DNA, cell division and genetics

• The genetic material (DNA) is contained in


  • Chromosomes
  • Nucleus of the cell
• Chromosomes


  • Occur as functional pairs except gametes
• Genes


  Short sections of chromosomes that control specific characteristics, therefore short lengths of DNA
• DNA


  The code-carrying part of genes and chromosomes that determine how individuals develop
• DNA structure


  • Two phosphate and sugar strands held together by bases linked by hydrogen bonds
  • Bases can only combine in the order Adenine-Thymine, Cytosine-Guanine
  • DNA is folded into a double helix
• How DNA works


  1. Codes for different amino acids

  2. Amino acids are linked together to form proteins
• Mitosis


  • Cell division that takes place throughout the body
  • Ensures new daughter cells have exactly the same arrangement as each other and the parent cell
  • Produces clones of cells - all identical
• Meiosis


  • Occurs in sex organs only
  • Produces gametes
  • Reduction division as it produces gametes with half the number of chromosomes
  • Each chromosome pair can combine with either chromosome from another pair in gamete formation
  • Independent assortment is a major cause of variation
• Gene


  Short section of chromosome that codes for a particular characteristic
• Allele


  A particular form of a gene
• Homozygous


  Both alleles of a gene are the same
• Heterozygous


  Two alleles of a gene are different
• Genetic crosses


  1. Punnett square can be used to determine offspring genotypes and phenotypes

  2. Test cross can be used to determine unknown genotype of an individual with a dominant phenotype
• Sex determination in humans


  • Humans have 22 pairs of autosomes and one pair of sex chromosomes (XX for females, XY for males)
  • Equal number of boys and girls are produced
• Sex linkage


  • Sex chromosomes can carry genes and alleles that control characteristics
  • Recessive alleles on the X chromosome in males cannot be masked and therefore show in the phenotype
• Haemophilia is a sex-linked inherited condition caused by a recessive allele on the X chromosome
• Cystic fibrosis is caused by a recessive allele, so affected individuals must be homozygous recessive
• Huntington's disease is caused by a dominant allele
• Down syndrome is caused by an extra chromosome, usually due to a gamete with 24 chromosomes being involved in fertilisation
• Genetic screening


  1. Amniocentesis involves taking a sample of amniotic fluid and checking the number of chromosomes

  2. Blood tests after 10-14 weeks of pregnancy can determine if there is a higher than normal risk of Down syndrome
• Genetic screening raises ethical and moral issues around decisions about termination of pregnancy and discrimination
• Genetic engineering


  1. Involves modifying the genome of an organism to introduce desirable characteristics

  2. Bacteria are genetically engineered to produce human insulin by inserting the human insulin gene into bacterial DNA. Usually involves adding a human gene to the DNA of another organism
• Advantages of genetically engineered human insulin include increased availability and reduced risk of allergies compared to animal insulin
• Genome 


  All the DNA is an individual
• How DNA works -

  • Bases are read in a sequence of 3. Each group of 3 is called a base triplet which codes for a particular amino acids.
  • After amino acids are coded, they are linked together to form a protein in protein synthesis.
  • Only one code strand of DNA is involved
• Mitosis

  Is important for growth, replacing worn out cells and repairing damaged tissue
• Meiosis

  • One cell produces 4 genetically different, haploid cells in 2 divisions.
  • Either chromosome in a pair can combine with either chromosome in another pair to ensure that there are millions of possible chromosome arrangements in the gametes
• Independent Assortment 


  A process that takes place during meiosis, in which chromosomes are re assorted in the formation of gametes
• Haemophilia 


  Caused by a problem with the blood clotting mechanism. Sufferers are at risk of excessive bleeding, even from very small wounds or bruising. It is a sex linked inherited condition caused by a recessive allele on the X chromosome. Most are males with the genotype XhY
• Cystic Fibrosis 


  Those suffering have frequent and serious lung infections and problems with food digestion. Caused by a recessive allele, so affected individuals must be homozygous recessive.
• Huntingtons Disease 


  Those suffering have progressive brain deterioration, which usually becomes apparent in middle age. It is fatal and there is no cure. Caused by dominant allele
• Down Syndrome 


  caused by extra chromosome on 23rd pair. Occasionally gametes are formed with 24 chromosome, so if one of these gametes is involved in fertilisation with a normal gamete then the child produced will have 47 chromosomes. They will have easily identified facial feature, reduced muscle tone and reduced cognitive development
• Genotype 


  Paired symbols showing the allele arrangement in an individual
• Phenotype 


  outward appearance of an individual
• Dominant 


  In heterozygous condition, the dominant allele overrides the recessive allele
• Recessive 


  recessive allele is dominated by the dominant allele- only shows itself in phenotype if they are 2 recessive alleles
• Genetic Screening 


  Can be used to identify the presence of genetic conditions e.g can be used to test for downs syndrome and cystic fibrosis in the foetus
• Testing for Down’s syndrome

  • Amniocentesis
  • Blood test
• Amniocentesis 


  involves taking foetal cells from the amniotic fluid, growing these cells in the lab and checking how many chromosomes each cell has. 1% chance of miscarriage
• Blood Test 


  Pregnant women are offered a blood test after 10-14 week of pregnancy. This will determine if possibility of this is higher than normal. Amniocentesis is then offered to higher risk mothers. Not as accurate but carries no risk to mother or foetus but helps identify those who should take the procedure