Topic 3 - Genetics

• A gene is a section or length of DNA that codes for a polypeptide

  
• Genes


  Heritable factors that influence specific characteristics (via the polypeptides produced)
• Characteristic


  A feature of an organism like height in pea plants or blood group in humans
• Heritable


  Genes are factors that pass from parent to offspring during reproduction
• Locus


  The specific position of a gene on a chromosome

  
• A gene consists of a length of DNA found in the nucleus that codes for specific polypeptides

  
• Each chromosome in a human cell nucleus contains one very long DNA molecule made up of thousands of specific nucleotide sequences called genes that code for specific polypeptides

  
• Loci


  The locations of genes on a chromosome

  
• Through experiments and genetic mapping techniques, scientists were able to work out the specific physical location of a gene on a chromosome

  
• Each gene occupies a specific locus so that the gene for a particular characteristic is always found at the same position on a particular chromosome

  
• Comparison of the number of genes in different organisms

  • Humans: ~20,000 genes
  • Dogs: 19,000 genes
  • Water flea: >31,000 genes
  • E. coli: 4,300 genes
  • Rice plant: 41,500 genes

  
• Alleles


  Alternative forms of a gene that can exist

  
• New alleles occur through mutations
• Homozygous


  When the two alleles at a locus are the same/identical

  
• Heterozygous


  When the two alleles at a locus are different

  
• Even a very small change in base sequence can bring about a large effect in gene function, with a large knock-on effect on the phenotype

  
• SNPs (Single Nucleotide Polymorphisms)


  The exact positions where bases differ between alleles

  
• Mutation


  A change in the sequence of base pairs in a DNA molecule that may result in a new allele

  
• Most mutations are harmful or neutral (have no effect) but some can be beneficial
• Substitution mutation


  A mutation that occurs when a base in the DNA sequence is randomly swapped for a different base

  
• A small change to a gene can have serious consequences for an organism, as seen in sickle cell anaemia
• The total of all the genetic information in an organism is called the genome of the organism
• Comparison of genome size in different organisms

  • Viruses and bacteria tend to have very small genomes
  • Prokaryotes tend to have smaller genomes than eukaryotes
  • Plant genomes can vary widely

  
• Human Genome Project (HGP)


  An international, collaborative, research programme to sequence the entire human genome
• The HGP discovered new data about non-coding DNA, suggesting that it plays an active role in the cell, and that it isn't just 'junk' DNA

  
• The sequencing of the human genome has shown that all humans share the vast majority (99.9%) of their base sequences, but also that there are many SNPs that contribute to human diversity

  
• Scientists have noticed a correlation between changes in specific genes and the likelihood of developing certain inherited diseases

  
• In scientific research, critical developments often follow improvements in scientific apparatus
• There have also been specific genes linked to the development of Alzheimer's disease
• Ethical, legal and social issues are generated by the genome sequencing project
• Genome sequencing techniques


  • Developments in scientific research follow improvements in technology
  • Gene sequencers, essentially lasers and optical detectors, are used for the sequencing of genes

  
• Distant objects in Space often remain undiscovered until a telescope (or some other piece of equipment) powerful enough to detect them is developed
• The fact that scientific research is often held back by a lack of sufficiently powerful or precise apparatus is a problem that will continue into the future
• In some ways, this is very exciting, as it suggests that our scientific knowledge and understanding of the universe will continue to expand as new scientific techniques and technologies are developed
• Investigations such as the Human Genome Project are dependent on the use of powerful computers and improvements in technology to store and analyse vast quantities of data
• To sequence a genome


  1. The entire genome is broken up into manageable pieces

  2. The fragments are separated so that they can be sequenced individually

  3. Single-stranded copies are made

  4. Nucleotides are each tagged with a differently coloured fluorescent marker, one for each base, adenine, cytosine, guanine and thymine

  5. Samples are separated according to length, by capillary electrophoresis machine

  6. This procedure is very high resolution and distinguishes DNA fragments that differ in size by only a single nucleotide

  7. A laser beam makes the fluorescent markers fluoresce

  8. An optical detector linked to a computer deduces the base sequence from the sequence of colours detected

  
• This process highlights the use of a database to determine differences in the base sequence of a gene in two species

  
• Gene sequencing


  Made possible by the use of optical laser technology, optical detectors, and computers

  
• Prokaryotic chromosomes


  • The DNA in prokaryotic cells is significantly different from the DNA found in eukaryotic cells
  • Prokaryotes do not contain a nucleus therefore the DNA is located in the cytoplasm of the cell
  • Prokaryotic DNA consists of a single, circular chromosome
  • It is sometimes referred to as a nucleoid
  • The DNA within prokaryotic cells is not associated with any proteins
  • Prokaryotic DNA is sometimes referred to as naked
  • Eukaryotic DNA associates with histone proteins

  
• Plasmids


  • Prokaryotes usually have one or more plasmids
  • Most eukaryotes do not contain plasmids
  • Yeast are the only types of eukaryotes that contain plasmids similar to those in prokaryotes
  • Plasmids are very small circular DNA molecules
  • They usually only contain a few genes
  • They are short, typically 100,000 base pairs in length
  • They are more accessible for proteins required for gene expression and therefore contain genes that are required often, quickly and/or in emergencies
  • Plasmids can sometimes be passed 'sideways' from one cell to another, outside of the normal inheritance pattern during cell division
  • They can also be used as a vector during genetic engineering to transfer DNA between species