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• Prokaryotes have a single, circular chromosome and smaller circular plasmids.

  
  

  Eukaryotes all have linear chromosomes, in the nucleus, which are tightly coiled and packaged with associated proteins(histones).

  They also contain circular chromosomes in their mitochondria and chloroplasts. Yeast is a special example of a eukaryote as it also has plasmids
• replication of DNA

  1. Prior to cell division, DNA is replicated by DNA polymerase. DNA polymerase needs primers to start replication. A primer is a short strand of nucleotides which binds to the 3’ end of the template DNA strand allowing polymerase to add DNA nucleotides
• replication of dna steps

  1. DNA is unwound as hydrogen bonds between the bases are broken
  2. DNA Polymerase adds DNA Nucleotides to the 3' end of primer
  3. DNA Polymerase can only add DNA Nucleotides in one direction, resulting in the Leading Strand being replicated continuously and the Lagging Strand being replicated in Fragments.
• PCR AMPLIFIES DNA using complimentary primers for specific target sequences
• PCR steps

  1. DNA is heated to between 92 and 98 to separate the strands
  2. cooled to between 50 and 65 to allow Primers to bind to target sequences.
  3. It is then heated to between 70 and 80 for heat tolerant DNA Polymerase to replicate the region of DNA.
• Practical Applications of PCR

  1. help solve crimes
  2. settle paternity suits
• Gene Expression involves the transcription and translation of DNA sequences. Only a fraction of the genes in a cell are expressed.
• Each triplet of bases on the mRNA molecule is called a CODON and codes for a specific amino acid.
• tRNA folds due to complementary base pairing. Each tRNA molecule carries its specific amino acid to the ribosome.

  A tRNA molecule has an anticodon (an exposed triplet of bases) at one end and an attachment site for a specific amino acid at the other end.
• Cellular differentiation is the process by which a cell expresses certain genes to produce proteins characteristic for that type of cell.

  This allows a cell to carry out specialised functions.
• Meristems are regions of unspecialised cells in plants that can divide and differentiate.
• embryonic stem cells can differentiate into all the cell types as all genes can be switched on so they're pluripotent

  
  

  Tissue stem cells are used for growth, repair of tissues and can't differentiate to all cell types so they are multipotent.

  eg blood stem cells in bone marrow

• ethical issues with using stem cells

  they involve the destruction of an embryo
• stem cells:

  therapeutic uses: bone marrow transplants for burns

  research: study how diseases develop, drug testing
• The genome of an organism is its entire hereditary information encoded in DNA
• A genome is made up of genes and other DNA sequences that do not code for proteins.
• Most of the eukaryotic genome consists of non-coding sequences.
• Mutations are changes in the DNA that can result in no protein or an altered protein being synthesised.
• Single gene mutations involve the alteration of a DNA nucleotide sequence as a result of the substitution, insertion or deletion of nucleotides.
• substitutions:

  • Missense mutations result in one amino acid being changed for another. This may result in a non-functional protein or have little effect on the protein. (mistake)
  • Nonsense mutations result in a premature stop codon being produced which results in a shorter protein
  • Splice-site mutations result in some introns being retained and/or some exons not being included in the mature transcript
• Nucleotide insertions or deletions result in frame-shift mutations which causes all codons and amino acids after mutation to be changed. This has effect on structure of protein
• Chromosome mutation

  • Duplication - section of chromosome is added from its homologous partner.
  • Deletion - section of chromosome is removed.
  • Inversion - section of chromosome is reversed
  • Translocation - section of a chromosome is added to chromosome, not its homologous partner.
• Duplication allows potential beneficial mutations to occur in a duplicated gene whilst the original gene can still be expressed to produce its protein
• Evolution — the changes in organisms over generations as a result of genomic variations
• Natural selection non-random increase in frequency of DNA sequences that increase survival and the non-random reduction in the frequency of deleterious sequences.
• In stabilising selection, an average phenotype is selected for and extremes of the phenotype range are selected against.
• In directional selection, one extreme of the phenotype range is selected for
• In disruptive selection, two or more phenotypes are selected for
• Horizontal gene transfer is where genes are transferred between individuals in the same generation.
• Natural selection is more rapid in prokaryotes as they can exchange genetic material horizontally, resulting in faster evolutionary change than in organisms that only use vertical transfer.

  Vertical gene transfer is where genes are transferred from parent to offspring as a result of sexual or asexual reproduction
• Speciation is the generation of new biological species by evolution as a result of isolation, mutation and selection.
• The importance of isolation barriers in preventing gene flow between subpopulations during speciation.
• Geographical - allopatric speciation

  behavioural/ecological barriers - sympatric speciation
• In genomic sequencing the sequence of nucleotide bases can be determined for individual genes and entire genomes.
• Computer programs can be used to identify base sequences by looking for sequences similar to known genes.
• compare sequence data:

  computer and statistical analyses (bioinformatics) are required.
• Evidence from phylogenetics and molecular clocks to determine the main sequence of events in evolution. The sequence of events can be determined using sequence data and fossil evidence.
• Comparison of genomes reveals that many genes are highly conserved across different organisms.
• Many genomes have been sequenced, particularly of disease-causing organisms, pest species and species that are important model organisms for research
• Phylogenetics is the study of evolutionary history and relationships.