B3

• 3.3 Explain the role of meiotic cell division

  each with half the number of chromosomes, and that this results in the formation of genetically different haploid gametes
• 3.4 Describe DNA's outside look

  DNA molecules are tightly coiled and packaged up with proteins to form chromosomes. is a polymer made up of two strands coiled to form a double helix.
• 3.4 how are the strands linked

  strands linked by a series of complementary base pairs (a-t/c-g) joined together by weak hydrogen bonds. The hydrogen bonds are formed because parts of DNA bases have very slight electrical charges.
• 3.4 what do the two strands look like

  nucleotides that consist of a sugar and phosphate group with one of the four different bases attached to the sugar
• 3.5 Describe the genome as the entire DNA of an organism and a gene as a section of a DNA molecule that codes for a specific protein

  The word genome describes all the genetic information (DNA) of a single organism. The human genome has been studied, which has improved our understanding of the genes linked to different types of disease, the treatment of inherited disorders and has helped in tracing human migration patterns from the past
• 3.6 Explain how DNA can be extracted from fruit

  A- dissolve 3g of salt in 100cm_3 of water. add 10cm_3 of washing up liquid and stir gently until the salt dissolves, (no foam). -detergent breaks down cell surface membrane and membranes around the nuclei.

  B- thoroughly mash 50g of peas. put the mashed peas into an empty beaker and add solution from step A. stir slowly for 1 min.

  C- place the beaker in a water bath at 60 degrees for 15min.

  D- filter the mixture and collect the filtrate in a small beaker.

  E- measure 10cm_3 of the filtrate and pour into a boiling tube

  F- add 2 drops of protease enzyme solution. (chromosomes also contain proteins, and the enzymes used break them down). swirl carefully and leave it to stand for 1min.

  G- tilt boiling tube slightly and pour ice cold ethanol down the side, letting it run down the inside of the tube very slowly. stop when there is the same amount of ethanol and filtrate. DNA is insoluble in ethanol so it forms a precipitate

  H- leave the tube for a few minutes. a white layer forms between the filtrate and ethanol. this is the DNA.
• 3.12 Explain why there are differences in the inherited characteristics as a result of alleles

  Alleles (different forms of the same gene) lead to differences in inherited characteristics. This is because different alleles code for different forms of the same protein - an allele that codes for a damaged form of a protein can cause illness
• 3.13 Explain what is a gamete

  An organism's reproductive cell (egg in female and sperm in males), which has half the number of chromosomes (23)
• 3.13 Explain what is a chromosome

  A structure found in the nucleus which is made up of a long strand of DNA.
• 3.13 Explain what is a gene


  A short section of DNA that codes for a protein, and therefore contribute to a characteristic.
• 3.13 Explain what is an allele


  The different forms of the gene - humans have two alleles for each gene as they inherit one from each parent
• 3.13 Explain what is a dominant allele

  Dominant allele: Only one (out of the two alleles) is needed for it to be expressed and for the corresponding phenotype to be observed.
• 3.13 Explain what is a recessive allele


  Recessive allele: Two copies are needed for it to be expressed and for the corresponding the phenotype to be observed.
• 3.13 Explain what is meant by homozygous

  Homozygous: When both inherited alleles are the same (i.e. two dominant alleles or two recessive alleles).
• 3,13 Explain what is meant by heterozygous

  Heterozygous: When one of the inherited alleles is dominant and the other is recessive.
• 3.13 Explain what is meant by genotype

  The combination of alleles an individual has.
• 3.13 Explain what is meant by phenotype


  The observable characteristics of an organism (seen just by looking - like eye colour, or found - like blood type)
• 3.13 Explain what is meant by zygote


  The stage of development immediately after fertilisation - a diploid (2n) cell formed from the fusion of two haploid g
• 3.19 What are most phenotypic features a result of?

  Multiple genes, rather than a single gene inheritance
• 3.20 Describe the causes of variation that influence phenotype

  genetic variation - different characteristics as a result of mutations of DNA and sexual reproduction

  environmental variation - different characteristics caused by an organism's environment (acquired characteristics)
• 3.21 Discuss the outcomes of the Human Genome Project and its potential applications within medicine

  - Produced a map of 3.3billion complimentary base pairs in one set of 46 human chromosomes. Completed in 2003Applications:- Improved understanding of genes linked to different types of disease; eg BRCA1 and BRCA2 identified as genes that can cause breast cancer if mutated, allows people to know if they have these mutations and some may choose to get a mastectomy as a result- Helped in the treatment of inherited disorders- understanding that alleles can affect how medicine works in the body- Indicates risk of developing diseases caused by alleles- Helped trace human migration patterns
• 3.22 What is genetic variation like in a species?

  there is usually extensive genetic variation within a population of a species and that these arise through mutations
• 33.23 Do most genetic mutations have an effect on phenotype?

  Most genetic mutations have no effect on the phenotype, some mutations have a small effect on the phenotype and, rarely, a single mutation will significantly affect the phenotype