DNA, Genes and Chromosomes
- Molecules are large and complex
- They carry genetic code that determines chacteristics of a living thing
- Each persons DNA is unique (except in identical twins)
- A gene is a short section of DNA
- Each gene has codes for a specific protein by specifying the order in which amino acids must be put together
- A cells nucleus contains chromosomes
- Chromonsomes are made up of large molecules of DNA
- It has a double helix structure
The chromosomes contain the cell’s genetic information. They are normally found in pairs in body cells. Body cells divide by a process called mitosis. Mitosis is used when plants and animals want to grow or replace cells that have been damaged.
1) In a cell thats not dividing the DNA is all spread out in long strings.
2) Ir the cell gets a signal to divide, it needs to duplicate its DNA so theres one copy for each new cell. The DNA is copied and forms X-shaped chromosomes. Each 'arm' of the chormosomes is an exact duplicate of the other
3) The chromosomes then line up at the centre of the cell and cell fibres pull them apart. The two arms of each chromosome go to opposite ends of the cell.
4) Membranes form around each of the sets of chromosomes. These become the nuclei of the two new cells.
5) Lastly, the cyto plams divides
So now you have 2 new cells, containing exactly the same DNA - theyr'e identical.
Gametes and Fertilisation
- Cells in the reproductive organs (testes and ovaries) divide to form gametes.
- Gametes are sex cells
- Male gametes are sperm
- Female gametes are eggs
- Gametes contain one set of genetic information (whereas body cells contain 2)
- The type of cell division that produces gametes is meiosis.
Fertilisation is the fusing of a male gamete and a female gamete.
When gametes join at fertilisation, a single body cell with new pairs of chromosomes (from the mother and father)
A new individual then develops by this cel repeatedly dividing by mitosis.
Differentiation and Stem Cells
Most types of animals differentate at an early stafe whereas many plant cells retain the ability to differentaite throughtout life.
In mature animals, cell division is mainly restricted to repair and replacement.
Cells from human embyros and adult bone marrow, called stem cells,, can be made to differentiate into many different types of cells, e.g. nerve cells.
They are able to develop into any kind of human cell.
Stem Cell Treatment
Stem cells have the potential to be used in new treatments for conditions such as Parkinson's disease and paralysis. There are social and ethical issues concerning the use of human embryonic stem cells in medical research and treatments.
A cell divides by meiosis to form gametes
- Copies of the genetic information are made
- The cell divides twice to form four gametes
Each gamete has a single set of chromosomes.
Genetic Variation and Genetic Disorders
Alleles are different forms of a gene. They can be dominant or recessive. Genetic diagrams help us to understand the possible outcomes when parents produce offspring. Polydactyly - having extra fingers or toes - is caused by a dominant allele. Cystic fibrosis is a disorder of the cell membranes caused by a recessive allele.
Gender Determination and Alleles
Human body cells have 23 pairs of chromosomes in the nucleus. One of these pairs controls the inheritance of gender - whether offspring are male or female:
- In males, the two sex chromosomes are different. They are XY
- In females, the two sex chromosomes are the same. They are XX
Some characterists (e.g. eye colour or shape of earlobe) are controlled by one single gene. Each gene may have diffferent forms called alleles.
Alleles can either be DOMINANT or RECESSIVE
- The characteristic controlled by a dominant allele develops if the allele is present on one or both chromosomes in a pair
- The characteristic controlled by a recessive allele develops only if the allele is present on both chromosomes in a pair
The cells of offspring produced by asexual reproduction are produced by mitosisfrom the parental cells. They contain the same alleles as the parent. However,sexual reproduction causes variation because when two gametes join together one of each pair of alleles comes from each parent.
Each person (apart from identical twins) has unique DNA. This can be used to identify individuals by DNA fingerprinting. This technique uses some of the small differences between the DNA from different people to make a picture rather like a barcode.
If enough parts of the DNA are tested, it is very unlikely that two identical DNA fingerprints would belong to two different people. This makes the method very useful for matching samples found at the scene of a crime to people suspected of committing the crime.
Gregor Mendel (1822-1884) studied the inheritance of different characteristics in pea plants. He found that when he bred red-flowered plants with white-flowered plants, all the offspring produced red flowers. If he bred these plants with each other, most of the offspring had red flowers, but some had white. This was because the allele for red flowers is dominant, and the allele for white flowers is recessive. Genetic diagrams help to show how this works.
In a genetic diagram, you show all of the possible alleles for a particular characteristic. There will be two alleles from one parent, and two from the other parent, making four altogether. You then draw lines to show all the possible ways that these alleles could be paired in the offspring. There will be four possible ways, but some or all of them could be repeated.
Polydactyly is an inherited condition in which a person has extra fingers or toes. It is caused by a dominant allele of a gene. This means it can be passed on by just one parent if they have the disorder.
Cystic fibrosis (a disorder of cell membranes) must be inherited from both parents. The parent smay be carriers of the disorder without actually having the disorder themselves.
It is caused by a recessive allele of a gene and therefore can be passed on by parents, neither of whom has the disorder
(Embryos can be screened for the alleles that cause these and other genetic disorders.)