BIOLOGY

• What is asexual reproduction? Reproduction without fertilisation, producing 2 clone daughter cell
• Features of sexual reproduction -Relatively slow -Requires mate -Variation in offspring
• What happens during fertilisation? One diploid cell from each parent produces 4 haploid gametes through meiosis 2 of these gametes fuse during fertilisation, forming a diploid zygote Zygote divides through mitosis forming embryo
• How does asexual reproduction work? A single diploid cell spilts in two through mitosis into two cells
• What does meiosis produce 4 daughter cells which are haploid gametes
• What happens in meiosis? -Starts with a normal diploid cell -It has 23 pairs of chromosomes -Each chromosome divide and becomes an X shaped chromatid -The cell divides into two, each cell having one set of 23 X shaped chromatids -Those cells (and the chromatids) divide into two, each cell having 23 chromosomes
• Why do gametes have to be haploid Because if they were diploid, then the zygote would have 86 chromosomes which is too many.
• Where are genes found -Each diploid cell contains 46 chromosomes -Each chromosome is a very long DNA molecule wound up tightly -A Gene is a specific section of the DNA
• Where is DNA found Tightly coiled as chromosomes
• The 4 bases of DNA Cytosine, Guanine, Adenine and Thymine C pairs with G and A pairs with T (known as complementary pairing)
• What is the overall structure of DNA Double Helix with weak hydrogen bonds
• what does the base contain contains nitrogen
• Pentose sugar in DNA called deoxyribose in RNA called ribose
• How is DNA extracted from an Onion -Onion cut up to increase surface area -Water, soap and salt added to onion which breaks down cell membranes -The mixture is put in a warm water bath to denature DNA-breaking enzymes and then cooled -Blended in a liquidizer to break cell walls -Filtered to remove cell walls, proteins, etc -Protease enzyme added to filtered onion mixture to break down soluble proteins -Mixture added to ethanol and DNA percipitates as white solid
• codon 3 bases on a DNA strand
• What is protein synthesis Making proteins
• How is the shape of a protein determined Through the amino acids in the protein
• How is the order of amino acids in a protein determined By the order of the bases in the DNA
• Transcription -In the nucleus -Produces messenger RNA mRNA
• Describe Transcription -RNA Polymerase attaches itself at a non-coding binding site -The RNA Polymerase unwinds the DNA -The RNA Polymerase moves along one strand and adds complementary RNA nucleotides which are floating in the nucleus -Uracil is used instead of Thymine -The RNA nucleotides are joined together into a mRNA strand
• Translation -In the cytoplasm / ribosome -Produces a polypeptide chain
• Describe the stages of translation -mRNA travels out of the nucleus and attaches to a ribosome -The ribosome moves along the mRNA a codon at a time -For each codon, a complementary tRNA with a complementary anti-codon that carries a specific amino acid lines up -As the ribosome moves along, it joins the amino acids carried by the tRNA and forms a polypeptide chain
• What effects do mutations have If a base changes, the amino acid coded could be changed and so there may be a different polypeptide chain produced during protein synthesis
• Who was Mendel A Monk who discovered the basis of genetics and developed the modern idea about genes
• What did Mendel do He bred peas with different phenotypes e.g. tall plants with short plants He found that the when he bred tall with short plants, all offsprings are tall, suggesting being tall is a 'more powerful factor' However, when he bred two of the tall plants, a quarter of the offsprings were short
• True breeding plants These are plants that always carry a characteristic to its offspring
• Blending of characteristics At Mendel's time, scientists thought there would be a 'blending' of characteristics from both parents (i.e. tall plant x short plant = medium plant)
• Why did other scientists not understand Mendel's findings They believed in blending of characteristics They did not have understanding of the nucleus and so could not find evidence to Mendel's claims
• Allele different versions of the same gene
• homozygous each chromosome has the same allele
• recessive an allele that is only expressed in the phenotype when there is no dominant allele
• Punnett square make sure to represent dominant allele as CAPITAL and recessive as lowercase
• Pedigree chart A way of representing the genetic information in a family Square = male, circle = female in exams make sure to include the key
• How does sex determination in humans work Egg cells only have X chromosomes Sperm cells can be either X or Y Depending on the specific sperm and egg, if the resulting zygote is XX, then it is a female, but if the resulting zygote is XY, it is a male
• Codominance When both alleles are expressed in the phenotype, with neither being dominant or recessive to each other
• What is the ABO blood group system Everyone's blood is categorised into 4 types, based on the antigens on their red blood cell A blood has only A antigens, has anti-B antibodies and can only receive A and O blood B blood has only B antigens. has anti-A antibodies and can only receive B blood AB blood has both A and B antigens, and can receive any blood (Universal Recipient) O blood has no antigens and both anti-A and anti-B antibodies therefore they can receive only O blood but is a Universal Donor
• Example of sex linked genetic disorders Colourblindness (men more likely to be colourblind)
• Why are some diseases sex linked Men have XY chromosomes, but Y very small and misses some alleles This means if they have a recessive allele in their X chromosome, in women it would be overpowered by the dominant allele in the other chromosome, but in men, they might just not have that allele because Y small and so they will have the recessive allele
• What are some characteristics controlled by multiple genes eye colour. One of the genes controlling eye colour also controls melanin levels
• Advantages of the Human Genome Project -You can know the risks of developing a disease based on your genes -You can identify which medicine works the best on someone