Inheritance AND Variation : DNA - triple

1. Inheritance AND Variation : DNA - triple Deoxyribonucleic acid (DNA) is the chemical that all of the genetic material in a cell is made up from. It contains the code for an organism. Your DNA determines what inherited characteristics you have. DNA is found in chromosomes, which come in pairs. • Genome • An organisms genome is the entire set of genetic material in the organism. • Scientists have worked out the entire human genome. This is important for science and medicine: • Diseases can be linked to specific genes • This can help us understand them better and develop more effective treatments • Scientists can trace the migration of certain populations of people across the world. The human genome is mostly identical in all individuals but as different populations of people migrated away from Africa, they gradually developed tiny differences in their genomes. Scientists can study when new populations split off and what route they took. The structure of DNA DNA is a polymer made up of two strands coiled together in the shape of a double helix. Nucleotides are the repeating units that make up DNA. They have a sugar and a phosphate to make up the backbone and one of four different bases. Complementary base pairing occurs. A always pairs with T and G with C. The order of these bases codes for amino acids. Genes and proteins A gene is a small section of DNA on a chromosome. Each gene codes for a particular sequence of amino acids which are put together to make a specific protein (every 3 bases codes for one amino acid). Only 20 amino acids are used, but they make up thousands of different proteins. The genes tell cells what order to put amino acids together in to make specific proteins, this determines what type of cell it is. Proteins are made in the cell cytoplasm on ribosomes. mRNA copies the code from DNA and carries the code to the ribosomes. The correct amino acids are brought to the ribosomes in the correct order by carrier molecules.

2. Inheritance and Variation : Mutations A mutation is a random change in an organism’s DNA. They can sometimes be inherited. Mutations occur continuously and spontaneously, however, the chance of mutation is increased by exposure to certain substances or some types of radiation. Mutations change the sequence of DNA bases in a gene (produces a genetic variance), sometimes this leads to a change in the protein it codes for. Most mutations have little or no effect but some seriously affect a protein by changing its shape. This can affect its ability to perform its function (e.g. an enzymes active site shape could change so the substrate no longer fits). A mutation in non-coding DNA can alter how genes are expressed. Types of mutation Insertions Where a new base is inserted into the DNA sequence. An insertion changes the way the groups of three bases are read which can change the amino acid they code for. This has a knock on effect on the bases further down the sequence. Deletions Where a random base is deleted from the DNA base sequence. This changes the way the groups of bases are read and can have a knock-on effect. Substitutions When a random base in the DNA base sequence is changed to a different base. This has no knock-on effect. CTG GAG CAT CTG CGA GCA T CTG GAG CAT CTG CAG CAT CTG GAG CAT CTG AGC AT

3. Inheritance and Variation : Reproduction Sexual reproduction Sexual reproduction involves the fusion of male and female gametes. Because there are two parents, the parents contain a mixture of their parents’ genes. The mother and father produce gametes by meiosis (e.g. egg and sperm). In humans, each gamete has 23 chromosomes – half the number of a normal cell (one of each pair). Flowers can reproduce this way too, producing egg cells and pollen cells. Asexual reproduction In asexual reproduction there’s only one parent. There’s no fusion of gametes, no mixing of chromosomes and no genetic variation between parent and offspring. The offspring are genetically identical to the parent – they’re clones. Bacteria, some plants and some animals reproduce asexually. Some organisms can reproduce by both methods Malaria parasite reproduces sexually in the mosquito and asexually in human hosts. Fungi release spores which are reproduced sexually and asexually. Strawberry plants produce seeds sexually and runners asexually that spread along the surface of the earth and produces new plants along the runner. Plants make bulbs, new bulbs can form from the main bulb and divide off and grow into new plants.

4. Inheritance, Variation and Evolution: Meiosis To make gametes which only have half the original number of chromosomes, cells divide by meiosis. This process involves two cell divisions. After two gametes have fused during fertilisation, the resulting new cell divides by mitosis to make a copy of itself. Mitosis repeats many times to produce lots of new cells in an embryo. These cells differentiate into the different types of specialised cell that make up a whole organism. X and Y Chromosomes There are 23 pairs of chromosomes in every human body cell. The 23rd pair are labelled XX or XY – they control whether you are male or female. The Y chromosome causes male characteristics – males have an X and a Y chromosome: XY. The lack of a Y chromosome causes female characteristics – females have two X chromosomes: XX. When making sperm, the X and the Y chromosomes are drawn apart, there is a 50% chance a sperm gets a Y chromosome and 50% chance it gets an X. When making eggs, the XX chromosomes are drawn apart, there is a 100% chance the egg gets an X. 50% female XX 50% male XY

5. Inheritance and Variation : Genetic diagrams Some characteristics are controlled by single genes. All genes exist in different versions called alleles (represented by letters in genetic diagrams. You have two versions (alleles) of every gene in your body – one on each chromosome in a pair. If the two alleles are the same then it’s homozygous for that trait. If they are different then it’s heterozygous for that trait. If two alleles are different, only one can determine what characteristic is present. The dominant allele will always be shown if present (‘CC’ or ‘Cc’), if there is no dominant allele then the recessive allele will be shown (‘cc’). Your genotype is the combination of alleles you have (BB). Your phenotype is what characteristic you have (brown hair) Superpower inheritance in hamsters

6. Inheritance and Variation : Inherited disorders Cystic fibrosis is caused by a recessive allele, it is a condition where the body produces a lot of thick, sticky mucus in air passages and in the pancreas. For a child to have the disorder, both parents must either be a carrier or have the disorder themselves. Cystic fibrosis Cystic fibrosis is caused by a recessive allele, it is a condition where the body produces a lot of thick, sticky mucus in air passages and in the pancreas. For a child to have the disorder, both parents must either be a carrier or have the disorder themselves. Polydactyly Polydactyly is caused by a dominant allele, it is a condition where a baby is born with extra fingers or toes. For a child to have the disorder, only one parent needs to carry the dominant allele Embryo screening Before an embryo is implanted into the mother in IVF treatments, a cell can be removed and it’s genes can be analysed. Many genetic disorders such as cystic fibrosis can be detected this way. It is also possible to get DNA from a baby in the womb and test that. This is controversial because of the decisions it can lead to (non-implantation, abortion, etc.). It can stop people from suffering though and prevent the Government from having to spend a lot of money supporting these disorders. At the moment it is tightly regulated.

7. Inheritance and Variation : Mendel - triple Mendel was an Austrian monk in the mid 19th Century. He studied how characteristics in plants were passed from one generation to the next. This became the foundation of modern genetics. Findings Mendel had shown that the height characteristic in pea plants was determined by separately inherited ‘hereditary units’ passed on from each parent. The ratios of tall to dwarf plants showed that the unit for tall plants was dominant over the unit for small plants. Characteristics in plants are determined by ‘hereditary units’. These are passed on to offspring unchanged from both parents (one unit from each) These units can be dominant or recessive – if the organism has a dominant unit and a recessive unit then the dominant characteristic will be expressed. People dismissed Mendel’s work as they didn’t know about genes, DNA and chromosomes. He influenced what we know now though: In the late 1800s, scientists learned about chromosomes by observing cell division. In the early 1900s, scientists discovered similarities between chromosomes and Mendels ‘units’. They proposed the units were found on chromosomes (now these units are known as genes). In 1953, scientists discovered the structure of DNA which allowed them to go on to find out exactly how genes work.