Genes & Chromosomes

• Genes:

  = a heritable factor that controls/ influences a specific characteristic (consisting of a length of DNA occupying a particular position on a chromosome (locus))
• Alleles:

  = a specific form of a gene. Differ from each other by ≥ 1 (few) bases. Found at the same locus. New alleles formed by mutations. 
• Genome:

  All the genes of a cell, tissue/ organism. Genome size = ttl no. of DNA base pairs in 1 copy of a haploid genome.
• Mutation:

  = permanent Δ in the base sequence of DNA. Cumulative efx from millions of mutations & natural selection → complex organisms (currently) evolving from simpler ancestors. Not all mutations cause disease but some do. Even tho DNA replication is protected by base-pairing, mistakes can still occur (this risk is ↑ by exposure to mutagens). The mutation may cause a gene to be wrongly expressed → genetic disease. 
• Base-substitution mutation:

  One base is substituted for another → Δ of a single amino acid in a polypeptide. There are 3 types of this:

  • Silent mutation (Δ in base seq has no effect on amino acid produced)
  • Missense mutation (Δ in base seq has caused 1 diff amino acid to be produced)
  • Nonsense mutation (Δ in base seq has caused a stop codon to be produced, so the polypeptide is shortened) [cystic fibrosis]
• Sickle cell anaemia

  Normal = Pro-GA-GA, Mutant = Pro - Val - GA. Glutamic acid in normal one is polar & valine is non-polar. → Sickling of red blood cells which block capillaries & blood flow → Sickled red blood cells cannot carry enough oxygen for the body → fatigue. Homozygous HbS/HbS state causes severe anaemia & death at low oxygen conc. It is only beneficial in preventing malaria. 
• Chromosomes:

  They are structural units made up of DNA & proteins. By coiling the DNA around the proteins many times a large amt of info can be stored inside the nucleus. All members of a species have = no. of chromosomes (each w the same genes in the same gene loci). Chromo no. doesn’t reflect the complexity of an organism. Chromo no. possessed by a species aka N number.
• Prokaryotes have 2 types of DNA: (*Both are circular)

  • Single chromosome (coiled up & conc in the nucleoid region) (cus there’s only chromosome there is only 1 copy of each gene) (A copy of this is made b4 cell division via binary fission)
  • Plasmids 
• Features of plasmids:

  • Naked DNA - not associated with histone proteins
  • Small circular rings of DNA
  • Not responsible 4 normal life processes (which is controlled by chromosome)
  • ≈ contain survival characteristics, e.g. antibiotic resistance
  • Can be passed btw prokaryotes
  • Can be incorporated into the nucleoid chromosome
• Eukaryotes:

  Nucleosomes = DNA wrapped around histones. Eukaryotic chromosomes may be up to 85mm in length. To fit such a length of DNA into a nucleus with a diameter of 10 μm it has to be coiled. Eukaryotes possess multiple chromosomes. All individuals of a species have = chromosomes, with the same gene loci. 
• The chromosome no. is an impt characteristic of the species. Organisms w diff no. of chromosomes are unlikely to be able to interbreed successfully. Chromosomes can fuse/ spit during evo – these are rare events (chromo no. ≈ same for a gazillion yrs). 
• Genetic modification: (aka genetic engineering)

  All living things use same bases & same genetic code. Each codon produces same amino acid in transcription & translation, regardless of the species. Soooo seq of amino acids in a polypeptide remains unchanged. ∴, we can take genes from one species & insert them into the genome of another.
• Bt Corn:

  Genes from Bt have been inserted into maize so GM plants can produce an insecticidal toxin & be resistant to pests.
• Clone:

  It’s a group of genetically identical organisms/ a group of cells derived from a single parent cell. Clone eg: Starfish, if dmg, can regenerate whole body from a single leg; asexual reproduction, such as binary fission in bacteria.  
• Runners are modified laterally growing stems used to reproduce asexually. Each new plantlet can separate to produce a new plant. Clones allow plants to quickly propagate (produce copies) successful plants (abv shows runners). Tubers, the swollen tips of underground stems, are storage organs in plants such as sweet potatoes. During winter the plant dies back, but in spring each tuber starts to grow producing separate plants, all clones of the parent plant.
• Artificial propagation:

  Many common plants root easily from stem cuttings producing full-grown (clone) plants quickly.
• Monozygotic twins:

  Embryos split & then continue to develop separately to form identical twins. This is possible because in embryonic development the cells are still unspecialised and can become any type of cell. 
• Cloning differentiated cells:

  To clone organism w desired traits is problematic as developed organism consists of specialised cells which are multipotent, unipotent/ cannot divide at all. For clone to develop somatic (diploid body) cells of the donor, organism need to be induced to become pluripotent (cells capable of dividing to become any type of cell). 
• Somatic-cell nuclear transfer made easy: 

  1. Remove differentiated diploid nucleus from the individual to be cloned. 
  2. Enucleate a donor egg cell. 
  3. Insert diploid nucleus into the enucleated egg cell. 
  4. Implant into the endometrium of a surrogate mother and gestate. 
  5. Newborn will be genetically identical to the donor nucleus parent. 
• Therapeutic cloning made simple:

  1. Remove differentiated diploid nucleus from the cell to be cloned. 
  2. Enucleate donor egg cell. 
  3. Insert diploid nucleus into the enucleated egg cell. 
  4. Stimulate it to divide and grow in vitro. 
  5. Resulting embryo is a rich source of stem cells which can be harvested/ cultured. 
  6. Outer layer of cells is removed, so only the inner cell mass is used to culture the tissues needed. 
• Uses of therapeutic cloning:

  • Create stem cells for transplants, such as in burns patients or leukaemia. 
  • Replace other dmg tissues such as nerves, pancreas cells etc. 
  • ↓ risk of rejection of cells cus they are genetically identical to the recipient. 
• Chromosomes (within the same species) can vary by:

  • Length/ no. of base pairs in the DNA molecule
  • Pos of the centromere