A gene is a small section of DNA that codes for a particular order or combination or amino acids, which make a special protein.
Everyone has unique DNA that can identify them using DNA fingerprinting.
Inheritance in Action
Human beings have 23 pairs of chromosomes, one pair being the sex chromosomes which determine if you are female (XX) or male (XY).
Some characteristics are controlled by a single gene.
Genes can have different forms called alleles.
Some alleles are dominant and some are recessive.
We can construct genetic diagrams (i.e. a punnett square) to predict characteristics.
Phenotype: physical appearance of the characteristc i.e. dimples
Genotype: the genetic make up - which alleles does the individual inherit - DD, Dd or dd?
Homozygous: both alleles are the same - DD (homozygous dominant) or dd (homozygous recessive)
Heterozygous: the two alleles are different Dd.
5.6 Inherited conditions in humans
Polydactyly is caused by a dominant allele of a gene and can be inherited from only one parent.
Cystic fibrosis is caused by a recessive allele of a gene and so must be inherited from both parents.
If a parent is heterozygous for polydactyly each child has a 50% chance of inheriting it.
If both parents are heterozygous for cystic fibrosis each child has a 25% child of inheriting the disorder