Cell division and growth
New cells are needed for an organism (or part of one) to grow, or to replace old, worn out cells. But they must have the same genetic information as the original to do the same job.
Gene - a packet of information that controls a characteristic
Allele - different forms of a gene
Chromosome - where genes are grouped together (hundreds or even thousands)
23 pairs of chromosomes (one of each from mother and one from father). Gametes (sex cells) only have 23 chromosomes, not paired.
In early development, cells are unspecialised. They differentiate later in life (in humans by the time they are born), meaning they now have a specific job (muscle cell, liver cell, skin cell).
Plant cells are able to differentiate all through their life and can even redifferentiate.
Mitosis and Meiosis
- Produces two identical cells wirh the same chromosomes.
- Asexual reproduction, cells of offspring produced by mitosis have the same alleles as parent.
- The cell produces new copes of the chromosomes in the nucleus and then divides
- Gametes are formed by meiosis, where the chromosome number is reduced by half.
- The chromosomes are copied so their are four sets of the chromosome before the cell divides.
- It divides twice in quick succession to form four gametes each with a single set.
- Each gamete is slightly different from the others as they contain random mixtures of the original chromosomes. This introduces variety.
Cell division in sexual reproduction
More variety is added when fertilisation take place. When two sex cells (egg and sperm) the single new cell has a full set of chromosomes.
The combination of genes on every newly fertilised ovum is unique, it now begins to divide by mitosis.
In asexual reproduction the offspring are produced as a result of mitosis so their is no variation in the genetic material.
In sexual reproduction the gametes are produced by meiosis so their is a lot of variation.
Stem cells are unspecialised and can differentiate to make any cell in the body.
Scientists believe it will be possible to culture embryonic stem cells and use them to treat diseases and injuries such as paralysis.
Many stem cells come from aborted embryos and spare embryos in fertility treatment. This raises ethical issues, questioning the use of potential human life as a source of cells, especially as the embryo cannot give permission. Also, it has been seen in mice that the use of stem cells can cause cancer, and also making stem cells is slow, difficult, expensive and hard to control.
Scientists have also found stem cells in the umbilical cord blood of unborn babies.
Stem cells found in bone marrow can only develop into a limited range of cell types.
Therapeutic cloning involves making a cloned embryo of the adult to use the stem cells to make organs that cannot be rejected by the original donors body
Inheritance in action
XX means you are female XY means you are male
Dominant alleles are shown even if there is an alternative allele present
Recessive alleles can only be shown if there is no dominant allele present
Homozygous is an individual with two identical alleles for a characteristic (DD or dd)
Heterozygous is an individual with different alleles for a characteristic (Dd)
Genotype describes the genetic makeup of an individual's characteristic (Dd/dd/DD)
Phenotype describes the physical appearrance of an individual's characteristic (no dimples/dimples