Genes, Chromosomes and DNA
Nucleus - contains genetic material arranged into chromosomes. Human = 23 pairs
Chromosome - long molecule of DNA coiled up
Gene - short length of a chromosome
Genes control the development of different characteristics and how an organism functions
Each gene is a code for making a certain type of protein. Having different versions of proteins = different characteristics
Structural proteins - Part of things like skin, hair, blood, cytoplasm. Collagen found in tendons, bones, cartilage
Functional proteins - Enzymes are proteins that help with digestion by breaking down food molecules.
Genotype - All the genes an organism has
Phenotype - Characteristics an organism displays
Characteristics can be controlled...
- Only by genes, either one gene or several working together. E.g eye colour, dimples
- Only by environmental factors. E.g how and where you live
- Both genes and environment. E.g weight
Sex cells have 23 single chromosomes. During sexual reproduction 23 chromosomes in sperm and egg combine.
2 chromosomes in a pair carry the same genes and each gene is found in the same place. Because the two chromosomes in a pair come from different parents, they may have different alleles of the same gene.
Half a child's chromosomes come from each parent. Look similar but not exact.
When people produce sperm or egg cells, their pairs of chromosomes separate and go into different cells
- 2 chromosomes in a pair are never identical because they have different alleles
- When they go into 2 differemt sex cells, each of the cells get different alleles
- Each of the 23 chromosomes pairs separates independently. Millions of different combinations that can be produced
- All sex cells produced by one individual will probably be genetically different.
Homozygous - Two alleles the same for that particular gene
Heterozygous - Two different alleles
Have 2 dominant alleles or 1 dominant and 1 recessive, the characteristic caused by the dominant allele will be shown
Both alleles have to be recessive to show characteristic
In genetic diagrams, dominant = capital letter, recessive = small letter
Genetic diagrams + Punnet sqaures can be used to work out a genetic cross
Men = XY, Women = XX chromosomes
Y chromosome carries a gene which makes an embryo develop into a male as it grows.
The gene that makes an embryo into a male causes a specific protein to be produced.
When the embryo's reproductive system begins to develop, this protein causes the development of testes.
The testes produce male sex hormones, which make the rest of the male reproductive system develop.
In females the protein is not produces, so the embryo develops ovaries and the rest of the female reproductive system.
Some disorders are inherited by one or both parents carrying a fault allele that passes on to their children.
- Faulty recessive allele. People with only one copy of the allele won't show the symptoms and are known as carriers. Both parents must be carriers or sufferers
- Symptoms = chest infections, difficulty in breathing/digesting food
- Caused by a dominant allele.
- 50% of child inheriting if one parent is a carrier
- Causes tremors, memory loss, mood changes, poor concentration
- Symptoms don't normally appear until after the age of 40
Embryos produced using IVF tested for genetic disorders ( Pre-implantation genetic diagnosis). Only healthy are implanted, others discarded. Important if one parent carries alleles for that disorder
Children/Adults checked to see if they carry alleles for genetic disorders. Also can be tested before certain drugs are prescribed to show how it is likely to affect the individual E.g effectiveness and safety
- Not 100% accurate. Errors due to sample contamination/ misinterpretation. False positve/negative
- Pregancy not 100% safe. Lead to miscarriage
- Positive - other members tested? Have children? Terminate preganancy? Cause discrimination by employers/ insurance
Clones - genetically identical organisms. Same genes and alleles. Difference due to environment.
- One parent and offpring are identical to each other and parent
- Bacteria reproduce by diving into two
- Horizontal stems (runners) move out from base of plant and form new plants at their tips
- Bulbs (underground fleshy structures) grow to form a new identical plant.
- Few animals can reproduce asexually. Eg. Greenfly lay eggs which develop into identical females
Identical twins = clones. (Cells of an embryo spilt)
- single egg fertalised by sperm
- embryo splits into two, two separate embryos devlop
- two embryos are genetically identical so identical babies are born
Scientists make animal clones in the lab
- Nucleus of an egg cell is removed - egg cell has no genetic info
- Nucleus from adult donor cell is inserted in it's place
- Cell is then stimulated so that it starts dividing as if it was a normal embryo (fertilised egg)
- Embryo produced is genetically identical to the donor cell
Stem cells - unspecialised cells which can develop into different types of cells depending on what instructions they're given.
Embryonic Stem Cells:
- Unspecialised - found in early embryos
- Stem cells removed from embryo, then embryo is destroyed
- Have the potential to turn into any kind of cell
Adult Stem Cells:
- Unspecialised cells - found in adult animals
- Involved in maintaining and repairing old and damaged tissues and can specialise into many cells (not all)
- Stem cells can be safely removed from adult patients
Can be used to treat many illnesses
- Medicine uses adult stem cells to cure disease. Bone marrow contains adult stem cells that can turn into new blood cells
- Embryonic stem cells can replace faulty cells in sick people. E.g. heart muscle cells for people with heart disease or insulin-producing cells for people with diabetes. Still being researched.