Genes, chromosomes and DNA
Most cells in the body have a nucleus- contains genetic material- arranged into chromosomes- human cell nucleus contains 23 pairs of chromosomes. Each chromosome is one v. long molecule of DNA that's coiled up. A gene is a short length of a chromosome. Gene control the development of different characteristics. Genes exist in different versions- alleles.
A gene codes for a protein. Structural- part of things like skin, hair, blood. E.g. collagen- structural proteins that is found in tendons, bones and cartilage. Functional proteins- enzymes are proteins that help with digestion. Amylase- a digestive enzyme that breaks down starch to maltose.
Organism's genotype shows genes it's got, characteristics that an organism displays are called phenotype.
Environmental factors- don't have anything to do with genes
Genetic factors-controlled by genes
Both- e.g weight.
Genes and Variation
Gametes have half the normal amount of DNA- 23 chromosomes
- when sperm fertlises egg, the 23 chromosomes from each combine to make 23 pairs
- two chromosomes always carry the same genes and each gene is always found in the same place on the two chromosomes
- they may have different alleles.
Genes are shuffled together randomly to make eggs and sperm- when people produce sex cells, their pairs of chromosomes separate and go into different cells.
- two chromosomes in a pair are never identical bc they have different alleles
- when they go into two different sex cells, each of the two gets different alleles
- each of the 23 chromosome pairs separates independently
- there are millions of different chromosome combinations
- means sex cells produced by one individual will probably be genetically different
- when a woman releases an egg it can be fertilised by any one of millions of sperm
- all this means that chances of identical siblings is miniscule.
Inheritance and Genetic diagrams
Combination of alleles determines the phenotype-when you have two copies of the same gene (one from each parent) usually only one of them can be expressed in the phenotype.
- Homozygous= two alleles the same for that particular gene
- Heterozygous= two different alleles for that particular gene
- Alleles can be recessive or dominant
- if you have two dominant alleles for a gene or one dominant and one recessive, only the characteristic caused by the dominant allele will be shown.
- To show a characteristic that is caused by a recessive allele, both alleles have to be recessive
- letters represent alleles- CAPITAL=dominant and lowercase=recessive
- can use them to work out percentage likelihood of a child adopting a characteristic
- Punnett square is a type of genetic diagram- pair them up in boxes to see possible combinations
There are 23 pairs of chromosomes in every human body cell- the 23rd pair are labelled XY. These are the sex chromosomes and decide the sex of the baby.
MALE= XY FEMALE=XX
The Y chromosome carries a gene which makes an embryo develop into a male as it grows, females who don't have this gene develop in a different way. 50% chance of being either sex.
The Y chromosome determines which sex organs you develop
- the gene that makes an embryo into a male causes a specific protein to be produced
- when the reproductive system begins to develop, this protein causes development of the testes, instead of the ovaries
- the testes then produce male sex hormones, which gradually make the rest of the male reproductive system develop.
- In females the protein is not produced, so the embryo develops ovaries and the rest of the female reproductive system.
Genetic Disorders- caused by faulty alleles
Some disorders are inherited, Cystic fibrosis and Huntington's are caused by a faulty allele of a single gene.
CYSTIC FIBROSIS-DISORDER OF THE CELL MEMBRANES- RECESSIVE ALLELES
- Symptoms: thick sticky mucus (gut, airways, pancreas), breathing difficulty, chest infections, difficulty digesting food.
- Because it is recessive, people with only one copy won't show the symptoms of the disorder- they are carriers
- Both parents have to have the disorder for the child to have a chance of inheriting it
HUNTINGTON'S DISEASE- DOMINANT ALLELE
- Symptoms: tremors, clumsiness, memory loss, mood changes, poor concentration
- no cure
- 50% chance of having it even if just one parent has it
- symptoms do not usually occur until approx. 40yrs, when it has usually already been passed on to children.
G.T can be used on Embryos, Children and Adults
- Embryos produced using IVF can be checked for genetic disorders- pre-implantation diagnosis. Only healthy embryos are implanted into the mother's womb, others are discarded.
- children and adults can be tested to see if they are carriers and see if their children are likely to inherit the disorder
- they can be tested before drugs are prescribed to show how a drug is likely to affect an individual (if it will be a good treatment or the opposite)
- not 100% accurate- errors due to misinterpretation/contamination
- tests carried out during pregnancy aren't always safe, genetic test causes miscarriage on 0.5-1% of cases.
- If positive, should other family be tested? Is it fair to terminate/ not have children?
- Discriminantion, employers may be reluctant to hire people suffering w. G.D.
- Insurance people may refuse to give life insurance/ charge more.
Clones- genetically identical organisms
Bacteria, some plants and some animals reproduce asexually to form clones (same genes and same alleles for those genes) Any differences in clones would be due to their environment.
Nature makes clones by asexual reproduction and when cells of an embryo spilt.
- Asexual reproduction: one parent, bacteria divide in two, multiply v. quickly. Plants reproduce asexually- some produce runners that move out from the base and form new clones. Others produce bulbs underground, these can grow to form an identical plant. Some animals can reproduce asexually- e.g the female greenfly-they lay eggs which develop into more identical females, they can also reproduce sexually.
- Identical Twins: a single egg is fetilised by a sperm and an embryo begins to develop normally, then sometimes the embryo splits and two seperate embryos begin to develop.
Scientists can make animals clones in a lab
- Nucleus of an egg cell si removed, a nucleus from an adult donor cell is inserted in its place.
- Cell is then stimlated so it starts dividing as if it was a normal embryo
- Embryo produced is genetically identical to the donor cell.
Stem cells can become other types of cells- most cells in the body are specialised for a particular job and become specialised during early development. Stem cells can develop into different types of cells, depending on instructions they are given. Main two types=
- Embryonic- unspecialised cells found in early embryos, these are removed from an embryo then the embryo is destroyed, they have potential to turn into any cell at all.
- Adult- unspecialised/ slightly specialised cells found in adult animals, involved in maintaining and repairing old and damaged tissues and can specialise into many, but not any, type of cell. These have to be safely removed from adult patients e.g. extracting bone marrow.
Stem cells could be used to treat many illnesses
- adult stem cells are already used in medicine to cure disease, e.g sickle cell anaemia can be treated by bone marrow transplants. Bone marrow contains A.S.C that can turn into new blood cells and replace old ones.
- Embryonic stem cells could be used to replace faulty cells in patients- this is still being researched but they could cure things like diabetes, paralysis etc.