DNA, genes and chromosomes

• A gamete is a sex cell, with sperm in males and eggs in females
• DNA is a large and complex polymer made up of two strands forming a double helix
• DNA determines the characteristics of a living organism, and each person's DNA is unique
• Chromosomes are structures found in the nucleus of most cells, occurring in pairs inherited from the mother and father
• Genes are small sections of DNA on chromosomes that code for specific proteins
• Alleles are different versions of the same gene, such as blue or brown eye color alleles
• Genotype is the collection of alleles that determine characteristics, while phenotype is the visible characteristics of an organism
• Dominant alleles are always expressed in the heterozygote, represented by a capital letter, and only one copy is needed for expression
• Recessive alleles are only expressed with two copies and in the absence of the dominant allele, represented by a lower-case letter
• Homozygous alleles are identical for the same characteristic, while heterozygous alleles are different for the same characteristic
• • DNA molecules are large and complex. They carry the genetic code that determines the characteristics of a living organism.
  • A gene is a section of DNA that codes for a specific protein. It is the unit of heredity, and may be copied and passed on to the next generation.
  • Chromosomes: The nucleus of the cell contains chromosomes. These are long threads of DNA, each made up of many genes.

  The diagram shows the relationship between the cell, its nucleus, the chromosomes in the nucleus, and genes v

  
• Base pairs

  • thymine, T
  • adenine, A
  • guanine, G
  • cytosine, C
• There are chemical bonds between the two strands in DNA, formed by pairs of bases. They always pair up in a particular way, called complementary base pairing:

  • thymine pairs with adenine (T–A)
  • guanine pairs with cytosine (G–C)

  
• Although many characteristics are controlled by multiple genes, some characteristics are controlled by a single gene, eg fur in animals, and red-green colour blindness in humans. Each gene might have different forms, and these are called alleles.
• The sequence of every amino acid in a protein is determined by the triplet code, which is a code of three bases. Therefore each specific amino acid will have a triplet code of different bases.
• Each human cell normally contains 46 chromosomes arranged into 23 pairs. Chromosomes contain all our hereditary information. Humans have 23 pairs of chromosomes because we are diploid. Diploid cells have two sets of chromosomes, one set coming from each parent.
• The height of pea plants is controlled by a single gene which has two alleles – tall and short.

  The tall allele is dominant and is shown as T.

  The short allele is recessive and is shown as t.

  
  

  The female parent plant is tall – TT.

  The male parent plant is short – tt.

  
• • males have two different sex chromosomes, XY
  • females have two X chromosomes, XX
• These diagrams are known as human karyotypes, and show all the chromosomes aligned in pairs.

  Chromosomes from a male

  The blue box shows the two sex chromosomes – these are different sizes, an X (the large chromosome) and a Y (the small chromosome).

  
• Chromosomes from a female

  The pink box shows the two sex chromosomes – these are the same size, two large X chromosomes.

  
• A zygote is a fertilized egg cell.
• In the human genome, there is a small amount of DNA that is unique to individuals
• By cutting a sample of DNA into fragments and separating the fragments by size, it is possible to make a characteristic profile of DNA bands for individuals
• DNA profiling can be used to determine paternity or help solve crimes where the suspect may have left a sample of body tissue at the crime scene
• Results of genetic engineering can include:

  • Crops being created that produce their own insecticide or are herbicide resistant
  • Crops being given genetic advantages that suit the areas in which they are grown to maximise yield
  • Crops being modified to produce particular substances, eg vitamin A, or energy-producing crops
• Genetic engineering involves taking genes from one organism and placing them into the genome of another
• Arguments against genetic engineering:

  • The escape of pollen from genetically modified crops could lead to the modification of other crops
  • Fear of creating 'super weeds' due to accidental pollination by genetically modified, herbicide-resistant plants
  • Some GM crops are owned by large companies, leading to price control
  • Less reliance on crops from developing countries could result from the use of GM crops
  • Concerns about the effects on the body of eating genetically modified crops
• In the example above, is the father of the baby Father A or Father B?

  
  

  Answer: Father A. Band 4 of the baby’s DNA profile is not found in the mother’s DNA profile, and only Father A possesses the same band. This also true of band 8.